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Article

Geospatial Model Suggests Sterilizing Free-Roaming Domestic Cats Reduces Potential Risk of Toxoplasma gondii Infection

by
Sue M. Neal
1,2,*,
Peter J. Wolf
3 and
Melanie E. Anderson
2
1
Department of Political Science, Arkansas State University, Jonesboro, AR 72467, USA
2
Center for GIS and Spatial Analysis, West Chester University, West Chester, PA 19383, USA
3
Best Friends Animal Society, 5001 Angel Canyon Road, Kanab, UT 84741, USA
*
Author to whom correspondence should be addressed.
Zoonotic Dis. 2025, 5(3), 24; https://doi.org/10.3390/zoonoticdis5030024
Submission received: 13 May 2025 / Revised: 16 August 2025 / Accepted: 20 August 2025 / Published: 27 August 2025
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Simple Summary

Humanely managing free-roaming domestic cats to reduce environmental contamination of the Toxoplasma gondii parasite would reduce the risk of infection for humans and animals. This is aligns with the One Health approach (i.e., the recognition of the interconnectedness of humans, animals, and environmental health). Combining geographic data obtained from a clinic providing free veterinary services to residents of Jefferson County, KY, USA, with previously published data for free-roaming domestic cats, we used a geospatial analysis to investigate potential environmental contamination with T. gondii. Our results show that trap-neuter-return (TNR) is a valuable harm reduction strategy in mitigating the risks of T. gondii infection. These results should be of interest to policymakers involved with the management of free-roaming domestic animals (e.g., city and county staff overseeing animal shelter operations) and public health administration sectors, with significant areas of overlap in the One Health framework.

Abstract

Although trap-neuter-return (TNR) is a popular method for managing free-roaming domestic cat populations, a common criticism is that sterilization fails to mitigate the public health risks posed by free-roaming cats. One of these risks is the environmental contamination of Toxoplasma gondii, a parasite that can be spread in the feces of actively infected felids (both domestic and wild). In healthy humans, toxoplasmosis tends to be mild or asymptomatic; however, the disease can have severe consequences (e.g., for pregnant women) and even be fatal in immunocompromised individuals. Previous research has examined the extent to which free-roaming domestic cats might contaminate sites frequented by young children (e.g., schools and parks). However, the model used included several assumptions that are not reflective of sterilized cats in an urban setting (e.g., smaller home range). By properly accounting for several key factors (e.g., reproductive status, home range), our modeling revealed considerably lower rates of potential incursions by sterilized free-roaming cats than those reported previously. More importantly, our results show that sterilization contributes to a considerable reduction in the risk of environmental contamination; TNR therefore appears to be a valuable harm reduction strategy in mitigating the risks of T. gondii infection.

Graphical Abstract

1. Introduction

Toxoplasma gondii is a ubiquitous coccidian parasite that can infect many warm-blooded animals, including humans. In humans, T. gondii infection can occur through the consumption of tissue cysts (e.g., in undercooked meat), from sporulated oocysts in the environment (e.g., soil contaminated with infectious cat feces or vegetables grown in such soil and not properly washed before eating), or, more rarely, congenitally [1]. Toxoplasmosis has been described as “an emerging and re-emerging zoonosis of global public health concern” [2] and also an “epidemiological paradox” because it is, arguably, “one of the most prevalent and most widespread parasitic infections, yet one of the most ignored of all human infections” [3]. The global infection rate is estimated to be 25.7% (range: 0.5–87.7%) and 17.5% in the United States and Canada [4]. Periodically, surplus serum from the National Health and Nutrition Examination Survey (NHANES) surveillance program has been tested for T. gondii antibodies to estimate seroprevalence rates in the USA [5]. An analysis of this data revealed a significant decrease in overall seroprevalence rates across three tracking cycles: 15.3% (2009–2010), 12.8% (2011–2012), and 11.9% (2013–2014) (p < 0.001) [6]. Sera has not been evaluated for T. gondii antibodies under the NHANES program since 2014 [7]. Among USA-born persons 12–49 years of age, age-adjusted seroprevalence rates have decreased in recent decades, from 14.1% (1988–1994) to 9.0% (1999–2004) to 6.7% (2009–2010) [5]. The most recent NHANES data for which seroprevalence rates are available (2011–2014) indicate a further decline: 4.5% for USA-born persons 12–49 years of age [8]. It has been suggested that these decreases are likely the result of improved hygiene standards and safer handling of meat products during cooking, as opposed to any connection to the number of cats in the USA [5].
The Centers for Disease Control and Prevention (CDC) considers toxoplasmosis to be “one of the neglected parasitic infections of the United States,” prioritized “for public health action… based on the number of people infected, severity of the illnesses, and ability to prevent and treat” the disease [9]. Nevertheless, toxoplasmosis is not “reportable” (i.e., requiring healthcare professionals to report diagnoses) at the national level; indeed, only eight states require that toxoplasmosis be reported to health departments: Arkansas, Delaware, Hawaii, Kentucky, Minnesota, Nebraska, Pennsylvania, and Wisconsin [10].
In healthy individuals, toxoplasmosis tends to be mild or asymptomatic; however, immunocompromised individuals “can develop potentially fatal encephalopathy” [10]. HIV/AIDS patients, in particular, can be at risk of toxoplasmic encephalitis (TE) due to reactivation of chronic infection [11]. The incidence of TE has declined dramatically since the introduction of advanced antiretroviral therapy; however, it remains a concern where treatment is unavailable or refused [12,13]. In addition, infection during pregnancy can “result in a range of adverse outcomes, including fetal ocular infection, cranial and neurologic deformities, stillbirth, and miscarriage” [10]. Adverse outcomes are significantly more common in countries that lack routine prenatal screening, pointing to an important but underutilized mitigation mechanism [14].

1.1. Environmental Contamination

Soil can become contaminated when felids shed infectious oocysts in their feces, and these oocysts can go on to contaminate drinking water [15]. Soil experiments have shown that sporulated T. gondii oocysts can remain infective for up to 18 months in temperatures ranging from −20 to 35 °C [16,17]. However, recent research has shown that oocysts were “completely destroyed” in 24 h or less under hotter, drier conditions [18]. Oocysts can be transported mechanically by cockroaches [19] and flies [20], possibly by earthworms [21], and perhaps in the fur of dogs [22,23]. In addition, T. gondii oocysts have been found in the feces of dogs infected both experimentally [24] and naturally [25], suggesting “that coprophagia with a subsequent intestinal passage by dogs plays a role” in environmental contamination [25].
The difficulties associated with detecting T. gondii oocysts in soil are well documented [26]. Microscopy, for example, is “poorly sensitive if sample purification is insufficient and weakly specific due to risk of confusion with morphologically related coccidia, Hammondia sp. and Neospora sp.” [17]. Real-time polymerase chain reaction (PCR, also known as quantitative PCR, or qPCR) can be useful for identifying T. gondii DNA in soil; however, a positive result can be caused by a cat’s consumption of infected prey rather than the shedding of oocysts [27,28].
Previous research has identified areas such as parks, playgrounds, schools, and community gardens as particularly important sites for potential exposure to T. gondii, especially for children [26,29,30]. This is because of the natural behavior of cats to deposit feces in loose soil (e.g., sandboxes), and because children, who may be less immunologically mature and prone to poor hygiene habits, often frequent these areas.

1.2. The Role of Domestic Cats

Much of the literature concerning environmental contamination of T. gondii focuses on domestic and, to a lesser extent, wild felids, as they are the only animals known to be definitive hosts; sexual reproduction of the parasite occurs within the intestines of cats [31]. It must be noted, however, that transmission routes can be more complex. Studies have shown that the parasite can be transmitted congenitally in wild mice [32] and sheep [33], for example, and that T. gondii infections have been detected in parts of the world where there are no domestic cats—the likely explanation being that the parasite was brought there by birds [34,35].
Laboratory studies suggest that the typical duration of oocyst shedding is “less than one week” [36], an estimate supported by surveys of fecal samples [1]. Fecal surveys suggest that 0.3–3.2% of domestic cats who rely at least in part on human provisions are shedding T. gondii oocysts at any given time [37,38]. However, little is known about the frequency of, or key factors involved with, re-shedding under natural conditions [36].
Following infection, cats seroconvert with antibodies apparent in testing 1–4 weeks post-exposure [39]. Re-shedding has been observed among cats infected in laboratory settings but “there are no data on naturally infected cats” [36]. In addition, re-shedding even under laboratory conditions is variable and has been found to be dependent upon the genotype strains of T. gondii exposure, with exposure to heterologous strains more likely to cause re-shedding events [40].
Sera tests are often used to evaluate infection rates among populations of domestic cats. Results generally reveal higher seroprevalences among older cats [36,41,42,43] and those living without anthropogenic food sources [38,42,43,44]. In a study of four sites along the central California coast, for example, the odds of detecting T. gondii DNA were nearly five times greater at sites where kittens were seen compared to sites without kittens [44]. In another study conducted along the California coast, researchers reported a seroprevalence rate of 16.8% among cats living close to humans and 81.3% among cats found in “undeveloped landscapes” [38].

1.3. Mitigation Measures

Concerns over transmission risk are sometimes accompanied by calls for more effective management of free-roaming cats. Trap-neuter-return (TNR) is sometimes dismissed, either implicitly [15,45,46] or, more explicitly, as “ineffective” [26] or “a poor choice” [47]. One long-time opponent of TNR has described T. gondii as “an environmental contaminant on the order of, if not worse than, DDT” [48]. A bill proposed during Hawaii’s 2022 legislative session cited free-roaming cats’ role in “the spread of diseases such as Toxoplasma gondii” as one reason to eradicate cats from Kauai, Maui, and Hawaii, and reduce their numbers by 50% on Oahu, by December 31, 2025 [49]. As the empirical evidence indicates, however, complete removal has rarely proven successful and incomplete removal efforts are prone to backfire [50]. There is little reason to think, therefore, that such efforts are likely to reduce the environmental contamination of T. gondii or any associated transmission risk.

1.4. TNR as Harm Reduction

The concept of harm reduction is common in public health, often as it relates to drug use. The concept was developed in recognition of the fact that the ideal state is often at odds with the practical, and that greater benefits can be achieved in recognizing the importance of reducing immediate, specific harms [51]. As applied to drug use, total harm is expressed as the “product of total use and the average harm per unit of use” [52]. Harm reduction has been recognized as being a both humane and pragmatic approach to public health issues [53].
If we accept that the complete elimination of T. gondii oocysts shed by free-roaming cats is not a realistic objective, then it becomes important to evaluate the harm reduction benefits of various management techniques. To eliminate environmental contamination of T. gondii associated with domestic cats, all cats born indoors would need to be kept indoors and those born outdoors would need to be removed (and likely euthanized). Neither scenario is realistic. Roughly one-third of pet cats spend at least some time outdoors [54]. In addition, given the challenges associated with eradicating cats even on small, uninhabited islands [50], it is unlikely that culling (or TNR) could eliminate all outdoor cats in any open system (e.g., nearly all USA cities and counties). Moreover, wild felids are potential sources of environmental contamination and have been linked to human outbreaks of toxoplasmosis [55,56].
Some have argued that there is a lack of research showing TNR to be a “positive form of managing disease, particularly within a One Health context” [57]. Indeed, absent from the literature is any evaluation of the potential mitigating impact TNR might have on the risk profile of specific public health concerns, apart from the complete elimination of risk through total elimination of the outdoor cat population—again, an unlikely outcome of any population management approach.
TNR likely mitigates the transmission risk of T. gondii due to reduced prey consumption (e.g., when regular feeding is included as part of ongoing management [58,59]), improved immunologic status [60,61], altered age profile of cat colonies (i.e., fewer kittens and young cats [62]), and reduced roaming [63,64]. This last point is important because roaming can lead to cats contaminating public areas, thereby increasing the likelihood of exposure. Thus, understanding how TNR might mitigate the likelihood that a cat would be found in an area where transmission to vulnerable populations is more likely is an important gap in the literature. The present analysis is intended to be an important first step in filling this gap.
This research evaluates the impact of TNR on the likelihood that free-roaming “community” cats would intrude into areas where transmission may be more likely to occur, comparing those results with those of unmanaged (i.e., intact) cats. We hypothesize that there is substantial reduction in this activity based on the impacts of TNR on roaming behavior. Taetzsch et al. [29] completed an analysis of the overall risk of cats coming into contact with specific points of interest in the metropolitan Richmond, VA, USA, area. Our work builds on this analysis using a substantially larger dataset and by considering an adjusted home range post-TNR. In doing so, we are evaluating TNR through a harm reduction lens—investigating the quantifiable impacts that TNR can provide in reducing the environmental contamination of T. gondii, thereby reducing potential exposure among at-risk populations (e.g., children).

2. Materials and Methods

Data cleaning, manipulation, and analysis was performed in Python (v. 3.13) and spatial analysis was conducted in the ArcGIS (https://www.arcgis.com/index.html, accessed on: 13 May 2025) suite. Map compositions were created using ArcGIS Pro (v. 3.4).

2.1. Study Area

Jefferson County is located along the western edge of Kentucky, in the eastern south-central region of the United States (Figure 1). With an estimated population of 773,399 across 986 km2 [65], it is the most densely populated county within the state and is representative of moderately sized metropolitan communities throughout the country. Human population density within the county is highest in the urban center of Louisville, with lower densities in the outlying suburban regions. Jefferson County is an appropriate site for the present study in part because of its widespread use of TNR as the official approach for managing free-roaming cats, as stipulated by ordinance. This ordinance, enacted in 2012, covers the entirety of Jefferson County, which has a unified government encompassing the metropolitan area of Louisville as well as its outlying suburbs [66]. Under agreement with the county, the non-profit organization Alley Cat Advocates (ACA) sterilizes, vaccinates, and ear-tips cats admitted to Louisville Metro Animal Services (LMAS) as “strays” lacking any signs of ownership (e.g., collar and tags). ACA also provides sterilization, vaccination, and ear-tipping for community cats brought by residents to their veterinary clinic. With few exceptions (e.g., the demolition and redevelopment of the site where a cat was originally captured), these cats are returned to where they were trapped. This same community has been the site of two previous studies of community cats, one of which focused on caregivers’ attachment levels [67] while the other focused on increased lifesaving at LMAS following the implementation of intensive TNR efforts [68].

2.2. Home Range

The home range size of free-roaming cats is influenced by a variety of factors, including sex, reproductive status, and resource availability. In addition, home range is negatively correlated with cat density, a reflection of both reproductive status and resource availability [69]. As noted previously, the present study draws on previous work estimating the environmental contamination of T. gondii associated with community cats in the Richmond, Virginia, area. Taetzsch et al. (2018) used the maximum home range of unowned male cats reported by Horn et al. [70] (i.e., 157.01 ha) to identify the proportion of community cats likely to travel into areas of potentially high exposure risk in the community (e.g., public parks and schools, playgrounds).
The difficulty in using ownership status as an explanatory factor for home range variance is that ownership status is a legal classification rather than a biological or ecological classification. For the purposes of this study, we will instead rely on the key factors mentioned previously (e.g., sex, reproductive status, and resource availability), ascribing home range values reported for unowned cats to intact cats with relatively little reliance on humans. Home range values reported for owned cats, on the other hand, will be ascribed to sterilized cats who rely heavily on humans for food (Table 1). These home range values are more representative of cats who have been sterilized as part of a TNR program and are being cared for by humans, as reflected by the fact that they were trapped and brought to ACA for sterilization and vaccination.
The surgical sterilization performed in TNR programs results in a reduction in sex hormones and has been found to reduce both overall activity levels and roaming behavior [63,64]. Ferreira et al. [64] monitored cats pre- and post-sterilization over an extended period of time (i.e., 6–12 months pre- and 4–12 months post-sterilization), documenting reductions of nearly 80% in home range size and nearly 30% in overall activity patterns post-sterilization. These researchers also provide a thoughtful discussion about why the impact of sterilization might have been masked or overlooked in earlier studies (e.g., the influence of resource availability). In addition, previous research has found that semi-feral (“unowned cats directly fed by a resident”) and owned cats exhibited significantly smaller home ranges than those of feral cats (i.e., “unowned cats not directly fed”) [71].
Following Taetzsch et al. [29], we chose to estimate the home range of Jefferson County’s free-roaming cats based on a study by Horn et al. [70] in southeastern Champaign-Urbana, IL, USA. This site is roughly comparable in its geography and built environment to Jefferson County, and the results include values for both owned and unowned cats. The spatial coverage of the home ranges was estimated based on the minimum convex polygon (MCP) mean and, for purposes of mapping, the MCP area was assumed to be a circle from which a radius could be calculated. The first phase of our modeling used the home range for unowned males (equivalent to a radius of 707 m), while the second phase used the home range for owned males (equivalent to a radius of 76 m). The unowned males served as proxies for intact cats while the owned males served as proxies for TNR cats. We chose to use the male home range to compare our results to previous research [29] that used the mean home range of unowned male cats reported by Horn et al. [70].

2.3. Points of Interest

Previous research has identified certain areas as particularly important sites for potential exposure to T. gondii (especially for children) resulting from environmental contamination associated with free-roaming cats. Public parks, playgrounds, schools, and community gardens are those occurring most frequently in the literature [26,29,30]. Again, following Taetzsch et al. [29], our study uses the location of each point of interest (POI) within the study area. For preschools and elementary schools, addresses were obtained from the Jefferson County Public Schools website [72]. Locations of public parks were obtained from a GIS Hub maintained by the Louisville/Jefferson County Information Consortium of Louisville Kentucky Metroparks [73], with one cemetery and 10 golf courses removed from the list as these did not meet the type of recreational land use generally associated with children’s infection risk. Community garden locations were obtained from the Jefferson County Extension Service [74].

2.4. Cat Data

This research uses data obtained from ACA, which provided location data for all cats trapped as part of the area’s TNR program during the study period (January 2020 through April 2023). We chose the study period through consultation with ACA leadership, as this represents the period for which they had the highest level of confidence in the accuracy and completeness of their records. The data includes the approximate age of each cat at time of surgery, sex, and trapping location (i.e., exact street address). Kittens were classified as any cat five months of age or younger, sub-adult cats were defined as any cat 6–11 months of age, and adults include all cats one year of age or older. Cats trapped at the same address were aggregated into colonies for efficiency in geocoding and mapping; however, the number of cats in each colony was used throughout the analysis.

2.5. Spatial Analysis

The location of each cat and each point of interest was geocoded and mapped in ArcGIS Online (AGOL, https://www.arcgis.com/index.html, accessed on: 13 May 2025). We then used the radius equivalent of home range (described above) and the Buffer function in AGOL to delineate the estimated home range of each cat. In addition, buffers were delineated around each POI. Once the buffers were created, the Aggregate function was used to return the number of unique POIs found within each cat’s estimated home range. The Summarize Within function was then used to sum the number of cats within the buffer of each POI, yielding the number of cats that have an estimated home range intersecting a POI. These results were used to calculate subtotals for each POI category as well as overall totals.
Buffers were treated under two scenarios. In the first scenario, buffers were treated as overlapping. This results in multiple buffers having some shared area in common. In the second scenario, buffers were “dissolved” so that areas where one or more buffers intersect are combined into a single buffer (Figure 2).

3. Results

Geocoding resulted in 3624 unique colony locations, representing a total of 10,750 cats, each of which was included in our analysis after successful geocoding. Of these cats, 5256 (48.9%) were male and 5494 (51.1%) were female (Table 2).
Figure 3 shows the locations of Jefferson County’s POIs as well as the number of cats residing in each census tract. Table 3 summarizes the number of POIs in each category (including a comparison to previous research), Table 4 summarizes the incursion counts for each POI category, and Table 5 provides descriptive statistics for incursions into each POI.
Table 6 shows the number of cats with potential incursions onto each POI type as well as incursions into any POI. This analysis uses dissolved buffers; thus, cats with incursions into multiple points of interest of a single type (e.g., cats who travel into more than one park) are counted only once. Similarly, cats with incursions into multiple POIs of different types (e.g., an elementary school and a public park) are also counted only once.

4. Discussion

This study revealed somewhat lower potential incursion rates by “unowned” cats (i.e., intact and with relatively few resources) than were reported in the previous study upon which our analysis was based [29]. In addition, a number of key factors suggest that incursion rates—and the associated risks for T. gondii infection—are, in fact, considerably lower. To our knowledge, this is the first study to model the risk reductions associated with TNR.

4.1. TNR Reduced Incursion Rates

We found that 53.6% of “unowned” cats had estimated home ranges that included at least one incursion site (Table 6), compared to the 81.5% found previously [29]. Direct comparisons are difficult, however, because Taetzsch et al. [29] reported an incursion rate only for 27 cats trapped and returned within Richmond (81.5%), not for the other 248 cats trapped elsewhere in the county. For the “unowned” cats in our analysis, the most common potential incursion sites were public parks (34%), which is consistent with previous research [29] in relative ranking if not in magnitude (70%). Other incursion sites included elementary schools (31%), community gardens (4%), and preschools (1%).
More important than any comparisons to the previous research conducted in Richmond, though, are the comparisons between intact and TNR cats. Using home range values for “owned” cats (i.e., sterilized and with relatively plentiful resources), we found considerably lower potential incursion rates for cats who are part of a TNR program. Indeed, fewer than 1% of these cats had estimated home ranges that included at least one POI (Table 6), 98.3% fewer than the potential incursions associated with “unowned” cats. This finding alone highlights TNR’s potential value as a harm reduction strategy (discussed in greater detail below).
As indicated in Figure 3, Jefferson County’s cats tend to be more concentrated in areas of higher human density, consistent with previous research [75,76]. Higher cat densities are in turn associated with smaller home ranges, largely due to greater resource availability [71,75,77]. Because the estimated home ranges we used were derived from areas likely to have lower human density and resource availability [70] than those in our Jefferson County dataset, our resulting rates of incursion are likely inflated. This is especially true for “owned” cats.
Previous research documenting the home range of free-roaming cats has revealed strong, generalizable relationships among some factors (e.g., cat density, resource availability) but also the complex, context-dependent relationships among others (e.g., kinship, environmental barriers). Summarizing the results of earlier research, for example, Liberg [75] illustrated the strong inverse relationship between cat density and home range, observing that “food abundance and distribution” is the primary factor for determining females’ home range whereas “female density and, even more so… female distribution” determine males’ home range [75]. Unfortunately, these observations reveal little about the home range of sterilized free-roaming cats.
Investigations into the home range of sterilized free-roaming cats have produced mixed or even contradictory results. As noted previously, Ferreira et al. [64] documented reductions of nearly 80% in home range size and nearly 30% in overall activity patterns following the sterilization of cats from two residential areas of a Brazilian island. Although the sample size was small (N = 8) and limited to male cats, the duration (12 months before and 12 months after castration), intensity of monitoring (i.e., 1620 points corresponding to the coordinates of 6 cats before and 1389 points following castration), and considerable difference in pre- and post-home ranges (mean MCP 68.91 ha vs. 14.25 ha) provide compelling evidence that sterilization alone (at least in male cats) reduces home range.
Others, however, have reported no meaningful differences in the home range of sterilized and reproductively intact cats [e.g., [70,78,79]]. As Ferreira et al. [64] have pointed out, though, these studies failed to account for the influence of food availability as a confounding factor. The sterilized share of cats monitored by Horn et al. [70], for example, was comprised mostly of owned cats (11 of 13), whereas the intact share was comprised entirely of unowned cats (12 of 14). Such sampling makes it virtually impossible to assess the impact of sterilization alone. Resource availability was also a key factor in a study of free-roaming cats in a suburban Texas community, where Schmidt et al. [71] documented mean home ranges of 3.3 ha (100% MCP) among “semi-feral” cats (i.e., intact, unowned cats fed by residents), less than a quarter of the mean home range of unowned cats not being fed (14.7 ha). It is perhaps worth noting that the use of MCP for estimating home range is, although common, prone to inflating an animal’s true home range: “Large areas that are rarely used by the animal may be included in the calculated home range” [80]. These findings provide further evidence that the rates of incursion we have reported are likely conservative (i.e., inflated).
Finally, less direct evidence for the ability of sterilization to be an important factor in reducing free-roaming cats’ home range comes studies documenting the number of deceased cats in various communities. In San José, California, a 20% reduction in “dead cat pick-up off the streets” was observed four years after the implementation of an RTF program [81]. Comparable results were observed in a study of six three-year, shelter-based TNR/RTF [82]. These results are not entirely surprising in light of the significant number of male cats sterilized, which are then less likely to roam [83].
These various factors contributing to smaller home ranges (e.g., sterilization, food availability) are typically part of TNR programs and related caregiving [84,85,86]. Indeed, caregivers often provide food daily and interact with the cats in their care in ways similar to the way cats owners interact with their pets [67], suggesting that the home range of these cats is probably not so different from that of pet cats with outdoor access.

4.2. The Impacts of Infection Risk and Shedding Prevalence

Studies reporting seroprevalence rates in free-roaming domestic cats reveal a negative association between T. gondii infection rates and resource availability [38,42,43]. A study conducted along the California coast, for example, found that 121 of 720 cats (16.8%) “collected from small to large colonies in close proximity to people, where they had access to provided food sources (e.g., commercial cat food or discarded human foods)” were seropositive, compared to 13 of 16 cats (81.3%) “removed from critical shorebird habitat by specialists through intensive trapping. These solitary, feral domestic cats living in undeveloped landscapes likely subsisted primarily on wild prey and had minimal association with humans” [38].
Seroprevalence rates reflect past infections rather than active oocyst shedding. Following initial infection, cats generally develop antibodies [40,87], and it is generally assumed that cats who have developed antibodies have already shed oocysts [1]. The rate and duration of oocyst shedding are important factors in understanding the public health impacts of environmental contamination with T. gondii. As with home range estimates, proximity to humans (and their resources) plays a key role in exposure to the parasite. The previously mentioned study of cats from coastal California found that 0.5% of cats living in close proximity to people were actively shedding oocysts, as opposed to a 5.9% rate among “solitary, feral domestic cats” [38]. A more recent review estimated the rate of shedding for cats spending at least some time indoors and who are “at least partially dependent on human caretakers” (29 studies) to be 0.3%, compared to 3.2% for cats living “exclusively outdoors” (34 studies) [37]. Unfortunately, this analysis was complicated by the inclusion of cats “fed by caretakers” and “independent cats eating hunted prey” in the “unowned” category. Moreover, the inclusion of some studies that relied only on PCR testing without also using microscopy to confirm the presence of oocysts [88,89] inflated the estimated shedding prevalence [37].
As noted previously, laboratory studies suggest that the typical duration of oocyst shedding is “less than one week” [36], an estimate supported by surveys of fecal samples [1]. Re-shedding has been observed among cats re-infected in laboratory settings [40,87]; however, the extent to which this might occur in natural settings is unclear [1,37].
Accounting for the lower risk of T. gondii infection and lower rate of shedding prevalence in populated areas [38,42,43,90] considerably reduces the potential threat of environmental contamination at incursion sites (Figure 4). This is in addition to the smaller home ranges associated with areas of higher population densities and relatively plentiful resources [71,75,77]. These key factors were not considered in earlier modeling exercises [29].

4.3. Additional Factors Leading to Reduced Infection Risk

Cats become infected with the T. gondii parasite when they consume infected prey or are fed under-cooked meat [91,92]. Conditions leading to a reduction in prey consumption therefore reduce the risk of initial infection and re-infection. Unsurprisingly, cats who are being offered commercial cat food have been found to consume significantly less prey than cats who are not provided for [58], with commercial food comprising a very large proportion (i.e., >80%) of their diets [59]. Field research confirms the connection between cats who are being cared for and lower levels of T. gondii infection [37,38,93]. One study in Brazil, for example, found no evidence of antibodies among cats mainly fed commercial or home-prepared diets (N = 32) regardless of their access to the outdoors, and attributed this to their consumption of foods other than prey animals [93].
A previous survey of cat caregivers in Jefferson County found that most caregivers (62.1%) fed the cats twice daily, with another 23.9% providing food once each day [67]. Nationally, an estimated 30% of USA residents feed free-roaming cats that they do not own [86]. Although such activities are often seen merely as acts of kindness, the available evidence suggests that feeding community cats improves public health by reducing environmental contamination and potential exposure to T. gondii.
Age is an important factor in toxoplasmosis epidemiology; older cats are more likely to have developed T. gondii antibodies than are kittens [36,41,42,43]. As one team of researchers noted, “a population of actively reproducing cats, with many young, nonimmune kittens will produce more oocysts than will a more stable population of older cats” [22]. One early study found greater infection rates and titer levels among kittens under two months of age compared with older cats [94]. The age stratification associated with oocyst shedding follows a similar trend, leading some researchers to conclude that cats over the age of 12 months may not be reliable subjects for investigations into oocyst shedding [94]. It has also been suggested that “the presence of kittens may encourage females to hunt intensively and share prey, in which case a single prey item may infect several cats” [95].
TNR programs influence community cat age demographics in two important ways: first, by reducing the number of kittens born and, secondly, by removing kittens for adoption. One 28-year study found, for example, that no new kittens were born into a campus cat population after two years of targeted sterilization efforts [62]. The removal of kittens (as resources allow) has been an integral part of TNR from its earliest days [85], and remains common practice today—again, as resources allow [96,97,98]. Another long-term study (i.e., 22 years) found that roughly half of the cats included in a TNR program (50.1%) were adopted [99]. Research modeling seroprevalence conversion of farm cats (N = 130) across seasons and age groups has suggested that reducing the number of kittens born, via sterilization, may be an effective strategy to reduce the environmental contamination of T. gondii [60]. Indeed, a study of pig farms in the Netherlands documented a “significant reduction in seroprevalence” within a year on three farms where cats were sterilized and kittens removed [61]. More than 70% of the cats in our study population were adults, perhaps reflecting the years of large-scale TNR efforts in this community. Although the cats included in TNR programs do not necessarily represent a random sample of the larger population and the true age stratification of community cats in Jefferson County is unknown, the reduction in kitten births resulting from intensive, prolonged sterilization efforts have almost certainly reduced the risk of T. gondii infection via environmental contamination.

4.4. TNR as Harm Reduction Strategy

The application of harm reduction principles can be observed in the One Health approach [100] and case studies have demonstrated the practical application of harm reduction strategies in various conservation contexts [101,102]. For instance, Stephen et al. [103] utilized a harm reduction approach in studying fish and wildlife health, illustrating how these principles can be adapted to address ecological concerns while simultaneously protecting public health. This adaptability underscores the potential for harm reduction strategies to inform environmental policy and practice, particularly in areas where human activities threaten ecological integrity.
One of the core tenets of harm reduction is the recognition that the complete elimination of risk is often impractical; therefore, strategies should focus on minimizing harm rather than pursuing an unattainable ideal. Others have pointed out the impracticality of removing all free-roaming cats “by any means necessary” [48] and of lethal removal efforts more generally [104]. Indeed, previous research has demonstrated the challenges associated with lethal removal efforts, as populations quickly rebound when new cats move in [105,106]. This instability almost certainly results in an increased risk of T. gondii exposure. TNR, by contrast, can reduce such risk, making it an attractive harm reduction strategy.
Another practical consideration is the T. gondii seroprevalence in prey species. Researchers modeling consumption of infected rodents found that even at low seroprevalence (e.g., 1%) and consumption (e.g., 0.2 rodents/day) levels, free-roaming cats face a high risk (e.g., ~50%) of exposure annually [107]. Again, this illustrates the futility of attempts to completely eliminate the risks of environmental contamination and the benefits of a more practical harm reduction strategy.
And finally, it has been suggested that TNR is itself a public health threat [46,47,98], leading some to call for an “end [to] TNR as a purported population management tool” [108]. These characterizations fail to account for the important aspects of TNR that make it an effective harm reduction strategy, as demonstrated by our results, or for its popularity [109] and practicality [50]. Calls to “ban (with enforcement) outdoor cat feeding sites” [108] are similarly misguided, given the reduced risk of environmental contamination resulting from the common activity of feeding cats [86]. Indeed, our analysis makes clear that the elimination of TNR would likely result in a greater risk of T. gondii infection due to increased environmental contamination.

4.5. Limitations

This study is not without its limitations. Although the dataset used is relatively large (N = 10,750), the extent to which key factors (e.g., age, location) represent the population of free-roaming cats in Jefferson County is unclear. In addition, our results might not be applicable in areas that are considerably different from Jefferson County (e.g., geographically). Points of interest were conceptualized as points although they are more accurately described as polygons; we used point locations to keep our methods consistent with the previous research [29]. And finally, the impact of TNR was assessed somewhat indirectly (i.e., using home range estimates) rather than directly (e.g., via seroprevalence comparisons or soil sampling).

5. Conclusions

In addition to its impact on reproductive capacity, TNR can reduce the home range and age profile of free-roaming cats. Taking these factors into account, our geospatial analysis suggests that TNR is a valuable harm reduction strategy in mitigating the risks of T. gondii infection.

Author Contributions

Conceptualization, S.M.N. and P.J.W.; methodology, S.M.N.; formal analysis, S.M.N.; investigation, S.M.N.; data curation, S.M.N. and M.E.A.; writing—original draft preparation, S.M.N. and P.J.W.; writing—review and editing, S.M.N. and P.J.W.; visualization, S.M.N. and M.E.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets presented in this article are not readily available because under an ethical obligation to protect the privacy of individual caregivers and the cats in their care. Requests to access the datasets should be directed to the corresponding author.

Acknowledgments

The authors would like to acknowledge the invaluable assistance from Alley Cat Advocates’ Karen Little, in the project design and initial data collection.

Conflicts of Interest

Author P.J.W. was employed by Best Friends Animal Society. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ACAAlley Cat Advocates
CDCCenters for Disease Control and Prevention
LMASLouisville Metro Animal Services
MCPMinimum convex polygon
NHANESNational Health and Nutrition Examination Survey
POIPoint of interest
TEToxoplasmic encephalitis
TNRTrap-neuter-return

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Figure 1. Study area: Jefferson County, Kentucky, in the United States (986 km2, estimated population 773,399) and its urban center, Louisville.
Figure 1. Study area: Jefferson County, Kentucky, in the United States (986 km2, estimated population 773,399) and its urban center, Louisville.
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Figure 2. Illustration of overlapping (A) versus “dissolved” (B) buffers using an example from the cat home ranges in the study area. Magenta dots represent cat locations. Note that both examples display the same cat locations and home range values; the only difference is the function applied. The different functions result in different incursion rates; in this case, treating buffers as overlapping (A) yields six inclusions while dissolving buffers (B) yields three.
Figure 2. Illustration of overlapping (A) versus “dissolved” (B) buffers using an example from the cat home ranges in the study area. Magenta dots represent cat locations. Note that both examples display the same cat locations and home range values; the only difference is the function applied. The different functions result in different incursion rates; in this case, treating buffers as overlapping (A) yields six inclusions while dissolving buffers (B) yields three.
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Figure 3. Estimated cat densities and POI locations throughout the study area. Densities are represented as quintiles.
Figure 3. Estimated cat densities and POI locations throughout the study area. Densities are represented as quintiles.
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Figure 4. Community cat locations in Jefferson County, KY, USA using data collected from January 2020 through April 2023. Gray dots indicate locations of cats (or colonies); magenta dots indicate cats/colonies (A) included in the full dataset used in the present analysis; (B) with at least one POI within their pre-TNR home range; (C) with at least one POI within their post-TNR home range (to reflect sterilization and resource availability); and (D) when only those likely to be actively shedding oocysts are accounted for. Note that the results shown in (D) are based on a shedding prevalence of 2% and represent hypothetical locations only due to the temporal nature of shedding (based on the literature described above).
Figure 4. Community cat locations in Jefferson County, KY, USA using data collected from January 2020 through April 2023. Gray dots indicate locations of cats (or colonies); magenta dots indicate cats/colonies (A) included in the full dataset used in the present analysis; (B) with at least one POI within their pre-TNR home range; (C) with at least one POI within their post-TNR home range (to reflect sterilization and resource availability); and (D) when only those likely to be actively shedding oocysts are accounted for. Note that the results shown in (D) are based on a shedding prevalence of 2% and represent hypothetical locations only due to the temporal nature of shedding (based on the literature described above).
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Table 1. Estimated home range of free-roaming cats, as reported by Horn et al. [70].
Table 1. Estimated home range of free-roaming cats, as reported by Horn et al. [70].
Ownership StatusSexNo. of Cats TrackedEstimated Home Range (95% MCP, ha)
IntactSterilizedTotalMeanMedian
OwnedM0331.830.68
OwnedF0881.920.42
UnownedM516157.0148.30
UnownedF911056.5934.31
MCP = minimum convex polygon.
Table 2. Number of cats by age and sex.
Table 2. Number of cats by age and sex.
Age Class *SexTotals (%)
Male (%)Female (%)
Kittens1414
(26.9)		1550
(28.2)		2964
(27.6)
Sub-adults25
(0.5)		5
(0.1)		30
(0.3)
Adults3817
(72.6)		3939
(71.7)		7756
(72.2)
Totals5256
(100)		5494
(100)		10,750
(100)
* Kittens: 0–5 months; sub-adults: 6–11 months; adults: ≥1 year.
Table 3. POIs in Jefferson County, Kentucky.
Table 3. POIs in Jefferson County, Kentucky.
Points of InterestJefferson County, Kentucky (%)“Central Virginia” *
Elementary schools108 (40.0)32 (19.5)
Preschools7 (2.6)
Public parks143 (53.0)125 (76.2)
Community gardens12 (4.4)7 (4.3)
Total270 (100)164 (100)
* From Taetzsch, Bertke, and Gruszynski [29] (who combined elementary and preschools).
Table 4. Incursions into each Jefferson County POI category.
Table 4. Incursions into each Jefferson County POI category.
POICount (%)
Elementary schools108 (40.0)
Preschools7 (2.6)
Public parks143 (53.0)
Community gardens12 (4.4)
Total270 (100)
Table 5. Statistics describing the potential incursions for each POI category using the overlap function.
Table 5. Statistics describing the potential incursions for each POI category using the overlap function.
Free-Roaming Cat ClassificationModel ResultsPOI Category
Elementary SchoolsPreschoolsCommunity GardensPublic Parks
IntactSites with potential incursions (%)10010091.787.7
No. of cats35981934405979
Mean no. of cats per site (range)33.3
(1–280)		27.6
(8–46)		36.7
(0–122)		41.8
(0–335)
Median no. of cats per site21293021
SterilizedSites with potential incursions (%)0.030.0033.312.6
No. of cats701278
Mean no. of cats per site (range)0.06
(0–2)		0
(0–0)		1
(0–0)		0.54
(0–0)
Median no. of cats per site0000
Table 6. Statistics describing the potential incursions for each POI category using dissolved buffers. Raw counts are shown with percentages in parentheses.
Table 6. Statistics describing the potential incursions for each POI category using dissolved buffers. Raw counts are shown with percentages in parentheses.
Free-Roaming Cat ClassificationModel ResultsPoints of Interest
Elementary SchoolsPreschoolsCommunity GardensPublic ParksAny Point
of Interest
IntactNo. of cats (%)3351
(31.16)		165
(1.53)		440
(4.09)		3688
(34.30)		5766
(53.62)
SterilizedNo. of cats (%)7
(0.06)		0
(0)		12
(0.11)		78
(0.72)		97
(0.90)
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Neal, S.M.; Wolf, P.J.; Anderson, M.E. Geospatial Model Suggests Sterilizing Free-Roaming Domestic Cats Reduces Potential Risk of Toxoplasma gondii Infection. Zoonotic Dis. 2025, 5, 24. https://doi.org/10.3390/zoonoticdis5030024

AMA Style

Neal SM, Wolf PJ, Anderson ME. Geospatial Model Suggests Sterilizing Free-Roaming Domestic Cats Reduces Potential Risk of Toxoplasma gondii Infection. Zoonotic Diseases. 2025; 5(3):24. https://doi.org/10.3390/zoonoticdis5030024

Chicago/Turabian Style

Neal, Sue M., Peter J. Wolf, and Melanie E. Anderson. 2025. "Geospatial Model Suggests Sterilizing Free-Roaming Domestic Cats Reduces Potential Risk of Toxoplasma gondii Infection" Zoonotic Diseases 5, no. 3: 24. https://doi.org/10.3390/zoonoticdis5030024

APA Style

Neal, S. M., Wolf, P. J., & Anderson, M. E. (2025). Geospatial Model Suggests Sterilizing Free-Roaming Domestic Cats Reduces Potential Risk of Toxoplasma gondii Infection. Zoonotic Diseases, 5(3), 24. https://doi.org/10.3390/zoonoticdis5030024

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