High-Mortality Outbreak of Staphylococcus aureus Mastitis Associated with Poor Milking Practices in a Goat Dairy

Simple Summary

Mastitis is a painful and economically significant udder infection that reduces milk production and can result in death in severe cases. During a kidding season, a commercial dairy goat herd experienced a severe outbreak of mastitis caused by Staphylococcus aureus, a contagious bacterium transmitted primarily during milking. Approximately one-third of the herd developed gangrenous mastitis, resulting in mortality. A substantial proportion of apparently healthy goats had S. aureus recovered from their milk, contributing to bacterial dissemination. Poor milking hygiene and milking equipment maintenance were identified as key risk factors for transmission. Goats with infections scored higher on an on-farm screening test, demonstrating its utility for early detection and management. Control measures, including improved milking hygiene, equipment maintenance, segregation of infected animals, and vaccination of replacement does, successfully reduced S. aureus to undetectable levels in the subsequent two kidding seasons, although environmental Staphylococci persisted. This case highlights the role of subclinical carriers in mastitis outbreaks and illustrates practical interventions that enhance animal welfare, milk quality, and farm sustainability.

Abstract

An outbreak of mastitis caused by Staphylococcus aureus occurred in a commercial dairy goat herd during kidding season, resulting in fatal gangrenous mastitis in approximately 30% of the herd. S. aureus was recovered from milk, mammary tissue, and other organs in does subjected to necropsy. The S. aureus milk culture-positive rate among does in the hospital pen was 58.3%, while whole-herd milk cultures of clinically normal mature does identified S. aureus in 15.0% with an additional 15.0% positive for coagulase-negative Staphylococcus (CNS), yielding a total culture-positive rate of 30.0%. The prevalence of CNS in subclinical animals was consistent with previous reports from U.S. dairy goats; in contrast, S. aureus isolation rates substantially exceeded previously reported prevalences. Poor milking hygiene and milking machine dysfunction were identified as major factors contributing to the spread of the S. aureus from goat to goat. California Mastitis Test (CMT) scores were significantly higher in culture-positive does compared with culture-negative animals (p < 0.05), demonstrating the value of CMT as a practical on-farm tool for early treatment decision making. Interventions focused on addressing milking hygiene and milking machine maintenance, as well as segregation and vaccination of replacement females. S. aureus dropped to undetectable in the next two kidding seasons, whereas the CNS culture rates remained unchanged, suggesting other factors may be contributing to CNS infection. This case highlights the role of subclinical intramammary infection and milking practice factors in transmission and control of contagious mastitis pathogens like S. aureus.

1. Introduction

Mastitis in dairy goats, particularly infections caused by Staphylococcus aureus, presents significant problems due to economic losses and impacts on animal welfare. S. aureus is one of the primary pathogens responsible for intramammary infections in goats, and it can cause both clinical and subclinical mastitis. Mastitis due to S. aureus can lead to decreased milk yield, reduced milk quality, loss of saleable milk due to treatment, mortality losses, and increased veterinary costs [1,2,3]. An outbreak of S. aureus mastitis can have devastating effects on dairy operations [4,5].

Clinical mastitis caused by S. aureus is often characterized by severe systemic illness, with severe damage to the mammary gland often described as peracute gangrenous or “blue bag” mastitis [2,5,6]. Acute clinical mastitis confined to the mammary gland can also present with abnormal milk, swollen painful mammary gland, and gangrenous lesions. Subclinical infections are harder to detect and frequently go unnoticed, allowing the pathogen to persist within the herd, primarily spreading during milking [7,8]. Despite recommended best practices for milking hygiene in dairy farms, consistent adoption of hygiene recommendations can vary widely between farm procedures [9,10].

Milk culture-based studies in U.S. dairy goats indicate that Staphylococcus aureus is isolated infrequently relative to other staphylococci. Anderson et al. [9] reported S. aureus in only 1.4% of milk samples, demonstrating a low prevalence of S. aureus-positive milk cultures in commercial herds. Similarly, Bernier Gosselin et al. [11] reported a 13.2% cumulative incidence of new staphylococcal intramammary infections over the dry period, of which only 0.5% were attributable to S. aureus, indicating that most staphylococcal infections detected by milk culture were caused by non-aureus species. Despite this low incidence, S. aureus remains clinically important due to its contagious nature and potential for persistence and outbreak within dairy goat herds. Furthermore, S. aureus can colonize extramammary sites, such as the nares and skin, creating reservoirs for reinfection even after apparent recovery from intramammary infection [3,9,12,13].

This case report describes an outbreak of S. aureus mastitis that occurred in a commercial dairy goat herd and resulted in significant morbidity and mortality. The clinical investigation identified management and milking practices that likely contributed to the spread of S. aureus within the lactating herd and identified disease comorbidities that may have increased susceptibility to infection. Notably, an unusually high frequency of S. aureus was isolated from milk of nonclinical animals, in contrast to the low incidence reported in prior U.S. milk culture studies, and likely contributed to sustained transmission within the herd. Data collected by the farm allowed assessment of the utility of diagnostic tools, including milk culture and the California Mastitis Test, for disease identification and control. Management interventions were implemented, and the outbreak trajectory was monitored to evaluate successful resolution.

2. Materials and Method

2.1. Farm

The affected farm is a small artisanal goat dairy that produces milk for on-site cheese production and is supported heavily by agritourism. At the time of the outbreak, the herd comprised Saanen and Alpine goats and crosses thereof. The 2023 kidding season started with a mature doe herd (all ≥ 2 yrs) of 92 does due to kid from February to April, and 16 yearling, primiparous does due to kid in April and May. The mature does were housed in a loafing barn bedded by straw with access to pasture and the yearlings were housed in a separate facility with a similar setup at the start of the outbreak. The farm had a history of pregnancy toxemia and clinical disease related to Mycoplasma mycoides subsp. capri and Caprine Arthritis Encephalitis Virus (CAE). The farm reported that 7 does had died that year, prior to the start of kidding season, due to presumed pregnancy toxemia.

2.2. Outbreak Description and Sampling

Approximately 2 weeks into kidding season, does in the mature lactating herd started experiencing severe acute illness rapidly progressing to recumbency and death. The does presented with decreased milk production that was asymmetric between halves, lethargy and anorexia at the time of milking; animals typically became recumbent or died by the morning after signs appeared. Only a few does were noted to have gross changes to their milk prior to signs of severe illness. In the first 2–3 weeks of the outbreak, prior to seeking veterinary advice, 20–25 of the 92 mature does died with these clinical signs.

On 1 March, three does showing clinical signs presented to the Veterinary Medical Teaching Hospital for diagnostics. Two had evidence of sepsis: tachypnea, tachycardia, altered mentation, scleral injection, recumbency and hypothermia. The mammary glands of both animals had signs of gangrenous mastitis including being swollen, with one half cold and purple, with a sharp line of demarcation between the abnormal and normal skin. The more affected halves had serosanguinous secretions; the other halves expressed grossly normal milk. Both animals were euthanized and submitted for necropsy and mammary secretions were submitted for aerobic and Mycoplasma spp. culture. The third doe was dead on arrival, with evidence of gangrenous mastitis, and was submitted for necropsy. Cultures (milk and mammary secretions) from all three animals grew S. aureus. Antibiotic sensitivity was obtained on the isolates (

Table 1. MIC data on the S. aureus isolates from the first three animals that presented to the VMTH.

Antibiotic	S. aureus MIC (μg/mL)
	Isolate A	Isolate B (Milk Plate)	Isolate C
Ampicillin	≤0.25	≤0.12	≤0.25
Ceftiofur	1	1	1
Chlortetracycline	≤0.50		≤0.50
Clindamycin	≤0.25		≤0.25
Florfenicol	4		4
Neomycin	≤4.0		≤4.0
Oxytetracycline	≤0.50		≤0.50
Penicillin G	≤0.125	≤0.125	≤0.125
Spectinomycin	64		66
Sulphadimethoxine	>256	128	128
Tulathromycin	4		4
Tylosin	2		2

).

Between 3–11 March, milk samples from all lactating does (n = 52: lactating string n = 40, hospital pen n = 12) were collected and submitted to the Veterinary Medicine Teaching & Research Center (VMTRC) Milk Quality Lab for aerobic and Mycoplasma spp. culture. Subsequently, additional milk samples were collected from newly fresh mature does and submitted for culture on 11 April (n = 5) and 19 April (n = 18). All milking yearlings had milk samples following kidding and these were submitted for culture on 15 May (n = 16). Additional whole-herd, individual animal milk cultures were submitted at the end of kidding season in the two subsequent years.

A veterinary visit was conducted on 8 March of the outbreak year to investigate possible contributing factors to the outbreak and develop an intervention and management plan for the farm. Milking facilities, equipment and procedures were evaluated; additionally, housing, isolation and biosecurity protocols were assessed. Two additional lactating does with evidence of gangrenous mastitis were euthanized and necropsies were performed on the farm.

2.3. Milk Cultures

Milk samples were collected by farm staff using aseptic techniques; samples were pooled from left and right halves in approximately equal volumes. The samples were frozen at −20 °C until submission and then shipped overnight on wet ice to the UC Davis Veterinary Medicine Teaching and Research Center Milk Quality Lab (Tulare, CA, USA). Milk samples were cultured according to the National Mastitis Council guidelines with modifications as follows. Briefly, milk samples were plated on quarter plates of TSA with sheep blood agar (Remel Inc., Lenexa, KS, USA) using sterile cotton swabs (Puritan, VWR, Visalia, CA, USA). Plates were incubated at 37 °C, read at 18–24 h, and again 48 h after culture. Culture bacteria were identified by colony morphology, catalase test, KOH reactions, and Gram stain. S. aureus was confirmed if suspects were visualized, using rabbit plasma coagulase (BD BBL, Fisher Scientific, Waltham, MA, USA). Any suspect organism that required further confirmation were submitted to the California Animal Health and Food Safety Laboratory for species identification by MALDI-TOF (Microflex LT/SH, Bruker Daltonics, Billerica, MA, USA). Mycoplasma culture was also performed alongside blood agar culture. Briefly, Mycoplasma culture of milk samples was performed by plating a sterile cotton swab of milk on a Mycoplasma plate (Myco, Biological Media Services). Swabs used for streaking were placed into an enrichment Mycoplasma broth (3 mL aliquot, Biological Media Services). Plates were placed in an incubator at 37 °C, 4% CO₂ for up to 7 days. Enrichment Mycoplasma broths were placed in an aerobic incubator for 48 h, streaked onto Mycoplasma plates, and placed in the incubator, with the same conditions as mentioned above for Mycoplasma growth. Plates were read twice, at 4 and 7 days of incubation. Results were recorded as no growth if, after 7 days, no colonies were visualized; contaminated if colonies other than Mycoplasma spp. were visualized; and positive if they showed any number of small translucent domes with dense centers (fried egg appearance).

2.4. California Mastitis Test (CMT)

Farm staff were trained to perform CMT according to the following guidelines: CMT concentrate (Jorgenson Laboratories Inc., Loveland, CO, USA) was diluted according to the manufacturer’s instructions. Equal volumes of milk and diluted CMT solution were mixed in a paddle well and gently rotated. Gel formation was assessed within 10–15 s and scored as follows. Negative (0)—mixture remains liquid, no evidence of thickening. Trace (0.5)—slight slime forms, reaction disappears within 10 s. Weak positive (1)—thickening without gel formation. Distinct positive (2)—immediate gel formation, mixture thickens with some central peak on swirling. Strong positive (3)—firm gel formation, adheres to the paddle with pronounced central peak. A CMT was performed and recorded for every goat once daily from March 11 until March 31. The numerical CMT value from the left and right halves were averaged, and a daily score was assigned. The animal groups were as follows. No growth (n = 20)—no evidence of mastitis and a negative culture. Subclinical coagulase-negative Staphylococcus (CNS) (n = 6)—no physical abnormalities of the mammary gland or milk, cultured positive ≥ 1 colony of CNS. Subclinical S. aureus (n = 6)—no physical abnormalities of the mammary gland or milk, cultured positive for ≥1 colony of S. aureus. Clinical S. aureus infection (n = 3)—abnormal milk or mammary gland, but without systemic illness, culture-positive for ≥1 colony of S. aureus. Animals with systemic signs of sepsis were euthanized or died and excluded from this analysis as their datasets were incomplete and animals that freshened after March 11 were excluded. Serial CMT measurements for does with culture results from early March were analyzed by two-way ANOVA to compare between groups and post hoc testing with correction for multiple comparisons was performed using Tukey’s test. Significance level was set at ≤0.05. Treatment data were collected from farm records on all does with serial CMT measurements.

3. Results

3.1. Necropsy

The initial three does submitted for necropsy on 1 March had similar gross pathologic findings of the mammary glands including areas of parenchyma of the affected half that were dark red–brown in color, which was sharply demarcated from pale pink sections of parenchyma (Figure 1B). The red–brown parenchyma was firmer and denser in texture when compared to the pale pink sections. The collecting ducts and gland cistern contained abundant red fluid (Figure 1A). One doe had markedly edematous lungs and a pale liver and hemorrhagic infarctions throughout both renal cortices (Figure 1C). S. aureus was cultured from the mammary glands and secretions of all three does, including from the apparently unaffected mammary halves. S. aureus was also cultured from the kidneys of the doe with renal infarctions. One doe had regionally extensive coagulative necrosis of the corticomedullary tubules with hemorrhage and neutrophils and occasional fibrin thrombi in the renal glomerular capillaries. Histological findings included neutrophilic exudation with necrosis in the mammary alveoli and ducts associated with large numbers of bacterial cocci in clusters. In the major collecting ducts and teat cistern there was widespread epithelial ulceration with fibrinonecrotic neutrophilic exudation on the surface. These findings were consistent with S. aureus septicemia.

3.2. Farm Visit

3.2.1. Milking Facilities

The milking parlor was a single parallel 15. The vacuum pump, which had been recently replaced, was running at 9 InHg (Figure 2B). The pulsation rate was 180/minute and the pulsators were BouMatic 24v with a 60:40 pulsation ratio (BouMatic, Madison, WI, USA). The milking line was a high-line approximately level with the back of the does (Figure 2A). They used a California-style claw with 0/300 short goat silicone inflation liners (Hamby’s Dairy Supply, Maysville, MO, USA); vacuum is stopped with a milk line clamp approximately 24 cm from the claw and inflation plugs were used as needed (Figure 2A). The system is cleaned using a clean in-place unit following requirements by the California Department of Food and Agriculture for dairy licensure [14]. A number of problems were noted upon inspection. The vacuum pressure was low; high-line systems usually require 13–14 InHg at the gauge to achieve the desired teat-end vacuum of 10–11.5 InHg for goats [15]. The pulsation rate was approximately double the recommended pulsation rate of 90 for goats [15]. The low pressure was leading to liner slippage, “squawking”, and back flow of milk into the inflations. The use of short silicon liners on the California claw failed to stop the vacuum if inflations fell off. The liners were old, warped and overdue for replacement (Figure 2A).

Milking procedure was performed as follows: foremilk was stripped out; a pre-dip 0.5% iodine (made by diluting 1.0% with tap water) was applied using a Sani-Sure returning Teat Dipper (Figure 2D) (Coburn, Whitewater, WI, USA); each animal was wiped with a new disposable towel; the inflations were attached; inflations were removed as milk stopped flowing; if one half finished before the other an inflation plug was placed in the unused inflation; after removal of the inflations the does’ teats were dipped with 1% iodine teat dip in an Ambic non-return dip cup (Figure 2E) (Neogen, Lansing, MI, USA). Many of the does leaked milk after fore-stripping, leading to milk contamination of pre-dip cup contents and pools of milk on the milking platform (Figure 2C). The inflation plugs had significant milk residue after use that was rinsed off in cold water between animals but not dried or disinfected. Milkers wore nitrile gloves and cleaned their hands/gloves between strings.

3.2.2. Animals

The entire herd was observed by the veterinary team both during milking and in the barns. The does were housed in a loafing barn with access to pasture. The hospital pen was separated from the lactating does by a 6 ft alleyway but shared a fence line with the maternity pen. Most of the does in the lactating pen had no clinical signs of mastitis, but 30–40% had large swollen joints. The entire herd was tested with the CMT and does with a high CMT were identified. Two does from the hospital were euthanized and necropsied: an Alpine doe with enlarged joints, a hard nonproductive left half and bloody milk on the right half (Figure 3) and a Saanen doe who was recumbent and unable to rise with clinical signs of septicemia, gangrenous mastitis. The Alpine had gross findings consistent with chronic CAE and Mycoplasma with acute-on-chronic infection of the right mammary gland with S. aureus. The Saanen doe had gross findings consistent with S. aureus septicemia including hemorrhagic mammary secretions, udder parenchyma that was dark red–brown in color, heavy congested lungs and hemorrhagic infarctions in her kidneys (Figure 1). The yearlings were housed on a different property and had no clinical signs of acute mastitis; however, there was evidence of chronic infections with Mycoplasma and CAE including hard bag mastitis, swollen joints, and chronic lameness.

3.2.3. Farm Recommendations

The following recommendations were made for the milking parlor. Increase the vacuum pressure to 13–15 inHg when the system is running. Replace all liners and change the claw assembly to an ITP207 individual milking claw with automatic shut-off valve (Interpuls S.p.A., Albinea, Italy) that controls the vacuum supply to each liner/teat during both milking and stop the use of inflation plugs.

The following recommendations were made for the milking routine. Create four milking strings and milk in the following order: fresh yearlings, mature does with CMT < 2, asymptomatic does with CMT ≥ 2, and the hospital pen last. The following milking procedure was recommended. Pre-dip in a designated non-return dip cup using undiluted 1% iodine teat dip, wipe with one towel per goat, strip into a strip cup, CMT, milk, post-dip. Empty and wash the dipping reservoir on the dip cups and hose down the milking platform to remove any milk between strings. Perform and record a CMT on every milker every day until clinical disease is reduced. It was recommended that the farm institute vaccination of the yearlings with VIMCO^® S. aureus bacterin vaccine (Laboratorios Hipra, S.A., Amer, Spain) per the label instructions, 60 and 30 days prior to kidding. It was recommended that the use of this vaccine be continued annually in all does and yearlings.

3.3. Milk Cultures

The initial milk cultures included 40 asymptomatic does housed in the lactating pen and 12 does from the hospital pen, of which most had clinical mastitis. At the initial sampling of the lactating herd 70.0% (28/40) of does had no growth, 15.0% (6/40) grew CNS and 15% (6/40) grew S. aureus (Figure 4A) for an overall culture-positive rate of 30.0% (12/40). In the hospital pen, 8.3% (1/12) had no growth, while 58.3% (7/12) of does cultured S. aureus and 33.3% (4/12) cultured CNS (Figure 4B). The additional samples obtained from newly lactating mature does or clinically normal does still in the first mature milking string, after management changes were implemented, demonstrated no growth in milk of 95.7% (22/23) of the does, with only 4.3% (1/23) cultured CNS, and no S. aureus was recovered from these does (Figure 4C). Results from lactating yearlings included 87.5% (14/16) with no growth and 12.5% (2/16) with CNS-positive cultures (Figure 4D). No S. aureus was detected from whole-herd milk cultures submitted in spring of 2024 and 2025; however, the CNS culture rate remained similar at 20.9% and 19.4% respectively. Mycoplasma spp. was not cultured from any milk samples; however, Mycoplasma mycoides subsp. capri was isolated from lungs and joints of kids in both the 2023 and 2024 kidding seasons, suggesting that it was being shed in the milk of some does.

3.4. California Mastitis Test (CMT)

Animals that had no growth on initial culture had significantly lower CMT scores than subclinical animals with a positive culture at most time points. There was no significant difference in CMT scores between the culture-positive groups at most time points (Figure 5).

3.5. Treatment

Does with subclinical mastitis and a positive S. aureus culture were treated with Tylosin (Tylan 200, Elanco Animal Health, Greenfield, IN, USA), 20 mg/kg SQ every 24 h for 3–5 days, and Ceftiofur hydrochloride intramammary (Spectramast LC Zoetis, Parsippany, NJ, USA) once daily for 3–5 days. Tylosin was selected for use because it had long been the first-line antimicrobial used on this farm for respiratory and mammary infections due to the known presence of Mycoplasma, and limited susceptibility testing of S. aureus isolates did not demonstrate resistance. Does with subclinical mastitis culturing CNS were treated with Ceftiofur hydrochloride intramammary (Spectramast LC Zoetis, Parsippany, NJ, USA)) once daily for 3–5 days. A request was submitted for withdrawal periods to the Food Animal Residue Avoidance Database for the above treatments and the following recommendations were made: Tylosin once daily for 10 days, with at least a 63-day meat and 9-day milk withdrawal interval, and intramammary ceftiofur once daily for 5 days, with at least a 7-day meat and 12-day milk withdrawal period. Any doe that developed clinical disease was treated with the same medications as does with subclinical S. aureus infections; however, the goal in clinical cases was to stabilize the animals so that they could be culled and sent to slaughter once their antibiotic withhold was complete. According to farm management by the end of the outbreak, most of the does with positive S. aureus cultures had died, been euthanized or were culled.

4. Discussion

This case report describes a severe outbreak of S. aureus mastitis in a commercial dairy goat herd that resulted in significant morbidity, mortality, and production losses. The peracute gangrenous mastitis and septicemia initial cases are consistent with previously described “blue bag” mastitis in goats, for which S. aureus is a common etiologic agent [1,2,4]. The rapid progression to recumbency and death within 24 h of clinical signs highlights the virulence of S. aureus in this case and the poor prognosis once involvement occurs [13,16,17].

The detection of S. aureus in both clinically and subclinically affected animals underscores the pathogen’s ability to persist in herds and spread efficiently during milking [3,8]. The incidence of S. aureus-positive milk cultures in subclinical goats exceeded previously reported incidence levels in the U.S., whereas the CNS culture rate was consistent with previously reported levels in dairy goats [9,11]. In this outbreak, disease transmission was likely facilitated by faulty milking equipment and suboptimal hygiene, which created conditions conducive to teat-end damage and intramammary infection [15,18]. Recent studies in the U.S. have shown that milking hygiene practices can vary widely between dairy goat operations and this case highlights the potential consequences of not following recommended best practices for milking procedures [9,10]. Implementation of corrective measures, including equipment replacement, improved teat disinfection, and adjustments to milking order, was associated with a reduction in clinical and subclinical S. aureus cases. CMT proved to be a rapid, useful on-farm screening tool for identification and early treatment of intramammary infections, although bacterial culture remained essential for differentiation of pathogens [1,4,7,8].

Control of S. aureus mastitis in dairy goats is challenging due to persistence in subclinical carriers and extramammary sites [3,12,16]. Nevertheless, long-term improvements were achieved in this herd through enhanced milking hygiene, segregation of high-risk animals, vaccination, and ongoing surveillance, with whole-herd milk cultures later confirming the absence of detectable S. aureus. In contrast, the interventions in this herd did nothing to reduce the culture rate of CNS in the herd, suggesting that other factors are influencing the development of subclinical CNS infections. Limitations of this report include its focus on a single herd and the absence of molecular typing which may limit broader extrapolation of these findings.

5. Conclusions

S. aureus mastitis can have devastating consequences in commercial dairy goat operations, with rapid disease progression, high mortality, and significant production and welfare impacts. This case demonstrates that although eradication is difficult, sustained control is achievable through a comprehensive management approach incorporating strict milking hygiene, functional equipment, early detection of subclinical infections, segregation of high-risk animals, vaccination, and continued culture-based surveillance. This case also highlights that, in contrast to S. aureus, CNS infections were not reduced by the implemented control measures, suggesting differing epidemiology and risk factors.