A Review of Salmonella and Squamates (Lizards, Snakes and Amphisbians): Implications for Public Health

Globally, there has been an increase in squamates (particularly lizards and snakes) being kept as pets. Additionally, urban spread has resulted in greater human encroachment and interaction with the natural habitat of wild squamates. A potential consequence of increasing human interaction with squamates is the increased potential for disease transfer. This review collates the literature describing clinical salmonellosis cases that were definitively linked to a squamate through testing of the animal and population-based studies which investigate the risk of salmonellosis linked to pet squamates. It was demonstrated that although squamate-acquired salmonellosis accounted for a small percentage of total cases, children under five were at greatest risk, with the clinical manifestations tending to be more severe. In many cases, it was noted that the patient was unaware of the risks associated with keeping squamates and did not practice proper hand hygiene after handling the animals or cleaning cages. This highlights the need for more education focused on informing the general public of ways to reduce the risk of salmonellosis from pet squamates. There is also the need for future research into the role of wild squamates in the spread of human salmonellosis, both directly and indirectly through cross contamination.


Introduction
Salmonella is a significant cause of disease, affecting both humans and animals [1]. It is the causative agent of salmonellosis, a gastrointestinal disease of public health significance [2]. Globally, it is estimated that there are 93.8 million cases of salmonellosis each year [3]. Infrequently, Salmonella is also responsible for more invasive diseases, such as bacteraemia with or without metastatic disease, skin and bone infections, urinary tract infections, meningitis and splenic abscess [4][5][6][7][8]. Primarily, Salmonella is considered a foodborne pathogen, with contaminated food attributed to 80 million cases of salmonellosis annually [3,9]. However, in the USA it has been estimated the 6% of sporadic salmonellosis cases and 11% of cases in people aged under 21 are caused by reptile and amphibian contact [10]. This estimate was supported by a literature review conducted by Sauteur et al. [11] that examined published studies from 1965 to 2012 describing reptile associated salmonellosis in children aged less than 18 years. A total of 182 cases were identified. The primary reptile associated with gastrointestinal salmonellosis was turtles; however, exposure to iguanas was significantly more prevalent in children with invasive Salmonella diseases (septicaemia or meningitis).
Exotic pets, including lizards and snakes, have become increasingly popular [1,12,13]. Salmonella is often detected from captive reptiles and reports of salmonellosis linked to reptile pets are increasing [14]. A study in Malaysia demonstrated that 83.3% of captive lizards (Iguanidae, Agamidae,

Results
The population-based studies that examined the likelihood of notified salmonellosis cases to be linked to lizards and snakes are presented in Table 1. The studies are limited to the UK [27], USA [28][29][30][31], and Germany [32]. The most recent study is from the UK, which identifies that over a quarter of salmonellosis cases in children under five years of age are linked to reptile exposure; however, the species of reptiles involved were not examined [27]. The focus on exposure of children is supported by a study from the USA that demonstrated the median age for reptile associated salmonellosis was 11 years old. This study also demonstrated that reptile exposure was attributed to 3.5% of all salmonellosis cases and of these lizards and snakes were attributed to 47% and 20%, respectively [28]. Additionally another study from the USA investigated the source of notified S. Marina cases, specifically, and found 81% of patients were infants (less than one year of age) and 88% of these cases reported exposure to iguanas [29].
The case reports from the last twenty years which identify and confirm snakes or lizards as a source of human Salmonella infection are presented in Table 2. Studies were from Switzerland [33], Germany [34], USA [6,[35][36][37][38], Australia [39], UK [7,[40][41][42][43], France [44], The Netherlands [8], and Canada [13]. The most commonly-identified lizard or snakes were bearded dragons [34,35,40,45] and iguanas [6,13,36,37,42,43]; although other species included corn snakes [37,44], a water dragon [41], a boa constrictor [38], and a gecko [7]. The cases described in Table 2 demonstrate a range of clinical presentations including gastrointestinal, sinus, blood, brain, bone, urinary tract, and spleen infections and primarily involved young children and immunocompromised patients. In several cases observed in children less than six months of age the Salmonella infection was fatal [36,37,41,42]. The most commonly-identified infectious agents were Salmonella Marina [6,13,37] and Salmonella Poona [13,36,42]. Interestingly, not all patients described direct contact with the lizard or snake, despite an identical isolate from both animal and patient [35,37,40,45], suggesting that indirect contact may play a role in the spread of lizard-and snake-associated salmonellosis. Indirect exposure could also apply to the cases involving young children less than six months of age who are unlikely to have had direct contact with the snake or lizard [7,36,37,[41][42][43]45]. This transmission route was speculated by Glick and Sherman [46] who suggested that the mother or another family member acted as the vector transferring the Salmonella from the Gecko to the four day old baby. The role of indirect exposure is illustrated by the outbreak involving a potluck dinner which was prepared in the cook's home and the Salmonella serotype responsible for the outbreak was found in her vacuum cleaner and from one of her pet bearded dragons which were kept in an adjacent room [35]. Similarly, in a case from Australia, the patient had no direct contact with the bearded dragon, but the vacuum cleaner contained the same Salmonella serotype, suggesting that this could be the route of exposure [45]. A common observation made in the case reports was that patients and the parents of patients were not aware of the risk associated with handling lizards and reported poor hand hygiene practices after handling of snakes and lizards and cleaning of enclosures. It was noted in a case involving a 29 years old male from Switzerland that the patient did no wash or disinfect his hands after handling snakes, feeding them, or cleaning their terrarium [33]. Eighteen cases (16 households) were identified. In eight of 16 case households reptiles were kept. In four of the other households alternative forms of reptile contact were reported. keeping of a reptile and "any reptile contact" were associated with Salmonella Tennessee infection (mOR 29.0; 95% CI 3.1±∞ and mOR 119.5; 95% CI 11.7−∞ ).
In two cases the identical strain of Salmonella Tennessee was identified from a reptile in the household. Direct contact between child and reptile was denied.

Discussion
Increasing interaction with snakes and lizards, both as captive pets and through encroachment of their natural ecosystem, may result in an increase in the transmission of salmonellosis [13]. This study demonstrates that snake-and lizard-associated salmonellosis is being reported across the globe. It also highlights the diversity of clinical presentation of Salmonella infections associated with snakes and lizards and their potential severity. Children were the primary demographic identified in squamate-associated salmonellosis cases and the clinical manifestations were typically more severe than other cases of salmonellosis. These findings support the US Centres for Disease Control and Prevention (CDC) recommendation that children under the age of five should avoid contact with reptiles and that these animals should not be kept in childcare centres [47].
The lack of the general public's knowledge regarding the dangers associated with lizards and snakes was also highlighted and is supported by the finding of previous studies as one of the main risks for reptile associated salmonellosis [13,37]. In 2003, the US CDC found that only 4/49 US state health departments interviewed required pet store owners to provide information regarding risk of salmonellosis with the purchase of a turtle and no state health department required salmonellosis information to be provided to a person purchasing a lizard of snake [48]. This demonstrates the need for more education aimed at informing snakes and lizards handlers of the associated risk of salmonellosis and the importance of good hand hygiene practices [37]. Good hygiene is particularly important in cases where persons handling snakes and lizards were identified as the potential vector transmitting Salmonella to children. Additionally, in cases where there was no direct contact with the lizard or snakes, vacuum cleaners were identified as a potential source. The mechanical disturbance and agitation of settled dust containing microorganisms by a vacuum cleaner can provide a mechanism for the dispersal of bioaerosols [49]. It has been demonstrated vacuum cleaners can disperse bacteria emissions at concentrations as high at 10 5 bacteria per minute [50].
No studies which definitively identified a case of human salmonellosis caused by wild lizards and snakes were identified. However, previous studies have demonstrated the potential for wild lizards and snakes to carry Salmonella and, as such, they could potentially be playing a role in human cases [23][24][25][26]. The presence of Salmonella in wild lizards and snakes also presents other issues relating to biosecurity and animal health. It has been demonstrated that Salmonella is an opportunistic pathogen of lizards and snakes with weakened immune systems [51] and can cause bone infection in snakes [52,53]. There is also the potential for lizards and snakes to transfer Salmonella to animal production facilities, such as poultry farms. This could have significant consequences, particularly with the spread of antibiotic-resistant strains between flocks and after disinfection processes [54,55].

Materials and Methods
The databases Scopus and Web of Science were searched for articles written in English over the last twenty years containing the keywords (Salmonella OR salmonellosis) AND (lizards OR lizard OR Squamata OR Squamate OR snake OR snakes OR Amphisbaenia). Figure 1 presents the systematic approach to article inclusion or exclusion. Articles were screened by reading titles and abstracts and initially excluded if they did not refer to human salmonellosis or if they were review articles. Articles were then read in full and excluded if they described a clinical case of Salmonella infection linked to a squamate which was not confirmed through testing of the animal. Articles were included if they were population studies investigating squamates as a potential risk for salmonellosis and clinical cases that were definitively linked to a squamate through testing of the animal and comparison of animal and human Salmonella isolates.

Conclusions
This review demonstrates that salmonellosis associated with lizards and snakes is an emerging global issue of public health concern. Although it only accounts for a small proportion of all salmonellosis cases, the evidence suggests that it predominately affects children under five years of age and the clinical manifestations can be severe. There is a need for greater education aimed at informing people who keep lizards and snakes as pets of the potential risks and the best ways to protect themselves.

Conclusions
This review demonstrates that salmonellosis associated with lizards and snakes is an emerging global issue of public health concern. Although it only accounts for a small proportion of all salmonellosis cases, the evidence suggests that it predominately affects children under five years of age and the clinical manifestations can be severe. There is a need for greater education aimed at informing people who keep lizards and snakes as pets of the potential risks and the best ways to protect themselves.