The Ecological and Immunological Relationships Between Salmonella and Tuatara
The transmission and expression of disease in wild animal populations is a complex interaction of host, pathogen and environmental factors. The individual fitness of a host may be negatively impacted by pathogenic bacteria in a number of ways including increased predation risk and reduced survival and reproductive output. Salmonellosis is an important zoonotic disease resulting in significant morbidity and mortality in populations of wild reptiles, birds and mammals throughout the world, and herpetofaunal species have often been implicated as shedders and transmitters of Salmonella globally. To better understand the unique threats to New Zealand native wildlife, I investigated spatio-temporal dynamics of Salmonella in an island ecosystem, and selected one species, tuatara (Sphenodon punctatus) (an endemic New Zealand reptile), for in-depth immunological analyses. I collected cloacal swabs and faecal samples from native wildlife on Stephens Island repeatedly between October 2009 and October 2011. While Salmonella was isolated from 6.5% of native skinks and 8% of the soil samples, intestinal carriage of Salmonella was not detected in the more than 600 cloacal swabs collected from wild tuatara, despite these tuatara living in close proximity to Salmonella-positive skinks or soil. In context, the lack of Salmonella detected in tuatara in this and other studies raises the question of whether tuatara are innately resistant to Salmonella. To test this hypothesis I examined aspects of innate and adaptive immune responses in tuatara serum. Immune measurements included in vitro anti-microbial activity of serum and antibody recognition of bacterial antigens. Serum was tested against three closely related enteric pathogens, including Salmonella, in order to establish the importance of cross-reactivity in the strength of immune responses observed. I found that tuatara possess antibodies which recognise Salmonella antigens by Western blot and flow cytometry. I also determined that the anti-microbial activity of tuatara serum was approximately 6-fold higher than donkey or mouse sera, but showed similar activity to other reptilian species tested. These findings are the first report of both environmentally-induced anti-Salmonella antibodies and anti-microbial activity in tuatara serum. Taken together, these studies investigating the distribution and seasonality of Salmonella within the environment and evaluating anti-Salmonella immune responses in tuatara will help to inform decisions about disease screening and animal movements to maintain the health of native fauna.