Leptospirosis is an infectious bacterial disease found in mammals, particularly associated with rodents. Common in the third world and tropical areas, of high rodent density, it may come as a surprise that New Zealand has one of the highest leptospirosis rates in the world. However, knowing the precarious history of New Zealand with introduced mammals, perhaps the high incidence of leptospirosis should not be surprising. Introduced rodents then provide a meeting place for both veterinarians and ecologists such as myself, investigating the role of rodents in ecosystems. This meeting place most recently happened to be a make-shift epidemiological laboratory in a shearing shed in New Zealand.A farm shed converted to a field epidemiological laboratory (Photo by James Russell)
There has recently been a spike in leptospirosis cases in rural New Zealand, associated with new strains emerging. Vaccination of livestock continues to play a critical role in preventing leptospirosis infection, but like in any ecological system, limiting one species allows another to flourish. It remains unknown what the reservoir is for these new emerging strains, and veterinarians are now looking beyond the farmland to the neighbouring forest fragments where multiple introduced mammal species all coexist, in regular contact with one another.
Marine Moinet from the Massey University mEpiLab is tasked with intensive study of farm sites where she samples across the fragmented landscape to capture the resident introduced mammals and sample them for leptospirosis presence and strain. At her latest site the surprising winner was the presence of mice in the fields at densities over 50 per hectare, approaching that of offshore islands such as Antipodes Island prior to mouse eradication. Understanding the complex dynamics of multiple candidate host species and strains will help unravel the puzzle of infection dynamics, and likely demonstrate that a Predator Free New Zealand would also have broad benefits to public health as well as biodiversity.