Comparison of Mycobacterium bovis Genetic Distances among Livestock and Wildlife in Michigan, USA and New Zealand

Abstract

Mycobacterium bovis (M. bovis) is a bacterium that causes the chronic and contagious zoonotic disease bovine tuberculosis (bTB) that primarily infects cattle as well as other livestock, wildlife and humans. Bovine tuberculosis in humans happens through the consumption of unpasteurized milk and meat from an infected cow. The number of human cases and deaths are underreported because of the lack diagnostic tools and awareness, which hinders the goal to eradicate tuberculosis worldwide. An infected cow means an economic burden for pastoralists who depend on their cattle for their livelihoods and for those who depend on cattle for meat and milk production. Cattle serve as the main reservoir for this disease, but for each country where bTB is endemic, there are also wildlife reservoirs and amplifiers that help sustain the disease among cattle. Therefore, controlling bTB in wildlife populations is vital to put an end to this disease. Previous studies have investigated transmission patterns at the livestock-wildlife interface, but there have not been many studies using minimum pairwise genetic distances. This tool serves as a proxy for transmission rate and it can be used to infer transmission patterns.

This study aims to understand the disease transmission patterns among cattle and white-tailed deer in Michigan, USA and cattle and feral ferrets in New Zealand. M. bovis genetic distance patterns between cattle and wildlife animals were calculated and visualized with graphical representations to compare the level of similarity. These results were used to assess whether there are consistent, repeatable patterns or heterogeneity of transmission at the livestock-wildlife interface across two countries.

The results of this study reveal expected transmission patterns between cattle and deer and unexpected patterns between cattle and ferrets. For cattle-deer interface, the degree of similarity of their M. bovis genetic distances were high, which is due to the fact that deer are the wildlife reservoir for Michigan and because free-ranging deer’s territory may have overlapped with farms. This increases the opportunities for indirect interactions and therefore transmission events. The degree of similarity between M. bovis genetic distances for cattle and ferrets in New Zealand was higher than expected. Although ferrets are not the wildlife reservoir for bTB, the results suggest a level of indirect contact with cattle. Possible explanations include the following: isolates were collected in an infected area that had ferrets sustaining the disease and isolates were sampled from ferrets that had traveled from an endemic location. Across countries, results showed more heterogeneity than repeatable patterns. This suggests that the livestock-wildlife interface is unique in their respective countries and wildlife ecology.

There are multiple limitations for these qualitative conclusions; however, this serves as a basis to infer transmission rates in the absence of case data and timeseries data, which are hard to obtain for wildlife. Future directions include creating a map to present network transmission between the different species and creating phylogenies. Future directions for the unexpected results between ferrets and cattle include further investigation in the prevalence of the disease in ferrets and understand why some populations can sustain the disease and transmit it while others cannot.