Disease transmission between and within species, and the implications for disease control.
Disease transmission can occur between and within species for diseases with multiple hosts. If these diseases are undesirable, for economic or health reasons, then the relative importance of each type of transmission should be determined. Bovine tuberculosis (Tb) caused by Mycobacterium bovis is one such undesirable disease. In parts of New Zealand, M. bovis infection is highly prevalent in feral ferret Mustela furo populations, and there is concern they may be acting as a reservoir of infection for domestic livestock, similar to the role played by brushtail possums Trichosurus vulpecula. We undertook a manipulative large-scale field experiment to test for and quantify interspecific transmission of M. bovis from brushtail possums to feral ferrets, and intraspecific transmission of M. bovis within feral ferret populations. Age-specific prevalence data obtained from cross-sectional surveys was modelled to estimate the effect of experimental reductions in possum population density on the force of M. bovis infection in ferrets. A simple analysis estimated the force of M. bovis infection in ferrets to be reduced by 88% at sites previously subjected to a possum population density reduction. A more detailed analysis, incorporating ferret survey data from both before and after possum population density reduction and controlling for the effect of ferret sampling, estimated that possum population density reduction decreased the force of M. bovis infection in ferrets by 29% at sites of high ferret density, and 88% at sites of low ferret density, compared with experimental control sites. There is clearly substantial possum-to-ferret transmission of M. bovis, and controlling possum populations is the logical first step to managing M. bovis infection in ferret populations, especially at sites with low ferret density. Intra-specific transmission is virtually absent in low density ferret populations though evident at higher densities. Synthesis and applications. These results have management implications for other multiple-host diseases around the world, such as bovine tuberculosis and rabies. Control of within-species transmission may not be as effective for disease control as a reduction in between-species transmission. The management decision should be based on empirical estimates of the magnitude of each form of transmission.