Mathematical modeling at the livestock-wildlife interface: scoping review of drivers of disease transmission between species

Modeling of infectious diseases at the livestock-wildlife interface is a unique subset of mathematical modeling with many innate challenges. To ascertain the characteristics of the models used in these scenarios, a scoping review of the scientific literature was conducted. Fifty-six studies qualified for inclusion. Only 14 diseases at this interface have benefited from the utility of mathematical modeling, despite a far greater number of shared diseases. The most represented species combinations were cattle and badgers (for bovine tuberculosis, 14), and pigs and wild boar [for African (8) and classical (3) swine fever, and foot-and-mouth and disease (1)]. Assessing control strategies was the overwhelming primary research objective (27), with most studies examining control strategies applied to wildlife hosts and the effect on domestic hosts (10) or both wild and domestic hosts (5). In spatially-explicit models, while livestock species can often be represented through explicit and identifiable location data (such as farm, herd, or pasture locations), wildlife locations are often inferred using habitat suitability as a proxy. Though there are innate assumptions that may not be fully accurate when using habitat suitability to represent wildlife presence, especially for wildlife the parsimony principle plays a large role in modeling diseases at this interface, where parameters are difficult to document or require a high level of data for inference. Explaining observed transmission dynamics was another common model objective, though the relative contribution of involved species to epizootic propagation was only ascertained in a few models. More direct evidence of disease spill-over, as can be obtained through genomic approaches based on pathogen sequences, could be a useful complement to further inform such modeling. As computational and programmatic capabilities advance, the resolution of the models and data used in these models will likely be able to increase as well, with a potential goal being the linking of modern complex ecological models with the depth of dynamics responsible for pathogen transmission. Controlling diseases at this interface is a critical step toward improving both livestock and wildlife health, and mechanistic models are becoming increasingly used to explore the strategies needed to confront these diseases.

Maintenance, spillover and spillback transmission of bovine tuberculosis in multi-host wildlife complexes: a New Zealand case study

The causative agent of bovine tuberculosis (bTB; Mycobacterium bovis) has a broad host range. The role of each animal species in spreading the disease depends on how transmission occurs, on the abundance of each host, and on the interactions between hosts. This paper explores differences in the roles individual host species can play in allowing M. bovis infection to persist and spread within a multi-species complex, using New Zealand as a case study. In New Zealand, four wild mammal species are frequently infected. Of these the brushtail possum is now regarded as the only true "maintenance" host. Red deer and ferrets can become maintenance hosts where their densities are exceptionally high, but more often they are "spillover" hosts, with most infection arising from moderately frequent inter-species transmission from possums. The latter situation is even more strongly the case for feral pigs. Spillover hosts may occasionally play a crucial epidemiological role by transmitting infection back to a potential maintenance host (spillback). Three key factors make spillback transmission far more epidemiologically important than its low frequency of occurrence might suggest--amplification of the reservoir of bTB, far greater spatial spread than by the maintenance host, and greater persistence of bTB in long-lived spillover hosts extending the risk of spillback far into the future. The risk of spillback is undoubtedly low, but it nonetheless determines the nature, scale and duration of management required. Eradication of the disease may require management of both the infection in maintenance hosts and reduction or elimination of any risk of spillback.