Emergency rabies control in a community of two high-density hosts

Abstract

Background: Rabies is a fatal viral disease that potentially can affect all mammals. Terrestrial rabies is not present in the United Kingdom and has been eliminated from Western Europe. Nevertheless the possibility remains that rabies could be introduced to England, where it would find two potentially suitable hosts, red foxes and badgers. With the aim to analyse the spread and emergency control of rabies in this two species host community, a simulation model was constructed. Different control strategies involving anti-rabies vaccination and population culling were developed, considering control application rates, spatial extent and timing. These strategies were evaluated for efficacy and feasibility to control rabies in hypothetical rural areas in the South of England immediately after a disease outbreak.

Results: The model confirmed that both fox and badger populations, separately, were competent hosts for the spread of rabies. Realistic vaccination levels were not sufficient to control rabies in high-density badger populations. The combined species community was a very strong rabies host. However, disease spread within species appeared to be more important than cross-species infection. Thus, the drivers of epidemiology depend on the potential of separate host species to sustain the disease. To control a rabies outbreak in the two species, both species had to be targeted. Realistic and robust control strategies involved vaccination of foxes and badgers, but also required badger culling. Although fox and badger populations in the UK are exceptionally dense, an outbreak of rabies can be controlled with a higher than 90% chance, if control response is quick and follows a strict regime. This requires surveillance and forceful and repeated control campaigns. In contrast, an uncontrolled rabies outbreak in the South of England would quickly develop into a strong epizootic involving tens of thousands of rabid foxes and badgers.

Conclusions: If populations of both host species are sufficiently large, epizootics are driven by within-species transmission, while cross-species-infection appears to be of minor importance. Thus, the disease control strategy has to target both host populations.

Figure 1

Success of rabies control in separate hosts. Success of rabies control in separate hosts: foxes (graph A) and badgers (graph B, filled circles, medium badger density (MD) as in South East England; open circles: high badger density (HD) as in South West England). In each control application, 40% of animals are treated.

Figure 2

Success of rabies control in host species community. Success of rabies control in host community. (Graph A: medium badger density in South East England, graph B: high badger density in South West England. The fox density is the same in both graphs.) In each control application, 40% of the population of a host species is treated as specified in the legend. Rabies starts in badger population.

Figure 3

Success of rabies eradication in host species community. Success of rabies eradication in species community. The fox density is the same in both graphs. Graph A: medium badger density in South East England; graph B: high badger density in South West England; control: fox vaccination 70%, badger vaccination 20%, badger culling 40%; varied radius of badger culling area (see legend); dashed line: 6 monthly control application; solid line: 2 monthly control application.

Figure 4

Disease dynamics of a rabies outbreak with different control applications. Disease dynamics in high badger density (South West England) in a 38 km x 38 km area over 5 years (rabies starts in month 9); control: 70% fox vaccination, 20% badger vaccination and 40% badger culling within 9 km radius; Results from 1000 simulation repetitions; (A) Number of simulation runs in which a rabid animal has already left the simulated area by the month indicated on the x-axis; Different line types indicate the number of repeated applications at two monthly intervals; (B) Median and 25% quantiles (error bars) of the accumulated number of rabid animals in the simulated area over the 5 years.

Figure 5

Flow chart of processes to simulate rabies spread for a single scenario. The flow chart of the simulation model showing the initial setup on the left, and the annual and monthly loops on the right.