Explaining vampire bat rabies persistence

A new study has paired field data with modelling to understand how rabies infections persist in vampire bat populations. The results have implications for the control of rabies and other emerging diseases transmitted by bats.

Cattle farms provide vampire bat populations with almost unlimited amounts of food. As farmers encroach into forest habitat, rabies outbreaks originating in vampire bat populations, have become a significant source of livestock losses in Latin America.

Rabies outbreaks in humans are also increasing in remote Amazonian populations. This may be linked to a combination of human encroachment into forested areas, a depletion of the bats’ natural prey, and improved detection. Control measures involving indiscriminate culling of bats and vampiricide (an anticoagulant which kills bats who ingest it after grooming other bats) have been used since the 1960s, but even when used extensively they do not eliminate rabies infections from an area.

This study used data from infection studies in captive vampire bats and a unique long term field study monitoring individual rabies exposures in 17 colonies across four areas of Peru.

The study tried to match the observed patterns of rabies infections with models assuming different factors relevant to transmission and also culling of colonies. 

The authors concluded that the best explanation for the data came from models that assumed

1. that most infections (around 90%) were non-lethal to the vampire bats and effectively immunized them, and 
2. frequent movement of bats between colonies enabled viruses to be spread amongst them and persist in an area.

The conclusions have implication for laboratory studies and for control efforts. Many experimental studies use high doses of virus to generate 50-90% mortality in captive bats which, this study suggests, is not a natural phenomenon.  

Selective destruction of individual colonies may well limit local spillover into cattle and humans, but it is unlikely to be an effective means to eliminate the virus from a region. 

If localized culling reduces previously immune individuals, or increases the dispersal of bats, then this could actually make a rabies outbreak worse.  Effective control is likely to require a better understanding of how human practices affect vampire bat movements and more spatially coordinated control efforts.

Summarised by Louise Taylor from Blackwood et al “Resolving the roles of immunity, pathogenesis, and immigration for rabies persistence in vampire bats” in PNAS, vol. 110 p20837-42, December 2013