Understanding the dynamics of bat rabies.
Our understanding of rabies dynamics in bats has lagged behind that in other hosts, and long-term datasets on natural bat infections are rare. Now a new study by Dylan George from Colorado State University and other US colleagues has suggested that bat hibernation is vital to the persistence of the rabies virus in these hosts.
In the US, rabies cases in bats show seasonal variations, peaking in spring and particularly autumn each year.
Bats can develop symptomatic and ultimately fatal viral infections, or abortive infections where immunity prevents symptoms and transmission. The team used data from a 5 year study on a Colorado population of big brown bats (Eptesicus fuscus). These bats tend to roost in the same places from year to year, do not migrate long distances, and rabies variants are usually confined to single species of bats, constituting an approximately closed system for disease dynamics.
In the spring and summer, female bats form large maternity roosts where they give birth. Under these crowded conditions, oftenin buildings near humans, the main peak of rabies transmission occurs. By mid October these roost are empty and the bats leaveto hibernate in rock crevices at higher altitudes. After 6 months, in early spring, the bats emerge and early transmission occurs aspartial hibernation persist and the maternity roosts are established. Key aspects of the host and virus dynamics, such as bat mortality,bat reproduction and the virus incubation period vary across these three different periods, and the team used experimental and field derived data to asses these parameters for each season. For example, during bat hibernation, body temperature falls close to ambient temperature and viral replication is dramatically reduced.
A mathematical model, composed of a sub-model for each season was designed to estimate the number of bats in each of 4 classes (susceptible, exposed, infectious and resistant) through time. It was validated by evaluating how well it fitted data on bat population size, the proportion of bats that were infectious, and the seasonal peaks of rabies infections. The model accurately predicted more infectious adult females early in the year, and more infectious juveniles in the late summer. Over the long term, the model predicted coexistence of bats and virus.
By altering the parameters away from those observed from the data, the importance of various parameters could be investigated. Bat and viral persistence were most heavily influenced by (i) the case fatality rate; (ii) the natural mortality rate of juveniles during the transmission season and (iii) the incubation period of the disease.
Models that did not include a hibernation period repeatedly predicted bat extinction, demonstrating that low mortality during the winter is vital to maintain the bat population until the newborns are born in spring. A viable bat population is necessary for viral persistence, but does not guarantee persistence. The model also suggests that the viral incubation period has a strong impact on viral maintenance. Fast developing infections kill their hosts too quickly to ensure sufficient spread between hosts before hibernation occurs. Cold temperatures during hibernation ensure that viral replication cannot kill too many bats before spring. The study also
showed that the observed case fatality rate of the infections fell between extremes where too few infections would be produced to
maintain the virus, or where so many bats died that the bats and the virus both became extinct.
Overall, the study demonstrated that hibernation maintains a reservoir of infected individuals, allowing the virus to persist until the next transmission season. It also provides a framework for further exploration of rabies and other important viral diseases transmitted by bats.
Summarised by Louise Taylor from DB George et al. (2011) Host and viral ecology determine bat rabies seasonality and maintenance.
PNAS June 21, 2011
vol. 108 p10208-10213.