Friday, 11 November 2011

Urbanization may influence transmission of zoonotic diseases

Disease has played a vital role in shaping the course of human history--a point famously addressed in Jared Diamond's 1998 Pulitzer Prize-winning book, Guns, Germs, and Steel. Because we have created effective vaccines for so many diseases, and have gone quite some time without a major deadly global pandemic, it is easy to forget that infections are still a real threat. Several years ago, outbreaks of avian influenza served as a powerful reminder that any number of zoonotic diseases--those that can be transmitted between animals and humans--could eventually cross species barriers and cause real harm to humans.

In some areas, emergence of zoonotic infections appears to be tied to anthropogenic activities. For instance, habitat destruction in Australia has reduced the availability of food resources for flying foxes--a type of bat that has been the source of 4 novel human pathogens since 1994. Where the foxes once covered large areas of natural habitat to look for patchily distributed, ephemeral food sources, they now often remain resident in urban areas, where they can forage in gardens and parks year round. This means that the bats contact each other, and the species to which they can transmit diseases, at evolutionarily new rates, thus altering transmission dynamics. This is particularly scary in the case of the lethal Hendra virus (HeV). Infected bats do not appear to suffer any ill effects of the disease, and merely pass it out of their systems via urine, saliva, feces, and placental fluids. However, where these come into contact with pasture, feed, or water, they can result in infections in horses, who then pass on the disease to humans.

(Grey-headed flying fox, Pteropus poliocephalus)

So far, there appear to be both seasonal and cyclical trends associated with the appearance of HeV in non-bat animals. In order to better understand where and when HeV is likely to emerge, how long it might last, and what factors are related to its "spill-over" into non-bat species, it is necessary to create and compare a variety of transmission models; these can be used to provide a framework for understanding what additional field data need to be collected to improve our understanding of HeV and generate useful outbreak forecasts and control plans. This was done recently by a group of collaborators from seven institutions across two continents. They created a series of models, based on both field and captive data collected for two species of flying fox (the grey-headed and the black; Pteropus poliocephalus and P. alecto, respectively), to explore the factors driving HeV emergence.

 (Black flying foxes, Pteropus alecto)

The models investigated the impacts of variations within local populations (where individual bats can be in one of four states: susceptible to HeV, exposed, infectious, or recovered from the disease), as well as relationships between populations, and across all populations within a region. By varying connectivity between and among groups of bats, the scientists were effectively testing the relevance of migration: Higher connectivity indicated migration across a large area of habitat (the "historical" behavior of the bats), while lower connectivity indicated long-term settlement of individual groups of bats ("herds") at separate locations (the "modern" behavior in response to urbanization). The researchers also explored the impacts of differences in demographic characteristics such as birth and death rates.

The output of the study was a series of models that predict HeV transmission under a variety of different circumstances. Cumulatively, the models suggest that anthropogenic habitat changes have encouraged the emergence of HeV by altering historical patterns of bat ecology, behavior, and movement. However, while humans have influenced where the disease is transmitted; when the infections occur is likely determined by seasonal patterns associated with reproduction (or, epidemiologically speaking, the introduction of new, uninfected bats into the population).

 (Nightly migration of flying foxes)

Because the models represent alternative potential scenarios responsible for the observed outbreak patterns, more work will need to be done to determine which scenario--if any--actually reflects the situation on the ground. For instance, spill-over into horses and humans may have resulted from continuous, low levels of HeV infection in flying foxes occupying urban roosts; on the other hand, it may also have resulted from infrequent, but high levels of infection. Although data from the 14 known outbreaks generally support the latter, additional field work will be needed to confirm this over a larger sample size, and/or to link actual outbreaks with immunological states of flying foxes. Other important efforts will include comparing HeV dynamics in urban and non-urban populations of flying foxes and investigating variations of immunity to HeV within individuals and across entire herds.

The authors of the study hope that their models can be used to provide a framework not just for HeV, but also for other zoonotic disease systems. Their research indicates that multiple landscape and demographic factors can simultaneously influence the spread of disease between humans and animals, which suggests that it may not be easy to quickly address any new infections that emerge in the future. However, the finding that urbanization has increased the strength of the epidemiological link between flying foxes and both horses and humans suggests that the next zoonotic outbreak may come from species that are already in our midst--not entirely surprising given the sources of historical zoonotic infections, but disconcerting all the same.

For additional images associated with this post, please visit the Anthrophysis pin board at Pinterest.

Plowright, R.K., Foley, P., Field, H.E., Dobson, A.P., Foley, J.E., Eby, P., and Daszak, P. 2011. Urban habituation, ecological connectivity and epidemic dampening: the emergence of Hendra virus from flying foxes (Pteropus spp.). Proceedings of the Royal Society B 278:3703-3712.

Thanks to the following websites for providing the images used in this post:

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