Predicting advancing wavefronts of vampire bat rabies
Our latest set of analyses of data from the Ministry of Agriculture of Peru, together with our own questionnaire data, show that rabies continues to spread to the fringes of the vampire bat distribution in surprisingly constant and predictable wave-like expansions. The ability to forecast when and where rabies will appear next provides a tantalizing opportunity to develop interventions like vaccination of humans and livestock (or perhaps even the bats themselves) before rabies arrives to new regions. Hopefully this puts us one step closer to preventative rather than reactive management of vampire bat rabies.
Forecasting traveling waves of vampire bat rabies using simple landscape-adjusted linear regression models. Pie charts show the percentages of farms reporting bat bites (green), indicating the presence of vampire bats ahead of wavefronts. Blue and red points are the locations of rabies outbreaks in livestock.
The paper is published free online at Proceedings of the Royal Society B.
Julio and I also wrote a short piece in The Conversation describing the results of this analysis against a broader context of vampire bat rabies in Latin America.
Benavides, J., Valderrama, W., & Streicker, D.G. Spatial expansions and travelling waves of rabies in vampire bats (2016) Proceedings of the Royal Society B. DOI: 10.1098/rspb.2016.0328
Land use, vampire bat feeding and rabies
A stable isotope analysis of bat hair, stomach contents and possible prey species showed some expected things, like that bats specialize on livestock prey when they don’t have any other options. But there were also some surprises. For example, on the coast of Peru vampire bats in the same colonies fed on very food resources spanning primary consumers to top predators. Similarly, in the Amazon rainforest, we found evidence of individuals that fed mostly or exclusively on livestock living next to individuals that fed mostly on wildlife. This led to some of the largest within-population variability in carbon and nitrogen stable isotopes ever recorded in animals, suggesting that when it comes to feeding, these bats have personality and stick to what they know. Another surprise was that the diversity of dietary strategies tended to decline with the local availability of livestock, diets were most diverse in places where there were some livestock and some wildlife available – these were also the kind of places where we found bats feeding on people. But, once livestock became abundant, bats seemed to switched entirely to this relatively easy prey source. In the Amazon, this seemed to be a good thing for human health: bat bites on people were common in places without livestock but totally absent where bats were specializing on domestic prey. All of this is interesting from a disease transmission standpoint because as land use change and human behavior alter bat feeding behavior, they also alter which species are at risk for rabies infection. Next up, we will conduct genetic analysis of bat blood meals to find out exactly which species are bitten and Dan will investigate whether dietary differences among bat colonies affects bat immunity and infection.
The paper is published free online at Journal of Applied Ecology.
Streicker, D.G. & Allgeier, J.E. From food web to disease ecology: foraging choices of vampire bats in diverse landscapes (2016) Journal of Applied Ecology. DOI: 10.1111/1365-2664.12690
2 new grants this week to study infectious disease dynamics in vampire bats
NSF Dissertation Improvement Grant: “Consequences of resource heterogeneity for immune defense, connectivity, and rabies dynamics in vampire bats” to Dan Becker
Dan Becker was awarded $19k from the US National Science for expanding his work on how resource provisioning affects health and disease transmission in vampire bats. The new grant will allow Dan to develop new studies focusing on how livestock intensification in Latin America affects antiviral immunity, bat population genetic structure and the spread of rabies. Collaborators are Daniel Streicker and Sonia Altizer.
CONCYTEC Basic Research Grant: “The role of the vampire bat in the transmission of bacteria resistant to antibiotics used in humans and livestock”
A new grant from the Peruvian Council of Science, Technology and Technological Innovation and the has awarded our team, composed of researchers from the University of Glasgow and Cayetano Heredia University in Lima (UPCH), Peru a grant of S/.400,000 (= £80,000) to study whether antibiotic resistant bacteria are transmitted between vampire bats and livestock. Collaborators are Julio Benavides and Daniel Streicker (Glasgow) and Carlos Shiva and Nestor Falcon (UPCH).
New causes of mass mortality in modern bats
By reviewing the literature on causes of mass mortality in bats from 1790-2015, we found changes in the reasons that bats are dying in large numbers. While in the past, intentional persecution (killing, poisoning etc.) was a big factor, collisions with wind turbines and the disease white-nose syndrome in North America now lead the reported causes of mass death in bats. Interestingly, we found very little evidence for other viral or bacterial diseases causing large die-offs.
The absence of viral or bacterial infectious diseases associated with die offs is somewhat surprising given that bats often live in dense aggregations which should be ideal for disease spread. So, is disease-induced mortality really uncommon in bats? I think there are a few possibilities. First, perhaps many bat infections occur endemically at low prevalence rather than in epidemics that cause large scale die offs? If that were the case, bats could be dying from viral or bacterial infections, but since deaths occur over months or years rather than days, we fail to take notice. Rabies virus is a good example there – no die offs, but individuals certainly die of infection. On the other hand, could bats be somehow “special” in not actually showing disease at the individual-host level? This is commonly argued for other viruses like Nipah, Hendra and Ebola, where it is suggested that exceptional immune systems or ancient co-evolved relationships with viruses could protect bats at the individual level in a way that is somehow different from other host species.
Teasing apart these explanations requires a lot of work. We need long term monitoring to look at sublethal effects of infections on bats and longer term effects of infection on lifespan and we need tools to be able to actually measure the health of these bats beyond very crude measures of alive, dead or nearly dead in the field. We need experimental infection studies that replicate natural conditions of exposure and infection. All of these things are costly, but necessary if we are to fully understand bat mortality and the relationship between bats and their natural pathogens.
O’Shea, T., Cryan, P., Hayman, D., Plowright, R., Streicker, D.G. (2016) Multiple mortality events in bats: a global review Mammal Review DOI: 10.1111/mam.12064
Check out the USGS Press release for this paper here
“Livestock-dense habitat functions as an ecological trap for vampire bats: immunological evidence”
4 January 2016
Society for Integrative & Comparative Biology Annual Meeting
Oregon Convention Center
Portland, Oregon, USA
Urbanization and agriculture cause declines for many wildlife, but some species benefit from human-provided food. The abundance and predictability of these provisioned resources can affect wildlife behavior and physiology and in turn alter infectious disease dynamics. Specifically, host condition and immune defense responses to resource shifts are key to understanding whether provisioning amplifies or dampens pathogen transmission. We here tested relationships between provisioning, host condition, and immunity through a cross-sectional study of vampire bats (Desmodus rotundus) in Peru and Belize. This hematophagous species has potentially benefited from intensification of livestock rearing, which could reduce starvation stress and energy spent during foraging, allowing bats to invest more in immunity. We predicted that bats captured in sites of high livestock density would be in better body condition, display less chronic stress and inflammation, and have higher investment in humoral immunity. Contrary to this prediction, we found increasing livestock density was associated with poorer condition, greater levels of stress and inflammation, and lower IgG, and that these negative health impacts were more severe in reproductively active bats. We also found that although livestock intensification was associated with impaired bat health, capture success increased with livestock density. Together these results suggest livestock-dense habitats could function as ecological traps by producing source populations of immunologically impaired vampire bats. Increasing the relative abundance of such habitat could have profound impacts on bat susceptibility to zoonotic pathogens and the spatial spread of infections.
Building on our previous paper studying the different mechanisms through which different host species can arise as “key hosts” for parasite transmission, this new paper with Andy Fenton, Owen Petchy and Amy Pedersen creates a quantitative framework for partitioning R0 among many host species. This is a really exciting advance because we’re able to assess how robust estimates are to uncertainty in rates of cross-species versus within-species transmission and everything is estimated from fairly standard parasitological data. Applying this framework to my undergraduate dataset on gut parasites in small mammals confirms the existence of key hosts even for outwardly multi-host parasites (cryptic specialization) and shows in a hypothetical way, how different control measures could be more or less effective when targeting based on species specific values of R0.
Data for the paper are available here: http://dx.doi.org/10.5061/dryad.972mv
Abstract: Many parasites circulate endemically within communities of multiple host species. To understand disease persistence within these communities, it is essential to know the contribution each host species makes to parasite transmission and maintenance. However, quantifying those contributions is challenging. We present a conceptual framework for classifying multihost sharing, based on key thresholds for parasite persistence. We then develop a generalized technique to quantify each species’ contribution to parasite persistence, allowing natural systems to be located within the framework. We illustrate this approach using data on gastrointestinal parasites circulating within rodent communities and show that, although many parasites infect several host species, parasite persistence is often driven by just one host species. In some cases, however, parasites require multiple host species for maintenance. Our approach provides a quantitative method for differentiating these cases using minimal reliance on system-specific parameters, enabling informed decisions about parasite management within poorly understood multihost communities.
Change in post!
Vicky Estacio has decided to stay on at Glasgow and will join the Masters in Quantitative Methods in Biodiversity, Consevation and Epidemiology.
“Impact of human-provided food resources on infectious disease dynamics”
8 July 2015
International Association of Landscape Ecology World Congress
Portland, Oregon, USA
“Heterogeneity in patch quality buffers metapopulations from pathogen impacts”
13 August 2015
Ecological Society of America Annual Meeting
Baltimore Convention Center
Baltimore, Maryland, USA
Complex effects of dietary provisioning on wildlife disease dynamics
Becker, D., Streicker, D.G., & Altizer (2015) Linking anthropogenic resources to wildlife-pathogen dynamics: a review and meta-analysis. Ecology Letters. doi: 10.1111/ele.12428
Abstract: Urbanisation and agriculture cause declines for many wildlife, but some species benefit from novel resources, especially food, provided in human-dominated habitats. Resulting shifts in wildlife ecology can alter infectious disease dynamics and create opportunities for cross-species transmission, yet predicting host–pathogen responses to resource provisioning is challenging. Factors enhancing transmission, such as increased aggregation, could be offset by better host immunity due to improved nutrition. Here, we conduct a review and meta-analysis to show that food provisioning results in highly heterogeneous infection outcomes that depend on pathogen type and anthropogenic food source. We also find empirical support for behavioural and immune mechanisms through which human-provided resources alter host exposure and tolerance to pathogens. A review of recent theoretical models of resource provisioning and infection dynamics shows that changes in host contact rates and immunity produce strong non-linear responses in pathogen invasion and prevalence. By integrating results of our meta-analysis back into a theoretical framework, we find provisioning amplifies pathogen invasion under increased host aggregation and tolerance, but reduces transmission if provisioned food decreases dietary exposure to parasites. These results carry implications for wildlife disease management and highlight areas for future work, such as how resource shifts might affect virulence evolution.
Table 1. Representative study systems and evidence for different mechanisms that might drive relationships between food and disease.
New media coverage from the Wellcome Trust
The Wellcome Trust asked me a few questions about science and turned it into a nice addition to their blog. You can find the post here: