Ecology Determines Rabies Infection in Bats

A new approach to rabies virus epidemiology in bats shows that the risk of infection is higher in large and multispecies colonies. The research, published on the journal PLOS ONE, was led by Jordi Serra-Cobo, professor from the Department of Animal Biology at the UB and the Biodiversity Research Institute (IRBio).

Bats are a large group of mammals that appeared in our planet around 65 million years ago (Figure 1). They have colonized many natural habitats —except the poles—, and act as primary predators of vast numbers of insects in ecosystems. They are also the mammals which present the widest variety of virus infection (rabies, SARS, Ebola, etc.). Moreover, they are able to neutralize virus and survive infections. “Chiropters, a quite ancient animal group, are major reservoirs for diverse infectious viral diseases”, highlights Serra-Cobo. They have co-lived with virus for a long time and their immunological responses are more effective. According to Serra-Cobo, “this fact opens new research lines on the organisms’ immunological response and strategies to fight against infectious diseases”.

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Figure 1. A vespertilionid bat from Spain, one of the more than 1,150 bat species.

It is the first time that a research analyses ecological factors that might affect the infection dynamics of the rabies virus in bat colonies. Between 2001 and 2011, 2,393 blood samples were collected from 20 bats species and 25 localities in Catalonia, Aragon and Balearic Islands. The research is centered on the detection of
European bat Lyssavirus 1 (EBL1), one of the twelve different groups of the genus Lyssavirus related to rabies, an emergent zoonosis that affects mammals all over the world.

Jordi Serra-Cobo explains that “EBLV-1 seroprevalence is strongly affected by colony size and species richness. Previous studies have analyzed other aspects such as the seasonal variability. Ecological factors play a relevant role in seroprevalence variability, but they were to date unknown” (Figure 2).

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Figure 2. Variation in the percentages of seropositive bats as a function of species richness and colony size. (from Serra-Cobo et al. 2013)


All bat species do not response in the same way to viral infections. This research proves that immunological response to rabies virus varies among species. “Order Chiroptera has been widely diversified along its evolutionary history —affirms Serra Cobo— and their responses to ultrasound orientation mechanisms, immunological defense, etc. vary with different lineages”.

There are more than 1,150 bat species all over the world and new specimens are described every year. However, the loss of natural habitats due to human activity and climate change poses a major threat to bats. “It is a process of environmental degradation which favors the formation of larger bat colonies, which have a higher probability of EBLV-1 infection”, remarks Serra Cobo.
 
Spain has been free of rabies in terrestrial mammals since 1977. Nevertheless, in some countries it continues to be a problem of public Health. To know risk factors involved in viral disease transmission is essential to improve preventive policies. The new article published on
PLOS ONE will provide new tools to know more about viral infections epidemiology and natural resources management.

Source: Modified from materials provide by The Universitat de Barcelona.


Reference

Jordi Serra-Cobo,, Marc López-Roig,, Magdalena Seguí,, Luisa Pilar Sánchez,, Jacint Nadal,, Miquel Borrás,, Rachel Lavenir,, & Hervé Bourhy (2013). Ecological Factors Associated with European Bat Lyssavirus Seroprevalence in Spanish Bats PLOS ONE DOI: 10.1371/journal.pone.0064467.t003


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Warm Spring Weather Favors Female Newborns in Bats

Source: from materials provided by University of Calgary.

There must be something in the warm breeze. A study on big brown bats (Figure 1) suggests that bats produce twice as many female babies as male ones in years when spring comes early.

USFWS Headquarters
Figure 1. A hibernating big brown bat (Eptesicus fuscus). (From USFWS headquarters)

The earlier in the spring the births occur, the more likely the females are to survive and then reproduce a year later, as one-year olds, compared to later-born pups, according to Robert Barclay’s research published in PLoS ONE (Figure 2).

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Figure 2. Seasonal variation in Eptesicus fuscus offspring sex ratio with birth date at three colonies in Medicine Hat, Alberta, Canada from 1990 to 2004. (From Barclay 2012)

“The early-born females are able to reproduce as one year olds, whereas male pups can't,” explains Barclay, professor in the Department of Biological Sciences.

“Thus, natural selection has favored internal mechanisms that result in a skewed sex ratio because mothers that produce a daughter leave more offspring in the next generation than mothers who produce a son.”

The length of the growing season has an impact on the ratio of female to male offspring and the time available for female pups to reach sexual maturity, the study found. This suggests that not only does sex-ratio vary seasonally and among years, but it also likely varies geographically due to differences in season length.

Barclay analyzed long-term data on the variation in offspring sex-ratio of the big brown bat,
Eptesicus fuscus, a common North-American species that consumes insects.

“In this species, more eggs are fertilized than eventually result in babies, so there is some mechanism by which a female embryo is preferentially kept and male embryos are resorbed early in pregnancy,” says Barclay. But, he adds, the biochemistry behind the skewed sex ratio is unknown.

“Some other mammals and some birds have the ability to adjust the sex ratio of their offspring,” says Barclay. “Even human-baby ratios vary—there is a study showing that billionaires produce more sons than daughters, for example.”

This is the first long-term study on sex ratios in bats, says Barclay and it “suggests some pretty interesting physiology.”


References

Barclay, R. (2012). Variable Variation: Annual and Seasonal Changes in Offspring Sex Ratio in a Bat PLoS ONE, 7 (5) DOI: 10.1371/journal.pone.0036344


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