How to peacefully coexist with potentially dangerous species

Light Switch

How to peacefully coexist with potentially dangerous species

Biodiversity keeps rabid vampire bats feeding on their natural prey – not on us

Vampire bats are terrifying, but probably not for the reason you think. The bats themselves aren’t particularly dangerous; they only drink a few tablespoons of blood at a time and most of their victims are animals. But vampire bats can carry a deadly passenger: rabies.

In recent years, there have been as many as 55 reported cases per year of human rabies acquired from vampire bats in South America. Vampire bats are an even bigger threat to livestock: rabies kills thousands of cattle each year in Central and South America. In a survey of agricultural communities in Southern Peru, it was estimated that rabies transmitted by vampire bats resulted in $170,000 in losses in a single year. Losses of this size can be devastating for small-scale farmers.

Worldwide rabies elimination efforts have focused on rabies transmitted to humans by dogs. Overall, these efforts have been quite successful: in the Americas, the rate of rabies transmitted by dogs has fallen dramatically since the 1980s. The World Health Organization seems to be making progress toward its lofty goal of eliminating dog-to-human rabies transmission by 2030.

But controlling rabies in bats has proved much trickier. Although vampire bats can technically be vaccinated against rabies, vaccinating wild animals in such enormous numbers is unlikely to be a feasible solution. Many countries use culling as a strategy to control vampire bat rabies transmission. However, recent research suggests that this method is largely ineffective for disease control and indeed, cases of rabies in bats don’t appear to be decreasing. Worse, methods of culling (which have included lethal doses of anticoagulants, dynamite, and cyanide gas) can have disastrous environmental effects.

Right now, it appears that vaccinating humans and livestock is the most effective way to protect against rabies transmitted by vampire bats. But designing mass vaccination campaigns is difficult, especially in countries that have relatively high poverty rates. Collecting data on populations (human and non-human) that are most at risk for contracting diseases is essential for building infectious disease models — computer programs that help us predict how diseases spread in the same way that meteorological models help us predict the weather. These models are necessary for designing cost-effective vaccination campaigns. Indeed, disease models have been used in the design of vaccination campaigns against everything from rotavirus (which causes severe diarrhea) to human papilloma virus (HPV; which can cause cancer).

A recently published paper has used a new DNA screening method to determine which animals vampire bats feed on (and therefore are at risk for bat-transmitted rabies). Researchers collected samples (either the stomach contents or feces) from bats in different regions of Peru. They then used a technique called DNA metabarcoding to analyze the samples. Metabarcoding allows researchers to look for specific pieces of DNA – in this case, they were looking for DNA from a finite number of mammals, birds, and insects that the bats might have eaten.

This is different from another common technique called metagenomic sequencing. With metagenomic sequencing, every single piece of DNA from a sample is analyzed. If the scientists had used this technique, they would have had to analyze a lot of extra information, including potentially millions of genes from the bacteria that live in the gut. Because metagenomic sequencing analyzes so many different genes, this technique is much more expensive per sample, which means researchers can’t analyze as many samples. Using metabarcoding instead meant that the scientists could analyze the diets of a much larger number of animals: in this study, scientists were able to survey more than 100 bats.

The researchers found that bats mostly fed on domestic animals. Bats seemed to prefer cows and pigs, but also fed on sheep, donkeys, horses, and chickens. In one region, bats also fed on wild tapirs. Despite being native to the Americas, vampire bats mostly feed on non-native domestic animals. Scientists have speculated that this could be because domestic animals are generally kept in the same location every night, making them a reliable food source. Although vampire bats sometimes feed on native mammals like brocket deer and tapirs, these species are being over-hunted in some areas, which could potentially also contribute to the bats' apparent preference for livestock. In their samples, scientists also found DNA from insects like ticks that the bats may have consumed during grooming or while feeding on tick-infested mammals.

Although these data are useful, it’s important to note that this study only focused on a few regions in Peru. It is not yet known whether the findings from this study will be applicable to the many other countries in Central and South America where vampire bats reside. Much more information on the feeding patterns of vampire bats will be needed before effective disease models (and, in turn, vaccination campaigns) can be developed.

Using DNA metabarcoding to assess vampire bats' diets could not only be important for developing vaccination campaigns but also for assessing how environmental changes or hunting practices affect the patterns of vampire bat attacks on humans and livestock. As mentioned above, researchers found that the bats sometimes fed on tapirs. The authors note that, “Tapir are commonly hunted by Amazonian communities where they occur and can be rapidly depleted due to their slow reproductive rates.” 

Indeed, it’s estimated that lowland tapir populations have fallen by 30 percent in the past 30 years. As tapir populations fall, do vampire bats increasingly turn to humans for food, thus increasing the risk of bat-to-human rabies transmission? Conversely, could environmental restoration efforts provide bats with more wild animal sources of food and thus reduce rabies risk for humans and livestock? This information could be useful for supporting habitat restoration efforts or cautioning against interfering with vampire bats’ native prey species.

Although bats often carry viruses that are dangerous to humans, they are also an important part of many ecosystems. Using techniques like DNA metabarcoding combined with infectious disease modeling, we can predict which populations are most in danger of contracting diseases and take precautions like vaccinating at-risk individuals. Thus, new techniques like the one described in this paper could help us to peacefully co-exist with even potentially dangerous species.