Personality and the Spread of Disease

Studies of the spread of infectious diseases have shown that behavior plays a strong role in which individuals are more likely to be infected and which ones aren't. For example, sexually transmitted diseases (STDs) are more commonly diagnosed in people that have more sexual partners. But despite our understanding of how diseases are spread among people, we know very little about the spread of diseases among wild animals. Do their personalities play a role in the spread of wildlife diseases?

Laurie Dizney and Denise Dearing at the University of Utah recently investigated the relationship between behavior patterns and infection status in wild deer mice, Peromyscus maniculatus. They focused on the spread of hantavirus, a genus of viruses that are a great public health concern for people worldwide and are commonly spread by rodents. Specifically, they focused on a hantavirus ironically called Sin Nombre Virus (SNV; translated as "unnamed virus"). Although SNV does not appear to cause symptoms in deer mice, it is estimated to have a 35% mortality rate in people (not something I would want to mess with). Among deer mice, it is thought to be spread through aggressive encounters.

Studying the behavior of mice in the wild is no small undertaking (think about it... How would you do it?), but Dizney and Dearling developed a sophisticated deer mouse surveillance system. This system, located in the Great Basin Desert of central Utah, consisted of nine camera stations arranged in a grid. Each camera station had an elevated infrared camera (to photograph the nocturnal activity of the deer mice) that took four pictures per second and sent these images to a central computer powered by a generator. Each camera was focused on a foraging tray that contained millet seed (a tasty food for these critters) in sand. Half of these trays were placed in open and exposed areas; the other half were placed in protected areas under sagebrush cover.

The researchers trapped deer mice in the area. When they caught one, they recorded its weight, sex, reproductive status and whether it had scars. They also collected a small blood sample to test for SNV antibodies. Before they released it, they implanted it with a passive integrated transponder (PIT) tag, which is like an internal barcode that can be used to identify and track individuals (kind of like the one spy Aaron Cross cut out of his side in The Bourne Legacy). A PIT antenna with a data logger was then placed under each foraging tray in the camera array system to identify which animals came to each tray and when.

When Dizney and Dearling observed the deer mouse behavior from the collected images and data loggers, they found that these deer mice forage alone and avoid one another 99% of the time. When they do interact, almost half of those interactions are aggressive. When the researchers looked at infection status, they discovered that infected deer mice were much more likely than uninfected ones to engage in risky behaviors: They travelled further, spent more time on the foraging trays (especially the exposed ones), and engaged in more aggressive interactions. These bold deer mice were three times more likely to be infected with SNV than shy deer mice!

Although there is definitely a relationship between these bold behaviors and SNV infection status, we can't tell from this data whether the animals that are bold are more likely to become infected or if the infection makes animals bolder. Both of these effects are possible. If SNV is transmitted to other deer mice through aggressive encounters, then animals that engage in risky behaviors that increase the chances of those encounters happening are more likely to contract SNV. But viruses that change the behaviors of their hosts to increase the likelihood of spreading the virus to new hosts are not unheard of. Rabies virus, for example, often makes their hosts more aggressive, which increases the chances that the infected animal will bite another and spread the virus through its saliva. In this case, it is also possible that both of these phenomena are occurring. The only way to know for sure would be to observe the behavior of deer mice both before and after they are infected with SNV.

Although we have known of how behaviors affect the spread of many diseases in humans, this is the first study to look at this relationship in wild animals. While many scientists are not even on the same page as to whether animals have personalities, this research applies this concept to learning more about how infectious diseases are spread among animal populations. This could be a valuable direction of research, not just in understanding animal ecology, but also the spread of emerging infectious diseases in us humans.
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Further reading:
Dizney, L. and Dearing, M.D. The role of behavioural heterogeneity on infection patterns: implications for pathogen transmission, Animal Behaviour, 86, 911-916 (2013). DOI: 10.1016/j.anbehav.2013.08.003.

Image Credits:
Peromyscus maniculatus by the CDC and the Partnership, Inc. at Wikimedia Commons