Why would a virus fatten an animal?

We know parasites such as toxoplasma change the behavior of their hosts [1]. That seems to make nice evolutionary sense. But why would a virus induce obesity in some animals?

Contagious obesity? Identifying the human adenoviruses that may make us fat | Science Blog

Ad-37 third virus implicated in animal obesity

The theory that viruses could play a part in obesity began a few decades ago when Nikhil Dhurandhar, now at Pennington Biomedical Research Center at LSU, noticed that chickens in India infected with the avian adenovirus SMAM-1 had significantly more fat than non-infected chickens. The discovery was intriguing because the explosion of human obesity, even in poor countries, has led to suspicions that overeating and lack of exercise weren't the only culprits in the rapidly widening human girth. Since then, Ad-36 has been found to be more prevalent in obese humans.

In the current study, Whigham et al. attempted to determine which adenoviruses (in addition to Ad-36 and Ad-5) might be associated with obesity in chickens. The animals were separated into four groups and exposed to either Ad-2, Ad-31, or Ad-37. There was also a control group that was not exposed to any of the viruses. The researchers measured food intake and tracked weight over three weeks before ending the experiment and measuring the chickens' visceral fat, total body fat, serum lipids, and viral antibodies.

Chickens inoculated with Ad-37 had much more visceral fat and body fat compared with the chickens infected with Ad-2, Ad-31 or the control group, even though they didn't eat any more. The Ad-37 group was also generally heavier compared to the other three groups, but the difference wasn't great enough to be significant by scientific standards.

The authors concluded that Ad-37 increases obesity in chickens, but Ad-2 and Ad-31 do not. "Ad-37 is the third human adenovirus to increase adiposity in animals, but not all adenoviruses produce obesity," the study concluded.

There is still much to learn about how these viruses work, Whigham said. "There are people and animals that get infected and don't get fat. We don't know why," she said. Among the possibilities: the virus hasn't been in the body long enough to produce the additional fat; or the virus creates a tendency to obesity that must be triggered by overeating, she said.

It certainly makes sense to try and figure out how host adiposity could benefit a virus, but for no good reason I suspect a tendency to produce fat is a side-effect that's irrelevant to the virus.

[1] BTW, why do some dogs compulsively eat grass? Since grass eating is associated with Giardia infection, one might consider that bug. Or maybe a parasite who's life cycle involves deer ticks and who causes colitis in dogs ...