Science’s image as an objective collection of facts took another hit this week, as researchers report that a common fish parasite may have skewed the results of thousands of behavioral science studies. They published their findings in the journal Fish Diseases.

Pseudoloma neurophilia is a teeny-tiny type of parasitic fungus called a microsporidium. Microsporidia are prolific and diverse, infecting almost every type of animal on Earth. Symptoms of a microsporidium infection vary by parasite and host species. Sometimes it’s fatal. Other times the effects are so subtle that you’d really have to be looking to notice them.

Fortunately, some scientists are looking. Researchers in the biomedicine and microbiology departments at Oregon State University (OSU) have been tracking P. neurophilia for years—specifically, its effect on Danio rerio, commonly known as the zebrafish.

If you follow science news, you’ve probably heard about zebrafish before. These unassuming little fish have become some of the most popular research animals in the world, thanks to their low-maintenance lifestyle, susceptibility to drugs and genetic changes, and enormous broods. They’re also incredibly social, which has led researchers to consider them a good model for people. Consequently, we use them to test pharmaceuticals, hunt for clues to genetic disease, and even explore the roots of human behavior.

Studies of D. rerio’s social activity often center around one specific behavior: huddling. Stressed zebrafish band together in groups called shoals, while healthy, calm fish tend to spread themselves wider apart. So researchers generally assume that clustering fish have been negatively affected in some way by experimental treatment, whether that’s a drug or a gene associated with disease.

But there’s more to it, say the OSU scientists. Their studies of P. neurophilia suggested to them that the parasite could be quietly changing zebrafish behavior. To find out, they brought 140 zebrafish into the lab and divided them into 12 tanks of 10 fish and one “sentinel tank” of 20. They set up cameras by the 12 test tanks and took still images at regular intervals.

Then, the team added water to all the tanks. Six exposure tanks, and the sentinel tank, were topped up with water from a tank of infected fish. The remaining six tanks got water from a parasite-free tank.

Once again, the researchers trained their cameras on the swimming fish, monitoring their movement. Analysis of the tank snapshots showed that fish exposed to the parasite were more likely to stick together than fish in the clean tanks. The researchers noted that fish in the parasite-treated tanks stuck even more closely together than fish in other studies who had been dosed with stress-inducing chemicals.

Postmortem examinations of fish from each group confirmed that the presence of P. neurophilia in the water was more than enough to infect a tank’s inhabitants. None of the fish from the control tanks were infected, but nearly all the fish from the parasite-treated tanks were.

The team’s earlier work has shown that infection with the microsporidium is very, very common in laboratory zebrafish populations. Lead author and veterinary surgeon Sean Spagnoli noted that some researchers might not even check to see if their fish are sick.

“I haven't seen a single paper that stated that ‘fish used were certified pathogen-free for P. neurophilia'," he told Nature.

This wouldn't be the first time scientists have overlooked a big variable. A study published earlier this year found that lab mice get chilled, stressed, and sick at normal laboratory temperatures. Another concluded that software design issues may have led to false positives in thousands of brain scan studies.

University College London geneticist Elena Dreosti isn't sure that's what's happening here. Speaking to Nature, she argued that the study’s results are statistically weak.

“Considerable additional work is needed to know if this is likely to have a significant impact on the type of behaviour research that is done by the community working with zebrafish,” she said in Nature. Other researchers have expressed doubts about the precision of measuring intra-fish distances through snapshots.

The OSU team stands by their methods and findings, but will continue to study the issue.

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