Scientists are trying to determine whether Zika has infected nonhuman primates in South America. Fecal samples offer a way to find out.
Most emerging infectious diseases affecting people are zoonotic—they make the jump from other animals to humans. Transmission, however, is a two-way street. These zoonotic diseases can also jump from humans to other animals. Even if we eradicate a disease in humans, it can live on in animals that act as reservoirs, ensuring that the risk of human infection never entirely goes away.
In an attempt to discern this risk for the Zika virus, researchers need a noninvasive way to test the animals for the virus.
“These animals could be reservoirs and humans could then become infected with the virus from the animals,” says Krista Milich, assistant professor of biological anthropology at Washington University in St. Louis.
This happens in related viruses, she says. “There are feedback cycles, where humans are being exposed through non-human primate reservoirs,” says Milich, who along with Benjamin Koestler, a postdoctoral fellow in molecular biosciences at the University of Texas, Austin, led the study in PLOS ONE. The researchers determined that the Zika virus can be detected using feces samples.
The virus can be found in a host of bodily fluids: semen, urine, blood, and saliva. Getting those samples, however, requires capturing the animals; a risky proposition for all primates involved.
In humans, “you’re not going to ask for a fecal sample when you can take a blood sample.”
The easier option would be to test for the virus in feces, but only one research team has reported success in doing so, and that team did not publish its methods. “Ours goes on to provide the method used so people can follow it if they want,” Milich says.
It’s not surprising that there is not yet a method for collecting the virus in feces. Although Zika has been present in Africa and Asia since the 1940s, researchers hadn’t been looking for it in the Americas until it arrived around 2015. Once researchers did start to look for the virus in the Americas, the tests available were of other fluids, not feces.
That, Milich says, is because wildlife research is often “limited by what assays and what methods have been developed for humans.” And for humans, she says, “you’re not going to ask for a fecal sample when you can take a blood sample.”
Wild creatures and former pets
The team also wasn’t sure if it would be possible for another reason. “RNA viruses can be fragile,” Milich says, referring to viruses, such as Zika, which have RNA as genetic material, as opposed to DNA.
“That’s the reason we weren’t sure if we’d be able to detect it,” she says. “In feces, the body is breaking things down. … And you have a lot more contaminants. In order to measure things in feces, you have to deal with those contaminants.”
Before heading to the field to test its methods, the team had to make sure its test was accurate. They procured blood, urine, saliva, and fecal samples from captive, infected squirrel monkeys that were part of an ongoing study of Zika virus in pregnant monkeys.
Using proven methods, researchers tested the blood, urine, and saliva for the virus. They then tested the feces, using RT-qPCR-based assay (More about the methods is available in the paper). The results matched; they were able to detect the virus in feces, though detection time windows varied between different sample types.
The group has now moved on to testing samples from the wild. So far, the researchers have collected multiple samples from nearly 50 animals. The samples are coming from sites as varied as the middle of the Amazon, where people aren’t around and there shouldn’t be Zika infections, Milich says, to, unfortunately, animals that had previously been kept as pets.
“We’re looking for diversity,” Milich says, “not just in terms of countries and species, but also in how close the animals are in proximity to humans.”
As of February, the team had collected samples from Ecuador, Colombia, and Brazil.
Ideally, Milich says, people should not be keeping monkeys as pets, nor should they be hunting monkeys, either. “In fact, you have a greater risk of sharing diseases with animals when you’re either living in close proximity with them or when coming into contact with their body fluids, which is what happens when you kill them,” she says, adding that this was how zoonotic diseases such as HIV spread to humans.
“If the primates are infected with Zika, hopefully the message will not be these animals are terrible we need to get rid of them,” she says. “But it will be, we need to respect these wild animals and not come into contact with them.”