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In the last major US monkeypox outbreak 19 years ago, a shipment of pet prairie dogs caught the virus while caged in close quarters with infected rodents. The disease ultimately spread to dozens of people who bought the playful and cuddly prairie dogs to keep as pets.
At the time, Lisa A. Murphy, a veterinarian, was attending a class in foreign animal diseases in Wisconsin, the same state where the first positive case was reported in 2003.
Out of nowhere, the room filled with a flurry of ringtones and vibrations as experts from top agencies like the USDA got notifications of the outbreak.
“The instructors’ cellphones started blowing up, and they started getting pulled out of the room,” said Murphy, who is now the associate director of the Institute for Infectious and Zoonotic Diseases at the University of Pennsylvania School of Veterinary Medicine.
Although veterinarians only treat animals, things can get a little bit blurry when it comes to diseases that can pass between species. Shortly after attending the training, Murphy received a call from someone with a sick pet prairie dog. She immediately warned them that their health might also be at risk.
“As veterinarians, we are trained to recognize zoonotic disease. And even though we are not human medical doctors, that training becomes a very important part of our job if there is a public health risk to human health,” she said.
In the 2003 US monkeypox outbreak, there were 47 cases in six Midwestern states over a period of about three months. By comparison, in approximately the same amount of time in this year’s outbreak, there have been 7,102 confirmed monkeypox cases in every state except Montana and Wyoming and 28,220 cases in 88 countries worldwide. Something has changed.
“What we’re seeing this year is very different from 2003, which was all animal-to-human transmission. The spread in this current outbreak seems to be purely human to human,” said Murphy. “That doesn’t mean human-to-animal or animal-to-human transmission hasn’t happened or couldn’t happen, but the whole flavor of this outbreak is just completely different.”
Public health officials are working hard to test and vaccinate people to limit the spread of the virus in the US and around the world. As part of the process, they are looking for the monkeypox virus in wastewater — essentially flushed toilet water that can contain virus shed in urine or fecal material. Experts do the same thing with coronavirus and other viruses, since it’s a good way to gauge how many people are really infected with a germ.
However, some questions remain: If the virus is in wastewater, can monkeypox spread to rats or other urban rodents known to consume waste? And if so, could the virus become permanently established in the US with rats or mice acting as a reservoir?
We spoke with experts to determine whether viral DNA found in sewage actually poses a risk for the spread of monkeypox, and what — if anything — people with the virus should be doing to limit the spread when they flush.
In some countries, all of them in Africa, monkeypox spreads freely among one or more local wild animal species, called animal reservoirs. If humans come into contact with those animals, the virus can sometimes infect people. In the midst of a current worldwide outbreak, there are concerns that animals could play a role in making this disease globally endemic as well, becoming permanent fixtures in countries where it hasn’t existed in the past.
The virus was discovered over half a century ago, in 1958, so much is already known about how it behaves. Even so, there are still a lot of unanswered questions, especially when it comes to the version that is currently spreading: Why are we seeing higher levels of human-to-human transmission than ever before? Which animals can and can’t get it, especially our pets, and can animals spread it back to us?
We know that many animal species are potentially susceptible to monkeypox. Less is known about whether they can get it from humans or vice versa.
Monkeypox can affect a particularly wide range of animal species. The CDC has warned of previous infections in monkeys, anteaters, hedgehogs, squirrels, shrews, and of course, prairie dogs. For other common species, including many kept as pets — dogs, cats, gerbils, guinea pigs, hamsters, mice, rats, and rabbits — their potential for contracting monkeypox is not yet known. However, all of these animals have previously been observed with other diseases in the orthopoxvirus family.
Not every animal that can get a zoonotic virus can necessarily contract it from every possible host. There is still much unknown about which animals can get monkeypox from humans and which, if any, animals can give the virus back to us.
“There’s a sort of an implication out there that disease is just freely moving between animals and people, but that’s not necessarily the case,” said Murphy. “Even with the 2003 monkeypox example, it seemed to go rodent–rodent–people. It didn’t then go from people to rodents or any other animal.”
Experts are studying whether monkeypox has undergone genetic modifications that might make it more infectious or likely to spread in humans. However, the strain affecting people around the world is “pretty much the same virus” as the one that caused the 2017–18 surge of the virus in Nigeria, said Heather Koehler, an assistant professor at Washington State University’s School of Molecular Bioscience who studies virus-host interactions in monkeypox. That outbreak also included human-to-human transmission and resulted in at least 122 confirmed or probable cases of the disease.
“I am not sure if we ever invested the resources to actually understand the transmission that happened there,” she said. “We know there’s an animal reservoir that is probably being driven closer to human populations, so there’s more spillover events. But, at some level, there has to be a critical threshold to where you’re getting enough people infected where you could be spreading it human to human. And maybe we just haven’t seen that until now.”
SARS-CoV-2 is the most contemporary example of the temperamental nature of zoonotic transmission. There are many instances in which humans passed the coronavirus to other species. The first was a dog in Hong Kong. Then, there were countless stories of animals at zoos and sanctuaries all over the world affected by the virus, including two tigers belonging to Carole Baskin, of Tiger King fame. Reports of wild deer with antibodies include concerns that the species could be reservoirs for the next great COVID infection. But so far, very few humans have contracted the disease from animals — only some with direct connections to mink farms.
“Some diseases just go one way to a sort of dead end. For COVID to go from just passing from deer to deer, something will have to happen that the virus changes. That’s the concern,” said Murphy. “Over time, as that virus is hanging out in deer, what is it picking up along the way that makes it possible to then jump back out and infect humans or other species?”
The Sewer Coronavirus Alert Network (SCAN), which tests wastewater solids for the presence of virus in areas near San Francisco and Sacramento, announced recently the detection of monkeypox in samples from the Bay Area. The idea of testing wastewater for the presence of viruses originated in the 1940s with polio. That disease, once a childhood scourge, paralyzed over 15,000 Americans each year in the 1950s before a vaccine was available. It was eradicated in the US in 1979, although there are occasional cases in travelers infected in other countries. However, poliovirus was recently detected in wastewater in New York, which may have come from an unvaccinated man who developed paralysis in July and was the first polio case detected in the US since 2013.
SCAN was launched in 2020 to detect the presence of SARS-CoV-2, but it has since expanded its effort to include tests for the presence of other infections, including respiratory syncytial virus (RSV) and the flu.
“This tool has existed for a long time, but it really experienced a renaissance with new investments in this as a public health tool during COVID,” said Marlene Wolfe, an assistant professor of environmental health at Emory University and coleader of SCAN.
News of monkeypox in wastewater has led some to spin hypothetical scenarios. In theory, if the wastewater infects rodents, and if rodents could then become forever reservoirs of the virus, the rodents spread the virus back to humans through their droppings, it would be…bad.
But experts say that’s a lot of ifs.
“I know that there will be much more research that will come out on how monkeypox can be spread on surfaces and things that people are much more likely to come in contact with than wastewater,” said Wolfe.
Some of these hype flames have been further fanned by new publicity granted to a 2007 study, which showed that orthopoxviruses could survive in stormwater for days or weeks, particularly in colder conditions. But Dr. Saahir Khan, an infectious disease specialist from USC Keck School of Medicine, pointed out that lab conditions are very different from what happens in the real world.
“While you may be able to have a virus survive on a surface and be cultured in a laboratory and still be viable, that does not mean it could be an actual source of human infections,” he said. “There was a lot of panic early in the COVID-19 pandemic because studies showed the virus could survive on surfaces for a long period of time, and everybody was washing their groceries. And, of course, we found that transmission from surface contact is incredibly rare.”
Furthermore, SCAN’s testing method looks only for genetic material, which does not necessarily represent live, infectious virus. Their test is very sensitive, amplifying the viral DNA by 1,000 times. For SARS-CoV-2, SCAN can detect even one or two cases in a population of 100,000. Their test’s reliability for monkeypox is still being determined. Murphy said the wastewater doomsday scenario is not totally impossible, but it requires too many unlikely conditions to be of primary concern.
“Not that it can’t, but probably not. Just because there is viral DNA or RNA in the wastewater, that’s very different than it being viable virus that is also a potential risk for infectious disease,” she said. “But even without the viruses in there, you wouldn’t want to be taking mouthfuls of that.”
Some folks subscribing to this theory have suggested that people with monkeypox pour bleach down their toilets as a protective measure for public health. Take it from Wolfe, an expert on virus in wastewater — don’t do that.
“I’m definitely pro cleaning your toilet, but I suggest that people focus on the transmission pathways that we have information about and work to interrupt those by following public health guidelines if they are affected,” she said. “Pouring bleach down your toilet is not in the guidelines at all.”
There is increased belief among infectious disease experts that the window to keep monkeypox from becoming globally endemic may have already closed. Khan believes any theory of how monkeypox might become embedded in global society should begin with the primary driver of new infections: humans.
“I haven’t seen convincing reports during this outbreak of someone getting monkeypox without being in close contact with a human who had a monkeypox infection,” he said. “I actually think it’s likely to become an endemic disease in the human population forever, even without an animal reservoir.”
Khan believes that this disease will not likely cause as much severe illness or societal disruption as SARS-CoV-2, but he has concerns for what an increased presence of this infection could mean for immunosuppressed individuals, like those with severe HIV or people who have received organ transplants.
“When, and at this point, I would say when it becomes an endemic disease, there’s going to be a subset that is at risk for significant complications from this virus,” he said. “And, you know, having another disease out there is never a good thing.”
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