Scattered in cat sandboxes and dog runs around the world, there’s a parasite invisible to the naked eye that mostly infects these creatures. It’s a tiny worm that lives in the animals’ muscles and digestive systems, but can spread to others.
Sometimes, though, it infects humans. Increasingly researchers are concerned that children are inadvertently consuming these parasites, known as Toxocara, and so they are studying places where humans and pets come into contact with one another: city parks.
In New York City, one of the locations of the studies performed in recent years, these parasites are present in parks in all five boroughs, and there are more of them in parks in more impoverished neighborhoods. Around the world other researchers are also finding Toxocara in parks.
The presence of Toxocara is not necessarily a cause for alarm, the researchers say. It’s not that easy for humans to become infected – they must literally consume infected dirt. But this means children are at a higher risk. Even so, the scientists say, most infections are asymptomatic, though sometimes symptoms may include fever, fatigue, coughing, rash, or abdominal pain. In very rare cases, the parasite can reach the eye, causing blindness, and cause neurological damage in the brain.
In the US each year, at least 70 people, mostly children go blind as a result of toxocariasis, the disease caused by Toxocara, but the researchers say that is probably an underestimate, as a general pediatrician might not easily recognize the illness.
“People, especially those with children who tend to play in the sand or soil, should be aware of Toxocara,” said Donna Tyungu, lead author of the New York paper and a pediatric infectious disease specialist who researches parasites at the University of Oklahoma. “Pet owners should be aware that their dogs can get infected by something they eat off the ground.”
In the New York study, published in 2020, the scientists took samples from 91 parks and playgrounds in the five boroughs. The highest number of positive samples came from the Bronx, where more than half of the population are people of color and the median household income was roughly $42,000 in 2020. Toxocara eggs were found in 66.7% – or 10 of 15 – of tested playgrounds.
Manhattan, by comparison, has the highest median income but the lowest percentage of contamination, with 29.6% – or eight of 27 – of sites testing positive for Toxocara eggs. In general, the eggs all over the city tended to come from a species of Toxocara specific to cats rather than dogs.
“In a lower socio-economic area, there’s a lot more stray dogs and cats roaming around,” Tyungu said. The higher contamination rate may also be related to the ability of pet owners to pay for regular veterinary check-ups and deworming.
“I think their finding of Toxocara is both important and unsurprising,” said Thomas Nutman, chief of the laboratory of parasitic diseases at the National Institute of Allergy and Infectious Diseases. “It’s really important for us to understand that these neglected tropical diseases, Toxocara being one of them, are still a huge problem the extent of which we don’t yet know, because we really haven’t looked.”
These parasite eggs can make it through New York’s brutal winters, thanks to their protective cellular layer that helps them stay alive for months or even years. “In the wintertime or when it’s cold, the eggs are pretty hardy,” said Rojelio Mejia, one of the co-authors of the paper and a pediatric tropical diseases researcher at Baylor College of Medicine. “They can survive quite extreme environments in the soil, or in their animal hosts.”
The New York City parks and health departments did not respond to requests for comment.
An estimated 5-14% of all Americans have had exposure to Toxocara, with minorities, people living in poverty, and those in rural communities exposed to it at higher rates. Toxocariasis is one of five neglected parasitic infections prioritized by the CDC as requiring urgent public health action in the US.
Researchers in Hanover, Germany sampled sand from 46 playground sandpits, finding eggs in up to 23.9% – or 11 of 46 – with higher rate of infective eggs in January and February. In another study in Valencia, Spain, Toxocara was not found in parks and playgrounds, probably due to restricted animal access, but the eggs were discovered in dog parks.
Ways to rid parks of Toxocara eggs include raising awareness among parents and pet owners, deworming pets, and installing more fencing to keep stray animals out. Tyungu says it is crucial for children to wash their hands with soap and water after playing outside, and especially before handling food or eating, noting that antibacterial sanitizers don’t remove the Toxocara eggs.
“Preferably use soap and water,” Tyungu said. “But if you have nothing else if you’re in the park having a picnic, just pour a bottle of water over the children’s hands to rinse off anything that might be there, then follow with sanitizer for the other things.”
Video clip of the Toxocara parasite moving under the microscope courtesy of Donna Tyungu and Rojelio Mejia