More Than Half of New York City's Stray Cats Are Carrying Parasites. Here's Why That Matters.

Walk through almost any neighborhood in New York City and you'll probably spot a free-roaming cat lounging under a parked car, darting through an alley, or quietly watching the world from a vacant lot. They're part of the city's landscape, and for most people, they're easy to overlook. But beneath that familiar urban scene is a story most people never see.

Researchers studying New York City's stray cat population recently discovered that more than half of these cats are carrying intestinal parasites capable of contaminating the environment—and in some cases, posing a risk to both pets and people. Even more surprising? A relatively small group of young male cats appears to be responsible for much of that environmental contamination.

What the researchers found

Between May and July 2023, researchers partnered with trap-neuter-return (TNR) programs throughout New York City to collect fecal and blood samples from 87 free-roaming cats.

Their findings painted a concerning picture.

More than 57% of the cats tested positive for at least one parasitic infection, making gastrointestinal parasites far more common than many people might expect in an urban environment.

The most frequently detected parasite was Toxocara spp., a roundworm found in 54% of the cats. Hookworms (Ancylostoma spp.) were identified in nearly 14% of the animals, while coccidia were detected in just over 11%.

Researchers also screened for protozoal parasites and found Giardia in approximately 12% of the cats and Cryptosporidium in a smaller number of samples.

While some of these parasites primarily affect cats, others have important public health implications because they can infect humans under the right circumstances.

Meet the "super-shedders"

One of the study's most interesting discoveries wasn't simply how many cats were infected—it was which cats were contributing the most to environmental contamination.

Young male cats stood out.

According to the researchers, male cats were more than four times as likely to carry Toxocara than females. Even more importantly, males younger than one year of age shed dramatically higher numbers of parasite eggs into the environment.

Rather than every infected cat contributing equally, a relatively small population of juvenile males appeared to be acting as "super-shedders," releasing large numbers of parasite eggs into the soil.

For epidemiologists, that's a significant finding.

It suggests that targeted parasite control strategies focused on the highest-risk animals could potentially have a much greater impact than treating the population uniformly.

Why veterinarians should care

For veterinary professionals, this study serves as an important reminder that parasite prevention extends far beyond individual patients.

Toxocara is a well-known zoonotic parasite. People become infected by accidentally ingesting microscopic eggs from contaminated soil or other environmental sources. Young children are considered particularly vulnerable because they're more likely to play in dirt, sandboxes, and parks before washing their hands.

Although human infections are relatively uncommon, they can lead to serious conditions such as ocular larva migrans, which may affect vision, or visceral larva migrans, which can involve multiple organs.

The detection of Giardia and Cryptosporidium also reinforces the importance of environmental hygiene, particularly in densely populated urban settings where people, pets, and wildlife share common spaces.

A new role for Trap-Neuter-Return programs

The study also highlights an opportunity many veterinarians may not have considered.

Trap-neuter-return programs have traditionally focused on controlling free-roaming cat populations through humane sterilization and vaccination. However, the same programs could also become valuable tools for disease surveillance.

Because these cats are already being captured, examined, and handled, collecting diagnostic samples requires very little additional effort. That information can help researchers monitor parasite trends, identify neighborhoods with higher environmental contamination, and guide public health recommendations.

Rather than viewing TNR solely as a population management strategy, studies like this demonstrate how it can also contribute to our understanding of urban ecosystem health.

What this means for client conversations

Urban veterinarians are uniquely positioned to translate these findings into practical advice.

Clients often assume parasite prevention is primarily about protecting their own pets. This research reminds us that environmental exposure matters, too.

Talking with owners about routine fecal examinations, year-round parasite prevention, proper disposal of pet waste, covering children's sandboxes, and encouraging good hand hygiene can all help reduce the risk of environmental transmission.

The study also reinforces the importance of preventing pets from hunting wildlife or interacting with free-roaming cats whenever possible.

Looking beyond New York City

Although this research focused specifically on New York City, the findings likely extend beyond one metropolitan area.

Most large cities have established populations of free-roaming cats, creating similar opportunities for parasites to circulate between animals and contaminate shared public spaces.

That doesn't mean people should fear every stray cat they encounter.

Instead, it highlights the importance of combining humane population management with ongoing disease surveillance and public education. Understanding where parasites are most common—and which animals contribute most to their spread—can help veterinarians, public health officials, and community organizations make smarter decisions about prevention.

The bottom line

Free-roaming cats are likely to remain part of urban life for the foreseeable future. The challenge isn't simply managing their numbers—it's understanding the diseases they carry and minimizing the risks those diseases pose to both animals and people. This study offers valuable insight into that challenge. By integrating parasite surveillance into existing trap-neuter-return programs, researchers demonstrated that we can learn far more about urban ecosystem health without changing the humane practices already in place. Sometimes the biggest public health discoveries aren't found in the laboratory. Sometimes they're quietly roaming the streets, waiting for us to pay closer attention.

Reference: This study evaluated fecal and blood samples collected from 87 free-roaming cats participating in trap-neuter-return programs in New York City between May and July 2023, investigating gastrointestinal and vector-borne parasites and their potential public health significance.