More than half of New York City's free-roaming cats are infected with at least one internal parasite capable of spreading to humans, according to a new study published in PLOS One this week — with Toxocara roundworms, a parasite whose larvae can migrate to the eyes and cause vision loss in humans, found in more than half of infected cats.
The study, conducted by researchers from Long Island University's College of Veterinary Medicine and Rowan University's Shreiber School of Veterinary Medicine, examined cats enrolled in New York City's Trap-Neuter-Return (TNR) program, according to EurekAlert's report on the findings. Of 57.5% of sampled cats who tested positive for at least one zoonotic parasite, the single most prevalent was Toxocara — present in 54% of infected cats — followed by hookworm (13.8%), coccidia (11.5%), and Giardia (11.6%).
Blood testing also revealed that 8.9% of cats had antibodies to Toxoplasma gondii — the parasite responsible for toxoplasmosis, which poses serious health risks to pregnant women, newborns, and immunocompromised individuals.
Critically, young male cats were found to be what researchers called "super-shedders" of Toxocara eggs, depositing dramatically higher concentrations of eggs into urban soil than other cats — eggs that can survive New York City winters and remain infectious for years.
Why This Matters
New York City's five boroughs contain some of the densest human populations on earth — and an estimated hundreds of thousands of free-roaming cats. These cats move through parks, community gardens, children's playgrounds, and sandboxes. Their feces deposit zoonotic parasites into soil that humans, particularly young children, frequently contact. Toxocara eggs in particular can survive in soil for years, including through freeze-thaw cycles, making contamination cumulative rather than seasonal.
"Because Toxocara eggs are highly robust, remaining infective in soil for years and withstanding the northeastern winter freeze-thaw cycles, environmental shedding by these cats presents a long-term public health risk," the researchers wrote, according to Gizmodo's coverage.
Children — who play in contaminated soil, put their hands in their mouths, and may directly contact cat feces in sandboxes — face the greatest risk of Toxocara infection. Pregnant women and immunocompromised individuals face distinct but serious risks from Toxoplasma. These are not theoretical concerns: prior research has detected Toxocara eggs in New York City public spaces in all five boroughs, with the highest concentrations found in the Bronx.
What We Know So Far
The study, authored by Nguyen V., Gurtowski E., and Chen J. and published in PLOS One, analyzed cats enrolled in NYC's TNR program — a population of free-roaming cats that are captured, neutered, and returned to their outdoor territory. This population is considered representative of free-roaming urban cats more broadly.
Key findings:
- 57.5% of sampled cats tested positive for at least one internal zoonotic parasite
- Toxocara (roundworm): Found in 54% of infected cats; larvae can migrate to human organs including eyes
- Hookworm (Ancylostoma) : Found in 13.8% of cats; can penetrate human skin and cause cutaneous larva migrans
- Coccidia : Found in 11.5% of cats
- Giardia : Found in 11.6% of cats; causes diarrheal illness in humans
- Toxoplasma gondii antibodies : Found in 8.9% of cats, indicating past or current infection
- Super-shedder pattern : Young male cats deposited dramatically higher concentrations of Toxocara eggs than other cats
- No heartworm antigen detected ; no Cytauxzoon felis detected
The researchers noted that the findings "underscore the public health risks associated with unmanaged feline populations in densely populated urban centers."
Where the Risk Is Highest
Prior research on Toxocara soil contamination in New York City, published in PLOS Neglected Tropical Diseases in 2020, found infective Toxocara eggs in public spaces in all five boroughs, with the highest contamination rates in the Bronx (66.7%) — a borough with higher rates of poverty, more stray cat populations, and fewer resources for routine deworming.
The current study's finding that young male cats are super-shedders has geographic implications: wherever young male cats concentrate — which in NYC may correlate with areas that have large feral cat colonies — the soil contamination risk is highest. TNR programs help stabilize and gradually reduce feral cat populations, but the cats returned to their territories continue shedding parasites in the interim.
Children's playgrounds and sandboxes are particularly high-risk environments because cats often use loose soil and sandbox material as litter areas, and because children in those spaces have behavioral patterns — hand-to-mouth contact, playing in dirt — that maximize ingestion risk.
What Researchers and Experts Say
Dr. Ayesha Bryant, clinical advisor at Alpas Wellness, told Newsweek that the study illustrates the One Health principle — the interconnection between animal, human, and environmental health. "This study is a good example of how animal, human, and environmental health are closely related to each other," Dr. Bryant said. "The fact that over 50 percent of sampled free-roaming cats had zoonotic parasites doesn't necessarily mean that 50 percent of humans will become infected with these parasites; however, it reinforces the need to reduce contact with contaminated soil and animal waste."
"The results of this study provide evidence for effective methods for prevention rather than cause for concern," Dr. Bryant added.
The research team emphasized prevention over alarm. TNR programs reduce cat populations over time; routine deworming of pet cats is highly effective; and straightforward hygiene measures — handwashing, keeping sandboxes covered, discouraging cat access to children's play areas — directly reduce human exposure risk.
What the Evidence Shows — and What It Does Not
This is an observational prevalence study, not an outbreak investigation. It documents the rate of parasite infection among a sample of free-roaming NYC cats, establishing a baseline of risk in the urban cat population. It does not document specific human infections caused by these cats, and it does not quantify how much of the observed soil contamination in NYC can be attributed specifically to free-roaming cats versus pet cats or dogs.
The finding that young male cats are super-shedders is novel and has practical implications for which cats in a population pose the highest shedding risk — a consideration relevant to TNR program design and prioritization.
MedicalDaily Evidence Check
- Study type: Cross-sectional prevalence study
- Published: PLOS One , July 2026; EurekAlert coverage July 8, 2026
- Institutions: Long Island University College of Veterinary Medicine; Rowan University Shreiber School of Veterinary Medicine
- Sample: Free-roaming cats enrolled in NYC Trap-Neuter-Return program
- Key finding: 57.5% of sampled cats infected with at least one zoonotic internal parasite
- Most prevalent parasite: Toxocara (roundworm) in 54% of infected cats
- Toxoplasma: 8.9% of cats had antibodies (past or current infection)
- Novel finding: Young male cats are super-shedders of Toxocara eggs
- What it does not show: Specific human infections attributed to these cats; quantified soil contamination from this sample
Who Is at Greatest Risk?
Children under 5: Play patterns (hand-to-mouth, contact with soil and sand) maximize Toxocara exposure risk. Toxocariasis in children can cause fever, abdominal pain, and — in the most serious cases — ocular larva migrans (larvae migrating into the eyes), which can cause permanent vision loss.
Pregnant individuals: Toxoplasma gondii can cross the placenta. Primary Toxoplasma infection during pregnancy can cause miscarriage, stillbirth, or congenital toxoplasmosis in the newborn, which can cause brain damage, eye disease, and other serious complications.
Immunocompromised individuals: People whose immune systems are suppressed — due to HIV, organ transplant medications, chemotherapy, or other conditions — face an elevated risk of severe toxoplasmosis if exposed.
Gardeners and urban farmers: People who work with soil in areas frequented by cats have extended, repeated exposure to potentially contaminated ground.
Symptoms and Warning Signs
Toxocariasis in humans:
- Most infections produce no symptoms (subclinical)
- Symptomatic cases may include fever, fatigue, abdominal pain, and respiratory symptoms
- Ocular larva migrans: blurred or reduced vision, eye pain, or floaters in one eye — can lead to permanent vision damage if untreated
- Most commonly affects children ages 1 to 4
Toxoplasmosis in immunocompetent adults:
- Often produces no symptoms, or mild flu-like illness
- May cause swollen lymph nodes and fatigue
Congenital toxoplasmosis (in newborns of mothers infected during pregnancy): can cause seizures, vision problems, developmental delays, and other serious complications
See a clinician promptly if a child develops unexplained vision changes or if a pregnant person or immunocompromised individual suspects any cat-related exposure.
What You Can Do Now
For parents:
1. Cover sandboxes when not in use to prevent cats from using them as litter areas.
2. Teach children to wash hands thoroughly after playing outdoors, especially before eating.
3. Keep children away from known cat feces in parks or gardens.
For pregnant individuals:
4. Avoid handling stray cats or soil in areas frequented by cats during pregnancy.
5. Do not change cat litter during pregnancy if at all possible; if unavoidable, wear gloves and a mask and wash hands thoroughly afterward.
6. Cook meat thoroughly (Toxoplasma can also spread through undercooked meat containing cysts).
7. Talk to your OB-GYN about your cat and soil exposure risk during prenatal visits.
For pet cat owners:
8. Keep pet cats indoors — indoor cats are not significant contributors to environmental parasite contamination.
9. Deworm pet cats regularly according to your veterinarian's schedule. Most broad-spectrum deworming products are effective against Toxocara and other common parasites.
10. Scoop litter boxes daily — Toxoplasma oocysts in fresh feces are not yet infectious; they require 1 to 5 days in the environment to become so.
For all urban residents:
11. Wash produce thoroughly before eating, especially items grown in gardens that cats may access.
12. Wash hands after any outdoor activity involving soil contact.
Cost and Access: What Patients Should Know
Routine deworming of pet cats is inexpensive (typically $10 to $30 per treatment) and widely available at veterinary offices and pet supply retailers. The American Association of Feline Practitioners recommends routine fecal examination and deworming as part of standard cat care.
Testing for Toxoplasma during pregnancy is available as part of routine prenatal bloodwork and is covered by most health insurance plans when ordered by an OB-GYN or midwife. For uninsured patients, federally qualified health centers provide prenatal care, including infectious disease screening, on a sliding-scale basis.
Toxocara treatment in humans — when infection is confirmed — typically involves antiparasitic medications (albendazole or mebendazole) that are available by prescription. Ocular larva migrans may require specialist evaluation by an ophthalmologist.
What Happens Next
This study establishes a baseline for parasite prevalence in NYC's free-roaming cat population. Future research could examine whether targeted interventions — such as deworming feral cats before return in TNR programs, or managing super-shedder male cats differently — could reduce environmental contamination. The researchers' focus on young male cats as super-shedders is a potentially actionable finding for TNR program design.
The NYC Department of Health and Mental Hygiene's guidance on free-roaming cats and zoonotic disease is available at nyc.gov/health and is periodically updated in response to new research.
The Bottom Line
A new study found that 57.5% of New York City's free-roaming cats harbor at least one internal parasite capable of spreading to humans, with Toxocara roundworms dominant and young male cats depositing unusually high concentrations of infectious eggs into city soil. Toxoplasma was also present in nearly 1 in 10 cats. This is not cause for alarm — it is cause for practical prevention: cover sandboxes, wash children's hands after outdoor play, keep pet cats indoors and dewormed, and talk to a prenatal care provider about Toxoplasma risk during pregnancy. The public health picture in an urban environment always involves animal-to-human transmission risks; knowing which ones are present makes it easier to protect against them.
