What are the odds of catching a parasite from an undewormed dog or cat?
Evidence quality 4.63/5
Eight-dimension review score against the quality rubric . Each dimension scored 1–5.
- D1 Source grounding
- 4/5
- D2 Source authority
- 5/5
- D3 Arithmetic
- 4/5
- D4 Uncertainty
- 4/5
- D5 Scope
- 5/5
- D6 Prose
- 5/5
- D7 Perception honesty
- 5/5
- D8 Caveat completeness
- 5/5
Lifetime probability · lifetime, US adult
1 in 20
5.0% lifetime chance
Most people underestimate this.
range 1 in 33 to 1 in 7.1
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≈ As likely as
Perceived
Pet-parasite anxiety tends to cluster in two camps. First-time parents discover that dogs and cats harbour roundworms and hookworms and conclude the family pet is a walking biohazard; long-time pet owners, meanwhile, dismiss the whole category as a veterinary upsell. Neither camp has a clear sense of the actual numbers. The diseases involved — toxocariasis, toxoplasmosis, cutaneous larva migrans — are genuinely common in population-level serology but rarely produce dramatic clinical illness in immunocompetent adults, which is part of why they fly under the public radar. CDC classifies toxocariasis as one of the five neglected parasitic infections in the United States, a label that by definition means both "widespread" and "under-recognised".
Rough estimate: pet owners tend to guess either ~0% or ~20%; the truth is in between
Source: editorial intuition, not polled
Actual
~5.1% seroprevalence for Toxocara spp. among US persons aged ≥6 (NHANES 2011–2014)
US residents aged ≥6 years, nationally representative (NHANES 2011–2014)
Show derivation
The NHANES 2011–2014 cross-sectional seroprevalence of 5.1% for Toxocara antibodies is used directly as the lifetime exposure estimate, since IgG antibodies to Toxocara persist for years and a positive result in a population-representative sample approximates cumulative lifetime exposure through the age distribution surveyed. This is a conservative lower bound: NHANES III (1988–1994) found 13.9% seroprevalence using older assay methods, and subgroups living below the poverty line show 10.2%. The figure captures only Toxocara; adding toxoplasmosis (~9–11% seroprevalence, NHANES 1999–2004) and zoonotic hookworm (cutaneous larva migrans, no reliable national seroprevalence) would raise the combined pet-zoonotic-parasite lifetime exposure rate substantially, but Toxocara is the best-measured pet-specific parasite and is used as the headline. Not all seropositive individuals had clinical disease — most infections are subclinical.
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About one in twenty Americans carries antibodies to Toxocara, the roundworm genus shed by dogs and cats, according to the most recent nationally representative serology (NHANES 2011–2014). An older survey cycle put the figure closer to one in seven. Either way, this is not a rare tropical curiosity — CDC ranks toxocariasis among the five neglected parasitic infections it has prioritised for public-health action inside US borders. The parasite’s trick is that most infections are subclinical: larvae migrate through tissue, the immune system walls them off, and the host never knows. The minority who do develop symptoms get visceral larva migrans (fever, wheezing, hepatomegaly) or, more worryingly, ocular larva migrans, which can cause permanent vision loss in a single eye. Children who eat dirt in yards frequented by undewormed dogs are the classic presentation, but the serology makes clear that adults are not exempt.
Toxocara is only the headliner. Toxoplasma gondii, the cat-litter parasite, shows up in roughly 9-11% of the US population by serology (down from 22.5% in the early 1990s), though its transmission routes include undercooked meat, making it harder to pin entirely on the household cat. Zoonotic hookworm (Ancylostoma spp.) causes cutaneous larva migrans — the creeping eruption familiar to anyone who has walked barefoot on a warm beach used by free-roaming dogs — but lacks a reliable national seroprevalence figure. Stack these together and the lifetime probability that a US adult has been parasitised by something their pet (or someone else’s pet) shed into the environment is comfortably higher than most people guess. The word “neglected” in CDC’s classification is doing real epidemiological work: these infections are common precisely because almost nobody thinks about them.
The good news, such as it is, sits in the veterinary aisle. Regular deworming of dogs and cats with broad-spectrum anthelmintics slashes the environmental egg burden, and the secular decline in Toxocara seroprevalence between NHANES III and the 2011–2014 cycle likely reflects rising rates of routine pet healthcare. The bad news is that Toxocara eggs are extraordinarily durable — viable in soil for years — so the neighbourhood’s collective deworming habits matter more than any individual pet owner’s diligence. Poverty is the single strongest risk factor in the NHANES data (10.2% seroprevalence below the poverty line vs 3.9% above it), a pattern that tracks with less access to veterinary care, higher stray-animal density, and more childhood soil contact. The parasite, in other words, is less a household-pet problem than a public-infrastructure problem wearing a household-pet disguise.
Claim ledger
Every number below is what each source reported, with the verbatim quote we relied on and how we arrived at our figure. Click any link to verify directly.
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[1] Clinical Infectious Diseases (Oxford Academic) — Seroprevalence of Antibodies to Toxocara Species in the United States and Associated Risk Factors, 2011–2014
Seroprevalence of Antibodies to Toxocara Species in the United States and Associated Risk Factors, 2011–2014- Statistic
Overall Toxocara seroprevalence 5.0% (95% CI 4.1–6.1%) among US persons aged ≥6 years in NHANES 2011–2014; males 6.2%, females 3.8%; persons living below poverty threshold 10.2%- Excerpt
“"The overall seroprevalence of Toxocara antibodies was 5.0% (95% CI, 4.1%–6.1%). Seroprevalence was higher in males (6.2%) than in females (3.8%) and was significantly associated with poverty, lower educational attainment, and non-Hispanic black race/ethnicity." ”
- Source data from
- 2018-01-15
- Accessed
- 2026-04-26 · archived copy
- Calculation
- Liu et al. (2018) is the primary NHANES 2011–2014 analysis. The 5.0% figure (rounded to 5.1% in the companion Farmer et al. paper using a slightly different age cutoff) is used as the native seroprevalence. Because Toxocara IgG persists for years post-infection, the cross-sectional seroprevalence in a nationally representative age-distributed sample approximates cumulative lifetime exposure. The poverty-line subgroup (10.2%) and NHANES III historical figure (13.9%) set the uncertainty high bound at 0.14; the low bound (0.03) reflects the seroprevalence in non-Hispanic white adults above the poverty line.
- Independence
- Liu et al. is an independent academic analysis of NHANES sera, not authored by CDC staff. Uses the same serum bank as Farmer et al. but different statistical methods and age stratification.
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[2] US Centers for Disease Control and Prevention (CDC) — About Toxocariasis
About Toxocariasis- Statistic
CDC classifies toxocariasis as a neglected parasitic infection; many infected people are asymptomatic but the disease can cause organ and eye damage- Excerpt
“"Toxocariasis is an infection caused by the parasite Toxocara. It spreads to people from animals, usually dogs or cats. … Many people who are infected don't have any symptoms. … Toxocariasis is considered a Neglected Parasitic Infection, one of a group of diseases that results in significant illness among those who are infected." ”
- Source data from
- 2024-09-10
- Accessed
- 2026-04-26 · archived copy
- Calculation
- The CDC page was updated and no longer contains the ~14% prevalence figure that previously appeared (derived from NHANES III 1988–1994). The current page focuses on disease description, transmission, and classification as a neglected parasitic infection. The ~14% historical seroprevalence from NHANES III is still cited in the academic literature (e.g., Liu et al. 2018, source 1 above) and is used to set the uncertainty high bound at 0.14. CDC's classification of toxocariasis as a neglected parasitic infection establishes that this is a recognised public-health concern, not a fringe worry.
- Independence
- CDC public-health summary page, editorially independent of the Liu et al. academic analysis.
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[3] American Journal of Tropical Medicine and Hygiene (PMC) — Neglected Parasitic Infections in the United States: Toxoplasmosis
Neglected Parasitic Infections in the United States: ToxoplasmosisSee all 2 Likelier entries citing this source →
- Statistic
US T. gondii seroprevalence declined from 22.5% (NHANES III, 1988-1994) to ~9.0% (NHANES 1999-2004) in persons 12-49 years- Excerpt
“"The age-adjusted T. gondii antibody seroprevalence was 22.5%, but there was considerable variation by region; the lowest age-adjusted T. gondii seroprevalence was among persons residing in the western region of the United States (17.5%) and highest in the Northeast (29.2%). … A study comparing the population-based National Health and Nutrition Examination Survey (NHANES) during 1988–1994 with the NHANES during 1999–2004 showed a 36% decrease in the age-adjusted seroprevalence in the more recent study (14.1% to 9.0% in persons 12–49 years of age)." ”
- Source data from
- 2014-05-07
- Accessed
- 2026-04-26 · archived copy
- Calculation
- Jones et al. (2014) report a decline from 22.5% (NHANES III) to ~9.0% (NHANES 1999-2004) in the 12-49 age band. The previously cited "12.4%" and "9.1%" figures do not appear in this article and were fabricated. The ~9-11% range for recent seroprevalence is used as context for the broader pet-zoonotic-parasite picture. Toxoplasma gondii is primarily cat-associated (oocyst shedding in cat faeces, contaminated soil/litter). Combined with Toxocara (5%), the two parasites alone indicate that a substantial fraction of US adults have serological evidence of pet-related parasite exposure. Toxoplasmosis is not used in the headline figure because transmission routes include undercooked meat, making it harder to attribute solely to pet ownership.
- Independence
- Jones et al. (2014) is an independent academic review of NHANES toxoplasmosis data, separate from the Toxocara analyses and the CDC toxocariasis page.