Intestinal zoonoses also infect your family

The invisible risks that sneak into your home without asking permission 🦠🏠

Many pet owners live with dogs and cats without realising that some intestinal parasites do not understand the concept of ‘species’ and can also affect humans. When we talk about zoonoses, we are referring to diseases that can be transmitted between animals and humans. In digestive parasitology, this risk is often underestimated… until the shock comes.

And it doesn’t usually come in the form of a “dramatic outbreak”, but rather as a silent trickle of exposure: hands touching contaminated soil, children playing in parks, litter trays cleaned in a hurry, water or surfaces with microscopic cysts. To put it in figures: a global meta-analysis on public soil contamination found a pooled prevalence of 21% of Toxocara eggs in public places (meta-analysis). In Spain, a study in urban parks detected 10.9% of contaminated soil and also points out that in our country, approximate average ranges of 5–8% have been reported in soil studies, with urban areas reaching higher figures (study in parks). In other words, the risk is not always visible, but it may be there, right where children build sandcastles.

The most vulnerable groups are young children, pregnant women and immunocompromised individuals. The key is not to live in fear, but to use common sense: know which parasites are zoonotic, how they are transmitted and what specific measures realistically reduce the risk.

The ‘usual suspects’: common zoonotic parasites and how they get in

Toxocara canis / Toxocara cati (roundworms)

These nematodes produce microscopic eggs that are eliminated in faeces and contaminate the environment. Human infection occurs mainly through accidental ingestion (hands to mouth after contact with soil or contaminated surfaces). In humans, the larvae can migrate through tissues, producing visceral or ocular larva migrans; in plain terms: “larvae that travel” through the body and can inflame organs… and in some cases affect the eye.

👶 Medical fact | The eye that “betrays” a past infection

In paediatric ophthalmology, some cases of unilateral vision loss in children are retrospectively diagnosed as ocular toxocariasis. What is disturbing is that the infection usually occurred years earlier, while playing in contaminated parks or gardens, without any previous digestive symptoms. The parasite disappears, but the inflammatory scar on the retina remains as the only clue to the episode.

To put numbers on the “invisible”: in addition to the global meta-analysis of 21% contamination in public soils (meta-analysis), human population studies have also detected significant exposures. For example, in the US, an analysis of national data estimated a seroprevalence of 5.1% (people with antibodies, a sign of previous exposure) (human seroprevalence). This does not mean that 5% of people are seriously ill, but it does mean that contact with the parasite is more common than people imagine.

Ancylostoma spp. (hookworms)

Here the story changes: the larvae do not need to be ingested; they can penetrate the skin, especially when walking barefoot on contaminated sand or soil. In humans, they cause cutaneous larva migrans, intensely itchy lesions that follow a “snaking path”. In simple terms: the larva moves under the skin and leaves a very characteristic inflammatory trail. For a clinical explanation and information on transmission, you can refer to recent dermatological reviews on the condition and its mechanism of transmission through contact with contaminated soil/sand (clinical review).

Giardia duodenalis (Giardia spp.)

This protozoan is transmitted by highly resistant cysts in water, on surfaces or on fomites (contaminated objects). It can cause diarrhoea, abdominal pain, gas and digestive discomfort, especially in children and in situations of close cohabitation.

In pets, the figures help to understand why it is so common in consultations: a review with meta-analysis estimated pooled prevalences of around 15.2% in dogs and 12% in cats, with higher values in young animals and depending on the diagnostic method (meta-analysis). And if we look at field data, there are studies with even higher prevalence rates in certain contexts: a Spanish study of the canine population reported an overall prevalence of 36.5% (study in dogs). This does not mean that “a third of dogs are always sick”, but rather that exposure to and circulation of the parasite can be very high depending on the environment and the population studied.

Toxoplasma gondii

In this case, the main focus in public health is on pregnancy and immunosuppression. The critical (and often misunderstood) point is that cats do not ‘spread the infection through petting’: the real risk lies in the oocysts eliminated in faeces when there is a recent infection and in their maturation in the environment.

According to the CDC, cats usually shed oocysts for 1–3 weeks, but they can shed large amounts, and these oocysts take 1–5 days to become infectious in the environment (i.e., they do not “infect instantly,” but after sporulation) (CDC – cycle and timing). This detail is useful: it explains why cleaning the litter tray daily greatly reduces the risk.

And, to gauge the relevance in humans in Spain, a review and meta-analysis estimated a pooled seroprevalence in pregnant women of around 24.4% (with variability between studies) (meta-analysis Spain). In addition, a recent study of more than 20,000 pregnant women reported a prevalence of around 26.98% and an incidence of 0.19% in the period evaluated (study in pregnant women). Translated: a significant proportion of women have already had previous contact, and prevention remains crucial for those who are not immunised.

🧠 Mini revelation | The parasite that reprogrammes fear

Experimental studies have shown that mice and rats infected with Toxoplasma gondii lose their fear of the smell of cats and even show attractive behaviour towards their natural predator. This change is not accidental: it makes it easier for the rodent to be hunted and for the parasite to complete its life cycle in the feline’s intestine.

In humans, various studies are exploring whether latent infection could be associated with subtle behavioural changes, such as changes in risk perception or certain neuropsychological traits, although these effects remain the subject of scientific study and debate.

Why do puppies and kittens increase the risk? (It’s not their fault, it’s biology) 🐾

Young animals are a parasitological ‘hot spot’ for three classic reasons:

Vertical transmission. In Toxocara, puppies can become infected before birth or during lactation, which means that some begin their lives already carrying parasites.

Immature immunity. Their immune system is still developing, which favours more intense infestations and, therefore, greater elimination of infectious forms into the environment.

Exploratory behaviour. Puppies explore with their mouths. They are vacuum cleaners of the world. If the world has eggs/cysts, puppies pick them up.

That is why modern protocols insist on starting early: expert guidelines on parasite control recommend starting treatment of puppies around day 14 of life and repeating every 2 weeks until after weaning; in higher-risk situations, continue with a regular schedule during the first few months (ESCCAP 2025 recommendation). This schedule is not “exaggerated”: it seeks to break the cycle before the animal begins to significantly contaminate the environment.

Family prevention: specific measures that do change the risk (and don’t complicate your life)

Effective prevention is based on three pillars: guided deworming, environmental hygiene and hand hygiene. The powerful thing is that most actions are simple, but they must be consistent.

Continuous and personalised internal deworming. There is no single guideline that applies to everyone: it depends on age, habits (park, countryside, hunting, residence, contact with children), and local risk. But the central idea is to follow risk- and evidence-based schemes, such as those proposed by international guidelines (parasite control guidelines).

Immediate faeces collection and frequent cleaning of the environment. With Toxocara, the problem is the egg that remains in the soil; with Giardia, the resistant cyst; with Ancylostoma, the larva in soil/sand. Cutting environmental contamination means cutting transmission.

🏛️ Historical fact | When parks became a public health problem

In the 1970s, several European cities began fencing off playgrounds, not because of vandalism, but because of parasitology. Pioneering research detected high concentrations of Toxocara eggs in play areas, leading to massive faeces collection and dog deworming campaigns. It was one of the first clear examples of urban public health linked to pets.

Cat litter tray: clean daily and wear gloves if pregnant. The key factor here is time: if oocysts need 1–5 days to become infectious, daily cleaning drastically reduces the risk, especially in households with pregnant women (CDC – sporulation). In pregnant women, preventive common sense also dictates: gloves, hand washing and, if possible, delegating this task.

Hand washing (properly). Not a 3-second “dip”: wash thoroughly after handling animals, trays, toys, beds, and especially before eating. Hand hygiene is the simplest way to prevent faecal-oral transmission.

Sources and recommended reading

Hotez, P. J., Wilkins, P. P. (2009) – Toxocariasis: America’s Most Common Neglected Infection of Poverty – PLoS Neglected Tropical Diseases – Comprehensive review estimating significant human seroprevalence and highlighting the silent impact of Toxocara as an underdiagnosed zoonosis –

https://pmc.ncbi.nlm.nih.gov/articles/PMC5805073/

Ma, G., Holland, C. V., Wang, T. et al. (2018) – Human toxocariasis: A meta-analysis of global prevalence – PLoS Neglected Tropical Diseases – Global meta-analysis demonstrating an average prevalence of 21% of environmental contamination by Toxocara eggs in public soils –

https://pubmed.ncbi.nlm.nih.gov/30142562/

Pineda, C., et al. (2022) – Contamination of public parks with Toxocara spp. eggs: A potential public health risk – Veterinary Sciences (MDPI) – Study in urban parks detecting 10.9% of contaminated soil and contextualising exposure in environments frequented by children –

https://www.mdpi.com/2306-7381/9/5/232

Ryan, U., Cacciò, S. M. (2013) – Zoonotic potential of Giardia – International Journal for Parasitology – Reference review analysing the zoonotic transmission of Giardia duodenalis and its impact on public health –

https://www.sciencedirect.com/science/article/pii/S0304401714006451

Gómez-Muñoz, M. T., et al. (2018) – Prevalence of Giardia duodenalis in dogs in Spain – Public Health Research (ISCIII) – Epidemiological study reporting a prevalence of 36.5% in dogs, highlighting the role of young animals as a reservoir –

https://repisalud.isciii.es/rest/api/core/bitstreams/a5a93ef8-5fea-4247-8f3c-9fe0087b44aa/content

Centres for Disease Control and Prevention (CDC) (2023) – Toxoplasmosis: Biology and Transmission – CDC – Official document detailing the biological cycle of Toxoplasma gondii, including the elimination of oocysts for 1–3 weeks and their sporulation in 1–5 days –

https://www.cdc.gov/dpdx/toxoplasmosis/index.html

Ramos, J. M., et al. (2024) – Seroprevalence of Toxoplasma gondii infection in pregnant women in Spain: a systematic review and meta-analysis – European Journal of Clinical Microbiology & Infectious Diseases – Meta-analysis estimating an average seroprevalence of 24.4% in pregnant women in Spain –

https://link.springer.com/article/10.1007/s44197-024-00258-w

García-Basteiro, A. L., et al. (2025) – Prevalence and incidence of toxoplasmosis in a cohort of pregnant women in Spain – Infectious Diseases and Clinical Microbiology – Study of more than 20,000 pregnant women describing a prevalence of 26.98% and an incidence of 0.19% –

https://www.sciencedirect.com/science/article/abs/pii/S0213005X25002071

European Scientific Counsel Companion Animal Parasites (ESCCAP) (2025) – Guidelines for the control of endoparasites in dogs and cats – ESCCAP – European reference guide recommending deworming puppies from day 14 onwards, with fortnightly treatment –

https://www.esccap.org/uploads/docs/biu0jhej_0778_ESCCAP_GL1__English_2025_v21_1p.pdf