THE potential for retinal damage caused by Toxocara spp. infection in humans, particularly children, is well-known amongst vets, health professionals and parents.
A recent presentation from Maggie Fisher, who chairs the UK and Ireland contingent of the European Scientific Counsel for Companion Animal Parasites (ESCCAP UK and Ireland), highlighted some other human health concerns associated with the parasite, including longevity of infective stages within the environment and proven links to other childhood conditions such as respiratory disease, allergies and even learning difficulties.
A common parasite of pet cats and dogs, Toxocara spp. adult worms reside in the gut and lay eggs that are passed in the faeces of the animal. One mature female adult worm can produce up to 84,000 eggs per day.
These eggs are not immediately infective, instead taking around two weeks or more to mature to this stage and can persist in the environment long after animal waste has been washed away or decomposed. This means that environments which appear clean and safe actually may not be.
Source of infection
Accidental ingestion of infective eggs found in animal faeces is the main route of infection for humans, although it is also possible to become infected via ingestion of encysted larvae found within undercooked meat or paratenic hosts.
Numerous studies have indicated towards there being only a weak or even absent link between owning a dog and an increased risk of Toxocara infection, suggesting that most infections are obtained from the environment.1,2
Studies have shown that children who make a habit of eating soil (geophagia) are significantly more likely to become infected with Toxocara.1
Larval migration
Heavy adult worm burdens in young animals can cause loss of condition and the typical “pot-bellied” appearance of a worm-infested puppy. In humans, where migration of the larval stages of the parasite is the cause of disease, it is quite a different picture, with three main disease presentations:
- Visceral larval migrans: occurs after ingestion of a large number of infective eggs that hatch into larvae in the gut and migrate through body tissues, causing damage and severe, acute illness (rarely seen in this country as the level of infection is generally not that high);
- Ocular larval migrans: small numbers of larvae migrate to the retina, causing damage and affecting vision, sometimes years after initial infection. The risk of developing other clinical signs, such as convulsions, is also increased.
- Covert toxocarosis: this is the previously relatively unexplored association between Toxocara infection and a wide range of clinical signs and conditions.
The tip of the iceberg
The results of a number of studies present convincing evidence that the extent of the consequences of human Toxocara infection may be much greater than is currently thought.
Due to the covert form of toxocarosis being associated with relatively commonplace illnesses, such as coughs, allergies and asthma, prevalence of this form of the disease may also be grossly underestimated.
Maggie’s presentation touched on just a few of the many studies that support the links between Toxocara infection and a wide range of common health conditions/concerns.
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Persistent cough
An Eastern European study 3 included 425 children displaying signs of a persistent cough and 1,600 asymptomatic children as controls. Blood tests confirmed that a statistically significant 32% of the symptomatic group were positive for Toxocara antibodies, compared to just 17% of the asymptomatic control group.
What’s more, when given a week’s course of an appropriate anthelmintic, the vast majority of symptomatic children had a greatly reduced and frequently absent requirement for inhaled corticosteroids to control their symptoms when re-examined by the study team a year later.
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Reduced lung capacity and cognitive ability
Another nationally representative study performed in America 2 showed that people who tested positive for Toxocara antibodies (i.e. had been infected at some stage in their life) had significantly reduced capabilities on forced expiratory volume tests when compared to people who were Toxocara negative on serology. The same group analysed the cognitive abilities of children and once again, those that were Toxocara positive scored lower than their serologically negative counterparts on tests assessing maths, reading, verbal reasoning and physical dexterity.
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Allergies
A study performed in the Netherlands 4 analysed blood samples taken from over 1,300 children aged between four and 12. Only those children with allergies diagnosed by a doctor were included in the study group, the rest acting as controls. The results demonstrated a significant link between positive Toxocara serology, asthma and allergies to animals upon contact.
The role of veterinary professionals
Veterinary professionals are perfectly placed to make sure that pet owners are aware of the risks and take precautions to protect themselves, their families and the wider general public.
Advising pet owners to regularly (at least four times a year) worm pets with an effective anthelmintic from an appropriately young age (two weeks of age for puppies and three weeks of age for kittens) is important, remembering that transfer of the parasite to puppies before birth and to kittens via milk is a feature of the parasite lifecycle.
Some higher risk groups may even require monthly treatment: faecal egg counts or risk assessment can help dictate appropriate treatment frequency. Reducing pets’ access to raw food and hunting activities can also help reduce pet infection, egg excretion and environmental contamination.
Dog waste should be picked up immediately and children’s sandpits covered when not in use. More general recommendations include cooking food well and, particularly important, making sure children wash their hands before eating.
Further advice about companion animal parasite control is available at www.escaapuk.org.uk.
References
- Taylor, M. R. (2006) Ocular Toxocariasis. In: Toxocara, the enigmatic parasite. Eds. Holland, C. V. and Smith, H.V. CABI Publishing, ch. 9, pp127-144.
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3rd US National Health and Nutrition Survey.
Walsh, M. G. (2010) Toxocara infection and diminished lung function in a nationally representative sample from the United Sates population. Int J Parasitol 41 (2): 243- 247.
Walsh, M. G. and Haseeb, M. A. (2012) Reduced cognitive function in children with toxocariasis in a nationally representative sample of the United States. Int J Parasitol 42: 1,159-1,163.
- Bede, O., Szénási, Z., Danka, J., Gyurkovits, K. and Nagy, D. (2008) Toxocariasis associated with chronic cough in childhood: a longitudinal study in Hungary. J Helminthol 82: 357-363.
- Buijs, J. et al (1997) Relationship between allergic manifestations and Toxocara seropositivity: a cross-sectional study among elementary school children. Eur Respir J 10: 1,467-1,475.
