Toxocara (roundworm), worming and the environment
Toxocara (roundworm), worming and the environment
Toxocara is an example of a One-Health issue where there is a potential conflict between the needs of animal health, human health and the health of the wider ecosystem.
Toxocarosis, the disease that can develop in humans as a result of transmission of infective Toxocara worm eggs from the faeces of cats, dogs or foxes, is rare in the UK. Consequently it’s not uppermost in the minds of most healthcare professionals. In contrast, protecting humans from toxocarosis is a prominent concern in veterinary practice and pet owners are recommended to worm their cats and dogs regularly.
As the health of adult cats and dogs is not usually affected by Toxocara, apart from kittens and puppies, worming cats and dogs to treat Toxocara infection is done in the name of protecting human health.
There has been a trend towards more frequent and widespread use of parasiticides in cats and dogs to treat Toxacara infection and also to treat and prevent other common parasite diseases (so-called blanket protection). British veterinary organisations have become concerned about this because of the potential harm to wildlife when parasiticides end up in the environment and now recommend that vets adopt a risk-based approach to prescribing the drugs.
Decisions about the use of a veterinary intervention to protect the health of humans that involves the treatment of a UK population of pet cats and dogs numbering more than 20 million surely demands collaboration between doctors, vets and environmental scientists. The topic requires an impartial and dispassionate examination of the evidence, and the benefits and costs of interventions (given that parasiticides represent a significant source of income for the veterinary pharmaceutical and healthcare industries). A starting point for deciding about the optimal use of parasiticides against Toxocara must necessarily include an up-to-date evaluation of the evidence on the risk of toxocarosis in humans. To help with this, Veterinary Prescriber together with the Drug and Therapeutics Bulletin commissioned an evidence review on toxocarosis.
What does the evidence say?
Main findings of the evidence review
Toxocara Infection is spread from faeces (from dogs, cats or foxes) containing Toxocara eggs that may contaminate soil and foods and are ingested by other animals or humans (e.g. young children ingesting contaminated soil).
Most pets show no clinical signs of infection with Toxocara, but some may vomit, stop eating, or lose weight. Rarely, heavy infections in young puppies and kittens may be fatal. Among animals, Toxocara infection rates are higher for:
puppies, kittens, pregnant bitches, lactating bitches and queens and geriatric animals
stray cats, working and rural dogs than pet cats and dogs
dogs and cats that are left outside or with unrestricted access to the outdoors and allowed to eat other animals
animals kept in kennels, shelters or breeding stations, living with other dogs or cats, or living in or travelling to different geographic areas.
Toxocarosis in humans is uncommon, and if infection does occur, it is usually mild. However, serious consequences can occur, particularly ocular and neurological complications. Infection has also been associated with asthma, urticaria and epilepsy. Among humans, Toxocara infection rates are higher for:
people living in hot humid regions where eggs survive better in the soil
people with low levels of education, living in poverty, or in areas with a lower Human Development Index (a composite measure of life expectancy, education and national income), poor sanitation and hygiene.
those who consume raw meat or drink untreated water.
owners of dogs or cats.
young children, those who play in sandpits or eat soil
people with poorer cognitive function and epilepsy possibly due to pica behaviours or falls.
Key methods to control the infection include:
education about hygiene practices (such as handwashing after touching soil or animals and before eating);
cleaning up faeces promptly;
reducing contamination of public areas (especially children’s playgrounds and sandpits);
controlling stray animals;
washing and/or thoroughly cooking potentially contaminated food;
regular deworming of pets (especially young, pregnant or lactating dogs and cats).
The review concludes that “good hygiene practices and prompt disposal of faeces are uncontroversial. Publicity and changes in laws about dog fouling may have led to a decrease in infections in the UK, although causation is hard to prove. Areas with anti-fouling signs and lower visible faecal contamination have correlated with lower soil contamination with Toxocara eggs. It is important to continue to educate pet owners and the public about potential infection and necessary control measures. The rate of infections among pet dogs is low in the UK, and in other countries even among pet dogs that have not received deworming treatments for over 12 months. It is important to bear in mind the potential unwanted effects of deworming control measures, which include contamination of the environment with parasiticides. British veterinary associations now advocate that a benefit/risk assessment of each individual case should determine the choice of deworming treatment and frequency of administration rather than blanket treatment of all animals, most of which are at low risk of suffering from Toxocara infection or transmitting illness to humans. Focusing deworming treatments on dogs under a year old is likely to have the greatest impact on reducing transmission of eggs into soil, while reducing the frequency of deworming treatments for older animals (at lower risk) could reduce the impact of parasiticides on the environment while likely having little effect on the very low numbers of cases of human toxocarosis in the UK.”
You can access the full review via this link.
What do UK health professionals think?
Listen to this podcast discussion (which starts at about 14 minutes in) in which James Cave, a British GP and David Phizakerley, a pharmacist and DTB’s deputy editor discuss toxocarosis in humans and the risk of environmental contamination from parasiticides used for deworming cats and dogs.
What do vets think?
Routine worming of cats and dogs has become embedded in veterinary practice in recent years. Now there are signs that the tide is turning with the profession moving towards adopting a more evidence-based approach to worming.
The biodiversity crisis demands that we make the best decisions about how we use parasiticides in the interest of the health of people, animals and the planet. There is a balance to be found.