What Are Roundworms?
Roundworms are one of the most widespread intestinal parasites found in dogs and cats across the United Kingdom. The two species most relevant to pet owners are Toxocara canis, which infects dogs, and Toxocara cati, which infects cats. Despite being different species, they share a broadly similar lifecycle and cause comparable problems in their respective hosts.
Adult roundworms are pale, smooth worms that can reach several centimetres in length. They live in the small intestine of infected animals, where they feed, reproduce, and shed enormous numbers of microscopic eggs into the environment through the animal's faeces. Understanding how these parasites spread is the first step towards keeping your pet — and your household — protected.
How Do Pets Pick Up Roundworms?
The routes of infection differ depending on whether we are talking about puppies and kittens or adult animals.
Infection in Puppies
Puppies are particularly vulnerable because Toxocara canis larvae can cross the placenta during pregnancy, meaning many puppies are already infected before they are born. Larvae are also passed through the mother's milk during nursing, so even puppies that were not infected in the womb may acquire roundworms during the first weeks of life. This is why virtually all puppies should be assumed to carry roundworms, regardless of whether their mother appears healthy.
Infection in Adult Dogs and Cats
Adult animals can become infected by ingesting roundworm eggs from contaminated soil, grass, or water. Cats are also frequently infected by hunting and eating prey animals such as mice and birds, which can harbour infective larvae in their tissues. Once ingested by an adult animal, the larvae do not always develop into adult worms in the gut; in many cases they simply migrate to body tissues and lie dormant, only reactivating — in female dogs — during pregnancy.
Symptoms of Roundworm Infection
The clinical signs of roundworm infection vary considerably between young animals and adults.
Signs in Puppies and Kittens
Young animals with a heavy burden of roundworms can become quite unwell. Common signs include a pot-bellied appearance, a dull or staring coat, failure to thrive, vomiting, and diarrhoea. In severe cases, worms may be visible in vomit or faeces — they look like strands of cooked spaghetti. A heavy roundworm burden places significant stress on a young animal's digestive system and can interfere with normal development if left untreated.
Signs in Adult Animals
Adult dogs and cats often carry roundworms with no obvious signs at all. This is one reason why regular preventative worming is so important — you cannot rely on your pet appearing unwell as a signal to act. Some adult animals may show mild digestive upset, but many will appear entirely normal while still shedding infective eggs into the environment.
The Zoonotic Risk: Roundworms and Human Health
Perhaps the most important reason to take roundworms seriously is the risk they pose to human health, particularly to children. Toxocara eggs shed in pet faeces can survive in soil for months or even years. If a person — most often a young child — accidentally ingests these eggs through contact with contaminated soil or sandpits, the larvae can hatch and migrate through the body in a condition called visceral larva migrans.
In most cases the larvae eventually die and the illness resolves, but in some instances they migrate to the eye, causing ocular toxocariasis. This condition can lead to inflammation, scarring, and in severe cases permanent visual impairment. Children are most at risk because of their tendency to play in soil and put their hands in their mouths without thorough handwashing.
Good hygiene practices — including washing hands thoroughly after handling pets or gardening, keeping children away from areas where pets defecate, and picking up faeces promptly — are essential steps in reducing this risk.
Worming Schedules: When and How Often?
Given how easily puppies are infected and how serious the consequences can be, established worming guidelines in the UK recommend the following schedule for dogs:
- Puppies should be wormed every two weeks from two weeks of age until they are eight to twelve weeks old.
- From twelve weeks until six months of age, worming should continue monthly.
- Adult dogs should be wormed at least every three months throughout their lives.
- Nursing bitches should be wormed alongside their puppies to reduce ongoing egg shedding.
For cats, kittens should follow a similar programme, and adult cats — particularly those that hunt — should be wormed at least every three months, or more frequently if they have regular access to prey animals.
If you have young children in the household or if your dog spends a lot of time in public spaces, many vets recommend increasing the frequency of adult worming to monthly.
Treatments for Roundworms
Several effective treatments are available for roundworms in dogs and cats. The most commonly used active ingredients include:
- Fenbendazole — available as granules or a liquid, often used in puppies and kittens due to its gentle action. It is typically given daily for three to five days.
- Pyrantel — frequently found in combination wormers and works by paralysing the worms so they can be expelled naturally.
- Milbemycin oxime — often found in combination products that cover multiple parasite types, including some lungworm species.
Many broad-spectrum wormers available from your vet or a registered online pharmacy will cover roundworms alongside other common worm species. Always check the product label to confirm it covers Toxocara, and speak to your vet if you are unsure which product is appropriate for your pet's age and weight.
Environmental Hygiene
Treatment alone is not sufficient if the environment remains contaminated. Roundworm eggs are sticky and can cling to grass, soil, and hard surfaces for long periods. The following measures help reduce environmental contamination:
- Pick up dog and cat faeces promptly — daily in the garden if possible.
- Bag faeces securely and dispose of them in a bin, never on compost heaps.
- Cover children's sandpits when not in use to prevent cats from using them as a latrine.
- Wash hands thoroughly after gardening, handling pets, or any contact with soil.
- Teach children not to put soil or unwashed hands near their mouths.
Keeping Everyone Safe
Roundworms are a manageable risk when pet owners are informed and consistent. A regular worming programme tailored to your pet's age and lifestyle, combined with good hygiene habits at home and in the garden, significantly reduces the chance of infection for both your pet and your family. If you are unsure what worming schedule is right for your pet, your vet is always the best source of personalised advice.