Toxocariasis (roundworm from dogs and cats)

Toxocara canis in dogs and Toxocara cati in cats shed eggs in faeces. The NHS describes toxocariasis as an infection caused by small worms found in the poo of some dogs, cats and foxes, which occurs when worm eggs from contaminated soil or sand get into the mouth. Eggs need weeks in the soil to become infective, so a fresh pile of faeces is not an immediate risk, but eggs persist for a long time in sandpits, playgrounds, parks and gardens.

The CDC adds that infection can also follow eating undercooked meat from animals that carry larvae, such as lamb or rabbit, though soil is the main route. Once eggs are swallowed, larvae hatch in the gut, penetrate the intestinal wall and travel to organs, but they cannot complete their life cycle in humans.

The NHS describes the condition as rare, with most people who ingest eggs not becoming ill, and notes that children and those with poor hygiene habits face higher risk. Young children who play in dirt and put their hands or objects in their mouths are the classic group, as are people with pica, a habit of eating soil. Owners of puppies and kittens who are not regularly wormed, and people who garden without gloves, are also exposed.

When larvae spread in the body, the NHS lists fever, cough, wheezing, stomach pain, an itchy rash, eye pain or redness and vision changes such as floaters or flashes. The CDC describes two main forms. Visceral toxocariasis involves internal organs such as the liver, lungs or central nervous system. Ocular toxocariasis affects the eye, causing inflammation, retinal damage or vision loss, typically in one eye.

Many infections are silent, and a blood test may incidentally reveal antibodies. Eye disease tends to occur in older children and young adults and can be mistaken for other eye conditions or tumours, so an eye specialist usually makes the diagnosis.

• Visceral form: fever, cough, wheeze, tummy pain, enlarged liver, rash.

• Ocular form: blurred vision, floaters, flashes, red painful eye, squint, often one eye.

• Covert form: vague symptoms such as tiredness, headache or abdominal discomfort.

The CDC explains that healthcare providers collect blood or eye fluid samples to detect antibodies and ask about exposure to dirt or pet waste, as the parasite cannot easily be seen in tests. Blood tests may also show a raised eosinophil count in visceral disease. Scans may be used to look for liver or brain lesions.

The NHS states that toxocariasis only needs to be treated if it causes symptoms. Medicine to kill the larvae is the main approach, and steroid eye treatments or surgery may be needed if the eye is affected. Eye specialists manage ocular disease, and early treatment can limit damage, although vision lost to scarring may not return.

Prevention is largely practical, as the NHS lists. Wash hands regularly, wear gloves when gardening, teach children not to eat soil, wash vegetables, pick up pet waste promptly and make sure pets receive regular worming treatment. Cover sandpits when not in use, keep dogs and cats out of play areas, and use a vet-recommended worming schedule, which is particularly important for puppies and kittens. The CDC emphasises the same measures, including having pets tested and treated for worms.

See a GP if a child has unexplained fever, cough and wheeze after exposure to dogs or soil, or an adult has persistent symptoms with a relevant history. See an optician or go to an eye casualty promptly for sudden changes in vision, floaters, flashes or a painful red eye. Call 999 for severe breathing difficulty or seizures.