Parasites are one of those topics that sound dramatic and are usually entirely manageable. They're more common in young children than parents realise, partly because toddlers explore everything with their mouths and partly because the standards of personal hygiene at age two are charmingly low. Knowing what to look for, and what's actually likely, takes most of the alarm out of the diagnosis. For a wider view, see our complete guide to child health.
Threadworms (Pinworms)
Enterobius vermicularis — small, white, around 8–13 mm long, looking like animated pieces of cotton thread — is the most common intestinal parasite in UK children. The female worm crawls out of the anus at night to lay eggs on the surrounding skin, which causes the characteristic itching.
What you'll see: Intense itching around the bottom, particularly at night and especially in the hour or two after lights-out (which is when the worms are most active). Children scratch in their sleep, wake up irritable, and sometimes have disrupted nights. In girls, the worms occasionally migrate forward and cause vulvovaginal irritation. You might see the worms themselves on the surface of stool, in nappies, or — with a torch and a bit of nerve — around the anus an hour after the child has fallen asleep.
How they spread: Hand-to-mouth, almost entirely. A scratching child collects eggs under their fingernails, then deposits them on toys, taps, towels, and back to their own mouth. The eggs are sticky and survive on surfaces for two to three weeks. By the time you spot threadworms in one child, every member of the household has likely been exposed.
Treatment: Mebendazole (Vermox, Ovex) is the standard. Single dose, available over the counter from age two. Treat the whole household at the same time — this is the bit people skip and then wonder why the threadworms are back in a fortnight. Repeat the dose after two weeks to catch any worms that hatched after the first round (mebendazole kills adult worms but not eggs).
For under-twos, mebendazole isn't licensed and the GP should be involved. Strict hygiene measures alone are usually sufficient because mebendazole effectively breaks the cycle in older household members and the toddler often clears within a few weeks once the household is decontaminated.
The hygiene routine that actually works:
- Wash bedding, pyjamas, and towels at 60°C on the day of treatment, and again two weeks later
- Vacuum and damp-dust the bedrooms (avoid shaking out bedding into the air — that aerosolises eggs)
- Cut nails short, discourage nail-biting and thumb-sucking for the next two weeks
- Morning showers (rather than baths) wash off any eggs deposited overnight
- Hand-washing with soap after the loo and before meals, every time
Giardia
Giardia duodenalis is a single-celled parasite that infects the small intestine. It's spread by contaminated water (untreated streams, swimming pools, the occasional contaminated water supply) and by faecal-oral routes — nappies, toilet contact, daycare outbreaks.
What you'll see: Persistent diarrhoea, often pale, frothy, or greasy in appearance and floating in the toilet. Bloating, smelly burps (sometimes described as eggy or sulphurous), abdominal cramps, and a degree of weight loss in prolonged cases. Symptoms can come and go in waves over weeks. Some children carry it without symptoms.
Diagnosis: Stool sample. Modern PCR-based testing is more sensitive than the older microscopy and is now standard in most NHS labs. Three samples on different days improve detection if the first comes back negative and suspicion is high.
Treatment: Metronidazole, typically a 5–7 day course in children. It's effective but tastes foul and commonly causes a metallic taste, mild nausea, and a coloured tongue — finishing the course matters. Tinidazole (single dose) is an alternative used in some specialist settings. Re-treatment is occasionally needed if symptoms persist.
Toxocara (Roundworm from Pets)
Toxocara canis (dogs) and T. cati (cats) live in the intestines of unwormed pets, particularly puppies and kittens. Eggs are shed in faeces and become infectious in soil after two to four weeks. Crucially, the eggs persist in soil and sand for years.
A toddler playing in a contaminated sandpit or garden, then putting their hands in their mouth, is the canonical exposure. In the UK, soil contamination rates in some urban parks are surprisingly high — surveys have found Toxocara eggs in 5–10% of public park samples.
What you'll see: Most often, nothing. The vast majority of children with positive Toxocara serology have no symptoms. In a small minority, the larvae migrate from the gut into other organs:
- Visceral larva migrans: fever, abdominal pain, cough, sometimes wheeze, and elevated eosinophils on a blood test
- Ocular toxocariasis: rare but important — granulomatous lesion in the eye that can cause reduced vision in one eye. Usually diagnosed by an ophthalmologist after a child fails a vision check.
Prevention: Worm pets routinely (puppies and kittens monthly until six months, then quarterly), pick up dog faeces straight away, cover sandpits when not in use, and supervise hand-washing after outdoor play and before meals.
Cryptosporidium
A waterborne parasite that causes profuse watery diarrhoea, abdominal cramps, low-grade fever, and sometimes vomiting. The classic exposures are:
- Farm visits and lambing season — calves and lambs are major reservoirs. Petting-zoo outbreaks happen most years.
- Swimming pools — the parasite is chlorine-resistant. Outbreaks in leisure centres are well documented.
- Contaminated drinking water — rare in the UK but periodically reported.
In healthy children it's self-limiting (typically one to two weeks) and treated with hydration. Children with weakened immune systems can develop severe, prolonged disease and need specialist input.
Childcare exclusion: Public Health guidance is exclusion from nursery until 48 hours after the last episode of diarrhoea, with no swimming for two weeks after recovery (the parasite continues to be shed even after symptoms have stopped).
Prevention Across the Board
The single biggest lever is hand-washing with soap — properly, for around 20 seconds — after using the toilet, after changing nappies, after touching pets, and before handling food. In children too young to wash their own hands well, supervised hand-washing is the work of the parent or nursery worker.
Beyond that:
- Wash fruit and vegetables before they're eaten raw
- Avoid drinking untreated water — particularly when travelling abroad
- Keep sandpits covered when not in use
- Worm pets on schedule and clear up faeces straight away
- Be mindful around farm animals — wash hands thoroughly after petting-zoo or farm visits, and don't eat or drink until you have
If you suspect a parasitic infection, the GP-arranged stool sample is almost always the right first step. Don't reach for unprescribed antiparasitic medication or "natural worming" remedies — most are ineffective, some are unsafe in young children, and the targeted prescription treatment is straightforward and free on the NHS.
Key Takeaways
Threadworms (pinworms) are by far the most common — bottom-itching at night, especially after lights-out, in a toddler who's at nursery. Treat the whole household with mebendazole (over the counter from age two) and repeat after two weeks. Giardia causes prolonged greasy or frothy diarrhoea and is diagnosed on a stool sample. Toxocara from dog or cat faeces is usually symptomless but causes the rare visceral or ocular cases that justify covering sandpits. Cryptosporidium typically follows a farm visit. Hand-washing — properly, with soap, after the loo and before food — does most of the prevention work.
