The symptoms of a gut parasite are non-specific. Watery diarrhoea, stomach cramps, bloating, nausea, tiredness and weight loss can all occur, and every one of them has more common explanations, from a passing stomach bug to a food intolerance.
What helps far more is timing. Diarrhoea that clears within a few days is usually caused by a virus or bacteria. Diarrhoea that is still going after two weeks makes a parasite genuinely worth considering. This guide covers the symptoms, why how long they last matters more than any single sign, and the points at which the right move is a GP appointment rather than another article.
What are the symptoms of a gut parasite?
The symptoms most often reported with the parasites that turn up in UK stool samples are:
- watery diarrhoea that keeps coming back
- stomach cramps or a dull ache
- bloating and wind
- feeling sick, sometimes with vomiting
- tiredness that does not lift with rest
- losing weight without trying
- pale, greasy stools that are difficult to flush
The NHS describes much the same picture for giardiasis, the most commonly identified of these infections in the UK: smelly diarrhoea, tummy pain or cramps, wind, bloating and weight loss. Pale, greasy stools are the one distinctive entry, and NICE's guidance on assessing long-lasting diarrhoea in adults treats them as a sign that fat is not being absorbed properly, which some gut infections can cause.
Nothing on the list is unique to parasites. The same symptoms appear with food poisoning, a food intolerance, IBS and ordinary stomach bugs. That is why no symptom list, including this one, can tell you what you have. It can only tell you what is worth ruling in or out, and that is a job for your GP and for testing, not for a checklist.
Why does how long symptoms last matter so much?
Duration is the one feature that genuinely changes the list of likely causes. A clinical review in JAMA of diarrhoea lasting 14 days or more sets out the logic doctors use. Diarrhoea that comes on quickly and clears within days is usually viral or bacterial, but once it persists past two weeks, parasites such as giardia and cryptosporidium move towards the top of the suspect list.
Travel raises the odds but is not required. The same review estimates that around 3% of people travelling to lower-income countries develop persistent diarrhoea. Plenty of UK infections, though, are picked up at home, in swimming pools, nurseries and households, so a clear passport does not rule a parasite out.
Fourteen days is a way of thinking about causes, not a waiting requirement. The NHS advises urgent help once diarrhoea has passed 7 days, and sooner than that for any of the red flags listed at the end of this guide. If you are past either mark, book the appointment first and read on while you wait.
Can you have a gut parasite with no symptoms?
Yes, and it is common. A review of giardiasis reports that around 50 to 75% of infected children have no symptoms at all. Blastocystis, an organism stool tests report often, was found in 30% of 616 healthy Belgian adults studied in the journal Gut, most of whom had no gut complaints. The NHS makes the same point about giardiasis. You can carry it, and pass it on, without feeling ill.
Having no symptoms is not always the end of the story. A 2025 review in Emerging Infectious Diseases reports that longer-term problems such as IBS-type symptoms and ongoing fatigue have followed giardia infections, occasionally even ones that caused no symptoms at the time. How often this happens, and why it happens to some people and not others, is not yet well understood, so it is a reason for awareness rather than alarm.
Which parasites are common in the UK, and how do their symptoms differ?
Our complete guide to gut parasites covers the full range, and a separate guide explains how gut parasites spread. The four below are the ones that come up most often for UK readers.
Giardia
Giardia spreads through contaminated water, food and close contact, and it is the parasite a UK lab is most likely to confirm. The picture is watery, often smelly diarrhoea with cramps, bloating and wind, and weight loss when it drags on. Our guide to giardiasis covers how it is diagnosed and how the NHS treats it.
Giardia is also the parasite most clearly linked with symptoms that outlast the infection. A Norwegian study that followed 1,252 people for ten years after a waterborne outbreak found IBS and chronic fatigue were still more common in the infected group a decade on. That comes from one outbreak and shows an association rather than proof of cause, but it is a large, careful study and worth knowing about if your symptoms started with an infection.
Cryptosporidium
Cryptosporidium causes watery diarrhoea with cramps and is one of the two parasites the JAMA review names as a cause of persistent symptoms. It does not need foreign travel. When UK cases jumped sharply in autumn 2023, a UKHSA and Public Health Wales investigation found swimming pool use and travel to Spain were the strongest associations, with children under five most affected. Our guide to cryptosporidium has the detail.
Threadworms
Threadworms are the odd one out, because their picture is genuinely distinctive: itching around the bottom, worse at night, mostly in children of primary school age, sometimes with fine white threads visible in poo. A 2025 review estimates around 30% of children worldwide are affected at some point, and that 30 to 40% of children who have them show no symptoms. Our guide to threadworms in children explains the NHS pathway for parents.
If you suspect threadworms in your child, speak to a pharmacist or your GP. A stool test is the wrong tool here, and that includes ours. Threadworms are found with a sticky-tape check around the bottom, not in a stool sample.
Blastocystis
Blastocystis is the grey area of this list. It appears on stool test results more often than any of the others, yet, as above, it is carried by a large share of people with no symptoms. A pooled analysis of 17 studies found people with IBS were about twice as likely to carry it as people without, but that is an association at a point in time, not evidence that it causes symptoms, and researchers genuinely disagree about whether Blastocystis actually causes symptoms at all. A result naming it is a conversation to have with your GP, not a diagnosis.
Can testing tell you more than symptoms can?
Testing can tell you something different. Symptoms cannot separate a parasite from the other causes above, but a stool sample can be checked for the organisms themselves. What a test cannot do is diagnose you. Finding an organism does not prove it is causing your symptoms, a single sample can miss an infection, and our guide to how stool tests for parasites work is honest about both limits.
If your symptoms have persisted past the two-week mark and parasites are a live question, testing and your GP are not either-or. An at-home test for gut parasites screens a stool sample for giardia, cryptosporidium and other common organisms, and works alongside your GP's assessment, never instead of it. No result, in either direction, is a reason to delay an appointment you already need.
The NHS route is also simpler than many people expect. If a GP suspects giardiasis or a similar infection, they can send a stool sample for testing and treat what it finds, and the people you live with are sometimes tested and treated too.
For children, home testing is not the route at all. A child with persistent gut symptoms should see a GP, and suspected threadworms can go straight to a pharmacist. Nothing in this guide is a reason to test a child at home.
When should you see a GP?
Following the NHS guidance on diarrhoea and vomiting, ask for an urgent GP appointment or call 111 if:
- you have had diarrhoea for more than 7 days, or vomiting for more than 2 days
- there is blood in your diarrhoea, or bleeding from your bottom
- you keep being sick and cannot keep fluids down
- you have signs of dehydration that rehydration sachets have not helped, such as peeing much less than usual
See a GP routinely, without waiting for any two-week mark, if you are losing weight without trying, your bowel habit has changed and stayed changed for more than three weeks, you have a weakened immune system, or symptoms began during or after travel abroad. Mention the travel when you book. It changes which tests a GP considers.
For a baby or child, act sooner. Call 111 if you are worried about a baby under 12 months, if a child stops feeding, shows signs of dehydration such as fewer wet nappies, or has blood in their poo. Children lose fluids faster than adults, and same-day advice is the right call, not an overreaction.
This guide is for information only and does not replace medical advice. A stool test cannot diagnose any condition, and persistent or worsening symptoms mean seeing a GP, whatever any test result says.
Sources
- DuPont HL (2016). Persistent Diarrhea: A Clinical Review. JAMA. PubMed | DOI
- Leung AKC et al. (2019). Giardiasis: An Overview. Recent Patents on Inflammation and Allergy Drug Discovery. PubMed | DOI
- Litleskare S et al. (2019). Quality of life and its association with irritable bowel syndrome and fatigue ten years after giardiasis. Neurogastroenterology and Motility. PubMed | DOI
- Miko S et al. (2025). Postinfectious Syndromes and Long-Term Sequelae after Giardia Infections. Emerging Infectious Diseases. PubMed | DOI
- Williams SV et al. (2025). Retrospective case-case study investigation of a significant increase in Cryptosporidium spp. in England and Wales, August to September 2023. Eurosurveillance. PubMed | DOI
- Leung AKC et al. (2025). Pinworm (Enterobius vermicularis) Infestation: An Updated Review. Current Pediatric Reviews. PubMed | DOI
- Tito RY et al. (2019). Population-level analysis of Blastocystis subtype prevalence and variation in the human gut microbiota. Gut. PubMed | DOI
- Rostami A et al. (2017). The role of Blastocystis sp. and Dientamoeba fragilis in irritable bowel syndrome: a systematic review and meta-analysis. Parasitology Research. PubMed | DOI
- NHS. Giardiasis. nhs.uk/conditions/giardiasis
- NHS. Diarrhoea and vomiting. nhs.uk/symptoms/diarrhoea-and-vomiting
- NHS. Threadworms. nhs.uk/conditions/threadworms
- NICE Clinical Knowledge Summaries. Diarrhoea - adult's assessment. cks.nice.org.uk
