2.2 Cryptosporidium and Giardia

If Module 2 has a headline act, it is this pair. Protozoa are microscopic single-celled parasites, and the two that matter in UK water hygiene are Cryptosporidium and Giardia. Both have a life cycle built for water: inside an infected gut they produce tough environmental stages - the Cryptosporidium oocyst and the Giardia cyst - which are shed in the host's faeces in huge numbers, survive for long periods in cool water and soil, and are ready to infect the moment someone swallows them. Great Ormond Street Hospital describes Cryptosporidium oocysts as very hardy, able to survive many months in the environment in water and soil. They are microscopic: you cannot see them, and they do not cloud, colour or flavour the water they contaminate.

Cryptosporidium and Cryptosporidiosis

Cryptosporidium causes cryptosporidiosis. The UK Health Security Agency lists the symptoms as severe watery diarrhoea, vomiting, stomach cramps, nausea, fever and loss of appetite, and the illness typically lasts around two weeks, often seeming to improve and then returning before you properly recover. There is no specific treatment for most cases: the advice is rest and plenty of fluids, and most people with healthy immune systems fight the illness off within about a month. For people with weakened immune systems it can cause serious, sometimes prolonged illness.

Two features make Cryptosporidium the defining organism of modern water hygiene:

• Chlorine resistance. The oocyst's thick protective wall makes it highly resistant to chlorine at the doses normally used in drinking water and swimming pools. Disinfection - the barrier that deals with bacteria and most viruses - does not reliably inactivate oocysts. Control therefore rests on physical removal by filtration at the treatment works and, above all, on keeping faecal contamination out of treated water in the first place.
• Animal reservoirs and a spring pattern. C. parvum infects farm animals, especially young calves and lambs, and human infections peak in spring, in April and May, tracking the lambing and calving season. C. hominis, which spreads between humans, peaks in autumn. UKHSA national data show 2024 was unusual, with a large spring spike driven by outbreaks linked to spring lambing events.

Giardia and Giardiasis

Giardia causes giardiasis. The NHS lists the main symptoms as smelly diarrhoea, tummy pain or cramps, flatulence, burps that smell like eggs, bloating and weight loss; greasy, foul-smelling stools are a classic recognised sign. Symptoms typically begin one to two weeks after the parasites are swallowed. Unlike cryptosporidiosis, giardiasis can be treated with antibiotics, and symptoms usually stop about a week into treatment, though they can last much longer untreated. You can also carry and spread Giardia with no symptoms at all. Giardia cysts are markedly harder to inactivate with chlorine than bacteria, so filtration and source protection matter here too, and untreated surface water plus person-to-person spread are the classic routes.

Comparison Table

The Flagship UK Example: Brixham, May 2024

In May 2024 the drinking water supply to Brixham in Devon became contaminated with Cryptosporidium. The Drinking Water Inspectorate's investigation traced the source to animal faeces entering the distribution network through a buried, damaged air valve on the main feeding Hillhead service reservoir, in a field grazed by cattle and sheep. There were 143 laboratory-confirmed cases, with hospitalisations, and 16,221 properties (about 39,000 consumers) were told to boil their water, for up to eight weeks in the worst-affected streets. South West Water pleaded guilty to supplying water unfit for human consumption and on 2 June 2026 was fined a record £1.853 million at Exeter Magistrates' Court following the DWI prosecution. Note the detail that matters most for your role: this was a treated, chlorinated network. Chlorine did not prevent the outbreak, because chlorine does not reliably kill oocysts. What stands between contamination and the customer in distribution is asset integrity and hygienic working by people holding the card you are studying for. Chapter 3 examines this case study in full.

Why Hygienic Working Around Treated Water Is Critical

Everything from Section 2.1 about the faecal-oral route applies doubly to the protozoa: oocysts and cysts travel from faeces to water to mouth; they survive normal disinfection; they are invisible; and the infectious dose is low. The practical implications on site are therefore absolute rather than proportionate:

• Keep faecal material, slurry, soil and animal contamination away from anything that will touch treated water.
• Treat every open excavation, chamber and reservoir as an asset where ingress equals a potential outbreak.
• Disinfect tools, pipe surfaces and fittings, and keep boots, gloves and clothing clean.
• Wash hands before eating, drinking, smoking or vaping - the last link of the chain is always you.

Site Scenario: Reinstatement Beside Lambing Fields in April

You are reinstating a main on farmland in April, at the very peak of the C. parvum spring season, with ewes and lambs in the next field and a wet forecast. The field may look clean, but spring livestock are the reservoir that drives the seasonal peak, and run-off from it heads for your excavation. So you check that the trench is bunded against run-off; you keep overalls and boots off ground that animals have used, or disinfect them before approaching the works; you disinfect every surface that will contact the water; and you wash your hands thoroughly before anything goes near your mouth. None of this is optional simply because the water in the main is chlorinated - oocysts shrug that off.

Common Candidate Traps

• 'Chlorine kills everything in water.' It does not. Cryptosporidium oocysts survive normal chlorination, which is exactly why filtration at treatment works and hygienic working in distribution carry the burden of control.
• 'Oocysts are visible to the naked eye.' They are microscopic. Contaminated water looks and tastes normal, so you cannot rely on your senses to detect it.
• 'You only catch these diseases abroad.' Brixham in 2024 was a UK drinking-water outbreak on a treated public supply. Travel-associated cases occur, but home-gained infection is very real.