Cryptosporidium
A protozoan parasite whose oocysts are highly resistant to chlorine at the doses used in drinking water treatment. It is the classic case where disinfection alone is not enough, and physical removal by filtration is the barrier that matters.
Key facts
- Contaminant group
- Biological
- World Health Organization — guideline
- No numerical guideline valueWHO uses a risk-management approach through water safety plans rather than a concentration limit.
- United Kingdom — legal limit
- Treatment and monitoring requirements at at-risk worksThe former numerical standard was replaced by a risk-assessment-based regime.
- United States — legal limit
- 99% removal required for filtered systemsA treatment technique requirement rather than a concentration limit.
Where it comes from
- Faeces of infected humans and livestock, particularly young cattle and sheep
- Agricultural runoff into surface water catchments after rainfall
- Sewage discharges and combined sewer overflows
- Contaminated groundwater where a borehole is under the influence of surface water
How it behaves in water
The oocyst is a robust, environmentally stable stage roughly 4–6 micrometres across. It survives for months in cold water and passes through chlorine contact tanks essentially unaffected. Peaks follow rainfall events that wash livestock faeces into watercourses.
How it is measured
Counted as oocysts per volume, usually per 10 or 100 litres, by filtration and immunofluorescence microscopy. Recovery efficiency is variable and the method does not distinguish viable from non-viable oocysts, nor human-infectious species from those that are not.
Regulatory position
| Jurisdiction | Value | Status | Notes |
|---|---|---|---|
| World Health Organization | No numerical guideline value | guideline | WHO uses a risk-management approach through water safety plans rather than a concentration limit. |
| United Kingdom | Treatment and monitoring requirements at at-risk works | legal limit | The former numerical standard was replaced by a risk-assessment-based regime. |
| United States | 99% removal required for filtered systems | legal limit | A treatment technique requirement rather than a concentration limit. |
Table sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- European Commission — Drinking Water Directive (EU) 2020/2184 and reporting. Open — attribution required · European Commission reuse policy (Decision 2011/833/EU)
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain
Health context
Causes cryptosporidiosis, a self-limiting diarrhoeal illness in healthy people lasting one to two weeks, but potentially severe and prolonged in the immunocompromised. The infectious dose is low.
What removes it
| Row | Effective | Not reliably effective |
|---|---|---|
| 1 | Membrane filtration or well-operated granular filtration to a strict turbidity target | Chlorination at drinking water doses |
| 2 | Ultraviolet disinfection, which is highly effective against Cryptosporidium at practical doses | Chloramination |
| 3 | Ozone, at sufficient contact time | Carbon filters without an absolute rated pore size |
| 4 | Boiling — the standard household advice during an incident | — |
| 5 | Point-of-use filters with an absolute pore size of 1 micrometre or less | — |
Occurrence
Major outbreaks — Milwaukee in 1993, and several UK incidents since — have consistently traced to filtration failure rather than disinfection failure.
Relationships
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted