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Guide 11 · Infection & prevention

Human parasites: a guide to infection, testing and care

A practical atlas of human parasites, exposure routes and the questions that guide testing and care.

4 minute read20 sourcesStreszczenie PL ↓
Giardia duodenalis cyst in a wet mount, differential interference contrast microscopy at 1000 times magnification.

G. duodenalis cyst, DIC microscopy, 1000×. CDC DPDx. Original diagnostic image; not AI-generated.

CDC DPDx · Public-domain CDC DPDx material; attribution retained · Original · Open full size ↗

Start with the organism and the exposure

Parasites include protozoa, helminths and organisms living on the skin. Giardia is a microscopic intestinal protozoan; a threadworm is a nematode; a scabies mite is an ectoparasite. Their life cycles determine how infection happens and what a useful test or treatment looks like. There is no single parasite test or universal deworming programme for every person. P01 P05

An exposure history often matters more than a long symptom checklist. Relevant questions include untreated water, travel, household itching, raw fish or meat, contact with soil and farm animals, and medicines that suppress immunity. Bloating, fatigue or a change in appetite can have many explanations. They do not establish infection or identify a species. P05 P09

A practical atlas

Organism or groupExposure and main concernDiagnostic direction
Giardia duodenalisFaecal contamination of water, food or hands; diarrhoea and malabsorptionAppropriate stool antigen, molecular or microscopy testing P04 P08
CryptosporidiumWater, animals and person-to-person spread; watery diarrhoeaStool testing which specifically includes this organism P23 P24
Enterobius vermicularisThreadworms spread easily in households; night-time anal itchingPerianal tape sampling, rather than relying on ordinary stool testing P06 P07
Ascaris lumbricoidesIngestion of infective eggs in contaminated environmentsExposure-informed stool assessment P11
Trichuris trichiuraSoil-transmitted whipworm; heavy infection can cause significant illnessStool examination in the appropriate setting P11
HookwormsInfective larvae can enter through exposed skin; blood loss in substantial infectionStool tests and assessment of anaemia P11
Strongyloides stercoralisSoil exposure; infection may persist through autoinfectionSpecialist-directed stool and/or antibody tests; tell the clinician about steroid use P10
Taenia soliumIntestinal infection after eating infected undercooked pork; tissue infection follows ingestion of eggsThe suspected form changes the investigations P21
EchinococcusIngestion of eggs; cysts may develop in organsImaging and specialised assessment, not a generic cleanse P22
TrichinellaInfected meat, including game; systemic illness after exposureClinical history and laboratory assessment P19
Toxoplasma gondiiMeat, soil, food or water contaminated with the organism; pregnancy and immunity matterTesting is selected for the individual clinical situation P20
PlasmodiumMosquito-transmitted malaria; fever after travel can be urgentPrompt clinical assessment and malaria testing P31
Sarcoptes scabieiScabies spreads through close contact; itching and rashExamination and coordinated management of relevant contacts P28
Head lice and ticksExternal parasites; ticks may also transmit other pathogensDirect examination and organism-specific advice P01 P18
BlastocystisFrequently detected organism with uncertain clinical significance in some casesA positive result alone does not prove the cause of symptoms P25

Symptoms, severity and the next step

Some infections cause no symptoms. Others lead to diarrhoea, itching, anaemia, fever or organ-specific problems. Severity depends on the organism, infection burden and host. A negative test may need interpretation against the method and timing; it is not a reason to assume hidden parasites indefinitely. P05 P09

Seek prompt medical advice for bloody or persistent diarrhoea, dehydration, severe pain or deterioration. Fever after a stay in a malaria area warrants urgent assessment even after preventive medication. Pregnancy, infancy and immune suppression lower the threshold for obtaining advice. If someone is severely unwell, use emergency services. P29 P31

What prevention can achieve

Safe water, appropriate food preparation and handwashing interrupt specific transmission routes. Hand sanitiser is not an effective substitute for soap and water against Cryptosporidium. Keep raw meat separate from ready-to-eat foods and wash produce. Management of threadworms or scabies may involve household contacts, but the measures are different for each infection. P07 P20 P23 P28

WHO mass-deworming programmes target defined populations in endemic settings. They should not be translated into a blanket recommendation for every reader in Ireland. The sensible question is which exposure and diagnosis apply to this person. P11

Cysts leave an infected host in faeces; Water, food, hands or surfaces contaminated; Another person swallows cysts; Small intestine: trophozoites multiply; Cysts formed and shed in faeces; Interrupt transfer: safe water + handwashing

Based on CDC DPDx giardiasis [P04]. Simplified educational diagram; not to scale. An infected person may have symptoms or remain asymptomatic.

[P04] · Open full size ↗

Sources & access limits

Results remain specific to the preparation, study population and outcomes described.

  1. [P01] CDC, About ParasitesPublic-health, institutional or source documentation.
  2. [P05] CDC, Diagnosis of Parasitic DiseasesPublic-health, institutional or source documentation.
  3. [P09] CDC Yellow Book, Post-Travel Parasitic Disease Including Evaluation of EosinophiliaPublic-health, institutional or source documentation.
  4. [P04] CDC, DPDx - GiardiasisPublic-health, institutional or source documentation.
  5. [P08] CDC, Clinical Testing and Diagnosis for Giardia InfectionPublic-health, institutional or source documentation.
  6. [P23] CDC, Preventing Crypto; 08.05.2025Public-health, institutional or source documentation.
  7. [P24] CDC, Treating Crypto; 04.06.2025Public-health, institutional or source documentation.
  8. [P06] CDC, Diagnosing PinwormsPublic-health, institutional or source documentation.
  9. [P07] HSE, Threadworms; przegląd 23.03.2026Public-health, institutional or source documentation.
  10. [P11] WHO, Soil-transmitted helminth infectionsPublic-health, institutional or source documentation.
  11. [P10] CDC, Clinical Overview of Strongyloides; aktualizacja 11.02.2026Public-health, institutional or source documentation.
  12. [P21] CDC, DPDx - CysticercosisPublic-health, institutional or source documentation.
  13. [P22] WHO, EchinococcosisPublic-health, institutional or source documentation.
  14. [P19] CDC, Clinical Care of TrichinellosisPublic-health, institutional or source documentation.
  15. [P20] CDC, Preventing ToxoplasmosisPublic-health, institutional or source documentation.
  16. [P31] HSE, Malaria - SymptomsPublic-health, institutional or source documentation.
  17. [P28] HSE, ScabiesPublic-health, institutional or source documentation.
  18. [P18] HPSC, Lyme disease factsheet - prawidłowe usuwanie kleszczaPublic-health, institutional or source documentation.
  19. [P25] CDC, Clinical Care of BlastocystosisPublic-health, institutional or source documentation.
  20. [P29] NHS, GiardiasisPublic-health, institutional or source documentation.

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