Medical parasitology
Parasitology: malaria, amoebiasis, leishmania, helminths, hydatid
Protozoa are single-celled parasites including Plasmodium, Entamoeba, Giardia, Leishmania and Toxoplasma; helminths include roundworms, tapeworms and flukes. Life cycles, vectors, diagnostic forms and drugs are tested heavily, especially for diseases common in Pakistan such as malaria, amoebic liver abscess, leishmaniasis and hydatid disease.
Malaria (*Plasmodium*)
- Vector: female *Anopheles* mosquito (bites at night). Sporozoites, then liver (exoerythrocytic schizogony), then merozoites, then red cells (erythrocytic cycle: ring, then trophozoite, then schizont), then gametocytes (taken up by mosquito; sexual cycle in mosquito — mosquito is definitive host, human intermediate).
| Species | Cycle (fever) | Hypnozoites | Key features |
|---|---|---|---|
| *P. falciparum* | Irregular/48 h (malignant tertian) | No | Severe malaria: high parasitaemia, cerebral malaria, severe anaemia, hypoglycaemia (also quinine), AKI (blackwater fever), ARDS, acidosis, shock, DIC; sequestration (knobs, PfEMP1 binding endothelium); banana-shaped gametocytes, multiple rings, appliqué forms, Maurer clefts; infects red cells of all ages |
| *P. vivax* | 48 h (benign tertian) | Yes (relapses) | Commonest in Pakistan; Schüffner dots, enlarged red cells, infects reticulocytes; needs Duffy antigen; splenic rupture |
| *P. ovale* | 48 h | Yes | Oval, fimbriated cells, Schüffner dots |
| *P. malariae* | 72 h (quartan) | No | Band forms, nephrotic syndrome, very late recrudescence |
| *P. knowlesi* | 24 h | No | Monkey malaria (South-East Asia); severe |
- Diagnosis: thick film (detects parasites — sensitivity), thin film (species and parasitaemia %), rapid diagnostic tests (HRP2, pLDH), PCR. Three negative films over 72 h to exclude.
- Protection: sickle cell trait, G6PD deficiency, thalassaemia trait, Duffy-negative (vivax), HLA-B53, ovalocytosis.
- Treatment: see antimalarials (ACT for falciparum; chloroquine + primaquine for vivax; IV artesunate for severe).
- Prevention: bed nets (insecticide-treated), indoor residual spraying, repellents, chemoprophylaxis, RTS,S and R21 vaccines (children in Africa).
Intestinal and other protozoa
| Organism | Disease and features | Diagnosis | Treatment |
|---|---|---|---|
| *Entamoeba histolytica* | Amoebic dysentery (flask-shaped ulcers in caecum/colon, bloody mucoid stools — less fever than shigella), amoeboma (mass), amoebic liver abscess (right lobe, single, anchovy-sauce pus, fever, tender hepatomegaly, raised right hemidiaphragm; young men; may rupture into pleura/pericardium/peritoneum) | Stool microscopy (trophozoites with ingested red cells; quadrinucleate cysts), stool antigen/PCR, serology (liver abscess), ultrasound | Metronidazole then luminal agent (diloxanide furoate, paromomycin); aspiration if large, left lobe or not responding |
| *Giardia lamblia* | Malabsorption, steatorrhoea, bloating, foul-smelling diarrhoea; contaminated water; IgA deficiency | Pear-shaped trophozoites ("falling leaf" motility, two nuclei — "face"), cysts; stool antigen; duodenal aspirate (string test) | Metronidazole, tinidazole |
| *Cryptosporidium* | Watery diarrhoea (severe chronic in AIDS) | Modified acid-fast oocysts | Nitazoxanide; ART |
| *Toxoplasma gondii* | Cat faeces, undercooked meat; congenital (chorioretinitis, hydrocephalus, intracranial calcifications — diffuse), ring-enhancing brain lesions in AIDS (CD4 < 100), lymphadenopathy, chorioretinitis | Serology, PCR, CT/MRI | Pyrimethamine + sulfadiazine + folinic acid; spiramycin in pregnancy |
| *Trichomonas vaginalis* | Frothy green-yellow discharge, strawberry cervix, pH > 4.5 | Motile flagellates on wet mount, NAAT | Metronidazole (treat partner) |
Leishmaniasis (sandfly — *Phlebotomus*)
- Amastigotes (Leishman–Donovan bodies) in macrophages; promastigotes in sandfly and culture (NNN medium).
- Cutaneous (*L. tropica*, *L. major* — common in Pakistan, Balochistan, KP, Afghan border): painless nodule becomes an ulcer with raised edge ("oriental sore", "volcano"); heals with scarring; intralesional antimonials (sodium stibogluconate/meglumine antimoniate), cryotherapy, thermotherapy.
- Mucocutaneous (*L. braziliensis*): nasal septum destruction.
- Visceral (kala-azar) (*L. donovani*, *L. infantum*): fever, massive splenomegaly, hepatomegaly, pancytopenia, weight loss, hyperpigmentation, hypergammaglobulinaemia; diagnosis by splenic aspirate (most sensitive), bone marrow, rK39 rapid test; treatment liposomal amphotericin B, miltefosine, antimonials; post-kala-azar dermal leishmaniasis.
Other blood and tissue protozoa
- Trypanosoma cruzi (Chagas; reduviid bug): Romaña sign, dilated cardiomyopathy, megaoesophagus, megacolon.
- *T. brucei* (sleeping sickness; tsetse fly).
- *Babesia* (ticks): Maltese cross in red cells.
Nematodes (roundworms)
| Worm | Transmission | Key features | Treatment |
|---|---|---|---|
| *Ascaris lumbricoides* | Ingestion of eggs | Commonest helminth; Löffler syndrome (lung migration: eosinophilic pneumonitis), intestinal obstruction (worm bolus in children), biliary ascariasis (obstructive jaundice, cholangitis, pancreatitis), appendicitis | Albendazole/mebendazole; pyrantel |
| *Enterobius vermicularis* (pinworm) | Eggs (autoinfection) | Perianal itching at night; Scotch tape (cellotape) test | Mebendazole (treat household, repeat in 2 weeks) |
| *Ancylostoma duodenale*, *Necator americanus* (hookworm) | Larvae penetrate skin (barefoot) | Iron deficiency anaemia (blood loss), ground itch, Löffler | Albendazole + iron |
| *Strongyloides stercoralis* | Skin penetration; autoinfection | Larva currens, eosinophilia; hyperinfection syndrome with steroids or HTLV-1 (Gram-negative sepsis) | Ivermectin (screen before steroids) |
| *Trichuris trichiura* (whipworm) | Eggs (barrel-shaped, bipolar plugs) | Rectal prolapse, dysentery | Albendazole |
| *Trichinella spiralis* | Undercooked pork | Myositis, periorbital oedema, eosinophilia, splinter haemorrhages | Albendazole + steroids |
| *Wuchereria bancrofti*, *Brugia* | Mosquito (*Culex*) | Lymphatic filariasis: elephantiasis, hydrocele, chyluria; nocturnal periodicity (night blood film); tropical pulmonary eosinophilia | Diethylcarbamazine, albendazole, ivermectin |
| *Onchocerca volvulus* | Blackfly | River blindness, skin nodules | Ivermectin |
| *Loa loa* | Deer fly | Calabar swellings, eye worm | |
| *Toxocara canis* | Dog faeces | Visceral larva migrans, ocular (retinal granuloma mimicking retinoblastoma) | Albendazole |
| *Dracunculus* | Water fleas (copepods) | Guinea worm (eradication near complete) | |
| *Anisakis* | Raw fish | Gastric pain |
Cestodes (tapeworms)
| Worm | Host details | Disease |
|---|---|---|
| *Taenia solium* (pork) | Ingest cysticerci in pork: intestinal tapeworm; ingest eggs (faecal–oral): cysticercosis | Neurocysticercosis: commonest cause of acquired epilepsy worldwide; cysts with scolex ("hole with dot"), ring-enhancing lesions, calcifications; albendazole + steroids (± antiepileptics); praziquantel for intestinal worm |
| *Taenia saginata* (beef) | Cysticerci in beef | Intestinal only (no cysticercosis) |
| *Echinococcus granulosus* | Dog = definitive host; sheep (and humans) = intermediate (ingest eggs from dog faeces) | Hydatid cyst: liver (right lobe, about 70%) > lung; "eggshell calcification", daughter cysts ("water lily sign" when membrane detaches), hydatid sand; rupture leads to anaphylaxis, dissemination; Casoni test (obsolete), serology (ELISA), ultrasound (WHO classification)/CT; albendazole pre- and post-operatively, PAIR (puncture, aspiration, injection of scolicidal agent — hypertonic saline/alcohol, re-aspiration), surgery (cystectomy; avoid spillage — pack with scolicidal swabs); avoid aspiration without cover |
| *E. multilocularis* | Fox | Alveolar hydatid: infiltrative, tumour-like liver lesion |
| *Diphyllobothrium latum* | Raw freshwater fish | Vitamin B12 deficiency |
| *Hymenolepis nana* | Direct (dwarf tapeworm) | Commonest tapeworm in children; autoinfection |
Trematodes (flukes)
| Worm | Features |
|---|---|
| *Schistosoma haematobium* | Snail host; skin penetration (swimmer's itch); terminal haematuria, bladder calcification, squamous cell carcinoma of bladder, hydronephrosis; terminal-spined eggs in urine |
| *S. mansoni*, *S. japonicum* | Periportal (pipe-stem) fibrosis, presinusoidal portal hypertension with preserved liver function; lateral-spined eggs (mansoni) |
| *Clonorchis sinensis* (Chinese liver fluke) | Raw fish; biliary obstruction, cholangiocarcinoma, pigment stones |
| *Fasciola hepatica* | Watercress; biliary disease |
| *Paragonimus westermani* | Undercooked crabs; lung fluke — haemoptysis mimicking TB |
- Treatment of flukes: praziquantel (except *Fasciola* — triclabendazole).
Eosinophilia
Helminths (tissue migration: Ascaris, hookworm, Strongyloides, Toxocara, Trichinella, schistosomes, filaria), allergy/asthma, drugs, Churg–Strauss, Hodgkin lymphoma, Addison disease, eosinophilic leukaemia. Protozoa usually do not cause eosinophilia.
How FCPS asks it
Recalls test: malaria life cycle and definitive host; species features (falciparum banana gametocytes and severe complications; vivax Schüffner dots, hypnozoites, reticulocytes, Duffy; malariae quartan and nephrotic); thick versus thin film; protective red cell traits; amoebic liver abscess site, pus, treatment with luminal agent; Giardia; Toxoplasma congenital and AIDS; kala-azar features and splenic aspirate; cutaneous leishmaniasis in Pakistan; Ascaris biliary and obstruction; Enterobius tape test; hookworm anaemia; Strongyloides hyperinfection with steroids; filariasis nocturnal periodicity and DEC; neurocysticercosis and epilepsy; hydatid hosts, liver right lobe, imaging signs, albendazole, PAIR, spillage anaphylaxis; *D. latum* B12; *S. haematobium* bladder SCC; Clonorchis cholangiocarcinoma.
Exam trap In hydatid disease the dog is the definitive host; humans are accidental intermediate hosts.
Exam trap *Strongyloides* can cause fatal hyperinfection with corticosteroids — screen and give ivermectin.
Exam trap Neurocysticercosis (*Taenia solium* eggs) is the commonest cause of acquired epilepsy worldwide.
Test yourself on this topic
5 of the 102 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A child from a rural area presents with colicky abdominal pain, vomiting and distension. X-ray shows a 'whirlpool' of worms in the small bowel. Which parasite is responsible?
Answer: Ascaris lumbricoides
Ascaris (roundworm) is acquired by swallowing eggs in soil-contaminated food. Larvae migrate through the lungs (Löffler syndrome: cough, eosinophilia) before maturing in the small bowel. Heavy loads can cause bolus obstruction, biliary ascariasis (cholangitis, pancreatitis) and malnutrition. Albendazole treats it; obstruction is first managed conservatively.
Ascariasis
- Eggs swallowed from soil
- Lung phase: Löffler syndrome
- Bolus small bowel obstruction
- Biliary ascariasis
- Treat: albendazole
Remember A ball of roundworms blocks the bowel.
Exam trap Worms can migrate into the bile duct and cause cholangitis.
Why the other options are wrong
- Enterobius vermicularis Perianal itch; tiny worms.
- Ancylostoma duodenale Anaemia; small worms.
- Trichuris trichiura Rectal prolapse in heavy infection.
- Giardia lamblia Protozoan.
A 9-year-old boy from a rural area has had fever for 3 months, weight loss and a hugely enlarged spleen. His skin has darkened. Hb 7.2 g/dL, WBC 2.1 ×10⁹/L, platelets 60 ×10⁹/L, and serum globulin is very high. Splenic aspirate shows small oval bodies inside macrophages. What is the diagnosis?
Answer: Visceral leishmaniasis
Visceral leishmaniasis (kala-azar) is caused by Leishmania donovani (and L. infantum), transmitted by the bite of the female sandfly (Phlebotomus). The parasite lives inside macrophages of the spleen, liver and bone marrow as amastigotes (Leishman–Donovan bodies). Typical features: prolonged irregular fever, massive splenomegaly, hepatomegaly, weight loss, pancytopenia (hypersplenism and marrow infiltration), polyclonal hypergammaglobulinaemia and, in South Asia, skin darkening (kala-azar means black fever). Diagnosis: amastigotes in splenic (most sensitive) or marrow aspirate, rK39 rapid antibody test. Treatment: liposomal amphotericin B, or miltefosine; older regimens used sodium stibogluconate. Post-kala-azar dermal leishmaniasis can follow treatment.
Kala-azar
- Leishmania donovani, sandfly (Phlebotomus)
- Amastigotes in macrophages: LD bodies
- Fever, massive splenomegaly, pancytopenia, hypergammaglobulinaemia
- Splenic aspirate most sensitive; rK39 test
- Treat: liposomal amphotericin B
Remember Black fever, giant spleen, parasites hiding in macrophages.
Exam trap Malaria parasites live in red cells; Leishmania amastigotes live in macrophages.
Why the other options are wrong
- Chronic malaria Chronic malaria can enlarge the spleen, but amastigotes in macrophages are diagnostic of leishmaniasis.
- Chronic myeloid leukaemia CML causes a high, not low, white count.
- Hydatid disease Hydatid disease causes cysts, not pancytopenia with intracellular parasites.
- Typhoid fever Typhoid does not cause months of fever with massive splenomegaly and pancytopenia.
A man from an endemic area has massive, firm, non-pitting swelling of one leg and the scrotum with thickened skin. What is the mechanism of the oedema?
Answer: Filarial lymphatic obstruction
Causes of oedema: raised capillary hydrostatic pressure (heart failure, venous thrombosis, fluid overload); reduced plasma oncotic pressure (nephrotic syndrome, cirrhosis, malnutrition); increased permeability (inflammation, burns, sepsis, anaphylaxis); and lymphatic obstruction (filariasis—Wuchereria bancrofti, Brugia malayi; malignancy; surgery or radiotherapy such as after axillary clearance). Lymphoedema is protein-rich, so it causes fibrosis, skin thickening and non-pitting swelling (elephantiasis). Treatment of filariasis: diethylcarbamazine, albendazole; mass drug administration.
Causes of oedema
- ↑ Hydrostatic: heart failure, DVT
- ↓ Oncotic: nephrotic, cirrhosis, malnutrition
- ↑ Permeability: inflammation, burns, sepsis
- Lymphatic: filariasis, cancer, surgery → non-pitting
Remember Protein-rich lymph does not pit.
Why the other options are wrong
- Low plasma albumin Gives soft pitting oedema, usually bilateral.
- Raised venous pressure Bilateral pitting oedema.
- Inflammatory capillary leak Red, painful, acute swelling.
- Renal sodium retention Generalised pitting oedema.
A young farmer from an irrigated area has haematemesis from oesophageal varices, a large spleen and normal liver function. Liver biopsy shows granulomas around eggs in portal tracts with 'pipe-stem' fibrosis. Which organism is responsible?
Answer: Schistosoma mansoni
Schistosoma mansoni (and S. japonicum) adult worms live in mesenteric veins; eggs carried to the liver lodge in portal venules and provoke a Th2-driven granulomatous reaction with eosinophils, followed by dense periportal ('pipe-stem', Symmers) fibrosis. This causes presinusoidal portal hypertension with varices and splenomegaly but preserved liver function. S. haematobium affects the bladder (haematuria, calcification, squamous carcinoma). Infection is from freshwater snails releasing cercariae that penetrate skin. Treatment: praziquantel.
Schistosomiasis
- Eggs → granulomas in portal tracts
- Pipe-stem fibrosis → presinusoidal portal hypertension
- LFTs preserved
- S. haematobium: bladder, squamous Ca; treat praziquantel
Remember Eggs build granulomas; granulomas build pipes.
Why the other options are wrong
- Echinococcus granulosus Hydatid cysts.
- Entamoeba histolytica Amoebic liver abscess.
- Fasciola hepatica Biliary disease from watercress.
- Clonorchis sinensis Biliary disease and cholangiocarcinoma.
A 22-year-old at 14 weeks has vaginal bleeding, severe vomiting, tremor and palpitations. Her uterus is the size of 20 weeks, no fetal heart is seen, and ultrasound shows a 'snowstorm' appearance. Free T4 is raised and TSH suppressed. What is the most likely diagnosis?
Answer: Complete hydatidiform mole
A uterus larger than dates, bleeding, no fetus and a snowstorm pattern indicate a complete hydatidiform mole. Molar trophoblast produces very large amounts of hCG, which stimulates the TSH receptor enough to cause clinical hyperthyroidism. The thyrotoxicosis resolves once the mole is evacuated and hCG falls. Graves disease would not explain the uterine findings.
Molar hyperthyroidism
- Uterus large for dates
- Snowstorm on ultrasound
- Very high hCG stimulates TSH receptor
- Cured by evacuation
Remember Big uterus plus thyrotoxicosis: think mole.
Exam trap Choriocarcinoma can do the same, but here the scan shows a mole.
Why the other options are wrong
- Graves disease in pregnancy Does not explain the uterus or the scan.
- Struma ovarii with thyrotoxicosis An ovarian teratoma; the uterus would be normal.
- Dichorionic twin pregnancy Raises hCG, but fetal hearts would be seen.
- Subacute granulomatous thyroiditis Painful thyroid; uterine findings unexplained.
Where this sits in the CPSP syllabus
- Paper 1: Microbiology, Parasitology: malaria, amoebiasis, leishmania, helminths, hydatid
Read next
Drug dose pages
Doses, cautions and practical tips for drugs in this chapter.