Parasitic infections

Malaria

• The female Anopheles mosquito injects sporozoites. They first multiply in the liver (exo-erythrocytic stage), then merozoites invade red cells (ring, trophozoite, schizont). Schizont rupture releases pyrogens and causes the fever spike. Some parasites become gametocytes; sexual reproduction occurs in the mosquito, the definitive host.
• Hypnozoites are dormant liver forms of P. vivax and P. ovale that cause relapses. Radical cure needs primaquine (or tafenoquine). Primaquine is an oxidant: in G6PD deficiency it causes intravascular haemolysis (back pain, dark urine, anaemia, raised unconjugated bilirubin), so test G6PD first.
• Why falciparum is severe: the parasite inserts PfEMP1 into knobs on the red cell membrane; PfEMP1 binds endothelial receptors such as ICAM-1 (brain) and CD36. This cytoadherence (sequestration) blocks microvessels, causing cerebral malaria, acute kidney injury, ARDS, lactic acidosis and hypoglycaemia (worsened by quinine). Rosetting and high parasitaemia add to it.
• P. malariae nephropathy: antigen–antibody immune complexes deposit in glomeruli, giving nephrotic syndrome that does not respond to antimalarials. Falciparum, by contrast, causes acute kidney injury (tubular necrosis, haemoglobinuria in blackwater fever).
• Splenic rupture is a real risk with the soft, rapidly enlarged spleen of acute malaria, especially vivax: left upper quadrant and left shoulder-tip pain (Kehr sign) with shock. Palpate gently.
• Anaemia is normocytic normochromic (haemolysis plus marrow suppression). Diagnosis: thick film (finds parasites), thin film (species), rapid antigen tests.
• Treatment: severe or falciparum malaria: IV artesunate, then an artemisinin combination (e.g. artemether–lumefantrine). Vivax: chloroquine or ACT plus primaquine. Sickle trait, G6PD deficiency, thalassaemia trait and Duffy-negative red cells (vivax) are protective.

Other blood and tissue protozoa

• Leishmania (sandfly vector): amastigotes (Leishman–Donovan bodies) inside macrophages. Cutaneous leishmaniasis (oriental sore, common in Khyber Pakhtunkhwa, Balochistan and Waziristan): painless ulcer with a raised edge; diagnose by slit-skin smear; standard treatment in Pakistan is intralesional meglumine antimoniate (or sodium stibogluconate). Do not excise it as a presumed skin cancer. Visceral (kala-azar, *L. donovani*): fever, massive splenomegaly, pancytopenia, hypergammaglobulinaemia; liposomal amphotericin B or miltefosine.
• Toxoplasma gondii (cat faeces, undercooked meat) is a protozoan. Congenital toxoplasmosis: hydrocephalus, diffuse (scattered) intracranial calcification and chorioretinitis. Congenital CMV instead gives microcephaly with periventricular calcification. In AIDS it causes ring-enhancing brain lesions.
• Acanthamoeba: free-living amoeba in tap water; contact lens keratitis with severe pain out of proportion and a ring-shaped corneal infiltrate.
• Trichomonas vaginalis: frothy discharge, strawberry cervix; metronidazole for both partners.

Filariasis

• *Wuchereria bancrofti*, spread by mosquitoes (Culex and others). Adults block lymphatics: lymphangitis, hydrocele, non-pitting lymphoedema and elephantiasis. Ruptured lymphatics into the urinary tract cause chyluria (milky urine after fatty meals).
• Microfilariae show nocturnal periodicity: take the blood film around midnight; a negative daytime film excludes nothing. Tropical pulmonary eosinophilia is cough, wheeze and very high eosinophils. Treatment: diethylcarbamazine, albendazole, ivermectin.

Intestinal protozoa

• A mature infective *E. histolytica* cyst has four nuclei; an eight-nucleate cyst is the harmless *Entamoeba coli*.
• Colonic complications: fulminant amoebic colitis with toxic dilatation (transverse colon over 6 cm), perforation, and amoeboma, a mass of granulation tissue in the caecum that mimics carcinoma or ileocaecal TB; biopsy shows trophozoites with red cells and it shrinks on metronidazole.

Amoebic versus pyogenic liver abscess

• Trophozoites invade colonic venules and are carried by the portal vein to the liver, mainly the right lobe (it receives most of the superior mesenteric stream). Many patients never had dysentery, and stool microscopy is often negative; serology helps.

• Drain (image-guided aspiration) an amoebic abscess when there is no response after 3–5 days of metronidazole, it is large (over 5–10 cm), or it lies in the left lobe or is about to rupture.
• Rupture: right lobe abscesses burst into the pleura or lung; left lobe abscesses threaten the pericardium, causing tamponade (raised JVP, muffled sounds, pulsus paradoxus). *Klebsiella* pyogenic abscess can seed the eye (endophthalmitis).

Helminths

• Pinworm diagnosis: sticky tape pressed on the perianal skin first thing in the morning, before washing. Eggs survive weeks on bedding, so treat everyone at home and repeat after two weeks.
• Ascaris obstruction without strangulation: "drip and suck" (IV fluids, nasogastric decompression); give the anthelmintic after the obstruction settles, since paralysed worms can form a firmer bolus. Operate for strangulation or failure.
• Strongyloides hyperinfection: steroids let larvae multiply and migrate massively, carrying gut bacteria with them: Gram-negative septicaemia, pulmonary infiltrates, serpiginous rash (larva currens). Eosinophilia may be absent on steroids. Screen or treat with ivermectin before immunosuppression in people from endemic areas.
• Praziquantel covers both schistosomes and *Taenia*. Lice are ectoparasites.

Hydatid disease

• *Echinococcus granulosus*, the dog tapeworm. Dogs are the definitive host; sheep are the usual intermediate host; humans are accidental intermediate hosts after swallowing eggs from dog faeces (shepherds, farmers).
• Larvae cross the gut into the portal blood, so the liver (about 70%, mostly right lobe) is commonest, then the lung (a round mass in a farmer's lung is classically hydatid).
• Layers: outer host pericyst (fibrous), acellular laminated membrane, inner germinal layer producing brood capsules, protoscolices (hydatid sand) and daughter cysts.
• Imaging: daughter cysts, eggshell calcification; a floating, undulating membrane (water-lily sign) means the endocyst has detached.
• Complications: rupture into the peritoneum (anaphylaxis, seeding), into the bile ducts (cholangitis and obstructive jaundice from membranes: colicky pain, fever, high ALP and eosinophils; ERCP shows white wrinkled membranes and clears them), into a bronchus, or secondary infection. Hydatid cyst does not raise cholangiocarcinoma risk.
• Never aspirate blindly. Treatment: albendazole before and after surgery or PAIR (puncture, aspiration, injection of hypertonic saline, re-aspiration), with packs soaked in scolicidal agent at surgery.

Remember Know each parasite by its door in, its home in the body and its drug.

Exam trap Humans are accidental intermediate hosts of hydatid; eating pork gives the tapeworm, swallowing *Taenia solium* eggs gives cysticercosis.

Exam trap Right lobe amoebic abscesses rupture into the pleura; left lobe abscesses rupture into the pericardium, so drain them early.

How FCPS asks about this

• Cerebral malaria mechanism: PfEMP1 on infected red cells binds endothelial ICAM-1 (cytoadherence).
• Relapse and primaquine: hypnozoites of vivax and ovale; G6PD deficiency causes haemolysis with dark urine.
• Quartan malaria: *P. malariae*, immune-complex nephrotic syndrome.
• Entamoeba: caecum and ascending colon; four-nucleate cyst; reaches liver by portal vein; anchovy-sauce pus.
• Parasites in children: pinworm tape test in the morning, treat household; *Trichuris* rectal prolapse; Ascaris obstruction is managed by drip and suck.
• Hydatid: dog definitive, humans accidental intermediate; water-lily sign; rupture into bile ducts causes cholangitis.
• Liver flukes and fish: *Clonorchis* cholangiocarcinoma; *Fasciola* from watercress; fish tapeworm B12 deficiency.