Describe Giardia lamblia with its pathogenesis, clinical features and diagnosis.

The Parasite

• Giardia lamblia (also called G. Intestinalis or G. Duodenalis) is a flagellate protozoan and one of the commonest intestinal parasites worldwide
• Trophozoite — pear- or heart-shaped, bilaterally symmetrical, with two nuclei giving a characteristic "face with two eyes" or monkey-face appearance, four pairs of flagella, and a ventral sucking disc by which it attaches to the duodenal mucosa. Its motility is described as "falling leaf"
• Cyst — oval, thick-walled, with four nuclei when mature; the infective form, resistant to chlorination and surviving in cold water for weeks
• Transmission — faecal-oral through contaminated water (the commonest), food, and person to person. The infecting dose is very low (as few as 10 cysts), which is why it spreads readily in nurseries and among household contacts. It is a zoonosis as well, with beavers, dogs and livestock as reservoirs

Pathogenesis and Clinical Features

• It does not invade — trophozoites attach to the duodenum and jejunum by the sucking disc, and the damage is caused by mechanical coverage of the absorptive surface, villous atrophy with crypt hyperplasia, disaccharidase deficiency and bile salt deconjugation
• Malabsorption is the hallmark — foul-smelling, greasy, bulky stools that float (steatorrhoea), with abdominal distension, flatulence, cramps and weight loss. There is NO blood, NO mucus, NO pus and NO fever, which distinguishes it from dysentery
• Secondary lactose intolerance is common and may persist for weeks after the parasite is cleared, so continuing diarrhoea does not necessarily mean treatment failure
• In children it causes failure to thrive and, with repeated infection, stunting — a significant contributor to childhood malnutrition in India
• Predisposing factors — IgA deficiency and common variable immunodeficiency (in whom infection is severe and persistent), hypochlorhydria and proton pump inhibitors, malnutrition, and cystic fibrosis

Diagnosis and Treatment

• Stool microscopy — cysts in formed stool and trophozoites in watery stool; but excretion is intermittent, so three specimens on alternate days are needed, and even then sensitivity is only about 70%
• Concentration methods improve the yield
• Duodenal sampling where stools are repeatedly negative — aspirate, biopsy, or the ENTERO-test ("string test"), in which a weighted gelatin capsule containing a nylon string is swallowed and withdrawn after some hours with adherent trophozoites
• Stool antigen detection by ELISA or immunochromatography is more sensitive than microscopy and is now widely used; PCR is the most sensitive
• Treatment — metronidazole or a single dose of tinidazole; nitazoxanide and albendazole are alternatives, and paromomycin is used in pregnancy
• Treat symptomatic household contacts, and consider treating asymptomatic carriers in a household with a young child or an immunodeficient member

Applied Aspects

• Consider giardiasis in chronic diarrhoea with weight loss and no blood, particularly in a child, a traveller or a patient with recurrent infections suggesting IgA deficiency
• Send three stool samples, not one; a single negative specimen is the commonest reason the diagnosis is missed
• Cysts resist chlorination, so treated municipal water may still transmit it; filtration or boiling is required, which matters for both outbreak investigation and travel advice
• Explain that lactose intolerance may persist after successful treatment, so that continuing symptoms are not mistaken for failure and treated with repeated courses
• Check immunoglobulins in recurrent or refractory giardiasis; selective IgA deficiency and common variable immunodeficiency both present this way
• Prevention is water and sanitation — safe drinking water, handwashing, and food hygiene; in India it is among the commonest parasitic causes of childhood diarrhoea and growth faltering