Giardiasis

Giardia lamblia, also known as Giardia intestinalis, is the most frequently isolated intestinal protozoa in the world. It is a common cause of diarrhoea and morbidity, especially in children,¹ but is easily missed due to the difficulty of finding it in stool microscopy samples. This was the first human parasitic protozoa to be discovered, seen in 1681 by van Leeuwenhoek.²

Giardia spp. are endemic where sanitation is poor, but can occur worldwide, including in daycare centres and institutionalized patients in developed countries.³ Giardia is a particularly significant pathogen for people with malnutrition, immunodeficiencies, or cystic fibrosis. Travellers to areas of high prevalence are at risk and so are some groups of homosexual men.¹

Incidence: in England and Wales there were around 1,000 cases notified in the first half of 2006.⁴

Worldwide prevalence: It occurs at any age but is common in young children, (estimated around 20% prevalence in young children in developing countries).³

• The life cycle of Giardia is very simple: the parasites (trophozoites) multiply in the duodenum, form cysts that are passed out in the faeces, and infect new individuals when they are swallowed in food or water.²
• The incubation period is 1-2 weeks.¹
• Transmission is usually via contaminated drinking water containing the cysts, which are resistant to standard chlorination.
• Faecal-oral transmission is also common.
• Transmission via food is rare.
• Many animals are host to the organism including pets, livestock and wild animals but it is not clear if they are a source of infection for man. Beavers may be an important reservoir host¹ (in Canada, giardiasis is nicknamed 'Beaver fever').

Symptoms

There is a broad spectrum of symptoms: most are gastro-intestinal. Remember that giardiasis is easily missed and is an important cause of treatable diarrhoea, especially in immunocompromised and palliative care patients.

• Gastrointestinal symptoms:
  • In a minority, there are acute symptoms: explosive, watery diarrhea, abdominal cramps, foul flatus, vomiting, fever and malaise - lasting a few days before settling into the subacute pattern.
  • The majority of patients have less acute symptoms, which are recurrent or persistent:
    • Malodorous, greasy stools or watery diarrhoea (which may alternate with soft stools or even constipation). Stools do not contain blood or pus.
    • Upper GI symptoms, including upper and central abdominal cramping, nausea, early satiety, sulphurous (egg-smelling) belching, bloating and heartburn.
    • Lactose intolerance may occur.
• Systemic symptoms:
  • Anorexia, fatigue, malaise, and weight loss are common.
  • Chronic illness may occur with adults presenting with long-standing malabsorption syndrome and children with failure to thrive.

Signs

• Generally there are few or no physical signs.
• In acute or severe cases, signs of dehydration or malnutrition
• Physical examination generally is unremarkable.
• Abdominal examination may reveal nonspecific tenderness

Unusual presentations

These include allergic manifestations such as urticaria, erythema multiforme, bronchospasm, reactive arthritis, and biliary tract disease.

Note: Where giardiasis is likely it may be easier to treat on suspicion than to investigate.

• Stool microscopy: this is not a straightforward method of detection because:
  • At least 3 stools taken at 2-day intervals should be examined for the trophozoa and cysts.
  • Trophozoites may be found in fresh, watery stools but disintegrate rapidly. If the stool is not fresh or is semiformed to formed, trophozoites will not be found.
  • Cysts are passed in soft and formed stools.Cyst passage is extremely variable, not related to clinical symptoms, and may lag behind the onset of symptoms by a week or more.
  • Antibiotics, enemas, laxatives, and barium studies may mask diagnosis; stool examination should be postponed for 5-10 days following these interventions.
• Stool antigen detection: Several commercially available tests are available which detect Giardia antigen in the stool. They have a sensitivity of 85-98% and a specificity of 90-100%.
• DNA probes for Giardia species are available.¹
• Other tests: duodenal samples for microscopy can be obtained using the 'string test' (swallowing a gelatin capsule on a string) or duodenal biopsy.¹

• Other causes of gastroenteritis,⁴ including cryptosporidiosis - all these will be more important in immunocompromised patients
• Other causes of malabsorption, e.g. coeliac disease
• Inflammatory bowel disease

General points:

• In areas where there is contamination of the water supply the treatment of asymptomatic patients is of dubious value as they will become re-infected.¹
• Treatment is required where there are symptoms, or where there is risk of infection of others who are at special risk. Some authors suggest that in non-endemic areas, everyone should be treated if found to be carrying the organism.¹
• Giardiasis is likely to be caused by food poisoning and should be notified as such.⁴

Drug treatment:

• If there is dehydration this needs to be corrected.
• Metronidazole is the drug of choice for treating giardiasis.⁴ In pregnancy and breastfeeding there are some concerns and cautions about using metronidazole; details are available.⁴
• Tinidazole and mepacrine are alternatives (mepacrine is unlicensed for this).⁵
• Mebendazole,⁶ quinacrine⁷ and chloroquine⁸ have also been used successfully in trials in developing countries. Nitazoxanide is a new drug which may be effective.⁹

• The prognosis is usually good, although both failure of treatment and re-infection can occur.
• Death is rare and usually from dehydration in those at high risk.
• In adults it can cause malabsorption.
• In children there may be growth retardation and failure to thrive.
• Lactose intolerance may occur.
• Occasionally, infective gastroenteritis may unmask other conditions such as coeliac disease or inflammatory bowel disease, so persisting symptoms should be investigated.¹⁰

• Handwashing and hygiene around infected people and in institutions.
• Chlorination, sedimentation, and filtration methods should be implemented to adequately purify public water supplies. Effective chlorine inactivation of Giardia cysts in water requires optimal water treatment procedures, which may be difficult to achieve, especially in swimming pools.
• Travellers to endemic areas should avoid eating uncooked foods.
• Drinking water can be purified by using filtration (pore size, <1 mm) or by boiling water for > 5 minutes.
• Chlorine or iodine water treatments are less effective than boiling or filtration, but are alternatives when other methods are not available.
• Breastfeeding is protective.