Hydatid disease is caused by the tapeworm Echinococcus granulosus which is transmitted by domestic dogs in livestock raising areas. The parasite eggs are passed out in the faeces of tapeworm infected canines. These are ingested by humans, the larva then penetrate the intestinal mucosa and travel via the circulation to the target organs. The mature cysts tend to form in the liver (50-70%) or lungs (20-30%) of patients but occasionally in the brain, heart, bones.
Cysts may remain asymptomatic until large or cause symptoms by eroding into local structures or by causing obstruction. Occasionally they can leak or rupture seeding daughter cysts into other areas of the body. This may be associated with an allergic reaction or with the consequences of disseminated disease. Often infection is detected incidentally on imaging studies.
The indirect haemagglutination test (IHA) has a sensitivity approaching 80% for hydatid disease of the liver and 40% for hydatid disease of the lung. A negative result does not exclude hydatid infection.
False positive results can occur, particularly in the presence of other helminth infections (e.g. neurocysticercosis). Assay specificity improves at higher titres. Titres of 320 or greater are considered diagnostically significant for hydatid disease. A stable titre of 160 in untreated patients should be confirmed by a second method (e.g. WB). Titres of 80 are not significant.The HA method detects all Immunoglobulin classes, but mainly IgG.