Manual of basic techniques for a health laboratory - libdoc.who.int

4. Parasitology 193clinical symptoms are absent. Patients with the chronic form of the diseasemay deny having ever experienced the acute form, possibly because it passedasymptomatically or because it occurred in childhood and has been forgotten.Sources of infection and modes of transmissionIn Chagas disease the parasite (Trypanosoma cruzi) is transmitted by bugs of thegenus Triatoma that become infected by ingesting the blood of infected humans oranimals. The parasite multiplies in the intestine of the triatomine bug. Humans areinfected when the wound at the site of a triatomine bite is contaminated with theinfected faeces of the bug.There is a serious risk that Chagas disease may be transmitted via blood transfusionif proper precautions are not taken.Diagnostic tests for Chagas diseaseTrypanosoma rangeli infects humans in almost the same areas as T. cruzi. AlthoughT. rangeli is not pathogenic, it must be identified and distinguished from T. cruzi forthe diagnosis of Chagas disease.Important: Motile trypanosomes are found in the blood during the acute phase ofthe disease, and rarely thereafter. During the chronic stage the diagnosis is basedessentially on immunological methods.The trypanosomes that cause Chagas disease are difficult to find in the blood. Thesame techniques are used as for African trypanosomiasis:— examination of a wet preparation (see page 186; rarely positive during thechronic stage of the disease);— examination of thick films (see page 187) repeated several days in succession;— examination of blood films prepared from centrifuged blood samples (seepages 187–188);— examination of dried blood samples for IgM and specific antibodies (see page187).Identification of Trypanosoma cruzi in thick blood films (Fig. 4.164)Length: about 15mm in broad forms and 20mm in slender forms.Shape: broad forms are C-shaped; slender forms are generally S-shaped.Cytoplasm: pale blue.Fig. 4.164 Appearance of Trypanosoma cruzi in thick blood films