Studies in epidemiology and seroepidemiology of visceral leishmaniasis in Iraq
A defined population was studied over a period of 7 months to elucidate the epidemiology of visceral leishmaniasis in Iraq, using serological methods as a screening test. Seroepidemiological methods were also used in the search for a canine reservoir of infection and the tests used were studied in defined animal systems and in confirmed human cases. A cross-sectional survey was made of the population of a defined rural area of 300 km2, south of Baghdad. It included 19 villages with 1,171 houses and a total population of 9,889. Houses were mapped and a census completed. The 3,403 persons under 7 years of age were screened using two serological tests for visceral leishmaniasis: indirect fluorescent antibody test (IFAT) and enzyme-linked immunosorbent assays (ELISA). Seropositive children were fully examined clinically and by the leishmanin test. The blood picture and serum proteins were determined and, in the absence of clinical signs, follow-up was by monthly serological examination. Symptomatic children were admitted to hospital for bone marrow biopsy. Results showed a range from subclinical cases defined only by sero-conversion through to severe disease needing hospital treatment and with a high mortality. A repeat survey of the same child population after 7 months showed serological changes following the main transmission season. 66 of the parasitologically confirmed sero-positive cases from this area and elsewhere in the endemic region were examined, and in some, monthly serology was determined at domiciliary follow-up. IFAT was found to be more sensitive than ELISA. 33% of cases of visceral leishmaniasis were found to revert to negative within 9 months of treatment. 435 hospital inpatients with a variety of diagnoses were studied to determine the specificity of tests. ELISA was found of greater specificity than IFAT. These cases included 124 clinically suspected leishmaniases of which 45 were subsequently culture-positive. A longitudinal serological study was carried out in inbred mice of varying genetically determined susceptibility to infection. All innately susceptible mice were seropositive by day 50 and the titre continued to increase until the end of the experiment at day 130 regardless of the parasitological course of infection. In a search for the postulated canine reservoir of visceral leishmaniasis 151 jackals and 65 dogs, largely strays, were studied parasitologically and serologically. Neither from these nor from a limited sample of rodents could the parasite be isolated, though several jackals were seropositive. The results clarify the epidemiology of visceral leishmaniasis but do not demonstrate an animal reservoir unequivocally. They do however define criteria which any satisfactory quantitative hypothesis of transmission needs to fulfil.