The epidemiology of visceral leishmaniasis in Teresina, Piaui State, Brazil, with special emphasis on diagnosis and transmissibility of canine infection
Visceral leishmaniasis (VL) is endemic in the semi-arid region of north-eastern Brazil. The causative agent is Leishmania chagasi (infantum). The domestic, peri-domestic and anthropophilic sandfly vector is Lutzomyia longipalpis. The domestic dog may be the principal reservoir of infection in endemic urban regions. The primary aim of this project was to study comparative diagnosis and transmissibility of natural and experimental canine VL in the city of T eresina, Piaui state, Brazil. Most (67 %) of human cases of VL in Teresina occurred in children under 5 years of age, adult VL predominated in males, and there was a seasonal increase in incidence of cases. No phenotypic diversity was found between L. chagasi isolates from dogs and humans, although limited comparisons were performed. Clinical diagnosis, parasitological diagnosis and serology were compared in a cohort of 209 dogs, comprised of both symptomatic and asymptomatic animals. Presence of clinical symptoms was not sufficiently sensitive to diagnose canine VL: only 42 % of dogs positive by reference standard criteria (RS positive) were symptomatic. Parasitology was less sensitive than serology for the diagnosis of canine VL and no parasitological test sho~ed more than 60 % sensitivity in comparison with RS criteria. The indirect fluorescent antibody test (IF A T) and DOT -enzyme linked imrnunosorbent assay (DOT -ELISA) were the most sensitive of the serological assays tested. The direct agglutination test (OAT) was highly specific but lacked sensitivity. Serum samples were more sensitive than filter paper blood spot samples. The Lmet2 DNA probe was generally less sensitive than traditional parasitological and serological methods for diagnosis of canine VL, although the probe was useful for screening sandflies for L. chagasi infections (below). The chance of demonstrating parasites in canine VL increased with the serological titre. Nevertheless, parasitoIogicalIy positive dogs could be found among those that were serologically negative. Large numbers of Lu. longipalpis were found in pigsties and chicken houses in the city of Teresina. The Lmet2 probe was shown to be effective for determining prevalence rates of L. chagasi infection in wild caught sandflies. Prevalence of natural infection in sandflies was particularly high when flies were caught in kennels where there were dogs with disseminated cutaneous infections. Experimental studies demonstrated that Lu. longipaJpis could be very readily infected with L. chagasi by feeding on dogs with canine VL and that transmissibility was associated with amastigote infection of the skin. Altered skin of symptomatic dogs was more infective than normal skin of symptomatic animals. Although symptomatic animals were more infective than asymptomatic animals, asymptomatic dogs with normal skin were still infective to large numbers of sandflies and asymptomatic dogs cannot, therefore, be excluded as a significant reservoir of infection. Transmission of experimental canine VL was demonstrated by a single infective sandfly bite. In a cohort of 25 experimental animals many dogs developed discrete, self-curing, cutaneous lesions, typical of cutaneous leishmaniasis. Seroconversion was the most sensitive test for canine VL, but seronegativity was not a reliable indicator of the absence of infection. Bone marrow positivity was only seen in dogs that were serologically positive. Apparent recovery from L. chagasi infection was seen, with serological reversIOn. Aminosidine, dependent on dose, duration of treatment and clinical status of the infected animal, was shown to be capable of producing clinical recovery and clinical cure in a small proportion of infected dogs, but could not be recommended as a systematic method of control. Single applications of ultra-low volume pyrethroid insecticide to individual animal pens was not effective for controlling Lu. iongipaipis. Nevertheless, pyrethroid insecticides had a high residual activity against Lu. iongipalpis when sprayed on to the walls of animal enclosures. Lambda cyhalothrine (ICON) was the most effective of three pyrethroid insecticides tested in the laboratory against Lu. iongipalpis. Overall, this project has produced unique observations on canine VL, supports the fundamental role of the dog as a reservoir host, and explains why culling of seropositive dogs is likely to have limited impact as a disease control strategy.