An infectious disease caused by an intracellular flagellate protozoan Leishmania donovani, common in rural parts of the tropical and subtropical countries of the world. The disease, also known as visceral leishmaniasis, is characterized by lesions of the reticulo-endothelial system, especially the liver and spleen, and is often fatal. Children are more susceptible to this disease. Kala-azar is transmitted to man by the bite of the infected female Phlebotomus sandfly. The incubation period is generally 2 to 6 months.
Kala-azar has been a public health problem in the Bengal moribund delta region since the early eighteenth century. It obtained an epidemic form in the late 18th and early 19th centuries. Due to Kala-azar, Burdwan and some other districts of West Bengal were losing population. Many people deserted the Kala-azar afflicted areas and move to East Bengal.
The protozoan parasite gets multiplied in the bone marrow, spleen and liver. It attacks lymph nodes or the body defence system, crippling the production of anti-bodies. This leads to unusual enlargement of the liver and spleen. Kala-azar also causes pneumonia, diarrhoea, loss of weight and appetite, discomfort in the abdomen, and often nasal and gum bleeding that may lead to death. Although there are several methods of detecting the Kala-azar parasite, the most reliable one is the direct agglutination test (DAT). The test is very expensive and in Bangladesh only five laboratories including one each at Pabna and Dinajpur have facilities for conducting it.
Phlebotomus argentipes is the recognized vector of Kala-azar in the eastern part of this subcontinent. A WHO report indicates that Kala-azar is a serious public health problem in the state of Bihar and West Bengal in India. After its resurgence in some parts of Bihar in the early seventies, the disease spread to 36 districts in Bihar, and 10 districts in West Bengal. There were 17,806 cases with 72 deaths till 1986, which rose to 77,102 cases with 1,419 deaths in 1992 in Bihar and West Bengal; at least 17,429 people were attacked and 255 of them died of the disease in 1997.
The sandfly breeds in soil with sufficient moisture and organic debris. It is found in cracks and crevices of dwellings and floors of cattle sheds. The larva takes about 20 days to 1.5 months to reach the adult stage depending upon temperature and availability of food. The female sandfly takes the blood of mammals. It can not fly but hops, is nocturnal in habit, and rests in humid, dark areas. High humidity favours the fly. Adult longevity is about two to four weeks depending upon prevailing environmental conditions.