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Abstract & Commentary
Synopsis: Leishmaniasis marches on in parallel with population migrations to urban areas in developing countries.
Source: Urbanization: An increasing risk factor for leishmaniasis. WHO Weekly Epidemiological Record. 2002;77:365-372. www.who.int/wer.
It is estimated that 12 million people are infected with Leishmania spp., with an annual incidence of 1 to 1.5 million new cases of cutaneous and 500,000 of visceral leishmaniasis. Furthermore, the geographical distribution of this disease is expanding, largely as the result of migration of rural diseases into urban areas in which both human and vector populations are concentrated, allowing increasing numbers of infections.
Examples of newly identified foci or of places where the incidence of zoonotic cutaneous leishmaniasis is increasing include Manaus, Brazil, and several urban areas of Colombia and Saudi Arabia. In Manaus, at the confluence of the Rio Negros and the Rio Solomonos forming the Amazon River in Brazil, there has been progressive intrusion of the urban area into the deforested rain forest. The level of intradomiciliary transmission of Leishmania spp. within these "suburbs" is closely correlated with the distance from the remaining remnants of rain forest in which the sylvatic animal reservoirs (opossums, anteaters, and 2-toed sloths) and sandfly vectors naturally reside.
Cities in southwestern Asia are the major foci of anthroponotic cutaneous leishmaniasis, mostly due to Leishmania tropica. It is estimated that 270,000 (13.5%) are infected in Kabul, the capital of Afghanistan, a city of 2 million people. The common characteristics of these endemic foci include high population density and poor sanitary conditions. A rapid increase in numbers of cases occurred in the poorest suburbs of Sanliurfa in southern Anatolia where cattle are frequently kept in house basements and the cow dung dried in the streets to be sold as fuel, providing ideal conditions for sandflies to breed and lay eggs.
Zoonotic visceral leishmaniasis is now reported from many areas of Latin America believed to have been previously free of this disease. Prolonged drought in northeastern Brazil has led to migration from rural areas to shanty towns on the outskirts of large cities where the sandfly vector and dogs are present in abundance, sanitary conditions are poor, and malnutrition is prevalent. In southern Europe, this form of leishmaniasis is spreading from rural areas into suburbs where the confluence of dogs and small gardens provide encouragement to the sandfly vectors. The confluence of conditions conducive to propagation of leishmaniasis with the HIV epidemic has led, in southern Europe and elsewhere, to emergence of large numbers of coinfection; these have now been reported from 34 countries around the world. While the availability of highly active antiretroviral therapy is associated with a decreasing incidence of coinfection, even in Europe, where such therapy is widely available, there has been a continuing increase of coinfections in injection drug users. Despite such therapy, a recent PCR examination of used syringes in 2 needle exchange programs in Madrid detected leishmanial DNA in 33% and 52%.
While anthroponotic visceral leishmaniasis primarily remains a rural disease, a link with urbanization has recently been suggested as the result of cases occurring in India in Bombay and in Patna, the capital of Bihar State.
Comment by Stan Deresinski, MD, FACP
Poverty and civil strife, both causes of population migrations, crowding, and poor sanitation and hygiene, are the handmaidens of many infectious diseases that afflict untold millions of inhabitants of planet earth. An example, not mentioned above, is the devastating surge of cases of visceral leishmaniasis in Sudan, a site of a seeming never-ending civil war, a condition certain to obliterate any efforts at control of the disease (email@example.com).
Since no vaccine of proven efficacy is available, methods of leishmaniasis control include control of the sandfly vectors and control of infected animals (in the case of zoonotic forms of the disease) and humans. Rodents and canines comprise most nonhuman animal reservoirs of infection. The sandfly vectors, of the genus Phlebotomus in the Old World and both Lutzomyia and Psychdopygus in the New World, rest and breed in a variety of sites, ranging from cattle sheds to termite hills. These tiny, yet dangerous, pests remain susceptible to standard residual insecticides. Their diminutive size allows them to pass through the mesh of most mosquito netting.
As with so many other diseases that plague the lesser-developed world, public health measures cannot be effectively applied in the absence of massive political, social, and economic changes.
Dr. Deresinski, Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor of Infectious Disease Alert.