What is leishmaniasis?
Leishmaniasis is a "vector-borne" infection, meaning that an insect from another host or "reservoir" (disease carrier) transmits it. Various animals, including dogs, rats, sloths, cattle, and humans can act as hosts for Leishmania parasites. Leishmania spreads to people through the bite of over 30 Phlebotomus genus of sand flies. These flies are only up to 2 mm long, they make no sound, and their bite can be painless. The females need a blood meal to produce eggs. Phlebotomine flies that transmit Leishmania live in the warm tropical and subtropical areas of the world. As climate changes and temperatures shift, tropical diseases are shifting, as well. Health researchers expect human leishmaniasis to follow the spread of Phlebotomus flies into regions of increasing warmth and humidity. Human migration and increasing urban encroachment into rural areas also offer increased opportunities for spread of these organisms.
While many Western doctors are unfamiliar with it, leishmaniasis has a tremendous human impact on a global scale. It is difficult to pinpoint how many cases occur globally, since most may occur in rural, poor areas without established systems of reporting, but the World Health Organization (WHO) reports over 1 million cases of cutaneous (skin) leishmaniasis in the last five years. About 300,000 cases of visceral (organ) leishmaniasis occur yearly, with over 20,000 deaths. Over 1 billion people live in areas where Leishmania is common. Despite its prevalence, few Western clinicians are aware of it, and it remains on the WHO's list of "neglected tropical diseases." Research, treatment, and eradication of diseases affecting very poor and resource-limited areas often require outside investment.
Leishmaniasis occurs primarily in South America, East Africa, the Middle East, Southern Europe, and Asia.
In traditional or older terminology, geography divides leishmaniasis. "Old World" (Eastern hemisphere) leishmaniasis occurs in tropical parts of Asia, the Middle East, East and North Africa, and southern Europe. The Old World species include Leishmania major, L. tropica, L. donovani, and others. "New World" (Western hemisphere) leishmaniasis occurs in tropical parts of Mexico and Central and South America. New World species include L. mexicana, L. amazonensis, L. braziliensis, and others.
Cases in the United States and Europe are usually occur in travelers or immigrants from these endemic areas. In fact, due to the increasing popularity of "adventure travel" and "ecotourism" to Central and South America, new cases of leishmaniasis are on the rise in the U.S. Given the general unfamiliarity and limited knowledge about this parasitosis, a comprehensive guideline for diagnosing and treating these cases appeared in the journal of the Infectious Disease Society of America Clinical Infectious Diseases (Clin Infect Dis) in 2016.
There have been autochthonous cases (those acquired without travel) described in the U.S. In 2012, 13 people acquired cases of cutaneous leishmaniasis in Texas and Oklahoma without travel. This suggests spread of the sand fly vector and parasite into the U.S. and/or spread of human habitats into more rural areas.
Large epidemics of leishmaniasis can occur among people displaced into crowded urban areas through war or migration, or when high rates of malnutrition weaken people in affected regions. Generally, dogs and rodents are the reservoir for most species of Leishmania. In poverty-stricken, densely populated areas, humans may become the primary "reservoir" and source of recurrent infection with leishmaniasis, particularly with L. donovani. This is "anthroponotic" transmission.
Figure 1: Picture of a sand fly biting a human arm. SOURCE: CDC/Frank Collins