LEISHMANIASIS & Sandflies
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Leishmaniasis disease involves more than 20 distinct species of Leishmania spp. parasites worldwide. Service (2008) reports that there are four main clinical forms: (1) Cutaneous, (2) Diffuse Cutaneous, (3) Mucocutaneous and (4) Visceral. The epidemiology involves different parasite species or strains and different reservoir hosts. Sandflies of the genera Phlebotomus and Lutzomyia are the principal insect vectors of the disease.
The life cycle begins when amastigote parasites are ingested by female sand flies during a blood meal. the parasites multiply in the sandfly intestines and turn into promastigotes, which are elongated and have a flagellum that attaches to the mid- or hind-intestinal wall where they multiply. Those that are not voided migrate to the anterior mid-intestines and then to the fore-intestines. There some parasites change into metacyclic forms. After 4-12 days after the sandfly has has an infected blood meal the metacyclic forms are found in the mouthparts from which they may be introduced to a new host during feeding. Transmission is maximized because infected sandflies probe more frequently. Because sandflies also feed on plants containing sugar, this aids parasite development. (Also see Life Cycle)
Most leishmaniasis are zoonoses, and the degree of human involvement varies in different areas (Service 2008). The epidemiology is mostly determined by the sandfly species, their ecology and behavior as well as the strains of Leishmania parasites. Sometimes the sandflies transmit infections mostly among wild or domestic animals with little human involvement. In other areas the animals may serve as reservoir hosts for human infection. The disease may also be transmitted between people by sandflies with animals taking no part in its transmission (e.g., India). Following are more details concerning the different forms of the disease as given by Service (2008):
Sometimes known as "Oriental Sore" this form of leishmaniasis occurs principally in arid parts of the Middle East to India, Asia and Africa. The principal parasites are Leishmania major, which are vectored by Phlebotomus papatasi, and Leishmania tropica is vectored by Phlebotomus sergenti. Leishmania major is mainly zoonotic and gerbils are the reservoir hosts. Leishmania tropica is in densely populated areas where humans are the principal reservoir hosts. In America the cutaneous form occurs mainly in forested parts of Mexico south to northern Argentina. It is caused by Leishmania braziliensis (= amazoniensis) and Leishmania mexicana. Dogs and rodents are probably the principal reservoir hosts. The main vectors are Lutzomyia wellcomei and Lutzomyia flaviscutellata.
This is a very severe form of the disease that occurs from Mexico to Argentina and is caused by Leishmania braziliensis. Dogs are reservoir hosts and Lutzomyia wellcomei a principal vector.
Cutaneous nodules all over the body characterize this form. It occurs in Venezuela, the Dominical Republic and the highlands of Ethiopia and Kenya. In South America the parasite is Leishmania amazonensis, which is vectored by Lutzomyia flaviscutellata. Spiny rats are reservoir hosts. In Africa the parasite is Leishmania aethiopica vectored by Phlebotomus pedifer and P. longipes. Rock hyraxes are reservoir hosts.
Also known as "kala-azar" this form is caused by Leishmania donovani donovani. It occurs in India, Sudan, Ethiopia and East Africa. Vector species include Phlebotomus argentipes and Phlebotomus orientalis. Wild cats, genets and rodents may be the main reservoir hosts. In the Mediterranean area and central Asia Leishmania donovani infantum is the parasite , which is vectored by Phlebotomus ariasi and Phlebotomus perniciosus. Foxes and dogs are reservoir hosts. This form also occurs at times in Central and South America where the parasite is Leishmania donovani infantum and is vectored by species of the Lutzomyia longipalpis complex (Service 2008).
Control or avoidance of the sandfly vectors is required to reduce incidence of infection (Alexander & Maroli 2003). Insecticidal control of vector sandflies is effective until resistance sets into the fly population. Therefore, the use of repellants is preferable. To reduce diseases caused by sandflies some efforts have been made to eliminate reservoir hosts from populated areas. Further efforts to control the vectors remain experimental, especially as the breeding sites of most sandflies are not easily found.
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