diseases caused by about 30 distinct species, subspecies and strains of
Leishmania parasites. Worldwide there are about 1–2 million cases a year,
with about 12 million people currently infected in 88 countries. The three
main clinical forms are cutaneous, mucocutaneous and visceral leishmaniasis. A fourth, less common form is diffuse cutaneous leishmaniasis,
while post-kala-azar dermal leishmaniasis is caused by Leishmania donovani
donovani following cure of the initial visceral form. The epidemiology of
leishmaniasis is complex, involving not only different parasite species but
different strains of parasites and different reservoir hosts.
Basically amastigote parasites ingested by female sand flies with a
blood-meal multiply in the gut and develop into promastigotes, which
are elongate, have a flagellum and attach to the mid-gut or hind-gut wall
and multiply rapidly. Many, however, are voided when the fly defecates.
After further development the survivors migrate to the anterior part of the
mid-gut and then to the fore-gut. Here some parasites become metacyclic
forms. Four to 25 days after the sand fly has taken an infective blood-meal
the metacyclic forms are found in the mouthparts, and are introduced into a
new host during feeding. Infective flies frequently probe more often than
uninfected flies, thus maximizing transmission of parasites during bloodfeeding. Previous feeding by females on sugary substances, mostly
obtained from plants, is essential not only for the survival of the sand fly
but also for the development of the parasites to the infective form.
Most types of leishmaniasis are zoonoses. The degree of involvement of
humans varies greatly from area to area. The epidemiology is largely determined by the species of sand flies, their ecology and behaviour, the
availability of a wide range of non-human hosts, and also by the species
and strains of Leishmania parasites. In some areas, for example, sand flies
will transmit infections almost entirely among wild or domesticated animals, with little or no human involvement, whereas elsewhere animals
may be important reservoir hosts of infection for humans. In India infections may be transmitted between people by sand flies, with animals taking
no identifiable part in transmission. The epidemiology of the leishmaniases
is complex, and only simplified accounts are given below.
CUTANEOUS LEISHMANIASIS (CL)
In the Old World, CL is known also as oriental sore. It occurs mainly in arid
areas of the Middle East to northwestern India and central Asia, in North
Africa and various areas in East, West and southern Africa. The principal
parasites are Leishmania major, transmitted mainly by Phlebotomus papatasi,
and Le. tropica, transmitted by P. sergenti. Leishmania majoris usually zoonotic
and in most of its range gerbils (e.g. Rhombomys opimus) are the reservoir
hosts; Le. tropica occurs in densely populated areas and humans appear to be
the main reservoir hosts. In the New World, CL is found mainly in forests
from Mexico to northern Argentina, and is caused by Leishmania braziliensis,
Le. amazonensis and Le. mexicana. Rodents and dogs appear to be reservoir
hosts. Vectors include Lutzomyia wellcomei and L. flaviscutellata.
MUCOCUTANEOUS LEISHMANIASIS (ML) (espundia)
A severely disfiguring disease found from Mexico to Argentina. It is mainly
caused by Leishmania braziliensis. Dogs may be reservoir hosts. Lutzomyia
wellcomei is an important vector.
DIFFUSE CUTANEOUS LEISHMANIASIS (DCL)
A form that causes widespread cutaneous nodules or macules over the body.
It is confined to Venezuela and the Dominican Republic and the highlands
of Ethiopia and Kenya. In South America the parasite is Le. amazonensis,
transmitted by Lutzomyia flaviscutellata, and spiny rats (Proechimys species)
are reservoir hosts. In Ethiopia and Kenya the parasite is Le. aethiopica,
transmitted by Phlebotomus pedifer and P. longipes, with rock hyraxes
(Procavia capensis) as reservoir hosts.
VISCERAL LEISHMANIASIS (VL)
Often referred to as kala-azar. It is caused by Leishmania donovani donovani in
most areas of its distribution, such as India, Bangladesh, Sudan, East Africa
and Ethiopia. Among the vectors are Phlebotomus argentipes and P. orientalis.
Rodents, wild cats and genets (Genetta genetta) may be reservoir hosts. In the Mediterranean basin, Iran and central Asia, including northern and
central China, Leishmania donovani infantum is the parasite, and the vectors
include P. ariasi and P. perniciosus. Dogs and foxes (Vulpes vulpes) are reservoir hosts. Visceral leishmaniasis also occurs sporadically in Central and
South America, where the parasite is Le. donovani infantum (Le. chagasi of
some authors), transmitted by species in the Lutzomyia longipalpis complex
PREVENTION MEASURES
- Personal protection can be achieved by applying efficient insect repellents such as DEET, piperidene-based ones and neem oil. Insecticide-impregnated bed-nets such as the long-lasting Olyset nets
- spraying houses with lambda-cyhalothrin has substantially
- reduced the vectors of cutaneous leishmaniasis.
- In areas where sand flies rest indoors, cattle sheds and chicken houses should also be sprayed.
