Organism. There are seven species of Leishmania and a number of subspecies:
Visceral L. donovani leishmaniasis (donovani, infantum, chagasi, archibaldi) L. braziliensis (braziliensis, peruviana), L. guyanensis (guyanensis, panamensis) L. mexicana (mexicana, amazonensis, pifanoi, garnhami, venezuelensis) L. major
L. tropica (killicki, tropica) L. aethiopica
New world cutaneous
Old world cutaneous Old world cutaneous
They are all transmitted by the bite of the sandfly and undergo the same simple life cycle. Promastigotes enter man with the bite of the sandfly, change into amasti-gotes (Fig. 15.18), and are engulfed by macrophages. They multiply and finally rupture the cell, invading other macrophages. When the sandfly takes a blood meal, they change into promastigotes. These multiply continuously so that the number produced can be so large as to block the fore-gut. When the insect next bites, it is forced to regurgitate promasti-gotes into the host before it can take a blood meal (Fig. 15.21).
In cutaneous leishmaniasis, the amasti-gotes remain at the site of introduction, contained by the macrophages of the skin. In visceral leishmaniasis, large mononuclear cells and polymorphonuclear leucocytes become invaded, which subsequently carry the parasites to the viscera, especially
Visceral L.. donovani
Mucocutaneous L.. braziliensis L. guyanensis
New World cutaneous L. mexicana
Old World L. major L. tropica L.. aethiopica
Fig. 15.21. Leishmania vector, parasite and life cycle.
the liver, spleen and bone marrow. The mucocutaneous form is intermediate, the parasite restricting its attack to the reticuloendothelial system of the mucus membranes of the mouth, nose and throat.
Clinical features There are three main clinical forms of the disease - cutaneous, mucocutaneous and visceral (kala-azar). The cutaneous infection starts with a papule and enlarges to become an indolent ulcer, which either heals or persists for many years. In the New World infections, a more aggressive form of mucocutaneous leish-maniasis (espundia, Chiclero ulcer) results in nasopharyngeal destruction and hideous deformities. The visceral form is a chronic infection with fever, hepato-splenomegaly, lymphadenopathy and anaemia. There is progressive emaciation and weakness with generally a fatal outcome if not treated. Post-
kala-azar dermal leishmaniasis can occur after apparent cure of the visceral case.
Like leprosy, host response largely determines the outcome of the disease and in any condition in which this response is minimized a more florid disease results. Cutaneous leishmaniasis is normally a self-limiting condition, but in some individuals diffuse cutaneous leishmaniasis, in which metastatic lesions are disseminated around the body can occur. The resulting nodular lesions resemble lepromatous leprosy and respond poorly to treatment. So any condition that compromises the host response, such as HIV infection, may lead to reactivation of latent disease or cutaneous disease progress to visceral illness. Leishmaniasis, like tuberculosis, is intertwined with HIV infection so that in areas where leishmania-sis is found, both conditions have a more serious outcome.
Diagnosis is by the detection of the intracellular-infected macrophages (Leish-man-Donovan bodies) in liver, spleen, bone marrow or cutaneous lesions by stained smear or culture. PCR techniques can also be used.
Transmission is by the minute and fragile phlebotomine sandflies, Phlebotomus and Lutzomyia. They are weak fliers, utilizing a hopping flight that only carries them a short distance from their habitat. This requires conditions of high humidity as found in animal burrows and moist tropical forests. Typical habitats are tree holes, new or old animal burrows, termite hills, rock crevices, foliage clumps and fissures that develop in the ground during the dry season.
The life cycle from oviposition to emergence of the adult can take 30-100 days depending on species and temperature, while the adult lives for approximately 2 weeks. Only the female sucks blood, but lizards, birds and mammals are satisfactory alternative food sources to humans.
Most species feed out of doors during the evening and night, or in the day when there is shade or the weather is overcast. If it is windy, they are unable to fly. They are not able to bite through clothing and mainly attack the lower parts of the body. The main vectors are summarized in Table 15.6.
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