Reservoir hosts of cutaneous leishmaniasis

Abstract

Cutaneous leishmaniasis (CL) is caused by several species of Leishmania, mainly L. tropica, L. major, L. aethiopica, and L. mexicana. Except for the former species, the host range of these parasites includes several groups of animals in addition to man; thus the disease they cause is termed a zoonosis. L. tropica, on the other hand, is basically anthroponotic affecting man only, but recent literature suggested that this parasite or some of its strains (zymodemes) could also be zoonotic and is discussed later. Primary reservoir hosts of CL are responsible for the long-term maintenance of the infectious agents and are basically mammals.1 Other types of hosts which play a secondary role as reservoirs include the incidental hosts and the liaison hosts. The incidental hosts are irrelevant to the long-term persistence of CL in an area, whereas the liaison hosts bring the infectious agents to the vicinity of man and facilitate their transmission.1 The relationship between parasites and their host(s) is usually complex. In zoonotic CL (ZCL), this relationship involves the parasite, the sandfly, and the mammalian host(s), and is considered as a self-sustaining, independent system. According to Ashford1 the reservoir host in such a system has the following characteristics: (1) is abundant; (2) forms a large proportion of the mammalian biomass; (3) is frequently a gregarious species; and (4) lives long enough to carry the disease into the next transmission season. WHO2 has set several criteria for incriminating reservoir hosts. In addition to what has been mentioned above, there should be intense host–fly contact, the course of infection in the reservoir should be long and relatively non-pathogenic, and the parasites in the reservoir must be indistinguishable from those in man. Very often, Leishmania parasites circulate among animal hosts in nature as they are transmitted through the bite of an infected sandfly. Man often becomes infected when he gets within that zoonotic focus and is fed upon by the infected sandfly. This has been the case in new settlement areas, road construction sites, new industrial projects, and during army maneuvers. Sandfly vectors belong to members of two genera, Phlebotomus in the Old World and Lutzumyia in the New World. Their distribution overlaps with the reservoir hosts and their relationship with the reservoirs is also complex. Some species breed in the burrows or nests of hosts where the fecal material of the animals provides the required nutrients for the larvae of the sandfly. The emerging females would then feed on the host itself. Others are attracted to their hosts. For example, Ototylomys phyllotis rodents were experimentally found more attractive to the vector, Lutzymyia flaviscutellata, than several species tested. The climbing habits of these rodents facilitate their exposure to the bite of the sandfly.3 Identification of the animal reservoirs of leishmaniasis depends on isolating the parasite from skin lesions or from the viscera. Various species of animals may become infected with Leishmania parasites and develop lesions but their roles as reservoirs need to be determined. New biochemical techniques such isoenzyme electrophoresis, DNA probes, and PCR techniques employed recently for the identification of Leishmania species isolated from man and animals have helped in improving our understanding about the role animal reservoirs play in the epidemiology of all forms of leishmaniasis. However, what may sometimes complicate this understanding is the difficulty of identifying the reservoir host itself. For example, the taxonomy of hyraxes which act as reservoir host of L. aethiopica is problematic.1 The same is true with several hosts of L. mexicana and L. brasiliensis. Accurate identification of reservoir hosts is important and the help of centers of expertise in the identification of animals is essential.2 The mammalian hosts belong to different phylogenetic groups such as rodents, sloths, primates, and carnivores. A given species of Leishmania may be maintained by several hosts species such as man, dogs, and rodents. For example, L. major has been isolated from man, Psammomys, Meriones, Rhombomys, and Arvicanthes.1,4 On the other hand, several species of Leishmania may be found in the same mammalian host. Man may be naturally infected with L. major, L. donovani, and L. From the Department of Biology, the Hashemite University, Zarka, Jordan; and the Amman Clinic, Amman, Jordan. Address correspondence to Prof. Elias Saliba, The Hashemite University, PO Box 150459, Zarka, 13115, Jordan.