As the first species of Leishmania encountered were the agents of human visceral and cutaneous leishmaniasis, it is understandable that studies on these parasites for a long time concentrated on those organisms commonly causing disease in man. Epidemiological studies over the past 20 years or so, however, have led to the inescapable conclusion that the genus Leishmania is comprised of numerous species of well adapted parasites, in a wide range of mammals, throughout most of those tropical and subtropical regions of the world where phlebotomine sandflies exist (Diptera: Psychodidae). Many of the leishmanias probably never gain entrance into man: due either to an incapacity to survive in his tissues, or (more likely) because the natural sandfly vectors do not feed on him. The leishmanias that do infect man are, nevertheless, among the greatest protozoological scourges of mankind, and a better understanding of their life-cycles may well help in future prevention or control of the diseases they cause. With few exceptions the leishmaniases are zoonoses, with a major source of infection in wild or domestic animals. In the Americas, the disease is essentially a rural one, and most commonly acquired by those penetrating forested or wooded regions. The following paper deals with the better known human leishmaniases of the New World, and some new ones, and discusses the major historical events in the laborious task of elucidating their ecology and epidemiology.

The American leishmaniases: some observations on their ecology and epidemiology

During observations on a focus of enzootic leishmaniasis of rodents in the Utinga Forest, Belem, Para, Brazil, active infections were encountered in 16 (17·98%) of 89 Oryzomys capito examined. Cutaneous lesions were restricted to the tails of these animals, and spontaneous recovery leaves characteristic scarring. Taking such scars into account, it is estimated that as many as 70% of the Oryzomys population may experience infection at some time during their lives. Leishmania were isolated from 3 specimens of Proechimys guyannensis captured in the same forest area. In addition to obvious lesions on the tail and foot, parasites were isolated from apparently normal ear tissue. The strains of Leishmania from Oryzomys and Proechimys appear to be identical. Insect traps were baited with Oryzomys, Proechimys and other rodents. During a period of 4 months they captured 2,774 phlebotomine sandflies, of which 2,731 were Lutzomyia flaviscutellata Mangabeira. In contrast, only 5 specimens (0·69%) of Lu. flaviscutellata were encountered among 724 sandflies caught of human bait during the same period, in the same area. Leptomonads were found in 8 of 2,706 Lu. flaviscutellata dissected (0·29%). Inoculation of these flagellates into hamsters produced typical leishmanial lesions in 6 cases. We conclude that Lutzomyia flaviscutellata is the major vector of the Leishmania sp., among Oryzomys and Proechimys in the Utinga Forest area of the Lower Amazon Region. Similarities are noted between this parasite of Brazilian rodents and Leishmania mexicana of rodents and man in Central America.

Leishmaniasis in Brazil: I. Observations on enzootic rodent leishmaniasis - incrimination of Lutzomyia flaviscutellata (Mangabeira) as the vector in the lower Amazonian basin.

Leishmanial organisms were cultivated from cutaneous lesions of British military personnel returning from Belize. Isoenzyme profiles of the freshly isolated organisms and 'marker' strains of New World Leishmania spp. were compared using 10 enzymes (ALAT, ASAT, ME, GPI, MPI, PGM, SOD, 6-PGDH, G-6-PDH and MDH), by starch gel electrophoresis. 19 of the 22 new isolates from Belize were isoenzymically indistinguishable from Leishmania braziliensis braziliensis (10 out of 10 enzymes) and clearly differentiated from L. b. guyanensis and L. b. panamensis (different in 6 out of 10 enzymes) and from L. mexicana mexicana and L. m. amazonensis (9 out of 10 enzymes). Two isolates closely resembled L. m. mexicana and one could not be positively identified. This is the first report of autochthonous human leishmaniasis caused by L. braziliensis group organisms as far north as latitude 16 degrees N.

The isolation and isoenzyme characterization of Leishmania braziliensis subsp. from patients with cutaneous leishmaniasis acquired in Belize.

Observations on a zoonosis: leishmaniasis in British Honduras.

Ecological niche models are useful tools to infer potential spatial and temporal distributions in vector species and to measure epidemiological risk for infectious diseases such as the Leishmaniases. The ecological niche of 28 North and Central American sand fly species, including those with epidemiological relevance, can be used to analyze the vector's ecology and its association with transmission risk, and plan integrated regional vector surveillance and control programs. In this study, we model the environmental requirements of the principal North and Central American phlebotomine species and analyze three niche characteristics over future climate change scenarios: i) potential change in niche breadth, ii) direction and magnitude of niche centroid shifts, iii) shifts in elevation range. Niche identity between confirmed or incriminated Leishmania vector sand flies in Mexico, and human cases were analyzed. Niche models were constructed using sand fly occurrence datapoints from Canada, USA, Mexico, Guatemala and Belize. Nine non-correlated bioclimatic and four topographic data layers were used as niche components using GARP in OpenModeller. Both B2 and A2 climate change scenarios were used with two general circulation models for each scenario (CSIRO and HadCM3), for 2020, 2050 and 2080. There was an increase in niche breadth to 2080 in both scenarios for all species with the exception of Lutzomyia vexator. The principal direction of niche centroid displacement was to the northwest (64%), while the elevation range decreased greatest for tropical, and least for broad-range species. Lutzomyia cruciata is the only epidemiologically important species with high niche identity with that of Leishmania spp. in Mexico. Continued landscape modification in future climate change will provide an increased opportunity for the geographic expansion of NCA sand flys' ENM and human exposure to vectors of Leishmaniases.

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Current and Future Niche of North and Central American Sand Flies (Diptera: Psychodidae) in Climate Change Scenarios

A study of human cases of dermal leishmaniasis shows that the infection is not confined to “high bush” but may be present in bush of any type. The advisability of employing prophylactic vaccination under certain conditions is worth considering. A rat was found to be infected with Leishmania mexicana, but there was no Leishmania among parasites obtained from lizards. 14 species of Phlebotomus were found, and preliminary observations were made on age changes and other features. Attempts to ascertain the natural infection rate, by inspection and by injection into hamsters, gave negative results, and possible reasons for this are discussed.

P. Williams

Papers

On dermal leishmaniasis in british honduras.