When it comes to sex differences on the incidence of cutaneous leishmaniasis, male bias can exist even when exposure time is comparable among men and women.
Cutaneous leishmaniasis (CL) is a tropical neglected disease that occurs in Central and South America, as well as in some regions of the African continent and the Middle East. Ulcerative lesions that develop at or near the vector’s bite site typically characterize CL, which is caused by parasites of the genus Leishmania, and transmitted by phlebotominae sand flies. According to the World Health Organization (WHO), in 2013 there were almost 100 thousand new cases reported all over the world. CL is considered a neglected disease because its mitigation receives less effort than it should, since the disease does not cause mortality. However, individuals presenting sings and symptoms of CL usually live in impoverished regions, can suffer from social exclusion because of the cutaneous nature of the lesions, besides having do deal with a medical treatment that can cause severe and debilitating side effects.
In South America, the disease cycle is mainly sylvatic, meaning that humans are accidental disease hosts. Generally, in forested areas, sand flies would feed on sloths, armadillos and other small mammals, and humans would get occasionally infected if they were exposed to habitats where both vectors and wild hosts are found. Since men are usually the ones that engage in activities that involve being in contact with habitats where transmission is likely, such as hunting and logging, disease incidence tends to be higher among males than their female counterparts. However, experimental infections in mice has also revealed that females, not only tend to develop the disease less often than males, but also present less severe lesions when clinical symptoms are present. Hence, one can ask whether male-biased disease incidence is solely due to differential exposure or is also a consequence of sex-related differences in the immune ability to cope with Leshmania spp. infection.
That was exactly the question I tackled for my MSc thesis research in the Instituto Nacional the Pesquisas da Amazônia. The study was just published on line in the journal Tropical Medicine and International Health, in which we investigated whether sex-biased CL incidence levels off when exposure time to CL vector habitats is comparable among sexes. We compared disease incidence between two populations from central Amazon: one composed by rural settlers, where exposure is male biased; and other composed by field researchers of the Biological Dynamics of Forest Fragments Project, where both males and females are similarly exposed to forested environments. Interestingly, at low levels of exposure disease incidence is higher among males in both populations, suggesting the existence of a sex-related and endocrine-mediated immunity against these parasitic infections. However, as exposure time increases, this suggested effect of higher immunity among females wears off, and disease incidence becomes comparable among sexes. Another relevant finding was that CL incidence among field researchers is eleven times higher than among rural settlers, which brings to attention a new disease risk group that deserves awareness.
This study was advised by Dr Gonçalo Ferraz, and co-advised by Dr Fernando Abad-Franch, in a collaboration between the Instituto Nacional de Pesquisas da Amazônia (INPA), the Biological Dynamics of Forest Fragments Project (INPA/Smithsonian Tropical Research Institute), and the Instituto Leônidas e Maria Deane (FIOCRUZ-AM). I am very grateful to all of those who answered my epidemiological questionnaire and made this study happen!
Soares, L., Abad-Franch, F. and Ferraz, G. (2014), Epidemiology of cutaneous leishmaniasis in central Amazonia: a comparison of sex-biased incidence among rural settlers and field biologists. Tropical Medicine & International Health. doi: 10.1111/tmi.12337