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Complexities of assessing the disease burden attributable to leishmaniasis

14 Nov 2008

Marcia Triunfol

Source: PLoS Neglected Tropical Diseases (see original article)

Citation: Bern C, Maguire JH, Alvar J (2008). Complexities of Assessing the Disease Burden Attributable to Leishmaniasis. PLoS Negl Trop Dis; 2(10): e313.

2008. This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

Can a disease already considered as being neglected become even more neglected? Yes, some researchers would say.

Leishmaniasis has some characteristics that make large-scale interventions, such as the use of mass drug administration, very complex, if not impossible. In addition, no vaccine against leishmaniasis yet exists. One factor that makes preventive and control programmes for leishmaniasis so difficult is that the infection can present in many ways and show different levels of severity. Also significant is the fact that several species of the protozoan genus Leishmania can cause the disease.

At least five different clinical manifestations of leishmaniasis exist. The most common form is cutaneous leishmaniasis (CL), which is commonly caused by L. major and L. tropica or by L. braziliensis and L. mexicana in the Americas. CL may be cured by spontaneous healing after months or years but it can also have serious consequences for patients, such as destruction of the nasal septum, palate and other mucosal parts. When mucosal areas are affected, the disease is known as mucosal leishmaniasis (MC). Cutaneous leishmaniasis can be limited to a single site in the body or it can be disseminated to several sites (disseminated cutaneous leishmaniasis – DCL). The latter is characterized by a diffuse non-ulcerating disease and by lesions that do not heal.

Besides the cutaneous form, leishmaniasis can also be visceral. Visceral leishmaniasis (VL), also known as kala-azar, is caused by L. donovani and L. infantun. It is characterized by fever, and several alterations in the blood, spleen and liver. Additional complications include an increased vulnerability for secondary infections. VL can be lethal when untreated. In those patients cured from kala-azar, a chronic skin rash remains and patients present complications such as erythematous or hypopigmented nodules. This post-kala-azar disease is known as post-kala-azar dermal leishmaniasis (PKDL) and in many cases does not require treatment.

The parasite can be transmitted to humans in sylvatic, domestic and peridomestic cycles that include cities, forests, and deserts. Furthermore, animal reservoir hosts can maintain enzootic transmission even if no disease occurs in humans, or until the contact between humans and the reservoirs is re-established. In domestic settings leishmaniasis has a powerful ally, as dogs are natural hosts of the parasite. Therefore, it is not surprising that a disease with so many clinical manifestations, hosts, vectors – and for which a vaccine is not available – is so hard to control. The same factors make it difficult to obtain a realistic estimate of incidence and burden.

In a review of the complexities of assessing the disease burden of leishmaniasis, Bern and colleagues include all the major 12 regions in the world that are foci for leishmaniasis and summarize the main characteristics associated with each region. These characteristics are the predominant clinical forms found in the region, the major infecting species, the major reservoir hosts, epidemiological pattern, and the setting (domestic, peridomestic, urban etc.). The authors also present some particularities found in the regions where leishmaniasis is more predominant. For instance, in the Horn of Africa (Sudan, Ethiopia, Kenya, and Somalia), epidemics of leishmaniasis have affected the many people displaced by recent wars in these countries. In this region, the association of epidemics with war, ecological disasters, famine, migration and population displacements is very strong.

In Asia, kala-azar kills more women than men – not because of a gender-preference by the parasite, but because women seek help less often than men. In Latin America, leishmaniasis is increasingly being transmitted to humans by dogs and – other than killing the domestic pet – no solution has been suggested so far.

Because of the considerable clinical and epidemiological diversity of leishmaniasis, it has been very difficult to assess the disease burden or to develop successful preventive or control programmes. Nevertheless, the authors suggest several measures that can be taken to better estimate the disease burden and to better monitor the impact of any programme. Most of these measures require developing improved data collection systems and ways to obtain better information through reviews of existing data, baseline field survey, and the establishment of surveillance programmes. Mortality data provided by hospitals and treatment facilities is also needed. Once the data is available, it should be used to evaluate the leishmaniasis burden and to better understand the dynamics of the disease. Although it is placed second in mortality and fourth in morbidity among all tropical diseases, no one seems to really know how big a problem it represents.