Later this year I will be visiting The Gambia in West Africa to work on a GirlGuiding community project. Before I go I will need to have some vaccinations to protect me from several of the diseases found there. Unsurprisingly, this has rekindled my interest in tropical diseases. This week, I have strayed from my usual subjects to write about the leishmaniases, a group of tropical diseases found across Africa, Asia and the Americas. Included in the World Health Organisation’s (WHO) list of Neglected Tropical Diseases, the leishmaniases are a big burden on public health services in many countries, and were responsible for an estimated 50,000 deaths worldwide in 2010 alone.
Leishmaniases are caused by parasitic single-celled organisms of the genus Leishmania. The parasites are carried between mammal hosts by several species of blood-sucking sandflies. The parasites live within the mouthparts of the sandflies and can be transmitted to mammals when the sandflies bite. The parasites exist in different forms in sandflies and mammalian hosts. In the sandfly, Leishmania cellsbecome promastigotes and move around the insect gut using string-like structures called flagella. Within a mammal host, the parasites “hide” within host immune cells (called macrophages) and lose their flagella to become amastigotes.
In more minor Leishmania infections, the parasites infect the skin, leading to sores and ulcers. Usually the infection is confined to the area around the insect bite, and clears up either by itself or in response to treatment. In some cases, the parasites can spread and cause sores on the skin in other parts of the body. The parasites can also move to the linings of the nose and throat and cause permanent tissue damage. In the second, more serious form of leishmaniasis, the parasites infect internal organs including the spleen and liver. The infection is usually fatal if left untreated.
No vaccines against Leishmania are currently available, so control of the diseases focuses on reducing the risk of people being bitten by infected sandflies. Several strategies are used including: the early diagnosis and treatment of disease cases, control of sandfly populations using insecticides and bed nets, monitoring of disease cases and control of populations of other host mammals e.g. dogs and rats (1). Efforts are also being made to educate local communities about the diseases and how they can alter their lifestyles to reduce their risk.
Unfortunately, our understanding of where the leishmaniases are found is poor, so it is difficult to target control efforts to those most at risk. In a research paper recently published in eLife, researchers compiled a database of disease cases across the globe from many sources including published literature, and online reports. The spread of diseases can be affected by environmental factors, such as climate, and by socioeconomic factors such as poverty, or whether an area is rural or urban. The researchers brought together disease case data with environmental and socioeconomic information and used statistics to create global risk maps for the leishmaniases.
The global risk maps highlight areas where leishmaniases are common, and also reveal areas at high risk that currently have few or no reported cases. This could be because the diseases are not present, or because cases are not being reliably reported. The information provided by the global risk maps can help target disease monitoring to at risk areas, identify target areas for disease control and help inform estimates of the burdens that may fall on public health systems in future.
Fortunately for me, leishmaniases are rare in The Gambia so my risk is minimal. Hopefully, the information provided in the risk maps can help to reduce the risks to people in regions that aren’t so lucky.
1) World Health Organisation (WHO) Media Centre. Fact Sheets http://www.who.int/mediacentre/factsheets/fs375/en/ (retrieved 17/07/14)
2) Pigott et al (2014) Global distribution maps of the leishmaniases. eLife