NTDs: Leishmaniasis – yet another disease transmitted by insects

This post is the 9th in a series highlighting the WHO’s list of 17 Neglected Tropical Diseases (now technically 18 as mycetoma was added to the list at the 69th World Health Assembly in May 2016). To read the previous posts in this series click here.

A sandfly, the leishmaniasis vector. Courtesy WHO/S. Stammers.

In continuing on with our Neglected Tropical Diseases (NTDs) theme, today we’ll discuss leishmaniasis. Leishmaniasis is one of those nasty diseases transmitted by insect bites. This time the insect doing the biting, and transmitting of the Leishmania parasites, is the sandfly, as opposed to the mosquito, kissing bug or tsetse fly. (As a side note, quite a few NTDs are transmitted by insects, now that I think about it) Over 90 species of sandfly transmit leishmaniasis. Leishmaniasis occurs in 98 countries, which means more than 350 million people are living at risk.

Leishmaniasis comes in three forms: mucocutaneous, cutaneous (the most common) and visceral (also called kala-azar and the most deadly form).

  • Mucocutaneous – leads to partial or total destruction of the mucus membranes. Almost 90% of cases occur in Bolivia, Peru and Brazil.
  • Cutaneous – causes skin lesions and ulcers which lead to scars and serious disability. Over 95% of cases occur in the Americas, Mediterranean, Middle East and Central Asia. An estimated 0.7 to 1.3 million new cases occur each year, mainly in Afghanistan, Algeria, Brazil, Colombia, Iran and Syria.
  • Visceral (kala -azar) – fatal in over 95% of cases if left untreated. It’s endemic in the Indian subcontinent and East Africa. There are an estimated 200,000 to 400,000 new cases each year.

Leishmaniasis is one of the NTDs that seems intrinsically linked to being poor. The WHO states that these are the major risk factors for acquiring leishmaniasis:

  1. Socioeconomic conditions – poor housing and sanitary conditions plus overcrowding is a sandflies dream habitat
  2. Malnutrition – infections are more likely to progress to kala-azar if infected folks are malnourished
  3. Population mobility – non-immune people migrate into endemic areas and get into the transmission cycle
  4. Environmental changes – urbanization, deforestation
  5. Climate change – changes in temperature, humidity, rainfall all affect sandfly habitat and force populations to migrate

However bad leishmaniasis sounds, it is treatable and curable, albeit with drugs that aren’t the most easily used for a variety of reasons. Some of the drugs are kind of toxic, require a long treatment duration, have infrastructure requirements like a cold chain or are just expensive.  But on a brighter note, recently WHO and Gilead Sciences made an announcement extending their partnership regarding donations of treatment for visceral leishmaniasis and support for expanded surveillance.

 

Courtesy WHO/C. Black

Millions of people are at risk of some form of leishmaniasis. Most of those at risk live in extreme poverty, which places them in direct contact with sandflies, the leishmaniasis vector. Some countries, such as Bangladesh, India and Nepal, are making great strides to reduce their number of cases. But with so many people at risk of some form of leishmaniasis, it will be a long time before we can add leishmaniasis to the list of diseases humanity is close to eradicating.

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