In honour of World Moquito Day, we have put together some information about LF in order to raise awareness about this potentially crippling NTD that still affects 120 million people in some of the world's poorest communities, primarily in Southeast Asia and Africa.
On August 20th, 1987, British doctor Sir Ronald Ross discovered that malaria is transmitted between humans by female mosquitoes. To mark the historic discovery, World Mosquito Day is celebrated every August 20th, and this year is no exception. The World Health Organization estimates there are 219 million cases of malaria every year, with approximately 660,000 cases resulting in death.
However, increased prevention and control measures have led to a reduction in malaria mortality rates by more than 25% globally since 2000. Additionally, early stage clinical trials of a malaria vaccine have shown positive results and will continue to be tested.
Although malaria is a leading cause of morbidity and mortality around the world, it is only one of many diseases transmitted by mosquitoes. Some neglected tropical diseases are also mosquito-borne, such as dengue and lymphatic filariasis (LF), or elephantiasis. The latter forms part of our work and we now have maps for many countries showing status and progress of LF elimination programmes. In honour of World Moquito Day, we have put together some information about LF in order to raise awareness about this potentially crippling NTD that still affects 120 million people in some of the world's poorest communities, primarily in Southeast Asia and Africa. Follow the links to find out more.
What are the effects of LF?
LF infections can cause disfigurement but are not directly fatal. Infection starts to appear in children of around five years and infection levels increase throughout childhood and early adulthood, levelling out at around 30 years. Virtually all infections have subclinical lymphatic damage and around 40% of infections present kidney damage. Chronic infection can cause hydrocele (accumulation of fluid in the sac surrounding the testicles) and lymphedema (elephantiasis), swelling of the legs and occasionally the genitals and female breasts. Read more about the effects of LF here.
How is LF transmitted?
People become infected with LF when mosquitoes carrying infective-stage larvae bite them. The larvae enter the skin and migrate to the lymphatic system, where they develop into adult worms over a period of 6-12 months. Once they reach adulthood, male and female worms form nests that can produce millions of microfilaria (larva-stage worms) over the course of several years. The microfilaria circulate in peripheral blood, from where it can be picked up by biting mosquitoes to continue the transmission cycle.
LF is caused by a species of filarial round worms, or nematodes, with up to 90% of all infections caused by Wuchereria bancrofti. Various mosquito species transmit these worms: Culex mosquitoes transmit W. bancrofti in urban and semi-urban areas, Anopheles mosquitoes transmit worms in rural environments, especially in Africa, and Aedes mosquitoes are responsible for transmission throughout the Pacific. The distribution of LF is strongly related to environmental factors that influence the distribution of mosquito species. LF is particularly prevalent in areas with hot and humid climates. Read more about LF transmission here.
The following video shows the transmission cycle of LF:
What is the global burden of LF?
LF is endemic in 83 countries and there are an estimated 120 million cases, including 25 million men with hydroceles and 15 million people, primarily women, with lymphedema. The disease occurs throughout the tropical areas of Africa, Asia, the Americas and the Pacific, with around 66% of the infection clustered in Southeast Asia and most of the remaining infection centred in Africa.
Numerous countries have implemented LF control programmes, with significant achievements in the Americas, the Pacific and Asia. A few countries in Africa are close to their elimination goals. Read more about the elimination of LF here.