Understanding diseases caused by mosquitoes

The leg of a person suffering from elephantiasis, a mosquito-borne disease. (Photo: File/Standard)
With a slender body, scaled wings and long legs, they are a nuisance mostly during the night.

When it buzzes in your ear, it is attracted to you by smelling the carbon dioxide you exhale.

The female species feed on blood whereas the male ones feed on nectar from plants that provide sugar that enables them to live.

Did you know that they do not spread disease by transferring blood from one victim to another but the disease is passed in the injected saliva? In an interview with Kenya Medical Research Unit Chief Research Officer Andrew Githeko, The Standard on Saturday takes you through nine diseases that are transmitted by the mosquito.

Malaria

It is a preventable and curable life-threatening disease caused by parasites that are mostly transmitted through the infected female Anopheles mosquito of the Plasmodium falciparum species, a most deadly species. In the just-released 2015 Kenya Malaria Indicator survey, malaria is described as a significant public health problem in Kenya, with more than 70 per cent of the population living in malaria risk areas around Lake Victoria and at the Coast facing the highest risk. Children under age five and pregnant women are most vulnerable to infection.

The disease is most serious in young children, non-immune pregnant women, people with HIV/Aids and international travelers from non-endemic regions. Anopheles mosquitoes lay their eggs in water which hatch into larvae, eventually emerging as adult mosquitoes. The female mosquitoes seek a blood meal to nurture their eggs.

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The initial symptoms of malaria are flu-like and include a high temperature (fever), headache, sweats, chills and vomiting. People with sickle cell anaemia traits rarely get malaria. Prevention includes use of long lasting insecticide-treated mosquito nets, antimalarial and indoor residual spraying. Treatment and management involves combination drugs known as artemisinin-based combination therapy (ACT).

West Nile

Mosquitoes of the genus Culex are generally considered the principal transmitters of the disease that was discovered in 1937 in West Nile District in Uganda. It affects the nerves, and in 80 per cent of the cases, it shows no symptoms but when severe can lead to death. Birds like crows host the virus. And when found in horses,  they become “dead-end” hosts, meaning that while they become infected, they do not spread the infection.

Yellow Fever

The Aedes aegypti mosquitoes carry the Yellow Fever virus from one host to another, primarily between monkeys, from monkeys to humans, and from humans to humans.

There are two cycles which are the urban and forest one. In the urban one, the disease is spread from small containers used to store water in the households whereas in the forest cycle, the Aedes species feeds from the monkeys.

There is no specific treatment for yellow fever but there is a life-long vaccine.

The symptoms include fever, muscle pain, backache, headache, shivers, loss of appetite, and nausea or vomiting. Yellow fever can be prevented through vaccination and mosquito control.

Chikungunya

It is caused by the Aedes species and is originally thought to have come from Tanzania in 1952.

It causes fever and severe joint pain. Other symptoms include muscle pain, headache, nausea, fatigue and rashes.

Joint pain is a common complaint in infected persons.

Zika

Zika virus is spread through the bite of an infected Aedes species mosquito. The disease was first identified in Uganda in 1947 in rhesus monkeys, with the first human cases recorded in Nigeria in the 1960’s.

These mosquitoes usually bite during the morning and late afternoon or in the evening.

Zika during pregnancy appears to increase the risk for several types of birth defects and miscarriages.

The study showed the virus has been found in the brains of affected babies.

There is no specific treatment or vaccine currently available.

Rift Valley fever

Rift Valley fever (RVF) is transmitted from animals to man and the Aedes species transmits the virus from infected female mosquitoes to offspring via its eggs. Humans can get RVF as a result of bites from mosquitoes. In Kenya, the disease is mainly found in the North Eastern region; however, it is now shifting towards the Central region.

Approximately one in every 100 humans that become infected with RVF dies. Apart from mosquito transmission, humans can be infected by consuming animal products from infected animals, a transmission pathway that is associated with most of the severe human cases. In humans, the disease presents after an incubation period of two to six days, with patients experiencing a sudden onset of fever, vomiting, headache, backache, muscle pains, and often the fear of light.  However, in most cases, the symptoms if mild fade in one week but serious infection could lead to jaundice, liver damage, permanent loss of vision and bleeding that could result in death.

Onyong’ Nyong’

This disease is named after one of its symptoms in Acholi language describing joint pain. It was first discovered in Uganda in 1959 during an epidemic that affected four million people. The virus is transmitted by the mosquitoes A. gambiae and A. funestusbut restricted to Africa. The first instance had spread from Uganda South to Mozambique and westward to Senegal. Comprising two million cases in East Africa alone, this first epidemic was ranked among the largest mosquito-borne virus outbreaks recorded.

The first imported case abroad was in October 2013, in a 60-year-old woman residing in Germany who had returned home two days before from a seven-week vacation in East Africa. She sought medical attention after experiencing a swollen face, feet and hands, and pain in joints. She is likely to have been infected in Kisumu where she had stayed. Ten days after symptom onset, the patient recovered spontaneously. Her husband had no symptoms of illness during travel or after return.

Elephantiasis

It is a painful and disfiguring disease also known as Lymphatic filariasis. It causes abnormal enlargement of body parts, causing pain and severe disability that leads to social stigma. Infection occurs when filarial parasites are transmitted to humans through mosquitoes; the Culex mosquito in urban and semi-urban areas, Anopheles, mainly found in rural areas, and Aedes. Interestingly in Kenya, it has been found in the Coast region. Other adverse effects of the condition include swelling of the scrotum in men, sometimes causing permanent disability.

Dengue Fever

The Aedes aegypti mosquito can transmit the viruses that cause dengue fever.

It is a disease of urban and semi-urban areas and bites early in the morning and in the evening just before dusk.

In Kenya, it is found in urban areas, especially in Nyanza and Western regions. In 2011, there was an outbreak of the disease due to storage of water in households. The virus incubation is four to 10 days and an infected mosquito is capable of transmitting the virus for the rest of its life and to multiple people during each feeding period.

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mosquitoessalivaZika virusYellow FeverDengue Fever