Mosquito Borne Disease

* "Human malaria is caused by one of four species of microscopic parasites: Plasmodium falciparum, P. vivax, P. malariae and P. ovale. The name malaria originally comes from the Italian word meaning “bad air”, reflecting the association of the disease with damp breezes from marshes.

The different malaria parasite species can be recognized in human blood by their size, shape and staining characteristics and clinically by the severity of infection and the pattern of fever and chills. The most severe form of the disease is caused by P. falciparum which can cause kidney failure, brain damage, severe anemia and death. However, this species lacks residual stages and does not relapse. A more common and less severe form of the disease is caused by P. vivax. However, this species characteristically has a residual liver stage that if untreated, may cause relapses for several years after the initial infection. The remaining two species, P. malariae and P. ovale, are limited in global distribution and cause comparatively less illness in humans.

Malaria parasites spend most of their life cycle within the liver or blood cells of humans, but require anopheline mosquitoes for sexual reproduction and biological transmission. Malaria parasites also can be transmitted mechanically either by contaminated blood products or hypodermic needles.

Transmission begins when the sexual stages of the parasite in the human blood stream are ingested with the blood meal by female Anopheles mosquito. In the midgut of the mosquito, the male and female parasites unite to form the invasive stage which burrows through the gut wall. On the outside of the gut, the parasites form a cyst and multiply asexually through cell division. After 8-14 days, depending on temperature, the mature cysts rupture and the liberated parasites pass through the body cavity to the salivary gland where they remain until the mosquito takes her next blood meal. If this meal is from a susceptible human, the parasites are transmitted to the human within the small amount of salivary fluid secreted by the mosquito prior to feeding. After being injected by the bite, the parasites are transported by the blood stream to the liver where they reproduce asexually within liver cells. After 7-14 days, the liver forms of the parasite are liberated into the blood stream where the parasite attacks the red blood cells. A cycle of red blood cell invasion, asexual reproduction and red blood cell destruction then begins, causing the characteristic fevers and chills associated with malaria. Later in the infection, the male and female forms of the parasite are produced and remain in the peripheral blood stream, available for mosquito ingestion.

Malaria (mostly P. vivax) was introduced into and became established in the Central Valley of California during the 1800s. Many older mosquito control districts originally were formed to combat this disease through anopheline  control. Case recognition and treatment, combined with successful mosquito abatement, essentially eliminated malaria as a major health concern in California by the early 1900s. However, persistent anopheline abundance and the repeated importation of malaria in infected immigrants and travelers continues to pose a risk of local transmission and re-establishment. Prevention and control of malaria emphasizes the diagnosis classification of parasite source, and rapid treatment of human cases by local health authorities. Mosquito abatement agencies become involved when cases are classified as being locally transmitted to persons with no known risk factors such as recent travel or intravenous drug abuse."

* From: The Biology and Control of Mosquitoes in California. 1996. Mosquito and Vector Control Association of California. Elk Grove, CA., CDPH

For more information on Malaria follow this link to CDC