Los Angeles County West Vector & Vector-Borne Disease Control District



falcip.gif (29159 bytes)    Malaria is a serious, sometimes fatal, disease caused by a parasite. There are four kinds of malaria that can infect humans: Plasmodium falciparum (plaz-MO-dee-um fal-SIP-a-rum), P. vivax (VI-vacks), P. ovale (o-VOL-ley), and P. malariae (ma-LER-ee-aa). 

    Malaria occurs in over 100 countries and territories.  More than 40% of the people in the world are at risk.  Large areas of Central and South America, Hispaniola (Haiti and the Dominican Republic), Africa, the Indian subcontinent, Southeast Asia, the Middle East, and Oceania are considered malaria-risk areas (an area of the world that has malaria).

    The World Health Organization estimates that yearly 300-500 million cases of malaria occur and more than 1 million people die of malaria.  About 1,200 cases of malaria are diagnosed in the United States each year.  Most cases in the United States are in immigrants and travelers returning from malaria-risk areas, mostly from sub-Saharan Africa and the Indian subcontinent.  Each year in the United States, a few cases of malaria result from blood transfusions, are passed from mother to fetus during pregnancy, or are transmitted by locally infected mosquitoes.

    In California, Anopheles freeborni, Anopheles hermsi, and Anopheles punctipennis (mosquito species) can transmit malaria.  Transmission begins when the sexual stages of the parasite in the human blood stream are ingested with the blood meal by a 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 blood stream, available for mosquito ingestion.


    Malaria has probably killed more people during man's recorded history than any other communicable disease.  In some countries it is still an important, often fatal disease.  Malaria was probably brought to California by fur traders, gold miners, and Spanish missionaries in the early 1800's, and remained epidemic in the Central Valley until the late 1800's.  By 1900 the level of malaria had been greatly reduced by the anopholine control efforts of many of California's mosquito control districts.  In the past, malaria was endemic throughout much of the continental United States; more than an estimated 600,00 cases occurred during 1914.  During the 1940's, a combination of improved socioeconomic conditions, water management, vector control efforts, and case management were successful in vastly reducing locally transmitted cases of malaria in the United States.

    Malaria is still a threat to California due to the large number of Anopheles species that are produced and infected humans who come into the State, especially the high numbers of international travelers and immigrants who visit Los Angeles County.   Three outbreaks of local mosquito transmission have occurred in California in the last 40 years.  In 1952, a number of campers visiting Lake Vera in Nevada County contracted malaria.  A veteran from the Korean war known to have had malaria camped nearby and presumably was the source of the infections.  Most malaria occurring in the United States each year is a result of people having relapses from former cases or from cases recently acquired in countries where malaria is endemic (introduced malaria).  

    During 1984-1986, locally acquired cases of malaria in the Sacramento Valley were traced to imported cases relapsing in immigrants from Punjab State, India.  An. freeborni was incriminated as the vector species in this outbreak.

    A similar third scenario occurred in 1986 and 1987 along riparian canyons near agricultural areas in San Diego County where cases transmitted locally by An. hermsi among farm workers from Mexico residing in primitive conditions went unnoticed until cases appeared in the resident population.

    During 1988, there were 1,023 malaria cases reported in the U.S., 991 of them were introduced cases.  In 1992, 910 malaria cases were reported in the United States, 251 in the State of California.  Seven of the 910 persons acquired the infection in the U.S.  Because malaria involves only the human population, control in California continues to focus on prompt diagnosis and treatment of imported relapsing cases.   

    Since malaria is transmitted from one human to another either through blood transfusion, congenital transmission, or through the bite of a mosquito, recent data indicates that malaria is present in persons in California, Anopheles species are present in the western portion of Los Angeles County, so the possibility of malaria transmission in the Los Angeles County West Vector & Vector-Borne Disease Control District exists.

 Click on these pictures for malaria distribution maps.

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    All travelers to malaria endemic areas of the world are advised to take an appropriate drug regimen to prevent malaria. However, regardless of the antimalarial drug regimen employed, it is still possible to contract malaria. Individuals who have the symptoms of malaria (headache, weakness, fever, and chills, which may occur at intervals) should seek prompt medical evaluation as soon as possible. Malaria symptoms can develop as early as 8 days after initial exposure or even months later after chemoprophylaxis has been discontinued. It is important to understand that malaria can be effectively treated early in the course of the disease, but delaying appropriate therapy can have serious or even fatal consequences. Your itinerary and degree of exposure to mosquitoes will determine your drug therapy. Remember, a single bite is sufficient to infect you.  You should see your health care provider 4-6 weeks before you plan to travel outside the United States or Canada to find out what prescriptions you and your family may need.


    Because of the nocturnal feeding habits of Anopheles mosquitoes, malaria transmission occurs primarily between dusk and dawn. Mosquitoes are also present in shady areas and on cloudy days. To reduce your contact with mosquitoes, remain in well-screened areas, use mosquito nets, and wear clothes that cover most of the body. It is recommended to use insect repellent on any exposed areas of skin. The most effective repellents contain a concentration of at least 30% N.N. diethylmetatoluamide (DEET), which is available in many commercial insect repellents. DEET may be absorbed through the skin into the systemic circulation, and toxic and allergic reactions have been reported. Prolonged or excessive application of any insect repellent should be avoided. The long-term effects are unknown. Read and follow all label instructions for repellents containing DEET.  Depending in its concentration, DEET may have use restrictions with respect to pregnant women and children.

    Travelers should also be advised to purchase a pyrethrum-containing flying insect spray to use in living and sleeping areas during evening and nighttime hours. (People with a ragweed allergy may have an allergic reaction using this spray.) Travelers can also buy permethin (Permanone) to spray on clothing for protection against mosquitoes.




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