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Transmission of Plasmodium vivax Malaria -- San Diego County, California, 1988 and 1989

Malaria transmission in the United States occurs infrequently; since 1950, 21 outbreaks of introduced malaria, all caused by Plasmodium vivax, have been identified. However, 14 of these occurred in California (seven during 1986 (1), 1988, and 1989). Four outbreaks--one each in 1986 (1) and 1988 and two in 1989--occurred in San Diego County. This report describes the outbreaks in San Diego County in 1988 and 1989.August 1988

On August 2, 1988, a migrant worker who lived in a canyon near the Lake Hodges reservoir (25 miles north of San Diego) (Figure

  1. was diagnosed with P. vivax infection. Twelve other workers who lived in the same area and had symptoms sug gestive of malaria were referred by a public health nurse for diagnosis; on August 3, P. vivax parasitemia was confirmed in all 12 workers.

An epidemiologic investigation identified a total of 30 persons who had symptomatic P. vivax infections with onset between July 24 and September 18. Two cases were in local permanent residents who had no apparent risk factors for malaria, and 28 were in migrant workers employed primarily in agricultural businesses near the Lake Hodges reservoir. All patients denied previous malaria infection, intravenous (IV)-drug use, and blood transfusions in the previous 3 years.

Nineteen of the migrant workers lived in the canyon area; seven lived on a farm directly south of the canyon; one pregnant woman worked at a tree nursery adjacent to the farm and was required to be outside at dawn and dusk; and one migrant worker had onset of symptoms after moving from the canyon. The two cases in local permanent residents occurred in a couple who lived in a development adjacent to the farm.

The migrant workers in the canyon had diverted water flow from a small channel to their camp; some of their shelters (consisting primarily of plastic tarpaulins and cardboard materials) were located within 5 feet of the water. Of the 39 migrant workers (20 of whom had malaria) in this canyon, questionnaires were administered to 31 (79%); the remaining workers could not be located. Cases and noncases were similar in terms of type of shelter used for sleeping (open vs. enclosed), average number of daily hours spent by open water (lagoon, river, or canal), bathing site, and time of day returning to the camp.

A possible index case for the outbreak was a migrant worker with documented P. vivax malaria who had spent several nights in the canyon at the end of June. Light traps placed near the canyon at that time caught 79 adult female Anopheles hermsi mosquitoes, a competent malaria vector.

On August 3, the San Diego County Department of Health Services began identification and treatment of persons with malaria. Chloroquine prophylaxis was administered to migrant workers in the canyon and on the farm directly south of the canyon. On August 4, traps placed in the canyon camp area caught from one to 11 adult female An. hermsi per trap; two traps placed next to an irrigation pond on the farm south of the canyon caught 194 An. hermsi mosquitoes. Vector-control efforts included the application of larvicides and insecticides. These efforts resulted in substantial reductions in mosquito populations.July 1989

On July 7, a migrant worker who lived in an open field in Rancho Penasquitos (20 miles northeast of San Diego) (Figure 1) had onset of chills and fever; P. vivax infection was diagnosed. A second migrant worker from this area became ill on July 30; P. vivax parasites were detected in a blood smear. On August 22, a permanent resident of this area became ill; P. vivax parasites were identified. This resident had no history of foreign travel, IV-drug use, or blood transfusions. Twelve light traps placed in the open field in Rancho Penasquitos on July 26 caught six An. hermsi mosquitoes. On August 31, 36 An. hermsi mosquitoes were caught in eight light traps placed near the resident's house. Control measures included treating infected persons and applying larvicides and adulticides. No further cases were reported from this area.August 1989

On August 6, a San Diego resident who lived north of the San Dieguito River (Figure 1) became ill with confirmed P. vivax infection. He had no history of foreign travel, IV-drug use, or blood transfusions. On August 10, a migrant worker who resided in a camp at the San Dieguito River (1 mile from the resident's house) became ill, and P. vivax infection was diagnosed. On August 11, P. vivax infections were diagnosed in three of 40 migrant workers living in the area around the camp. On August 10, 27 An. hermsi mosquitoes were caught in five light traps in the area. Control measures included the application of larvicides and adulticides and the administration of chloroquine chemoprophylaxis to migrant workers in the area. No further cases were reported from this area. Reported by: S Hunt, D Maher, M Ginsberg, MD, M Mizrahi, MS, C Peter, PhD, M Bartzen, M Thompson, DPH, D Ramras, MD, San Diego County Dept of Health Svcs, RR Roberto, MD, Infectious Disease Br, California Dept of Health Svcs. Div of Field Svcs, Epidemiology Program Office; Malaria Br, Div of Parasitic Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The outbreak of malaria in 30 persons in San Diego County in 1988 was the largest outbreak of introduced malaria in the United States since 1952, when 35 P. vivax infections were reported in members of a girls' club in California (2). A second large outbreak in 1986 involved 27 migrant workers and one local resident in Carlsbad, 35 miles north of San Diego (1).

Common features of the 1986 outbreak and later instances of malaria transmission in San Diego County were 1) the limited access to medical care for migrant workers with malaria, resulting in delays in identification and treatment of parasitemic persons and in institution of control measures; 2) the lack of sanitary water and waste water disposal facilities and adequate shelter for the workers; and 3) the presence of an effective Anopheles mosquito vector and a susceptible population. Specific sources of infection for the mosquitoes could not be identified. However, in all of these outbreaks, agricultural workers from countries with endemic malaria had been present in the affected areas.

Other clusters of introduced malaria in the San Diego area may have gone undetected because a diagnosis of malaria might not have been considered in symptomatic patients with no history of travel abroad and because malaria may be undetected in migrant workers and other populations who have limited access to medical care. Anopheline malaria vectors are found in many parts of California; therefore, locations that facilitate introduction and transmission of malaria cannot be readily predicted.

Reduction of the risk for malaria in migrant workers requires improved access to medical care and early detection of malaria cases. For workers living in remote areas, disease surveillance, health education, and early recognition and medical evaluation of illness are needed to reduce the risk for malaria transmission.


  1. Maldonado YA, Nahlen BL, Roberto RR, et al. Transmission of Plasmodium vivax malaria in San Diego County, California, 1986. Am J Trop Med Hyg 1990 (in press).

  2. Brunetti R, Fritz RF, Hollister HC. An outbreak of malaria in California, 1952-1953. Am J Trop Med Hyg 1953;3:779-88.

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