Two clusters of malaria involving 27 patients were identified in San Diego, California, in the period August 8-September 30, 1986. The initial patient identified (Patient A) in the outbreak was a 58-year-old resident of Carlsbad, a coastal community of 35,000 in San Diego County, who was seen by his local physician because of high fever and diarrhea. He was initially diagnosed as having a viral illness, but when his symptoms worsened on August 11, he was admitted to a local hospital. Three days later a blood smear was positive for Plasmodium vivax malaria. Treatment with chloroquine and primaquine led to his recovering without complications. He had no history of intravenous drug use, blood transfusion, or travel to areas with endemic malaria. The patient lives in a residential area across the street from a marsh that empties into a salt-water lagoon, and in July he took frequent evening walks through the marsh area.

In response to this report of P. vivax malaria in the area, on August 16 an effort was begun to identify all cases of malaria reported in San Diego County since January 1, 1986. The San Diego County Health Department records were reviewed, and local hospitals and physicians were contacted to detect unreported cases. As a result, an additional 26 cases of P. vivax (smear-positive) malaria were identified as having occurred in the Carlsbad area in the period June 18-September 20. The epidemic curve shows a bimodal distribution with a 24-day interval between the two clusters (Figure 2).

The first cluster of cases involved six patients who became ill between June 18 and July 2 and were identified as five Mexican migrant agricultural workers and one San Diego County resident who lived 20 miles from Carlsbad. This 30-year-old male (Patient B) had gone swimming on May 31 and June 7 in a lake 3 miles southeast of the lagoon area frequented by Patient A during his evening walks. In September 1985, Patient B had traveled to an area north of Puerto Vallarta, Mexico, where he slept on the beach. He denied any previous malaria infection, intravenous drug use, or blood transfusion.

The second cluster of cases involved 21 patients who became ill between July 26 and September 20 and were identified as 20 Mexican migrant workers and the local Carlsbad resident (Patient A) discussed above. Twenty of the 25 infections involving Mexican migrant workers from both time periods were reported by the same local hospital. The other five were diagnosed during an active case-detection survey involving interviews with 319 migrant workers on the three agricultural farms in the lagoon area.

Eighteen of the migrant-worker patients were interviewed. None of them had a history of intravenous drug use or blood transfusion, and only one had a history of malaria infection. All were males 17-30 years of age. They were employed in a variety of work situations, came from five different states in Mexico, and had arrived in the United States 2 weeks-20 months before becoming ill. Eleven of these patients had been in the United States at least 2 months.

On August 14, a baited light trap placed in the marsh area was found to contain 115 adult female Anopheles freeborni mosquitoes, a competent vector of malaria. On August 18, after the San Diego County Vector Surveillance Unit had applied adulticide/larvicide to the area, a baited light trap placed in the area contained 16 adult female An. freeborni. No An. freeborni were found in the trap on August 22, and subsequent trapping efforts led to counts of 0-10 An. freeborni per light trap in the marsh area. Reported by J Turley, Tri-City Hospital, Oceanside, E Orellana, S Hunt, M Mizrahi, MS, M Ginsberg, MD, M Thompson, DrPH, G Reaser, MD, D Ramras, MD, San Diego County Dept of Health Svcs, T Smith, MS, Vector Surveillance and Control Br, R R 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: This two-cluster outbreak of P. vivax malaria involving 27 patients and occurring within a 14-week period represents an unusually high number of reported cases of malaria in San Diego County in such a short time. That is, in all of calendar year 1985, only 20 cases of imported malaria among civilians were reported to the San Diego County Health Department. From January through August 1986, only two cases in addition to the outbreak described here were reported among civilians in San Diego County. One of these cases represented importation from India, and the other, from Papua New Guinea.

Some or all of the 27 cases of P. vivax malaria in these two clusters were acquired by local transmission in San Diego County. For example, the P. vivax infection of Patient A, who had no other identified risk factors, indicates local transmission. Furthermore, the occurrence of two clusters in time and space suggests that some of the cases among Mexican migrant workers represent P. vivax infection acquired in the marsh area rather than imported from Mexico. Data from the vector surveillance indicate that adult female An. freeborni were present in large numbers in the lagoon area during the period that transmission occurred. The 24-day interval between the two clusters is consistent with the time required for development of the parasite in the infected mosquito (range 11-20 days) and the incubation period of P. vivax in humans (12-17 days) (1). Furthermore, the case investigations indicated that the common factor shared by the Mexican migrant patients was that they all slept in the open on a hillside bordering the marsh.

This represents the largest outbreak of introduced malaria* in the United States since 1952 (2). Only 14 isolated episodes of introduced malaria have been reported in the United States since 1950, despite periodic increases in the number of imported malaria cases in the same period. Seven of these 14 episodes occurred in California. In the period 1966-1971, there were 16,872 reported cases of malaria imported by American military personnel returning from Southeast Asia. In the same period, only three episodes of introduced malaria could be related to Vietnam veterans (3). In 1979-1981, there were 1,571 cases of malaria reported among refugees from areas of Southeast Asia with endemic malaria. No cases of introduced malaria have been attributed to this influx of immigrants (4). However, the outbreak of introduced malaria described in this report may be related to the increased importation of malaria by migrant workers from Mexico who do not have authorization papers and may therefore be reluctant to seek medical care. The number of cases of malaria imported from Mexico into California rose from 20 in 1983 to 75 in 1985. In Mexico the number of reported malaria cases has doubled in the past 4 reporting years--from 42,104 in 1981 to 85,501 in 1984 (5).

Introduced malaria is quite rare in the United States despite the presence of competent anopheline vectors in California (An. freeborni), the states that border Mexico, and the entire Southeast (An. quadrimaculatus) during the warmer months. Transmission of malaria requires that a susceptible female mosquito feed on an infected person. The mosquito must survive long enough to allow the parasite to mature and then must find an available host to infect. In the United States, the likelihood that this sequence of events will occur is low. In Carlsbad, the influx of carriers of malaria parasites into an area with a susceptible population and large numbers of competent mosquito vectors created the rare juxtaposition of events required for a malaria outbreak.

If Patient A's malaria infection had not been diagnosed and reported, some of the locally acquired P. vivax infections among the Mexican migrants might not have been recognized. Medical personnel should be aware that introduced malaria may affect a susceptible population of migrant workers, as well as the resident population. A complete history of recent travel, malaria infection, time of arrival in the United States, intravenous drug use, and blood transfusion should be obtained. Regardless of the characteristics of the population of patients, a cluster of malaria cases should trigger an investigation to determine whether local transmission has occurred. Prompt reporting of malaria cases will assist in assuring that introduced malaria does not grow to be a substantial public health threat in the United States.

References

1. Bruce-Chwatt LJ. Essential malariology. New York: John Wiley & Sons, Inc., 1985:59.

2. Brunetti R, Fritz RF, Hollister AC Jr. An outbreak of malaria in California, 1952-1953. Am J Trop Med Hyg 1954;3:779-88.

3. CDC. Malaria surveillance annual summary. Atlanta, Georgia: US Public Health Service, 1971.

4. CDC. Malaria surveillance annual summary. Atlanta, Georgia: US Public Health Service, 1979, 1980, 1981. 5 Pan American Health Organization. Status of malaria program in

the Americas. Epidemiological Bulletin 1986;7:2. *Defined as malaria acquired by mosquito transmission in an area in which malaria does not occur regularly.

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