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Epidemiologic Notes and Reports Mosquito-Transmitted Malaria -- California and Florida, 1990

In 1990, two persons--one each in California and Florida--were diagnosed with malaria classified as cryptic*; their infections may have been acquired in the United States through bites of mosquitoes that became infected after biting parasitemic migrant workers. California

On July 30, a teenaged male resident of Oceanside in north San Diego County presented to a physician's office with an 11-day history of fever, malaise, and myalgia. Plasmodium vivax parasites were identified during a blood smear examination. On hospital admission, the patient had splenomegaly and a hemoglobin level of 5.9 gm%. He was treated with chloroquine and primaquine and recovered.

The San Diego Department of Health Services conducted an epidemiologic investigation to determine the source of his infection. The patient had no history of foreign travel, intravenous (IV)-drug use, or blood transfusions. He lives in a suburban housing development within mile of the San Luis Rey River. The open area between his house and the river is flat, with heavy vegetation near the river. In the evenings, he frequently visited a nearby park within 150 yards of the river. Several encampments of migrant workers employed at local farms were identified along the river. No history of malaria-like illness was elicited from migrant workers in these encampments; no malaria cases were reported among these migrant workers or among other residents of Oceanside.

Entomologic investigations along the river during August 1-6 identified larvae and adult mosquitoes of Anopheles hermsi, a competent mosquito vector for malaria. No anopheline mosquitoes were identified near the patient's residence. Control measures consisted of larviciding mosquito breeding sites with oil and fogging with pyrethrins along the riverbed. Florida

On June 8, a female resident of Bay County in the Florida Panhandle consulted a physician because of a 5-day history of remittent fever, chills, myalgia, and headaches. P. vivax parasites were identified on a peripheral blood smear. She was treated with chloroquine and primaquine and recovered.

The Florida Department of Health and Rehabilitative Services conducted an epidemiologic investigation to determine the source of her infection. The woman had no history of foreign travel, blood transfusion, or IV-drug use. A survey of medical-care providers in Bay County and neighboring Gulf County did not identify other cases of malaria or unexplained febrile episodes within the previous 3 months. The patient and her family had spent the nights of May 19 and 27 sleeping outdoors in a campground in Gulf County, 30 miles from her home. Mosquito activity and biting at night was reportedly intense.

A door-to-door survey of residents of this campground and follow-up visits with the owner of the campground did not identify any suspected cases of malaria. In May, a large fish farm contiguous to the campsite had employed approximately 40 migrant workers, many of whom came from Mexico and Central America. None of the migrant workers were known to have had symptoms compatible with malaria. Health-care providers in the area had not treated any patients with malaria-like symptoms. Efforts to trace and survey the migrant workers were unsuccessful.

On June 14, approximately 50 A. quadrimaculatus, a competent mosquito vector of malaria, were caught in light traps near the campsite. Control measures included ultralow-volume spraying with malathion. Reported by: M Ginsberg, MD, S Hunt, M Bartzen, A Caudillo, MD, D Ramras, MD, M Mizrahi, San Diego Dept of Health Svcs; RR Roberto, MD, Infectious Disease Br, GW Rutherford, III, MD, State Epidemiologist, California Dept of Health Svcs. S McClellan, MD, T Smith, MD, Panama City; P Sylvester, MD, J Cerosimo, MD, BW Clements, RA Calder, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. Malaria Br, Div of Parasitic Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Both of the malaria cases described here were classified as cryptic. However, both persons may have acquired their infections in the United States through bites of mosquitoes that became infected after biting parasitemic migrant workers.

Transmission of mosquitoborne P. vivax malaria in San Diego County has occurred intermittently since 1986 (2,3). These episodes have shared several common features: 1) identification of the initial case(s) usually in residents; 2) limited access to medical care for migrant workers from countries with endemic malaria, resulting in delays both in identification and treatment of parasitemic persons and in institution of control measures; 3) presence of standing water and lack of adequate sanitary facilities and shelter in migrant workers' encampments; and 4) proximity of competent Anopheles vectors and a susceptible population. In contrast, although A. quadrimaculatus is widespread in Florida, no cases of suspected or confirmed mosquitoborne malaria infections have been identified since 1948.

In other states, conditions may be similar to those in Florida and California (i.e., large populations of migrant workers and conducive environmental conditions), especially in the Southwest and along the Gulf of Mexico. Health-care providers should be aware of the potential for introduced malaria in both migrant workers and local residents. In these areas, malaria should be included in the differential diagnosis of any patient with a fever of unknown origin. When malaria infection is diagnosed, physicians should inquire about recent travel, previous malaria infections, IV-drug use, and blood transfusions. Prompt reporting of confirmed malaria infections will aid health departments in immediately investigating potential local transmission.


  1. World Health Organization. Terminology of malaria and of malaria eradication, 1963. Geneva: World Health Organization, 1963:32.

  2. CDC. Transmission of Plasmodium vivax malaria--San Diego County, California, 1988 and 1989. MMWR 1990;39:91-4.

  3. Maldonado YA, Nahlen BL, Roberto RR, et al. Transmission of Plasmodium vivax malaria in San Diego County, California, 1986. Am J Trop Med Hyg 1990:42:3-9.

    • An isolated case of malaria ascertained by appropriate epidemiologic

    investigation not to be associated with secondary cases (1).

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