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Introduced Autochthonous Vivax Malaria -- California, 1980-1981

Two cases of locally introduced Plasmodium vivax malaria have recently been reported from the Central Valley of California. The case histories are described below.

Case 1. On August 20, 1980, a 55-year-old truck driver left his home in San Bernardino County to haul grapes from vineyards to wineries in the Central Valley of California. He remained well until October 2, 1980, when he had onset of malaise, nausea, myalgias, and drowsiness; 5 days later he also experienced chills, high fever, and profuse sweats. He attributed his symptoms to "the flu" but noted they recurred almost each evening between 5 p.m. and 7 p.m. When the symptoms occurred, he would lie down in the truck and cover himself with blankets; by morning he usually felt well enough to continue driving. On October 20 he felt so ill that he returned home. The next day a laboratory technologist at Loma Linda University Medical Center identified P. vivax on a routine peripheral blood smear, and malaria was diagnosed. The patient responded promptly to antimalarial treatment with chloroquine and primaquine.

Investigation of possible sources of infection revealed no patient history of blood transfusions, IV drug or shared-needle usage, military service or travel in areas endemic for malaria. By the time he became ill, he had traveled several thousand miles in 11 counties of the Central Valley, plus Napa and Sonoma counties. His usual routine was to arrive at a vineyard at night, sleep in his unscreened truck until dawn, load grapes, and then deliver them to a winery. He recalled receiving mosquito bites but paid little attention to them.

Of the 14 areas visited by the patient and surveyed by the state's Vector Biology and Control Section, only the light trap near Artois in Glenn County showed substantial Anopheles activity during his periods of possible exposure. Evaluation of a ranch near Artois, which he had visited 6 times in the period September 9-19, indicated that Anopheles would have been abundant at that time; however, no malaria-like illness occurred among ranch workers. Rice fields, a prime habitat of

  1. freeborni, were within 1 mile of the vineyard. Many farm workers who lived or worked near the ranch had recently arrived from malaria-endemic areas of India and Mexico, but an investigation in areas where the patient traveled failed to reveal any other unreported or suspected cases of locally transmitted malaria.

Case 2. On September 5, 1981, a 46-year-old long-term resident of Yuba County became ill with fever, chills, headache, sore throat, nausea, and abdominal pain. On September 8, after pharyngitis was diagnosed at a local emergency room, he was treated with IM penicillin. On September 14, he returned to the same emergency room because of persistent symptoms, and ampicillin was prescribed. On September 17, he went to the Yuba General Clinic complaining of nausea and fever and shaking chills that recurred daily at about 3:00 p.m and lasted 30-45 minutes. On examination, he was found to be jaundiced with a tender liver and abdomen. Malaria was suspected and quickly confirmed at the Sutter-Yuma County Public Health Laboratory by demonstration of P. vivax on peripheral blood smears. Treatment of the patient with chloroquine and primaquine resulted in prompt recovery.

The patient had no history of blood transfusion, drug abuse, or travel outside the United States. The only recent travel outside the area had been an automobile trip to Pampa, Texas, between August 26 and September 1, 1981.

The patient lives 3 miles south of Marysville in a semi-rural setting next to the Feather River and within 1/4 mile of rice fields and orchards. He had not been employed regularly since December 1980. In the spring and summer of 1981 he did extensive fishing and camping throughout Sutter and Yuba counties, and often received many mosquito bites.

Malaria smears and serum specimens from the patient's wife, 2 children, and 3 members of an adjacent household were all negative for P. vivax. Intensive surveillance in the Sutter-Yuba area failed to reveal any other cases of malaria that could have been acquired locally through the mosquito-borne route. The Sutter-Yuba Mosquito Abatement District reported that A. freeborni (an efficient P. vivax vector) was abundant in Sutter and Yuba counties, including the patient's neighborhood, throughout the summer. Mosquito-control efforts in the area had included hand spraying with insecticide around the patient's house and insecticide cold-fogging on 3 occasions for a 1/2-mile radius from the house. Reported by AF Taylor, S Gaspers, LE Mahoney, MD, San Bernardino County Health Dept, T Rowsell, MD, Loma Linda University Medical Center; LE Pine, D Dragoni, MD, Glenn County Health Dept, K Whitesell, Glenn County Mosquito Abatement District (MAD), J Buckingham, Diablo Valley MAD, A Hibbard, MD, Yuba General Clinic, Marysville, L Eberhardt, J Hornstein, MPH, P Stotler, RN, M Cusick, MD, Sutter-Yuba County Health Dept, E Kauffmann, MPH, Sutter-Yuba MAD, RR Roberto, MD, D Womeldorf, MD, J Chin, MD, State Epidemiologist, California Dept of Health Svcs; Malaria Br, Parasitic Diseases Div, Center for Infectious Diseases, Quarantine Div, Center for Preventive Svcs, Field Svcs Div, Epidemiology Program Office, CDC; G Grodhaus. EE Lusk, R Yescott, Vector Biology and Control Section, California Dept of Health Svcs.

Editorial Note

Editorial Note: Considering the average 14-day incubation period for vivax malaria, and the travel history of the 2 patients, the infections reported here were most likely acquired in the northern part of California's Sacramento (Central) Valley. Historically, mosquito-transmitted malaria in California has been confined to the Central Valley where ecologic habitats provided by irrigated farmlands--including fruit orchards and rice fields--are ideal for the breeding of A. freeborni, a highly susceptible vector of vivax malaria. In addition, non-refugee agricultural workers from malarious countries provide a reservoir of vivax parasites in such areas as Sutter and Yuba counties.

Mosquito transmission in non-endemic areas (introduced malaria) requires only the presence of susceptible mosquitoes together with gametocytemic individuals. However, the chances for introduced malaria transmission are exceedingly low in this country because an exact sequence of interacting events involving mosquito, reservoir, and host must occur, and because there is a low probability that infected mosquitoes will survive beyond the maturation time required for the parasites (12-14 days for vivax malaria).

Thus, despite periodic large influxes of imported malaria cases, only 13 isolated episodes of introduced malaria have occurred in the United States in the past 30 years. In 1970, when the number of imported cases reached the highest level in recent years (4,247) due to American military personnel returning from Southeast Asia, 1 instance of introduced malaria was identified. This case was reported from Texas where a Mexican migrant was identified as the probable source of vivax gametocytes.

Like the previous 11 episodes, the 2 cases reported here were isolated events. Introduced malaria does not pose a substantial public health threat provided current malaria surveillance procedures and malaria awareness among medical personnel are maintained.

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