Probable Locally Acquired Mosquito-Transmitted Plasmodium vivax Infection -- Georgia, 1996
Endemic, mosquitoborne transmission of malaria was interrupted in the United States during the 1940s. Since then, 57 small localized outbreaks of probable mosquito-transmitted malaria in the United States have been reported to CDC (1,2). This report summarizes the investigation of a case of Plasmodium vivax infection in a resident of Georgia who had never lived in or visited a malarious area. The results of this investigation suggest that this case probably was acquired through the bite of a locally infected Anopheles sp. mosquito, although a probable source of infection for mosquitoes was not confirmed.
On June 22, 1996, a 53-year-old man residing in Tift County, Georgia, was admitted to a hospital because of a 12-day history of fever, chills, fatigue, and myalgias. Physical examination on admission revealed a temperature of 105.6 F (40.9 C) and mild tachypnea. Initial laboratory examinations demonstrated only moderate anemia (hemoglobin: 10.5 g/dL) and thrombocytopenia. The tentative diagnosis was fever of unknown origin.
On June 26, examination of a peripheral blood smear revealed intracellular parasites consistent with P. vivax. This diagnosis was subsequently confirmed at CDC by examination of a blood smear and serologic testing. The patient was treated with chloroquine phosphate (2500 mg total dose, divided over 3 days) and primaquine phosphate (26.3 mg daily for 14 days). All symptoms resolved and subsequent examination of peripheral blood smears showed clearance of the parasitemia.
The patient was born in Piedras Negras, Coahuila State, Mexico (on the Texas-Mexico border), approximately 500 miles from the nearest malarious area. He had emigrated to the United States during the mid-1980s, working for 2 years as a migrant farm worker in California and Florida before moving to Tift County to work on a hog farm. He had made one return visit to Coahuila, Mexico, in August 1993, during which he traveled only within an area 70 miles south of the Texas border and never entered any area where malaria transmission is known to occur. Since his return, he has remained continuously in southwestern Georgia. He had never received blood or blood products and denied use of parenteral drugs.
The patient reported that during May-June 1996, the period during which he probably became infected, he had spent his nights in Tift County at either a mobile home park, where many migrant farm workers reside, or a small encampment of trailers contiguous to the hog farm. In both locations, he slept in rooms with open, unscreened windows.
Active Case Detection
No other cases of malaria had been reported to the Georgia Department of Human Resources (GDHR) from southwestern Georgia since January 1, 1996. To identify potential unreported cases, a telephone survey was conducted of hospital infection-control practitioners in all hospitals and clinical laboratories serving southwestern Georgia; no additional smear-positive malaria infections were identified from May 1 through July 1, 1996 -- a period defining the time interval required for two complete parasite life cycles and during which climatic conditions would have supported local, mosquitoborne transmission. In addition, a telephone survey of all physicians in Tift County who specialized in internal medicine, family practice, and/or pediatrics did not identify any persons with malaria or unexplained fever.
Two potential sources of mosquito infection were considered: persons who recently had immigrated from regions where malaria is endemic, including migrant farm workers, and travelers returning from countries where malaria is endemic. An estimated 8000 migrant and seasonal laborers work in Tift County and neighboring Colquitt County (GDHR, unpublished data, 1994). In Georgia, approximately 95% of migrant laborers are natives of Mexico. In Tift and Colquitt counties, most migrant laborers are natives of Guerrero, Oxaca, Michoacan, and Chiapas states, in which malaria transmission is endemic. Most other migrant workers are natives of Guatemala or the United States. Migrant clinics in Tift and Colquitt counties did not report any patients with malaria or a fever of unknown origin during January-June 1996.
Data about the numbers of persons who have immigrated to southwestern Georgia and area residents returning from travel to countries with endemic malaria were unavailable. Previous reports have documented malaria transmission by mosquitoes unintentionally transported by aircraft from areas where malaria is endemic ("airport malaria") (3); however, the nearest airport receiving international flights is in Atlanta, approximately 175 miles from Tift County and beyond the radius of travel for a mosquito.
Environmental and Entomologic Investigation
Larvae of Anopheles quadrimaculatus -- species A, competent mosquito vectors of malaria, were identified in a creek located 0.2 miles from the mobile home park where the patient usually slept. Several adult An. quadrimaculatus mosquitoes were captured in a CDC light trap left overnight in a wooded area in the mobile home park. Larvae of multiple Culex sp., but not anophelines, were identified in one of many man-made pools on the hog farm. Two blood-fed, adult Culex sp. mosquitoes were aspirated from the corners of the bedroom of the trailer where the patient slept; however, no anophelines were found by aspiration or by overnight trapping.
The mean daily minimum and maximum temperatures for Tift County in May were 63.5 F (17.5 C) and 86.0 F (30.0 C) and, in June, were 65.2 F (18.4 C) and 85.3 F (29.6 C) (Coastal Plain Experiment Station, College of Agricultural and Environmental Sciences, University of Georgia, unpublished data, 1996). Although rainfall in May and June was only 1.2 and 2.0, respectively, farms that surround the mobile home park and the trailer encampment use overhead spray irrigation with multiple drainage canals to collect runoff -- preferred habitats for the breeding of Anopheles sp. mosquitoes. In addition, multiple small ponds and creeks in the surrounding area provide a suitable environment for breeding of anophelines.
Reported by: M Dawson, MD, PT Johnson, Tift General Hospital, Tifton; L Feldman, MD, R Glover, MPH, South Health District; J Koehler, DVM, P Blake, MD, KE Toomey, MD, State Epidemiologist, Epidemiology and Prevention Br, Div of Public Health, Georgia Dept of Human Resources. Entomology Br, and Malaria Section, Epidemiology Br, Div of Parasitic Diseases, National Center for Infectious Diseases, CDC.
Editorial Note: This investigation confirmed a single case of P. vivax infection in a person residing in Tift County, Georgia. Based on a consideration of five factors, this person probably acquired infection in southwestern Georgia through the bite of a locally infected Anopheles sp. mosquito. First, although the patient was born in Mexico, he had never lived in or visited an area where malaria is endemic. Second, the patient did not report other risk factors for malaria, including blood transfusion or use of injecting drugs. Third, both adult- and larval-stage An. quadrimaculatus mosquitoes were identified near the location where the patient usually slept in a room with open, unscreened windows. Fourth, environmental or climatic conditions were suitable for promoting development of the parasite in the mosquito (sporogonic cycle) (the speed of development is directly related to increasing ambient temperature) (4); a more rapid sporogonic cycle would increase the likelihood that the parasite would develop into the stage that is infective to humans during the mosquito's lifespan. Fifth, widespread use of overhead spray irrigation by farms in this area and multiple small ponds and creeks provided potential habitats for the breeding of Anopheles sp. mosquitoes in the absence of rainfall.
Although active case-detection efforts failed to identify persons with malaria who might have been the source of mosquito infection, migrant farm laborers -- including some from malarious areas of Mexico and Central America -- worked and lived in Tift and Colquitt counties when this patient became infected. Reasons for failing to identify potential source-patients among this group could include self-treatment with an antimalarial drug, receipt of an antibiotic that has antimalarial activity (e.g., trimethoprim-sulfamethoxazole) for another presumptive condition, or diagnosis of and treatment for malaria outside the area of active surveillance in a person who subsequently visited Tift County before parasitemia had cleared.
Two additional routes of infection in the patient cannot be ruled out by the findings of this investigation. First, an infected mosquito could have been transported in the baggage of a traveler returning from a visit to a malarious area ("baggage malaria") (5). This route has been implicated as the possible source of outbreaks in Europe, but no published evidence suggests that mosquitoes can survive long trips in baggage. Second, the patient's illness may have resulted from relapse of infection acquired during his trip to northern Mexico in 1993. Relapses of P. vivax can occur up to 4 years after the primary infection. Although the U.S.-Mexico border area is not considered an area with ongoing malaria transmission, previous investigations identified three persons with malaria who had traveled only in the border region (6; CDC, unpublished data, 1995), suggesting the possible occurrence of sporadic malaria transmission in northern Mexico. However, the patient did not recall a febrile illness during or after that visit, and asymptomatic malaria infection is rare among persons who do not reside in areas of intense malaria transmission (e.g., sub-Saharan Africa).
This report is the 11th documented episode of probable mosquito-transmitted malaria in the United States since 1986 (2,6-9); the frequency of such episodes has increased since 1976. Factors that may be contributing to the reemergence of locally acquired malaria include increased travel by U.S. residents to areas where malaria is endemic and shifting patterns of immigration to the United States. For immigrant populations originating from malarious areas, limitations in their access to health care (e.g., financial, cultural, and legal barriers) in the United States also may contribute by extending the duration of parasitemia in an infected person. Strategies to improve the detection and treatment of malaria among migrant and immigrant populations include clarification of current practices in the management of febrile illness, addressing obstacles to health care, development of appropriate educational messages about malaria, and encouraging appropriate use of medical services. Finally, malaria should be considered by all physicians who provide care for persons with unexplained fever -- regardless of travel history and particularly during summer months. Additional information is available from CDC, telephone (404) 332-4559.
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