In the United States, malaria transmission was eliminated in the 1940s, and malaria eradication was certified in 1970 (1). Since then, 60 small localized outbreaks of probable mosquito-transmitted malaria have been reported to CDC (2--6). Before 1995, the number of imported malaria cases reported to the Suffolk County (New York) Department of Health Services ranged from zero to eight per year. Since 1995, seven to 17 cases per year have been reported. In all of these cases, a history of residing in or traveling to an area with endemic malaria outside the United States was confirmed. This report describes the investigation of two cases of Plasmodium vivax malaria that occurred in Suffolk County in August 1999; the patients had no history of travel outside of the United States.

Case Reports

Case 1. On August 18, an 11-year-old boy residing in Suffolk County was seen by his physician with a 5-day history of fever, rigors, abdominal pain, arthralgias, and vomiting. Intracellular parasites consistent with P. vivax were noted on a complete blood count. The patient was admitted to a local hospital on August 21 with a temperature of 102.0 F (38.9 C), hepatosplenomegaly, and several healing maculopapular bite lesions. Initial laboratory examinations revealed leukopenia (white blood cell count: 2,800/mm³ [normal: 4,500--13,500/mm³]), anemia (hemoglobin: 9.8 g/dL [normal: 11.5--15.5 g/dL]), and severe thrombocytopenia (platelet count: 21,000/mm³ [normal: 150,000--400,000/mm³]). Serology was negative for Lyme disease and babesiosis. Serum electrolytes and chest radiograph were normal. Urinalysis demonstrated a slightly elevated urobilinogen. Examination of peripheral thick and thin blood smears at the New York State Department of Health (NYSDH) and CDC confirmed P. vivax infection. The patient was treated with chloroquine phosphate, quinine, clindamycin, and primaquine and was discharged from the hospital on August 25.

The patient's parents reported he had never traveled to a malarious area or had a history of a blood transfusion or organ transplantation. During August 1--7, the patient spent 1 week at a summer camp 20 miles from his hometown. He slept in a tent and went swimming in the camp pond. After his return home on August 7, the patient attended another camp in Massachusetts for 2 days.

Case 2. On August 22, an 11-year-old boy residing in Suffolk County was seen by his physician for a 12-day history of vomiting, diarrhea, fever, chills, and fatigue. On August 27, a complete blood count showed malarial ring forms; the boy was admitted to a hospital the following day. Physical examination on admission revealed a temperature of 100.0 F (37.8 C), no splenomegaly, and multiple healing maculopapular bite lesions. Initial laboratory examinations revealed leukopenia (white blood cell count: 4,300/mm³), severe anemia (hemoglobin: 8 g/dL), and thrombocytopenia (platelet count: 134,000/mm³). Routine blood and urine cultures were negative. Serology was negative for babesiosis. Urinalysis and chest radiograph were normal. Examination of peripheral thick and thin blood smears at NYSDH and CDC revealed intracellular parasites consistent with P. vivax (<1% parasitemia). The patient was treated with chloroquine phosphate and primaquine and was discharged from the hospital on August 29.

His parents reported he had never traveled to a malarious area or had a history of a blood transfusion or organ transplantation. The boy spent the same week at the same summer camp as case 1, which is 15 miles from his hometown. During the week he slept in a tent and participated in numerous outdoor activities. On August 10, he began having fevers ranging from 101.0 F to 104.0 F (38.3 C to 40.0 C) with rigors and sweats.

Epidemiologic Investigation

No other unexplained cases of malaria were reported to NYSDH during July 1--August 31, 1999. To identify potential unreported cases, a field investigation was conducted that included 1155 telephone interviews with boys who attended the camp, members of their families and the camp staff, and interviews with residents living within 1 mile of the camp. Sixty-three of 375 boys who attended the camp and members of their families who were interviewed reported having a fever during the defined time period. Fourteen of these persons had unexplained fevers; however, no malaria parasites were shown on peripheral blood smears on any of these persons. Two of the approximately 150 residents who lived within a 1-mile radius of the camp who were interviewed reported a fever during the specified time period. No malaria parasites were shown on their peripheral blood smears. Of 52 farm workers interviewed who had immigrated from Mexico, Guatemala, Honduras, El Salvador, and Bangladesh and who resided in three farms near the camp, three reported a recent history of fever; their blood smears did not reveal parasites.

Entomologic and Environmental Investigation

Routine mosquito trapping by the Suffolk County health department for eastern equine encephalitis during early August (the time these cases would have been transmitted) from sites 7 miles from the summer camp yielded Anopheles quadrimaculatus and An. punctipennis. Trapping from the campsite in eastern Long Island from August 24 to 31 yielded primarily An. quadrimaculatus and a few An. punctipennis. No mosquitoes (222 of 248 were tested) from the campsite or the boys' hometowns tested positive for Plasmodium species. Mosquito control measures to kill larvae and adults were performed at the camp. The adjacent state park was closed temporarily by the health department until surveillance indicated low numbers of mosquitoes.

Reported by: CB Bradley, MD, MH Zaki, MD, DG Graham, MD, M Mayer, MD, V DiPalma, MSN, SR Campbell, PhD, S Kennedy, Suffolk County Dept of Health Svcs, Hauppauge, New York. MA Persi, DO, Dept of Preventive Medicine, State Univ of New York at Stony Brook, Stony Brook, New York. A Szlakowicz, MA, P Kurpiel, J Keithly, PhD, J Ennis, P Smith, MD, State Epidemiologist, New York State Dept of Health. O Szlakowicz, Mayo School of Medicine, Rochester, Minnesota. Malaria Epidemiology Br (proposed), Entomology Br, Biology and Diagnostics Br, Div of Parasitic Diseases, National Center for Infectious Diseases; and an EIS officer, CDC.

Editorial Note:

The two cases presented in this report represent the third episode of possible mosquito-borne malaria in New York during the preceding 7 years (4,5,7) and the 24th episode in the United States since 1985. The possibility of autochthonous (i.e., locally acquired) mosquito-borne malaria transmission in the United States remains a concern because of the frequency of international travel, the presence of gametocytemic persons (i.e., persons with malaria parasites in the blood stream that can infect mosquitoes) in the United States, the presence of competent mosquito vectors, and the occurrence of environmental conditions that favor transmission. This investigation confirmed two epidemiologically linked cases of P. vivax infection in children residing and camping in Suffolk County, who probably acquired their infections in eastern Long Island through the bite of one or more locally infected Anopheles mosquitoes, a competent vector for malaria.

Neither patient had risk factors for the acquisition of malaria infection, such as travel to a disease-endemic area or history of intravenous drug use. Neither had ever had a blood transfusion or organ transplantation. Other potential sources of infective mosquitoes, such as international airports, were too distant from the presumed site of infection. However, Anopheles mosquitoes were identified in the recreational area that both patients had visited during the month of August 1999. In addition, potentially gametocytemic persons were living near this recreational area, and environmental conditions were suitable for the development of the parasite in the mosquito (sporogonic cycle) and larvae into adult mosquitoes. Although case finding and contact tracing activities did not identify persons with malaria who might have been the source of the infection, this does not preclude local transmission, which may have occurred weeks before the investigation.

Suffolk County is one of the most heavily mosquito-infested areas in the northeast. In 1999, the northeastern United States experienced one of the warmest and driest summers in history (8). However, heavy rainfall shortly before the two boys arrived at the camp may have resulted in a large population of adult female mosquitoes. Dry weather followed by heavy rains, in addition to resulting in conditions conducive for mosquito breeding, could have reduced the mosquito predator population.

Gametocytemic persons still may be present in the community and constitute a potential reservoir for future episodes of mosquito-borne malaria. Thousands of travelers return to the United States each year from areas where malaria is endemic, and many fail to take adequate chemoprophylaxis. Reintroducing malaria transmission on a small scale in selected areas in the United States is possible. This cluster underscores the need for ongoing surveillance for vector-borne diseases, including malaria. Prompt recognition and adequate treatment of malaria, including improved access to diagnosis and treatment for migrant populations, rapid reporting of malaria cases to public health authorities, and implementation of appropriate control measures, are indicated. Finally, malaria should be considered in the differential diagnosis of illness in any patient with unexplained fevers, regardless of travel history.

During the summer months, persons should follow personal protective measures that reduce contact with potentially infective mosquitoes. These include the use of protective clothing and insect repellants, and sleeping in screened or air-conditioned enclosures. Repellant products containing N,N-diethylmetatoluamide (DEET) are more effective than other compounds.

References

1. Wernsdorfer WH, McGregor I. Malaria: principles and practice of malariology. Edinburgh, New York: Churchill Livingstone, 1988:2 v. (xv, 1818).
2. CDC. Probably locally acquired mosquito-transmitted Plasmodium vivax infection---Georgia, 1996. MMWR 1997;46:264--7.
3. CDC. Mosquito-transmitted malaria---Michigan, 1995. MMWR 1996;45:398--400.
4. Zucker JR. Changing patterns of autochthonous malaria transmission in the United States: a review of recent outbreaks. Emerg Infect Dis 1996;2:37--43.
5. Layton M, Parise ME, Campbell CC, et al. Mosquito-transmitted malaria in New York City, 1993. Lancet 1995;346:729--31.
6. Brook JH, Genese CA, Bloland PB, Zucker JR, Spitalny KC. Malaria probably locally acquired

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