Mosquito-Transmitted Malaria -- Michigan, 1995

During the 19th and early 20th centuries, malaria was endemic in many areas of the United States. Although indigenous transmission was interrupted by the 1940s (1), recent outbreaks in New Jersey, New York, and Texas have underscored the potential for reintroduction of mosquitoborne transmission of malaria in the United States (2-4). This report summarizes the investigation of a case of Plasmodium vivax malaria diagnosed during September 1995 in a resident of Michigan with no history of international travel; the findings of the investigation indicated that the route of transmission was probably through the bite of a locally infected Anopheles spp. mosquito. Case Investigation

On September 3, 1995, a 31-year-old man was hospitalized because of a 5-day history of fever, chills, sweats, and headache. On physical examination, the only abnormality identified was a temperature of 103 F (39.4 C). A complete blood count on admission identified neutropenia (white blood cell count: 3100/mm3 {normal: 4000-10,000/mm3}) and thrombocytopenia (platelet count: 69,000/mm3 {normal: 150,000-450,000/mm3}). On September 6, examination of the patient's peripheral blood smear identified intracellular red blood cell parasites consistent with Plasmodium spp. The diagnosis of P. vivax infection was confirmed by slide examination, serologic tests, and DNA amplification using polymerase chain reaction. The patient responded to treatment with chloroquine and primaquine and was discharged on September 9.

The patient had no history of travel outside the United States, receipt of blood transfusions, injecting-drug use, or previous malaria infection. He lived in a suburban area, approximately 5 miles from the Detroit Metropolitan-Wayne County Airport. The patient cited three locations where he had been outdoors at night and could have been exposed to anopheline mosquitoes: a rural campground in southeast Michigan where he slept outdoors on August 18-19, a suburban golf course south of Detroit where he played golf regularly in the evening, and his backyard.

On September 19, adult female A. quadrimaculatus and A. punctipennis, both competent vectors for malaria, were recovered from dry ice-baited CDC light traps placed overnight at the campground. In addition, anopheline larvae were identified in a small swamp 10-15 meters from the site where the patient had camped. No adult or larval anophelines were recovered from the golf course or the yard. Weather data for three cities in the area indicated that the average evening temperature in August 1995 was 75.6 F (24.2 C), exceeding the 30-year average by 5.9 F (3.3 C). Active Case Detection and Investigation

A survey of laboratories, infection-control practitioners, infectious disease physicians, and local health departments was conducted to identify all cases of malaria diagnosed by physicians in residents of the area during June 1, 1995-September 19, 1995. Because the patient's campsite was adjacent to an automobile racetrack visited by persons from states in the surrounding region, the survey included 16 counties in southern Michigan, three in northwestern Ohio, and three in northeastern Indiana. The survey identified 10 additional cases of malaria that had been diagnosed in persons living in Michigan and two in Indiana; all 12 of those persons had histories of recent travel to malaria-endemic countries. The species identified were P. vivax (six cases), P. falciparum (five), and P. malariae (one). Only two of the 10 additional cases in Michigan had been reported to public health authorities by September 19.

Possible sources of infection for the case described in this report also included infected anopheline mosquitoes inadvertently transported to Michigan on aircraft ("airport malaria") (5) and unrecognized or unreported malaria infections among recent immigrants, migrant workers, and travelers from malaria-endemic countries. Follow-up investigation of the diagnosed malaria cases in the survey area did not establish epidemiologic links with the case in this report. Authorities at the Detroit airport reported that no direct flights into Detroit originate from known malaria-endemic areas. Based on information provided by the U.S. Immigration and Naturalization Service, of the 8736 immigrants to the Detroit metropolitan statistical area in 1994, 42% had arrived from countries with areas of endemic malaria transmission. In addition, an estimated 26,000 migrant farm workers enter Michigan each summer, including some who may have arrived from malaria-endemic areas of Mexico and Central America.

Reported by: J Sunstrum, MD, Oakwood Hospital, Dearborn; D Lawrenchuk, MD, K Tait, MPH, Wayne County Health Dept; W Hall, MD, D Johnson, MD, K Wilcox, MD, State Epidemiologist, Michigan Dept of Community Health; E Walker, PhD, Michigan State Univ, East Lansing. Malaria Section, Epidemiology Br, Div of Parasitic Diseases, National Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The findings of this investigation indicate that the patient probably acquired his malaria infection in Michigan, most likely at the campground. This conclusion is based on at least five factors. First, the patient had no history of foreign travel or other possible exposures identified. Second, he had camped within a few meters of an Anopheles spp. breeding site 11 days before the onset of illness -- a period consistent with the 8-14-day incubation period of P. vivax malaria. Third, the above-average temperatures in southeastern Michigan during August 1995 may have facilitated malaria transmission because warmer external temperatures shorten the reproductive (sporogonic) cycle of the parasite and prolong Anopheles spp. survival (1). Fourth, potential sources of mosquito infection include an average of 30 cases of imported malaria reported annually in Michigan and unrecognized or unreported cases among immigrants, migrant workers, and travelers from malaria-endemic countries. Fifth, because no airline flights arriving at the Detroit airport originated in areas where malaria is known to be endemic, transmission was unlikely to have occurred from infected anophelines inadvertently transported to Michigan on aircraft (5).

In the United States, mosquitoborne malaria has not occurred this far north since 1972. From 1957 through 1994, a total of 76 cases of locally acquired malaria were reported in the United States; of these, P. vivax was the species identified most frequently (59 {80%}) (1). Since 1986, local outbreaks of malaria have been identified in California, Florida, New Jersey, New York, and Texas (2-4,6-8). The outbreaks in California and Florida were associated with rural exposure, while those in New Jersey, New York, and Texas occurred in suburban or urban environments.

Although the investigation in Michigan did not identify any additional locally acquired cases, only 20% of the imported cases identified by active detection efforts had been reported through passive surveillance by the time of the investigation. The detection of local outbreaks -- which could indicate endemic mosquitoborne transmission of malaria -- requires sensitive and timely surveillance. The findings of this investigation underscore the need for enhanced surveillance systems for malaria and the role of laboratory-based case reporting to ensure the prompt identification, reporting, and investigation of all malaria cases. The Michigan Department of Community Health is planning a system of laboratory-based electronic disease reporting for laboratory-confirmed notifiable diseases.

The increasing number of persons in the United States who travel to or immigrate from malaria-endemic areas increases the likelihood of imported cases and locally acquired malaria. Therefore, health-care providers should consider malaria in the differential diagnosis of persons with unexplained fever, particularly during the summer months, initiate appropriate treatment on diagnosis, and promptly report cases to public health officials. Persons can protect themselves from the bites of mosquitoes that transmit malaria and other infectious diseases by using insect repellents containing N,N diethylmethyltoluamide (DEET), wearing long-sleeved clothing, and sleeping in a screened enclosure or under an insecticide-impregnated mosquito net.

References

1. Zucker JR. Changing patterns of autochthonous malaria transmission in the United States: a review of recent outbreaks. Emerging Infectious Diseases 1996;2:37-43.

2. Brook JH, Genese CA, Bloland PB, Zucker JR, Spitalny KC. Malaria probably locally acquired in New Jersey. N Engl J Med 1994;331:22-3.

3. Layton M, Parise ME, Campbell CC, et al. Mosquito-transmitted malaria in New York City, 1993. Lancet 1995;346:729-31.

4. CDC. Local transmission of Plasmodium vivax malaria -- Houston, Texas, 1994. MMWR 1995;44:295-303.

5. Isaacson M. Airport malaria: a review. Bull World Health Organ 1989;67:737-43.

6. 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.

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

8. CDC. Mosquito-transmitted malaria -- California and Florida, 1990. MMWR 1991;40:106-8.

   
   

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