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Multifocal Autochthonous Transmission of Malaria --- Florida, 2003

The majority of malaria cases diagnosed in the United States are imported, usually by persons traveling from areas where malaria is endemic (1). However, small outbreaks of locally acquired mosquito-borne malaria continue to occur (2--4). During July--September 2003, an outbreak of malaria (eight cases of Plasmodium vivax malaria) occurred in Palm Beach County, Florida (5). During the same period, two patients were evaluated for malaria in neighboring Okeechobee County, approximately 75 miles from the Palm Beach County transmission area. One patient was thought to have acquired infection with the same parasite species (P. vivax), and concerns were raised about a possible link. To determine whether infection was acquired in Okeechobee County and whether a possible link existed to the Palm Beach County outbreak, the Florida Department of Health (FDOH) initiated an investigation. This report describes that investigation, which determined that although initial laboratory results suggested local transmission, subsequent evaluation and testing confirmed the case as imported malaria. These findings underscore the importance of a rapid and thorough investigation of any malaria case suspected to be acquired through local mosquito-borne transmission.

In August 2003, two men were evaluated for malaria at an Okeechobee County clinic after 4 days of fever, chills, myalgias, fatigue, nausea, and headache. The first patient was a native of Uganda (patient 1) who had arrived in the United States 2 months before onset of symptoms; the second patient was a U.S.-born Florida resident (patient 2) with no recent foreign travel to areas where malaria is endemic or other risk factors for malaria. Both patients were treated presumptively for symptoms with doxycycline and diclofenac, and thick and thin blood smears were obtained for testing. A private laboratory identified P. vivax on the smear from patient 1; no malaria parasites were identified on the smear from patient 2. Because of increased malaria awareness from the Palm Beach County outbreak, smears for both patients were forwarded to the Florida State Laboratory (FSL) and CDC for confirmation.

After microscopic examination of both smears, FSL and CDC observed that the smears had been prepared poorly. However, malaria parasites were confirmed on the smear labeled as collected from patient 2 rather than patient 1, and P. vivax was identified as the most likely species. Because neither patient reported travel to the Palm Beach County transmission area, investigators considered the possibility of local mosquito-borne transmission in Okeechobee County.

Discrepancies in the smear results reported by the private laboratory, FSL, and CDC prompted investigators to suspect the smears were switched en route to FSL. An audit was conducted to trace the positive smear to the correct patient. In addition, blood smear microscopy, serology, and polymerase chain reaction (PCR) were conducted on specimens drawn from both patients after treatment was started. The audit revealed that the positive smear originated from patient 1. No evidence of previous or current infection was confirmed in specimens for patient 2. For patient 1, serology confirmed either previous or recent infection with malaria, and PCR revealed current infection with P. ovale, not P. vivax. These conflicting results prompted a review of the original microscopic diagnosis of P. vivax. Because whole blood specimens from the original positive smear were not available, PCR analysis of material scraped from the original positive blood smear confirmed infection with P. ovale as the diagnosis for patient 1.

In September, additional case finding was initiated to determine whether local mosquito-borne transmission occurred in Okeechobee County. Medical charts were reviewed for 232 patients with unexplained febrile illness during the 2 weeks before symptom onsets for patients 1 and 2; no other cases of malaria were found. After P. ovale was identified in patient 1, whose only risk factor was recent travel to malaria-endemic Uganda, investigators concluded this was a case of imported malaria, and measures to control local transmission of malaria were decreased. Patient 1 received additional treatment with primaquine to prevent relapse of P. ovale. Education materials about recognizing symptoms of malaria and preventing mosquito-borne diseases were distributed to neighbors of patient 1. Patient 2 recovered from his symptoms without further treatment, and a nonspecific viral syndrome was diagnosed.

During September--October, Okeechobee County physicians continued to evaluate patients with febrile illness for malaria; no additional cases were found. Mosquito trapping and testing in the county confirmed the presence of competent vectors (Anopheles sp.), but no mosquitoes tested positive for malaria.

Reported by: C Selover, C Thacker, M Hill, MPH, Okeechobee County Health Dept, Okeeechobee; F Lugo, MPH, Palm Beach County Health Dept, West Palm Beach; M Lo, MPH, M Pawlowicz, PhD, J Schulte, DO, C Blackmore, DVM, D Ward, L Crocket, MD, Florida Dept of Health. LM Causer, MBBS, M Parise, MD, J Barnwell, PhD, A DaSilva, PhD, Div of Parasitic Diseases, National Center for Infectious Diseases; DF Argüello, MD, S Filler, MD, EIS officers, CDC.

Editorial Note:

Approximately 60 cases of malaria are reported in Florida annually; nearly all are imported. Since the 1950s, only three outbreaks of local mosquito-borne transmission (Bay County 1990, one case of P. vivax; Palm Beach County 1996 and 2003, two and eight cases of P. vivax, respectively) have been reported (5--7). Because of its climate, proximity of human and mosquito populations, and the frequent travel of its residents and visitors from malarious areas, Florida often is perceived as vulnerable to the reestablishment of endemic malaria. However, endemic malaria was eradicated from Florida since the early 1950s, and reestablishment of endemic malaria is unlikely as long as present socioeconomic conditions and current health-care, mosquito-control, and public health infrastructures remain intact (2,4). Nevertheless, conditions exist for small outbreaks of locally acquired mosquito-borne transmission to occur sporadically.

This investigation demonstrates the need for proper smear preparation and accurate microscopic evaluation in confirming a case of malaria suspected as acquired through local mosquito-borne transmission. Errors in specimen handling and improper smear preparation complicated the process of identifying infection of patients and deciding whether control efforts were needed to prevent local mosquito-borne transmission. Investigators relied on additional laboratory methods (i.e., PCR and serology) to determine that the Okeechobee County case was not related to local mosquito-borne transmission.

Although additional testing was required to confirm diagnosis in the Okeechobee case, microscopic examination of thick and thin blood smears remains the standard for malaria diagnosis. Once a case of malaria is confirmed microscopically, a rapid and thorough case investigation must ensue to establish whether acquisition occurred locally or abroad. Control efforts must proceed on the basis of smear results and epidemiologic evidence and should not be delayed for additional tests. Local and state health officials should recognize that a patient who has malaria with no risk factors for malaria should be considered to have acquired the infection locally through mosquito-borne transmission until proven otherwise and investigated immediately.


  1. CDC. Malaria surveillance---United States, 2001. In: CDC Surveillance Summaries (July 18). MMWR 2003;52(No. SS-5).
  2. Zucker JR. Changing patterns of autochthonous malaria transmission in the United States: a review of recent outbreaks. Emerg Infect Dis 1996;2:37--43.
  3. CDC. Local transmission of Plasmodium vivax malaria---Virginia, 2002. MMWR 2002;51:921--3.
  4. Humphreys M. Malaria: Poverty, Race, and Public Health in the United States. Baltimore, Maryland, and London, England: Johns Hopkins University Press, 2001.
  5. CDC. Local transmission of Plasmodium vivax malaria---Palm Beach County, Florida, 2003. MMWR 2003;52:908--11.
  6. CDC. Mosquito-transmitted malaria---California and Florida, 1990. MMWR 1991;40:106--8.
  7. CDC. Malaria surveillance---United States, 1996. In: CDC Surveillance Summaries (March 30). MMWR 2001;50(No. SS-1).

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