Malaria --- Great Exuma, Bahamas, May--June 2006

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Malaria in humans is caused by four distinct protozoan species of the genus Plasmodium (P. falciparum, P. vivax, P. ovale, and P. malariae). These parasites are transmitted by the bite of an infective female Anopheles mosquito (1). In the Caribbean region, malaria has been eliminated from all islands except Hispaniola, the island consisting of Haiti and the Dominican Republic. Elimination of malaria elsewhere resulted from a combination of integrated control measures, socioeconomic development, and close public health surveillance. However, even Caribbean islands where malaria is no longer endemic remain at constant risk for reintroduction of the disease because of their tropical climate, presence of competent malaria vectors, and proximity to other countries where malaria is endemic. This susceptibility was underscored by the recent outbreak of malaria on the island of Great Exuma in the Bahamas; during May--June 2006, a total of 19 malaria cases were identified. Four of the cases, in travelers from North America and Europe, are described in this report; such cases of imported malaria can signal the presence of a malaria problem in the country visited and thus assist local health authorities in their investigations. On September 19, after 3 months with no report of new cases, CDC rescinded its previous recommendation that U.S.-based travelers take preventive doses of the antimalarial drug chloroquine before, during, and after travel to Great Exuma.*

Case 1. On May 24, 2006, a man aged 33 years from the United States received a diagnosis of malaria in a hospital emergency department in Virginia. The patient had intermittent fever, sweats, abdominal discomfort, nausea, and vomiting, which had begun during a May 4--7 visit to Great Exuma, where the patient had stayed in a resort hotel. The patient had no history of exposure to malaria. Blood smears on May 24 indicated P. falciparum. After outpatient treatment with chloroquine, changed later to quinine and doxycycline, the patient recovered uneventfully.

Case 2. On June 6, a woman aged 29 years from Germany received a diagnosis of P. falciparum malaria in a hospital in Germany. She had experienced fever, headache, nausea, and vomiting since May 30, near the end of a May 18--31 visit to Great Exuma. After her return to Germany, the woman was treated initially with antibiotics for suspected sinusitis. However, her illness persisted, and she was hospitalized on June 6 with high fever and neck stiffness. Diagnostic tests included magnetic resonance imaging of her head, a lumbar puncture to exclude meningitis, and a blood smear that revealed P. falciparum. She was treated with artemether-lumefantrine and recovered.

Case 3. On June 16, a man aged 20 years from Canada had P. falciparum malaria diagnosed. The man had been born in the Bahamas and had visited friends and relatives there during April 19--June 11, spending most of his time in Georgetown, the most populous city on Great Exuma. On June 14, the man experienced fever and chills and went to an emergency department for evaluation after learning that his cousin had been treated recently for malaria on Great Exuma. The diagnosis of P. falciparum malaria was confirmed by blood smear on June 16. He was treated on an outpatient basis with chloroquine followed by atovaquone-proguanil and recovered uneventfully.

Case 4. A man aged 66 years from the United States, who lived on a boat, received a diagnosis of P. falciparum malaria on June 19. The man, who had not recently visited any area that was endemic for malaria, stayed in Great Exuma from late April to late May. In early May, he began experiencing fever, chills, sweats, headaches, and fatigue but did not seek medical care; he left Great Exuma to sail to other Bahamian islands. On June 18, on his return to Great Exuma, the patient learned of the outbreak and went the next day to the district medical clinic, where he received a diagnosis of P. falciparum malaria. He was treated with chloroquine and primaquine and recovered uneventfully.

After report of the first case in Virginia, the Bahamian Ministry of Health (MOH) initiated epidemiologic and entomologic investigations with the technical assistance of the Pan American Health Organization. MOH also heightened mosquito-control activities that were already being conducted on Great Exuma in conjunction with the Bahamian Department of Environmental Health Services.

Active case detection was conducted on Great Exuma during June 6--30; however, no case of malaria was diagnosed later than the June 19 diagnosis in case 4. Persons examined at primary-care clinics who had a history of fever and a temperature of >99.0^ºF (>37.2^ºC) and contacts of persons who received diagnoses of malaria were screened using thick and thin blood smears stained with Wright's stain. On Great Exuma, 15 persons were determined infected with P. falciparum. Ages ranged from 16 to 66 years (median: 36 years); 84% were males. Most of these patients were residents of the Bahamas, clustered around the areas of Georgetown and Bahama Sound, and living in close proximity to a community of immigrants from Haiti; most said they had not recently traveled to Haiti or any other area endemic for malaria. All patients were initially treated with chloroquine and doxycyline; the latter was subsequently replaced by primaquine to eliminate gametocytes and thus prevent further transmission. All 15 patients recovered.

A parasite prevalence survey was conducted on Great Exuma in a community of immigrants from Haiti, from which anecdotal reports of illness had been received. Of 159 persons who consented to testing, 29 adults were determined infected with P. falciparum. This finding prompted mass treatment with chloroquine and primaquine of 203 persons within that community.

Entomologic surveys were conducted in multiple sites near bodies of fresh water identified by ground and air surveys in Great Exuma. Human bait and CDC light-trap collections yielded large populations of mosquitoes, of which only five were adult Anopheles albimanus. Surveys of potential breeding sites indicated few areas favorable for breeding of An. albimanus larvae, with five confirmed An. albimanus larvae collected from three breeding sites. Mosquito-control interventions were intensified beginning May 30. These measures included spraying 1) at all potential breeding sites, 2) within a quarter-mile radius of patients with confirmed cases, and 3) within a half-mile radius of patients detected through contact tracing, initially with a water-based pyrethroid insecticide, and later with malathion 96.5%. In addition, all bodies of fresh water on Great Exuma, neighboring Little Exuma, and surrounding cays (reefs) were treated with temephos to eliminate larvae.

As of September 19, no additional cases of malaria had been identified on Great Exuma or any other island in the Bahamas, despite intense epidemiologic surveillance. Mosquito-control measures were being continued throughout the Bahamas.

Reported by: M Dahl-Regis, MD, Ministry of Health, Bahamas. C Frederickson, PhD, Caribbean Epidemiology Centre; K Carter, MD, Y Gebre, MD, Pan American Health Organization, World Health Organization. B Cunanan, Arlington County Dept of Human Svcs, Arlington, Virginia. C Mueller-Thomas, MD, Klinikum rechst der Isar, Munich, Germany. AE McCarthy, MD, Ottawa Hospital--General Campus, Ottawa; M Bodie-Collins, Public Health Agency of Canada. P Nguyen-Dinh, MD, Div of Parasitic Diseases, National Center for Zoonotic, Vector-Borne, and Enteric Diseases (proposed), CDC.

Editorial Note:

The Bahamas is an archipelagic nation in the northern Caribbean Sea, consisting of approximately 700 islands and 2,400 cays stretching between Florida and Haiti (Figure). Persons from Hispaniola and other countries have emigrated to the Bahamas, where malaria is not endemic and only one imported case was reported in 2005. However, because of frequent travel and relocation among countries, health-care providers in the Bahamas and other countries where malaria is not endemic should remain alert to the risk for this disease, especially in travelers and immigrants. Introduced malaria is much less common than imported malaria but of greater epidemiologic significance. Imported malaria usually occurs when travelers acquire the infection while visiting areas where malaria is endemic. Introduced malaria typically occurs when infected travelers return home and transmit the infection to local Anopheles mosquitoes, which subsequently transmit it to local residents. Left unchecked, this process can result in reestablishment of endemic malaria in countries that have previously eliminated the disease because these areas have climatic conditions favorable to transmission and Anopheles species that are receptive to malaria parasites. In the United States, 1,320 cases of imported malaria were reported in 2004 (1), and 63 episodes of introduced malaria were detected from 1957 to 2003, the year when the latest episode occurred in Florida (2--4).

Available evidence indicates that during May--June 2006, Great Exuma experienced an outbreak of introduced malaria that was successfully contained and terminated. The observations that all cases were caused by P. falciparum and a substantial proportion of patients were immigrants from Haiti suggest that malaria was introduced by those immigrants. All patients treated with chloroquine responded to the treatment, which is a further suggestion that the parasites originated from Haiti, where P. falciparum has remained sensitive to chloroquine. P. falciparum causes 99% of malaria cases in Haiti and the Dominican Republic (MD Milord, Ministry of Public Health and Population, Haiti, and JM Puello, National Center for Control of Tropical Diseases, Dominican Republic, personal communication, 2006), which share the only Caribbean island still endemic for malaria. Conversely, P. vivax causes 94% of cases in Mexico and Central America (5).

The successful containment of this malaria outbreak is attributable to several factors. The first identified case, detected in a foreign tourist returning from the Bahamas, was promptly reported to the Bahamian MOH, which responded with several complementary interventions, including identification and treatment of patients and asymptomatic parasite carriers and institution of mosquito-control measures. Fewer than 30 days elapsed between diagnosis of the first identified case in Virginia and diagnosis of the last case on Great Exuma. Since June 19, no additional cases have been noted, despite intensive ongoing surveillance among febrile patients.

In view of these findings, CDC has rescinded recommendations made on June 16, 2006, that travelers take preventive doses of chloroquine before, during, and after travel to Great Exuma. As of September 19, CDC no longer recommends that travelers to Great Exuma take antimalarial prophylaxis.

This malaria outbreak illustrates the importance of vigilance by health-care providers and rapid response by public health authorities for successful containment (2) and also might provide incentive for measures to eliminate malaria from all Caribbean islands, including Hispaniola. Recently, the International Task Force for Disease Eradication recommended that Haiti and the Dominican Republic work jointly to eliminate from Hispaniola both malaria and lymphatic filariasis, two vectorborne parasitic diseases that have been eliminated from all other Caribbean islands (6). Agreements reached in July 2006 between the ministries of health of Haiti and the Dominican Republic represent a first step toward achieving this goal.

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