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Public Health Case Study 1

Palm Beach County, Florida

In Palm Beach County, Florida, during the summer of 2003 a cluster of eight cases of malaria was detected. The background incidence is 60 cases per year (Florida) and 10 cases per year (Palm Beach County).

Blood smear from a patient in the Palm Beach County outbreak.

All patients were infected by the same parasite species shown in the figure.

1. What is that species?

• Plasmodium falciparum
• Plasmodium vivax
• Plasmodium ovale
• Plasmodium malariae

All patients were male. Their ages varied from 17-45 (median 37). None had a history of prior malaria infection, recent history of travel in a malaria-endemic area, blood transfusion or organ transplantation or intravenous drug use. They were from very diverse backgrounds, including one homeless man living near a drainage canal, two undocumented workers, and one affluent banker who liked to golf in the evening). All lived within 10 miles of Palm Beach International Airport.)

2. Among the modes of infection below, which one(s) need to be given the most serious consideration?

• Infection during travel overseas
• Blood transfusion/Transplantation
• Needlestick contamination
• Congenital malaria
• Infection by local Anopheles mosquito
• "Airport malaria"

Epi curve of malaria cases, July - September 2003, Palm Beach County, Florida

The dates of onset of symptoms, between July 12 and September 14, are shown in the epi curve above.

3. Based on that curve and what is known of the biology of P. vivax, the most likely occurrence is that all patients were infected by:

• One individual Anopheles
• Several Anopheles during a short time window (2 weeks or less)
• Several Anopheles during a longer time window (more than 2 weeks)

The microscopic diagnosis of P. vivax was confirmed by PCR. Parasite multilocus genotyping indicated that all parasites were the same strain, further supporting that the 8 infections originated from the same source. In that scenario, a person infected in an endemic country with P. vivax arrived in Palm Beach County and was bitten by local Anopheles mosquitoes, leading to the 2-3 cycles of local transmission in local residents ("autochtonous malaria").

4. What would be the next steps of the epidemiologic investigation? (one or more answers may apply)

• Search for additional cases (including possible index case) by blood surveys in 10% of the population
• Search for additional cases (including possible index case) by review of medical/laboratory records and by surveillance for malaria and FUOs
• Searchfor additional cases (including possible index case) by canvassing community
• Search for additional cases (including possible index case) by screening immigrants and undocumented workers
• Confirm presence of Anopheles in areas where the patients live
• Test local Anopheles for infection with malaria parasites

Surveys of local health facilities and hospital charts, and community canvassing did not identify any additional cases, or index case.

Mosquito traps were set within a one-mile radius of the homes of the patients, and Anopheles quadrimaculatus and An. crucians, both potential vectors of malaria, were found near at least 3 homes (figure). None of the mosquitoes were positive for malaria (by sporozoite ELISA).

Mosquito traps placed near homes of 8 patients; in 3 locations Anopheles mosquitoes were caught

In addition to the investigations described above, the local health authorities:

• conducted sensitization campaigns in local communities, including groups at increased risk of infection (eg, homeless camps)
• intensified surveillance for suspected malaria at local health facilities (including hospitals and physician offices)
• continued existing mosquito control practices, and conducted additional insecticide spraying (both ground and aerial) within a radius of 3 miles around the suspected transmission areas.

Main points

1. In the United States, malaria has been eliminated but it can be reintroduced because travelers and immigrants may have malaria and local Anopheles mosquitoes can transmit malaria.
2. The risk is particularly high in areas where local climatic conditions allow survival of the Anopheles during a large part (or the totality) of the year, and where a large population of travelers or immigrants from endemic areas are found.
3. The most practical way to prevent re-introduction is through vigilant surveillance and investigations of all cases of malaria without known exposure (such as those described here).
4. Once local transmission is identified, aggressive surveys and canvassing of hospitals and communities are needed to identify and treat all cases, and thus interrupt local transmission.
5. Vector control measures (eg insecticide spraying) might play an additional, adjunct role.

This outbreak was published in Local Transmission of Plasmodium vivax Malaria --- Palm Beach County, Florida, 2003 Vol 52, No 38;908 09/26/2003