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Local Transmission of Plasmodium vivax Malaria -- Houston, Texas, 1994

Malaria was endemic in the United States until the late 1940s; since then, most cases of malaria reported in the United States has been acquired during international travel or has occurred in persons who had resided in countries where malaria is endemic. This report summarizes the investigation of three persons who acquired Plasmodium vivax infection in Houston, Texas, by presumed mosquitoborne transmission during 1994. Case Reports

Case 1. On July 8, a 62-year-old man was hospitalized with an 8-day history of fever chills, sweats, and vomiting. His temperature on admission was 104.0 F (40.0 C). P. vivax parasites were identified on a blood smear on July 11. The patient recovered after treatment with chloroquine and primaquine.

Case 2. On July 18, a 37-year-old man sought care in an emergency department at another hospital because of a temperature of 102.8 F (39.3 C) and a 3-week history of nausea, vomiting, fever, chills, sweats, headache, and shortness of breath. P. vivax parasites were identified on a routine peripheral blood smear on July 18. He recovered after treatment with chloroquine; although primaquine was not initially prescribed, he received it during the investigation in August.

Case 3. On December 4, a 50-year-old man was admitted to the same hospital as in case 2 because of altered mental status, fever, and headache of 2 weeks' duration; his temperature on admission was 100.0 F (37.8 C). P. vivax parasites were identified on a routine peripheral blood smear on December 6. He recovered after treatment with chloroquine and primaquine. He had had similar symptoms with onset during late July and early August and had been admitted to two different hospitals during August. During the second hospitalization, viral meningitis was presumptively diagnosed; evaluation included one thick blood smear on August 23 (which was reported as negative for malaria parasites), and acute and convalescent immunoglobulin M enzyme-linked immunosorbent assay titers for St. Louis encephalitis (both titers were 1:10). The blood smears from August 23 were unavailable for review. However, tests of serum specimens from the August and December hospitalizations for malaria antibody by an indirect immunofluorescent assay were positive for P. vivax (titer of 1:64 on August 23, 1:256 on August 30, and 1:256 on December 6). These results indicate P. vivax malaria infection before December, and that the December episode most likely was a relapse from dormant liver stages (hypnozoite), which result only from mosquitoborne inoculation with sporozoites and not from person-to-person transmission (e.g., through blood transfusions or injecting drugs). Case Investigations

Case-patients 2 and 3 had never traveled outside of the United States; case- patient 1 had traveled outside the United States only before 1956. None had a history of blood transfusions, tattoos, malariotherapy for Lyme disease, recent injecting-drug use, or previous malaria infection. They lived within a 3-mile radius, were not acquainted, and had not been in the same locations. However, all had prolonged nighttime exposure to mosquitoes, either through working outdoors at night or sleeping in housing without window panes and/or with unscreened windows and doors. They lived 10 miles from the nearest international airport, and there are no prevailing winds in Houston that would carry anophelines beyond their maximal flight range of 1-2 miles (1). Active Case-Finding

Medical record reviews at all clinical laboratories and hospitals and contacts with infectious disease physicians identified 21 additional malaria patients in Houston and Harris County during June 1-August 22. At the time of the investigation, four (19%) of these patients had been reported through the existing passive surveillance system; 17 (81%) were identified by contacting laboratories in the Houston area. All 21 had traveled to countries where malaria is endemic; however, two of the 21 had visited only parts of northern Mexico where malaria transmission has not been reported. Of the 24 total patients, 10 (including cases 1-3) were infected with P. vivax; three of the 10 were treated with chloroquine only and had not received primaquine to prevent a relapse infection.

The Harris County Mosquito Control District identified adult female Anopheles quadrimaculatus, a competent vector of malaria, in mosquito traps placed near the residences of patients 1 and 2 on August 4. Although possible breeding sites were identified near these residences, mosquito larvae were not found. Rainfall was below average during July-August, and many potential breeding sites were dry. Reported by: R Bell, PhD, J Cousins, W McNeely, MPH, P Rogers, PhD, A Payne, DrPH, M desVignes-Kendrick, MD, Houston Dept of Health and Human Svcs; J Billodeaux, R Jones, Harris County Mosquito Control District, Houston; J Taylor, MPH, K Hendricks, MD, J Perdue, Bur of Communicable Disease Control, D Simpson, MD, State Epidemiologist, Texas Dept of Health. Div of Field Epidemiology, Epidemiology Program Office; Div of Parasitic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The findings of the Houston investigation indicate that the P. vivax infections for patients 1-3 most likely were acquired locally (in Houston) as the result of mosquitoborne transmission. The course of illness in case 3 strongly supports mosquitoborne transmission and possible secondary transmission. Airport malaria (i.e., inadvertent transportation of infective anophelines on airplanes) is unlikely.

This cluster of patients with locally acquired P. vivax malaria in an urban setting occurred 1 year after identification of an outbreak of locally acquired P. falciparum infection in New York City (M. Layton, New York City Department of Health, personal communication, 1994). Local transmission in densely populated areas represents a change in the epidemiologic pattern of malaria: until 1991, when local transmission was reported in a suburban area of New Jersey (2-4), local transmission had occurred predominantly in rural areas.

Although malaria is a notifiable disease in all states, only seven (29%) of the 24 cases identified in this investigation had been reported to the health department in Houston. The lack of reporting of and information about these cases delayed the investigation and efforts to identify other possible locally acquired cases. For example, the two cases in persons who had traveled only to northern Mexico may have been either imported or locally acquired; however, because they had not been reported, they were not investigated promptly. In addition, although most hospital laboratories have the capacity to conduct malaria smear examinations, limitations in the experience of staff may decrease the likelihood of detection.

To improve surveillance of all notifiable conditions, the Texas Department of Health has begun an educational campaign and is implementing an enhanced toll-free telephone reporting system aimed at all health-care practitioners; in addition, the Houston Health Department has distributed newsletters to physicians and infection-control practitioners informing them of the locally acquired cases, the proper treatment for cases, and the importance of reporting. The Harris County Mosquito Control District will enhance vector surveillance for anopheline vectors, which will be linked to active malaria case detection this summer.

Malaria continues to be a leading cause of morbidity and mortality worldwide, particularly because of the development of drug-resistant strains, and is a continuing concern in the United States because of increased international migration, travel, and commerce. The basic requirements for local transmission of malaria- -including persons (who may or may not be ill) with malarial gametocytes in their blood (as was documented in Houston), competent vectors, and conducive weather conditions -- exist in many areas of the United States. Important strategies for preventing the re-establishment of malaria as an endemic disease in the United States are prompt recognition and reporting of cases of malaria; appropriate treatment of all malaria cases, including primaquine for P. vivax and P. ovale infections to prevent relapse; and implementation of appropriate control measures.


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

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

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

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

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