South Texas Sees Rise in Dengue Fever Outbreaks

0 Comments | Family Practice News, Feb 15, 2000 | by Guang-Shing Cheng

Dengue fever continues to creep northward into the United States.

But the most recent outbreak in southern Texas, which has claimed the life of 1 girl an sickened more than 50 others, has not yet convinced epidemiologists that dengue is now endemic in the United States.

Dengue is still very much an August-to-December disease in southern Texas, according to Gary Clark, Ph.D., chief of the Centers for Disease Control and Prevention's dengue branch in San Juan, P.R. The causative arbovirus depends for its transmission on the Aedes aegypti, a household mosquito that feeds during the day and does not travel more than a few hundred feet from where it hatches, Dengue is also transmitted by another species, A. aldopictus, which is also prevalent in the southern United States, but is not as effective a vector.

The incubation time for the disease is anywhere from 3 to 14 days after infection, and the viremic window for a mosquito to pick up the virus is only 5 days.

Given these limitations on transmission, the more likely epidemiologic scenario is that small outbreaks will continue to occur as consequence of people bringing dengue across the border, Dr. Clark said. The southern Texas climate cools considerably in the winter, precluding mosquito activity and, hence, the necessary human-to-human transmission needed to maintain the virus in the U.S. population year-round.

Local transmission of the arbovirus us was increased in frequency and scope during the most recent outbreak, compared with previous ones. But "there have to be more cases with transmission when mosquitoes are biting during dengue season before the disease can be considered entrenched [in Texas].... It's a slow process, but I think it's happening," said Julie Rawlings , M.P.H., an investigator with the Texas Department of Health, Austin.

The outbreak occurred in a region where thousands of people, some on a daily basis, cross the border area between the United States and Mexico. And a preceding hurricane left southern Texas ripe for mosquito breeding, Ms. Rawlings said.

Acting on a tip from Mexican public health officers, who had seen several cases of dengue, Texan health officers began notifying physicians about dengue fever in late July. The peak of the outbreak occurred in August and September; several cases were identified through retrospective chart reviews of patients who were seen in emergency departments or admitted for a febrile illness of unknown origin.

Dr. Carlos Casas, medical director of the Gateway Community Health Center in Laredo, saw several patients with the disease. A long-time physician in the Texas border town, he has seen dengue outbreaks every few years. Most of his patients had recently visited relatives in Nuevo Laredo, the Mexican city right across the border. Some of the patients were from the same household; all were healthy young adults.

He was not surprised to see dengue return, but like other physicians, he is concerned that the steady increase in cases that has characterized successive dengue outbreaks will continue and result in cases of dengue hemorrhagic fever. In early December, a girl in another town died of this rare and devastating consequence of the viral infection.

Officials said that she probably acquired the virus during a visit to Mexico, Pier death is the first fatality due to dengue in Texas, although the e have been other deaths in the United States from imported cases.

As of Jan. 4, there were 55 laboratory-confirmed cases in the most recent outbreak, including 17 locally acquired ones. All were affected by dengue virus serotype 3. The patients ranged in age from 13 to 86 years; the median age was 32.

Since 1980, the border area has experienced several small outbreaks, all associated with outbreaks in adjacent Mexico, where the disease is endemic. In 1980 came the first appearance of dengue in Texas in more than 30 years; there were 63 cases including 23 locally acquired ones. In 1986 there were 17 cases, 8 locally acquired. The last outbreak in 1995 involved 29 cases, 7 of which were locally acquired and were confined to two border counties.

Increased local transmission in the most recent outbreak implies that the disease has become more widespread. Local transmission has been reported in five counties, including nonborder counties. One patient with no travel history was identified 50 miles from the Texas-Mexico border, which is a "remarkable finding, because we haven't seen [dengue] that far from the border," said Richard Campman, Ph.D., an epidemiologist with the Texas Department of Health in Harlingen.

"There's the possibility that the virus has been present here for some time... Physicians only find what they look for," Dr. Campman added. Until this outbreak, dengue fever surveillance in the area was dependent on voluntary physician reporting. Now the public health department is conducting active surveillance for cases.

"I wouldn't be surprised if [dengue] is endemic," said Dr. Diane Griffin, who does research on arboviruses at the Johns Hopkins University School of Public Health, Baltimore. "It's only a matter of time" before the virus is firmly established in the states where there are Aedes mosquitoes.