Dengue Virus Spreads North to Texas

CHAPEL HILL, North Carolina, June 27, 2003 (ENS) - A new study from the University of North Carolina describes the emergence and spread of a virulent form of dengue virus from the Indian subcontinent to Latin America, including Mexico.

Dengue (pronounced den-GAY) includes four distinct viruses, all transmitted via the bite of Aedes mosquitoes, two of which, Aedes albopictus and Aedes aegypti, are common in the southeastern United States.

The study, which appears online this week in the journal “Emerging Infectious Diseases,” a publication of the U.S. Centers for Disease Control and Prevention, used viral genetics to demonstrate the global movement of a virus associated with severe disease.

Given that the major mosquito vectors of dengue are well established here, there is concern that dengue viruses could be introduced into the United States and transmitted to people, said Dr. Aravinda de Silva, assistant professor of microbiology and immunology at University of North Carolina School of Medicine and senior author of the study.

de Silva

Dr. Aravinda de Silva is assistant professor of microbiology and immunology at University of North Carolina School of Medicine (Photo courtesy )
A handful of cases have been documented in south Texas, close to the Mexican border, but may not spread farther.

"I don't think dengue would ever become a serious problem in the United States because humans are the main reservoir of this virus," de Silva said.

The particular dengue virus subtype studied by de Silva and his team has been responsible for epidemics of deadly dengue hemorrhagic fever in Sri Lanka, East Africa and Latin America.

Dengue virus is characterized by internal hemorrhaging that sometimes leads to shock - a drop in blood pressure and failure of blood cells to meet the metabolic demands of the body

Dengue virus is a leading cause of death among children in Southeast Asia.

Mosquitoes become infected with dengue after biting a dengue infected person. People infected with dengue virus develop dengue hemorrhagic fever or the less serious dengue fever.

Dengue fever is also known as breakbone disease because of severe headache and joint pain associated with it. In recent years hemorrhagic dengue has become increasingly prevalent in tropical America.

mosquito

Aedes aegypti mosquito on human skin (Photo courtesy Pagina Zero)
"Historically, illness associated with the dengue serotypes native to the Americas has been very mild, and the virus that was responsible for hemorrhagic fever in this region was a dengue serotype 2 strain introduced from Southeast Asia," he said.

A serotype is a group of closely related microorganisms distinguished by a characteristic set of antigens such as toxins or bacteria.

"It was, therefore, surprising to learn that the spontaneous outbreaks of dengue hemorrhagic fever in Nicaragua and Panama in 1994 were caused by dengue serotype 3," Dr. de Silva said.

Although dengue serotype 3 had been identified in the Americas more than 25 years ago, illness linked to it had been mild, he said.

"So those outbreaks associated with hemorrhagic fever meant one of two possibilities - either the native virus had mutated to become more severe or dengue serotype 3 from another part of the world had been introduced into Latin America," said de Silva.

The answer came from research that found the virus had moved across the globe to Latin America. The new study describes the most likely route of spread and the relationship to virus strains established in Asia and Africa.

"Our original intention had been to study how dengue hemorrhagic fever suddenly emerged in South Asia in the late 1980s," said Dr. William Messer, a doctoral graduate of the university’s ecology department.

Hemorrhagic dengue fever has continued to be a public health problem in Thailand, Indonesia and Malaysia since the 1950s, Messer said. All four dengue serotypes have been documented there.

"It was puzzling that all four serotypes also circulated in South Asia, in Sri Lanka and parts of India, yet dengue hemorrhagic fever was extremely rare," Messer said.

Then, abruptly, beginning in 1989, dengue hemorrhagic illness became common in Sri Lanka.

The researchers then wanted to find out if the virus had changed. They isolated dengue virus from pediatric patients in Sri Lanka. In those with hemorrhagic disease, serotype 3 was prevalent. Patients with the milder disease, dengue fever, showed infection with the other serotypes.

patient

Patient with dengue hemorrhagic fever in Southeast Asia World Health Organization (Photo courtesy )
"We then focused on serotype 3, collecting virus isolates from Sri Lanka and other parts of the world," de Silva said.

"Messer found that serotype 3 from Sri Lanka formed two genetically distinct groups. In one group were viruses of serotype 3 that were related to dengue disease prior to 1989, while the second group contained only viruses related to disease after 1989."

These findings provided the clue that a genetic change in the dengue serotype 3 virus circulating in Sri Lanka was responsible for the unexpected emergence of severe disease, de Silva said.

The researchers then genetically sequenced dengue virus isolated from Latin America and compared them to isolates from South Asia.

"We found that the group of Latin American isolates linked to severe disease were more closely related to the same isolates associated with dengue hemorrhagic fever in Sri Lanka," Messer said. "And that virus has been isolated all through Latin America and associated with dengue hemorrhagic fever from Mexico down to Brazil."

Unlike the West Nile virus, which is difficult to control because birds are the main reservoir, human-mosquito exposure is a lot less here because so many people live in air conditioned housing, de Silva said

“I doubt you can have the frequent, year round human-mosquito transmission required for establishment of the virus that occurs in poorer countries.”

A report of the research can be found online at: http://www.cdc.gov/ncidod/EID/vol9no7/03-0038.htm.