WASHINGTON, Aug 9 (Reuters) - Doctors and health officials in parts of south Texas should be on the lookout for dengue fever, a tropical virus that can cause internal bleeding and death, the U.S. Centers for Disease Control and Prevention said on Thursday.

They found an unexpected number of people had apparently been infected with the mosquito-borne virus in a survey launched after a Brownsville, Texas, woman became seriously ill.

"Health authorities along the Texas-Tamaulipas border should consider strengthening surveillance for dengue fever, given the potential for future outbreaks," the CDC said in its weekly report on death and disease.

Only a few hundred cases of dengue are reported in the United States each year, almost all of them imported. But the findings suggest the virus may occasionally be found inside U.S. borders, the CDC said.

The report describes the case of a Mexican-born woman who had lived in the border town of Brownsville for 16 years. In June 2005 she developed dengue and was treated in both Mexico and Texas.

A few months later, health officials in Mexico's Tamaulipas state, which borders that part of Texas, reported an outbreak of dengue fever.

The CDC and Mexican health authorities tested the blood of hundreds of residents on both sides of the border.

They found 1,200 Mexican cases of dengue-like fever, and 18 percent of these were dengue hemorrhagic fever -- the most dangerous form that includes bleeding.

"In Cameron County, Texas, where Brownsville is the county seat, (Texas health department officials) identified 24 additional cases of laboratory-confirmed dengue fever, including two additional cases of locally transmitted dengue and 22 cases associated with travel to Mexico," the CDC reported.

Blood tests of 141 local residents showed 51 had evidence of having been infected with dengue virus at some point and at least six had never left the United States, the CDC said.

"Early recognition and diagnosis of dengue hemorrhagic fever and careful fluid management can reduce the case fatality rate in cases with shock to less than 1 percent," the CDC said.

Dengue is diagnosed from symptoms such as fever and muscle ache, bleeding from the eyes or skin and certain blood tests. Globally, it infects 50 to 100 million people every year and 5 percent of those who get the hemorrhagic form die, usually from shock.

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