CARACAS, Jan. 6 (Xinhua) -- Cases of West Nile virus have been found in areas north of Venezuela's Orinoco River, and researchers suspected that the virus may have been brought in by migratory birds.

    A joint U.S.-Venezuela study found birds and horses in seven Venezuelan states carrying antibodies to West Nile virus, which means the virus has been transmitted by migratory birds to the animals, said Juan Carlos Navarro, a researcher at Central University of Venezuela.

    Navarro ruled out the possibility of a pandemic of West Nile virus in South America, given that most inhabitants in the region have either been vaccinated against yellow fever or have obtained immunity to the virus after contracting Dengue Fever.

    Medical experts are investigating whether there are human cases of the virus, whose symptoms may be mistaken for those of Dengue Fever, according to local media.

    The first case of West Nile, a tropical disease carried by mosquitoes, was found in Uganda in 1937. The disease typically causes mild flu-like symptoms, but can also induce inflammation of the brain and spinal cord or the loss of function of limbs, and even death.

    The first confirmed West Nile case in South America was found in 1999 in the United States. The virus has spread rapidly in recent years in the region and has so far been found in Mexico, Central America and the Caribbean region.