MEXICO declared a state of emergency last week after confirmation that West Nile virus has reached the country. Doctors and scientists fear that the impact there and in other countries further south could be far worse than it has been in the US and Canada.
The virus is spread by mosquitoes and birds, which could make Latin American countries ideal breeding grounds, because of their warmer climate, large bird populations and year-round mosquitoes. Millions of Mexico鈥檚 poorest people who use horses as pack animals could be hard hit if the animals are stricken by West Nile. And people who become infected with the virus could fare worse than their counterparts in the US and Canada as exposure to other viruses common in the region may have left them at increased risk of disease. Ecologists are worried about the effect on the diverse animal and bird life in the tropics.
West Nile virus is common in Africa, the Middle East and parts of Europe. A particularly virulent strain from Israel somehow reached New York in 1999 and has now spread through much of the US and Canada. Last year in the US, 4156 people were diagnosed with the infection and 284 died from brain inflammation caused by the virus. More than 230 animal species, including 138 birds, are known to have been infected. Horses have been particularly badly affected, with 14,000 animals falling ill last year.
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Now what seems to be the same virulent strain has spread south of the border. When a team led by Barry Beaty of Colorado State University in Fort Collins tested 24 horses from Coahuila state in northern Mexico it found that 15 had antibodies to the virus (Emerging Infectious Diseases, vol 9, p 857). In the Yucat谩n in southern Mexico three horses out of 252 tested positive. This region is a principal landfall for many species of migratory birds and a likely place from which the virus could spread throughout Latin America.
A prolonged drought in most of Mexico at the moment is probably helping spread the disease. As ponds and puddles shrink they become richer in nutrients, making them ideal places for mosquitoes to breed. They would be further helped by reduced numbers of predators such as frogs, which succumb to the heat. The high temperature also speeds up the life cycle of West Nile virus within mosquitoes. To add to this, birds may congregate near the few remaining water sources, making it more likely that they will be bitten by mosquitoes. The severe droughts of 1999 and 2002 in the US were linked with 鈥渢remendous amplification and spread of this disease鈥�, says Paul Epstein of the Center for Health and the Global Environment at Harvard Medical School.
By last week, West Nile had spread to six states, according to the Mexican government. It says that widespread vaccination will be needed to protect the horses, mules and donkeys that people depend on as beasts of burden. Animals infected with the virus may get fever, become drowsy and develop walking problems. Beaty says that West Nile would have 鈥渁 major impact鈥� on a family if their horse or mule becomes ill and dies.
Horses are the only animals for which there is a vaccine against West Nile, and immunisation is starting to be used more widely in the US. But in remote parts of Latin America vaccinating horses will not be easy. Animals have to be given two shots 3 to 6 weeks apart, and immunity does not develop until several weeks later. In areas where mosquitoes are a year-round menace, animals need a booster shot every 6 months.
Another big question is whether people in Latin America will be hit harder by the virus than people in the US and Canada. This is not simply because these are poorer countries with less effective healthcare systems. West Nile is a flavivirus, and there are concerns that people鈥檚 immune response to it will be affected by previous encounters with other flaviviruses endemic to the region. Many people there already carry antibodies to dengue, yellow fever and St Louis encephalitis viruses. While it is possible that these might provide some cross-immunity to West Nile, it is also known that infection with one strain of dengue virus can be worse after a previous infection with another dengue strain. This is the result of a misplaced immune system response known as immune enhancement. 鈥淲e have no idea how West Nile is going to present itself in the presence of all these antibodies,鈥� says Beaty. 鈥淚f we are lucky, it will be somewhat protective. [But] it might even be a more severe disease.鈥�
Ecologists are worried about the region鈥檚 bird life. 鈥淲e suspect that the virus was introduced into the Yucat谩n by migratory birds,鈥� says Beaty. His team has found that many species of migratory songbirds in the Yucat谩n are infected.
West Nile has also spread to the Caribbean, it was announced this month. Peter Marra of the Smithsonian Environmental Research Center in Edgewater, Maryland, and colleagues have found antibodies to the virus in migratory birds in Jamaica. Eleven resident species, including the white-chinned thrush, the black-faced grassquit and the northern mockingbird, were infected (Emerging Infectious Diseases, vol 9, p 860).
Other unpublished research has found antibodies to the virus in birds in the Dominican Republic and El Salvador. 鈥淵ou would expect horses to start dying, given what鈥檚 going on in Mexico now,鈥� says Marra.
There is concern among researchers about the impact of West Nile on all kinds of wildlife south of the US. 鈥淲ildlife in the Caribbean and Central America is already being bombarded from a variety of other stressors, habitat destruction being the primary one,鈥� says Marra. 鈥淪omething like West Nile virus could bring them down to dangerously low levels if an outbreak occurred in a really important area for a specific population.鈥�
This may already be happening. Doug Causey, a biologist at Harvard University, has reported a disturbing drop in the numbers of migratory songbirds in Costa Rica over the past year, and suspects West Nile is responsible. Epstein says that any drop in populations of birds of prey would result in an increase in rodent numbers that would eventually threaten humans. 鈥淚n South America, a whole bunch of emerging diseases are rodent-borne,鈥� says Epstein.
Researchers are also concerned about the virus鈥檚 effect on South American wildlife. 鈥淕iven the birds that West Nile infects, and their migratory habits, South America is only a hop, skip and jump away,鈥� says Marm Kilpatrick of the Consortium for Conservation Medicine in Palisades, New York. The organisation is setting up West Nile monitoring stations in Brazil.
