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West Nile virus predicted to increase in California

West Nile virus threatens wildlife populations and poses a serious health risk to humans
West Nile virus threatens wildlife populations and poses a serious health risk to humans
Robin Wulffson, M.D.

The West Nile virus can infect humans, birds, mosquitoes, horses, and some other mammals. In 1999, the virus occurred in the Western Hemisphere for the first time, with the first cases reported in New York City. Since then, West Nile virus is considered an emerging infectious disease in the US, as it has spread down the East Coast and many Southern and Midwestern states. The disease threatens wildlife populations and poses a serious health risk to humans. In 2012, more than 5,500 human cases of the disease were reported in 48 states, the highest number in more than a decade. Researchers from the Center for Tropical Research at UCLA’s Institute of the Environment and Sustainability have developed a model to help predict where the disease may occur under future climate change. The published their findings on February 27 in the journal Global Change Biology.

“To our knowledge, this is the first continental risk assessment of West Nile virus based on nearly a decade of data,” noted Ryan Harrigan, a postdoctoral researcher at the center who, along with center director Thomas Smith, collaborated with researchers from Germany’s University of Tubingen and Britain’s University of Leeds. He added, “What we are presenting is the first North American predictions of the disease under present and future climate conditions.”

The researchers used the relationship between current climate conditions and cases of West Nile virus to generate models that could predict where the disease may occur under future climate conditions. Their results suggest that higher temperatures and lower rainfall result in a higher probability of West Nile virus cases in humans, birds and mosquitoes; furthermore, as a result climate change, the disease will spread northward into previously unaffected areas. In regard to California, Dr. Harrigan noted, “In California, we estimate approximately 68% of the state’s area will have an increase in the probability of West Nile virus by 2050.” He said that their models identified current and future hotspots of West Nile virus transmission; thus, they offer an important new approach for monitoring the risk of this and other vector-borne diseases. He explained that the, although the models identified areas that are expected to have outbreaks, the severity of those outbreaks is still unknown.

The investigators’ review of data on West Nile virus patterns, together with observed climate data and a variety of species distribution models for the years 2003–2011 revealed an interesting finding. “Our model did a good job of predicting where we saw cases in 2012, including new cases in places that had never before had recorded cases, including Maine, which experienced its first human case in 2012,” explained Dr. Harrigan. Based on this study, the researchers went a step further and collected data related to future climate realities and developed models that reveal possible future activity in 2050 and 2080. This allowed them to make the projections for California.

The researchers noted that the most important climate variables that could predict West Nile virus occurrence were the maximum temperature of the warmest month and measures of annual and seasonal rainfall. Dr. Harrigan noted that their study did not take into account factors such as future land-use changes, insect control efforts, socioeconomic conditions, host-switching by West Nile carrying insects, or host community diversity, which are difficult to predict in the long-term. Despite that, he said the information is useful to a variety of groups, including: the general public, who could use it to better understand their personal geographical risk; local vector control agencies, which can identify potential “hot spots” in various counties and districts; and wildlife biologists, who can use it to determine need and risk in certain areas and how this may impact animal populations. Dr. Harrigan explained, “The intent of the research is not to cause alarm but rather to raise awareness of the existence of the disease and where the risks are to encourage people to take precautions.” He added that there is no cure for the virus; however, symptoms can be decreased, if caught early, through supportive therapy.

West Nile virus occurs in late summer and early fall in temperate zones; however, it can occur year-round in southern climates. Usually, the West Nile virus causes mild, flu-like symptoms. However, the virus can cause life-threatening illnesses, such as encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain and spinal cord), or meningoencephalitis (inflammation of the brain and its surrounding membrane).

According to the CDC, West Nile virus infection in humans is rare. Most individuals infected with West Nile virus experience only mild, flu-like symptoms that last a few days. Symptoms usually appear within three to 14 days of infection. Approximately 20% of the people who become infected will develop West Nile fever. The following are the most common symptoms of West Nile fever. However, each individual may experience symptoms differently. Symptoms may include:

  • Fever
  • Headache
  • Body aches
  • Skin rash on trunk of body
  • Swollen lymph glands

The more severe form of the West Nile virus (West Nile encephalitis, West Nile meningitis, or West Nile meningoencephalitis), that occurs in one out of 150 cases and primarily in seniors, occurs when the virus crosses the blood-brain barrier. Symptoms of West Nile encephalitis, West Nile meningitis, or West Nile meningoencephalitis may include:

  • Headache
  • High fever
  • Neck stiffness
  • Stupor (a state of impaired consciousness, extreme lethargy, and reduced reactivity to external stimuli)
  • Disorientation
  • Coma
  • Tremors
  • Convulsions
  • Muscle weakness
  • Paralysis

The symptoms of West Nile virus may resemble other conditions or medical problems. Therefore, one must always consult a physician for a diagnosis.

West Nile virus is transmitted to humans through the bite of an infected female mosquito. The mosquitoes acquire the virus through biting infected birds. Crows and jays are the most common birds associated with the virus; however, at least 110 other bird species also have been identified with the virus.

According to the CDC, West Nile virus does not spread between humans. However, in recent developments, several cases were documented of organ transplant recipients who contracted the disease from their donors. Health officials suspect the organ donor acquired the virus through a blood transfusion. As a result, the government is working to develop a blood-screening test for West Nile virus. However, the risk for contracting West Nile from blood is significantly lower than the risk of forgoing any procedure that would call for a blood transfusion.

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