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West Nile Virus in the U.S.
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The latest viral outbreak of note in the U.S. is West Nile virus (WNV). It is a mosquito-borne arbovirus belonging to the genus Flavivirus in the family Flaviviridae (http://en.wikipedia.org/wiki/West_Nile_virus). The virus is found in temperate and tropical regions of the world.
West Nile virus lives in and multiplies in birds. Mosquitoes become infected when they bite and draw blood from infected birds. Mosquitoes may then transmit the virus to humans and animals.
WNV was first identified in the West Nile subregion in the East African nation of Uganda in 1937. Prior to the mid 1990s, the disease occurred only sporadically and was considered a minor risk for humans, until an outbreak in Algeria in 1994, with cases of WNV-caused encephalitis, and the first large outbreak in Romania in 1996, with a high number of cases with neuroinvasive disease.
WNV has now spread globally, with the first case in the Western Hemisphere being identified in New York City in 1999; over the next 5 years the virus spread across the continental United States, north into Canada, and southward into the Caribbean Islands and Latin America. WNV also spread to Europe, beyond the Mediterranean Basin. A new strain of the virus has been identified this year in Italy.
WNV is now considered to be an endemic pathogen in Africa, Asia, Australia, the Middle East, Europe and the United States, the latter currently experiencing one of WNV's worst outbreaks (see below).
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West Nile virus usually causes either no symptoms or mild symptoms of headache, fever, body aches, and sometimes a rash and swollen lymph nodes. In rare cases it can lead to inflammation of the brain (encephalitis) or the tissues surrounding it and the spinal cord (meningitis).
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When West Nile virus affects the brain, symptoms may include headache, high fever, stiff neck, reduced attention to surroundings, disorientation, tremors, convulsions, muscle weakness or paralysis, and coma.
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No specific treatment is available for West Nile virus infection. Treatment for severe infection involves supportive care in a hospital to help the body fight the illness on its own. In a few cases the infection is fatal.
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People usually recover fully from the mild form of West Nile virus infection, but permanent problems may develop in those who have encephalitis, especially children and older people. They may have seizures, memory loss, personality changes, tremors, trouble with walking or balance, or brain damage.
THURSDAY, Sept. 27, 2012 (MedPage Today) - West Nile virus is "disturbingly unpredictable, disagreeable, and difficult to control," CDC researchers wrote recently in a new perspective piece.
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As of Sept. 25, the agency had received reports of 3,545 confirmed cases of infection with the virus, including 1,816 cases of neuroinvasive disease - which often leads to long-term functional impairment - and 147 deaths. The cases came from 47 states and the District of Columbia; only Hawaii, Alaska, and Maine have remained untouched.
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"To judge from past reporting trends, these figures suggest that this year's West Nile virus outbreak will be among the largest ever recorded," Lyle Petersen, MD, MPH, and Marc Fischer, MD, MPH, of the CDC's Division of Vector-Borne Diseases in Fort Collins, Colo., wrote online in the New England Journal of Medicine.
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West Nile virus was first detected in New York City in 1999. One in every four or five people who are infected develop symptoms, and less than one in every 150 cases develops into neuroinvasive disease.
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The virus's initial march westward led to large outbreaks in the Central States in 2002 and the Mountain States in 2003, but rates generally have been declining since then.
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"Until this year's resurgence, many experts questioned whether West Nile virus would remain a substantial public health concern," Petersen and Fischer wrote.
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Their characterization of the virus as hard to handle stems from the difficulty of identifying the specific ecological factors responsible for any given outbreak, as well as the lack of effective treatments and vaccines.
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Many factors influenced by the weather play a role in determining the size of an outbreak, including the numbers and distributions of susceptible birds and vector mosquitoes and the rate of virus replication in mosquitoes.
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Heat may also be a contributor, but not the sole culprit, because West Nile virus outbreaks are not always related to heat waves. This year, for example, most areas in which the virus is endemic that experienced heat waves do not have outbreaks.
"The complex spatial and temporal relationships among temperature, rainfall, and the underlying geographically variable ecologic factors promoting arboviral transmission have historically been difficult to sort out," Petersen and Fischer wrote. "Furthermore, even under similar circumstances, the stochastic nature of arboviral amplification may result in a wide range of outcomes in any given location simply as a result of chance. Thus, generating long-range predictions regarding West Nile virus outbreaks will be a formidable, if not impossible, task."
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Dealing with West Nile virus is also complicated by the lack of effective treatments related to practical obstacles.
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"The sporadic incidence and geographically dispersed distributions of cases and outbreaks of West Nile virus necessitate the participation of hundreds of clinical trial sites if researchers are to have a reasonable hope of achieving a sufficient sample size," Petersen and Fischer wrote. "As a result, all randomized clinical trials attempted to date have failed to enroll sufficient numbers of patients to demonstrate efficacy ... ."
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They cited similar barriers to the development of vaccines.
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Because of the difficulties presented by West Nile virus, surveillance on a national scale is key, the authors wrote, noting that the ArboNET surveillance system was set up to track West Nile and other arboviruses.
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When outbreaks are detected, actions can be taken to minimize the risk. Those include applications of insecticides either with truck-based or aerial spraying to control adult mosquito populations and public campaigns to raise awareness about personal protection measures.
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"Unfortunately, we cannot predict the distribution or incidence of West Nile virus for the next year, let alone the next decade," Petersen and Fischer wrote. "However, our 80-year experience in the United States with St. Louis encephalitis virus suggests that West Nile virus will cause sporadic cases and unpredictable outbreaks big and small for decades to come."
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