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West Nile Virus Q & A With Mosquito Squad of Diablo Valley Owner Eugene Murphy

Q: What is West Nile Virus?

A: West Nile Virus is a type of arbovirus, meaning it is a virus that is transmitted by arthropods, arthropods are the vectors. More specifically the virus is transmitted by mosquitoes (class Insecta, order Diptera). WNV is the leading cause of arboviral disease in the United States. It causes disease in humans, horses and other mammals but of primary concern are humans and to a lesser extent horses (viable horse vaccines exist for WNV). WNV spread rapidly across the United States since its arrival in New York in 1999. By 2003, the virus reached California and was detected in all 58 counties within a year, it reached establishment in 2009. WNV is at epidemic levels in all 48 contiguous states and the District of Columbia and in all provinces of Canada. It can cause WNV fever and the more serious neuroinvasive disease West Nile Encephalitis (WNE). WNV fever is characterized by symptoms similar to the Flu. The neuroinvasive disease progresses into mental confusion and coma, or death, survivors commonly suffer permanent brain damage.

Q: How likely are my family and I to get West Nile Virus?

A: That is a very difficult question to answer directly as there are an enormous amount of variables involved in the probability of an individual to contract WNV, therefore, I will present the statistical data and risk information. As of 2013 (1999-2013) it is estimated that more than 3 million people have contracted WNV. Of those 3 million it is estimated that 780,000 people developed WNV fever, (about 25%). In 2013 2,469 cases were reported to the CDC. Of those 2,469 cases that were reported 1,267 were Neuroinvasive (51%), 1,494 were hospitalized (61%), and 119 died (5%). The median age of those reported deaths was 78. There were six states that comprised about half (51%) of the reported neuroinvasive cases, one of those six was California. There are many restrictions that hinder accurate reporting of WNV. It is estimated that only 1%-3% of cases are actually reported and that 30-70 non-neuroinvasive cases occur for every neuroinvasive case reported. Extrapolating that estimate would mean that there were an estimated 38,000-88,500 non-neuroinvasive cases in 2013. Additionally, 2013 was a mild year for WNV. The two largest epidemics of WNV occurred in 2003 and 2012. You are most likely to contract WNV from dusk to dawn in the summer months. Most WNV cases occur June through September with the majority occurring in late August and early September. You are more likely to develop the neuroinvasive disease if you over 50 and the risk increases dramatically with age. Higher temperatures correlates to a greater risk of contracting WNV.

Q: Is there vaccine, treatment or cure for West Nile?

A: Currently there is not a registered WNV vaccine for humans. There are several registered vaccines for horses. There is no cure for WNV. Treatment is mostly supportive.

Q: How do I protect myself from getting West Nile Virus?

A: “Without an effective human WNV vaccine, the only way to prevent human infection and disease is to prevent infected mosquitoes from biting people”-CDC. Person to Person transmission of WNV has been observed outside of the lab only through blood transfusions and solid organ transplants. A case or two of aerosol transmission of WNV among laboratory workers has been documented but not considered likely to occur outside of a lab setting. The most effective way to protect yourself is to prevent mosquito bites. The most effective prevention measure would therefore be local mosquito control where you spend the most time outside. When you are not in a protected area, bug repellents with Deet or another EPA registered mosquito repellent should be applied and/or protective clothing should be worn.

Q: Is West Nile Virus spreading more or is it starting to go away?

A: Collectively epidemiologists agree that WNV is likely to continue to spread. There will likely be more frequent and more intense outbreaks in the future. There are many variables that contribute to WNV incidence. Increased temperatures have been shown to be the most significant variable in explaining fluctuations in WNV transmission rates. With global temperatures expected to continue to rise it is highly probable that WNV incidence will also continue to rise.

Q: Why do some people get the neuroinvasive disease while others just get West Nile fever or don’t get sick at all?

A: More research needs to be done on this in order to give a statistically significant definitive answer. Research suggests that the individual’s immune response determines who becomes infected after exposure and to what degree. Age has also been shown to be a significant factor. People over the age of 65 are 16 times more likely to develop WNV fever or neuroinvasive disease. In the end there are a plethora of variables that come into play to determine who gets sick and to what degree, currently it is not well understood.

Q: Why should I be more concerned about WNV from dusk to dawn when I get mosquito bites during the day all the time?

A Biting behavior varies depending on the species of mosquito. Most mosquito species primarily seek hosts (blood meals) during the hours between dusk and dawn. WNV is primarily transmitted by a specific group of mosquitoes, mosquitoes in the Culex genus, these mosquitoes are not considered to be day biting species. Therefore, you are much more likely to be bitten by an infected mosquito from dusk to dawn as that is when the Culex species are seeking hosts.

Q: Are all arboviruses transmitted by mosquitoes?

A: No, arboviruses are viruses transmitted by arthropods. The vast majority of arboviruses are transmitted by mosquitoes, ticks, and fleas. There are a variety of arboviruses that cause disease, including, Zika Virus, Chikungunya virus, Yellow Fever virus, La Crosse encephalitis virus and many more. Arboviruses are grouped under the broader category, vector borne diseases.

Q: Will West Nile Virus mutate and spread even more rapidly than it is now?

A: It would be extremely difficult to predict viral mutation outcomes in the future. WNV did mutate and evolve into a different strain (NY99 evolved and produced WN02) after it arrived in New York in 1999. The WN02 strain appears to be better adapted for transmission by the Culex mosquitoes and for transmission at higher temperatures. In theory pathogens should evolve to increase fitness over time so it would make sense for WNV to mutate and thus evolve in the future again. Viruses make constant mistakes in their replication cycle which results in altered nucleotide sequences. Epidemiologists and virologists use these variations in nucleotide sequences of the viral RNA strand to trace a virus’s origin and classify the virus taxonomically. Predicting which nucleotides will mutate is likely a tale only evolution can tell with certainty right now.


West Nile Virus (WNV) Facts

  • First Discovered in the Western Nile region of Uganda in 1937
  • First Discovered in North America in New York, 1999 through an infected traveler or migratory birds
  • WNV is transmitted by mosquitoes in a bird to mosquito to bird cycle with humans serving as a dead end host
  • The primary amplification hosts (hosts that spread the virus through the mosquito vector) are passerine birds, birds of a large order (Passeriformes) who have feet adapted for perching, this includes all song birds.
  • The Culex variety of mosquitoes (mosquito species of the Culex genus) are the primary vectors of WNV
  • Infectious WNV can be detected in mosquitoes saliva and legs 12 hours after feeding on an infected blood meal
  • WNV is a single stranded positive-sense RNA virus(Positive RNA is RNA that can act as mRNA and be translated)
  • WNV’s (a flavivirus) genome consists of a single strand of RNA 11,000 to 12,000 nucleotides long
  • The original strain that arrived in New York in 1999 (NY99) was most closely related to a strain detected in Israel in 1998
  • Since approximately 2002 the original WNV strain, NY99, has been displaced by a new strain, WN02 that is better adapted for infection, dissemination, and transmission by the Culex mosquitoes
  • The WN02 strain has a distinct advantage in warmer climates over NY99
  • WN02 strain was detected in 2001 and spread rapidly across North America
  • WNV can be passed on to mosquito eggs by their mother (vertical transmission) and this is likely how the virus is maintained in the northern states
  • Culex mosquitoes can overwinter as adults in temperate climates contributing to the maintenance of WNV
  • 2003 and 2012 were the two most significant epidemics of WNV in North America to date
  • Fluctuations in WNV incidence is likely a product of temperature fluctuations
  • Increased temperatures are strongly correlated to increased transmission of WNV
  • An estimated 3 million people from 1999 to 2010 were infected with WNV with 780,000 of those developing WNV fever
  • Most important Mosquito Species in Regards to WNV transmission:
    • Culex Pipiens (Northern Half of the Country)
    • Culex Quinquefaciatus (Southern Half of the States)
    • Culex Tarsalis (Western States)
    • Culex Salinarius (Northeastern States)
  • American Robins can play a key role as WNV amplification hosts even at relatively small population sizes
  • Increasing Weekly maximum temperatures and cumulative weekly temperatures were observed to be significantly associated (35-83% higher rates) with incidence of reported WNV infection over the following month
  • Warmer temperatures, elevated humidity, and heavy precipitation increased the rate of human WNV infection in the U.S.
  • Broad regional trends suggest that prior drought contributed to the initial US WNV outbreak
  • Intergovernmental panel on climate change (IPCC 2007) projects that North American future decades will have warmer temperatures, shorter winters, increased rain instead of snow, and increased frequency of heavy rainfall
    • If temperatures and precipitation increase the incidence of WNV will likely increase with the climate changes
  • A 5⁰C increase in mean max weekly temperatures was associated with a statistically significant, 32%-50% higher incidence of WNV
  • Viral evolution and temperature help shape the distribution and intensity of the transmission of WNV and provides a model for predicting the impact of temperature and global warming on WNV transmission
  • Both strains of WNV (WN02 & NY99) had sharply accelerated transmission with increased temperatures
    • Even a relatively small increase in temperature had relatively large effects
  • Increases in temperature have a two-fold impact on WNV transmission
    • Increases in disease is most likely contributed to increased viral replication
  • Viral replication cycle is completed at higher rates with higher temperatures
    • Leads to greater concentration of infectious virus
    • Is true for replication in all tissues
      • Therefore, increased temperatures affect not only WNV infection kinetics but dissemination kinetics as well

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