Monday, February 7, 2011

West Nile Virus


This week I am introducing another arbovirus: West Nile virus. If you remember from the series on dengue fever, an arbovirus is an arthropod-borne virus. The relevant arthropod for West Nile virus (WNV) is once again a mosquito, and the virus is once again a Flavivirus. Wondering where the name Flavivirus comes from? Well, flavus is Latin for yellow. The Flaviviruses are named after the virus that causes yellow fever, which is also, of course, a Flavivirus. Yellow fever, when it presents as clinically severe, can cause jaundice due to liver damage. And, thus we get the name yellow fever, whose "yellow" virus name was subsequently shared with all the Flaviviruses (though not all Flaviviruses cause jaundice).

So, West Nile virus. This is a single-stranded RNA virus that is transmitted, generally, though not exclusively, by the bite of Culex mosquitoes. This is a new genus of mosquito we have not discussed before, but some of the unique aspects of its ecology will be described below. Here is a look at the virus:



There are two major lineages of this virus, though a few others do exist. Lineage 1 has a wide range across the Middle East, parts of Africa, eastern Europe, North America, and Australia. Lineage 2 viruses are present in sub-Saharan Africa and Madagascar. The Lineage 1 viruses are associated with the greatest pathogenicity in humans, whereas the Lineage 2 viruses typically are asymptomatic or are associated with mild disease in humans. The spread of WNV into the western hemisphere, which began in New York in 1999, is due to Lineage 1.

The virus was first identified in 1937 in the West Nile district of Uganda, but during this time was considered a minor concern because it was typically asymptomatic, occasionally causing minor illness in humans.


However, in subsequent decades WNV began to be identified with more far reaching sporadic outbreaks of mortality in horses and encephalitis in humans in South Africa, India, the Middle East, and Europe. This map from the Center for Disease Control and Prevention (CDC) shows the global distribution of WNV in 2000 in blue. Notice New York newly represented here:



The sporadic nature of WNV across this large geographic region in the eastern hemisphere has been consistent over much of its epidemiologic history there, and continues in this way today. On the other hand, the pattern of virus spread has been quite dramatic in North America. Annual epidemics occurred in increasing areas of geographic space, moving from east to west with chronological time, since it's introduction in 1999. This sequence of CDC maps shows the westward spreading distribution of WNV in the US:

1999
2000

2001
2002
2003
2004
2005
2006


This series of maps shows quite dramatically the rapid spread of WNV across the US, which is in contrast to the pattern in Africa, the Middle East, and Europe where localized sporadic activity following favorable conditions has been the norm.

What could have led to this dramatic difference in the spread of the virus in these two distinct geographic regions? We also have to ask, IS there a difference, or was the western hemisphere simply comprised of entirely naive populations, such that the spread in North America appears more dramatic? Or can differences in pattern emergence be attributed to differences in surveillance?

Let's first look more specifically at who and what the virus infects, the disease it can cause, and the vector ecology that is important to transmission.

Birds serve as the primary hosts for WNV. While humans and horses are commonly infected, the avian hosts are able to maintain levels of viremia high enough to infect new mosquito vectors. Humans and horses can become infected and develop disease, but their viremia is generally too low to infect new mosquitoes so they are "dead end hosts" for the virus. Therefore, birds are the reservoir for WNV. Moreover, both migratory and non-migratory birds can serve as hosts for WNV, with the former obviously creating the potential for wide geographic distribution of the virus as long as a suitable vector is also present across the migratory landscape.

In humans, infection with WNV is most often asymptomatic. The low viremia established in humans following infection may be related to this low pathogenicity, however this is uncertain. Nevertheless, aymptommatic infection in humans is not a concern for future transmission of the virus because, as mentioned earlier, the viremia is too low to infect biting mosquitoes. Among infected individuals that do demonstrate symptoms, the presentation is typically a flu-like illness with fever, headache, myalgia, and sometimes diarrhea and vomitting. Most of these patients will recover in 7 to 10 days. However, about 1 person of every 150 infected will develop severe neuroinvasive disease. This condition can be classified into three, non-mutually exclusive, syndromes: encephalitis, meningitis, and acute flaccid paralysis. The highest neurologic complications and the greatest mortality are associated with the occurrence of encephalitis, which is an inflammation of the brain. Encephalitic complications can include headache, drowsiness, mental confusion, loss of memory, and convulsions. This map based on CDC surveillance illustrates the incidence of WNV neuroinvasive disease in 2010:


There is a very important risk factor associated with increased risk for severe WNV infection: AGE. This graph published by the CDC's Morbidity and Mortality Weekly Report in July of 2010 demonstrates this relationship nicely:

Number of incident cases of neuroinvasive disease per 100,000 population in 2009

Back to the vector. It's another mosquito. Mosquitoes do not serve as the exclusive vectors for WNV, but they are, by far, the most important for bird, and, likewise, human transmission. The genus of primary importance for WNV transmission is Culex, and the important species are Culex pipiens in the eastern United States and Culex tarsalis in the west.

Culex mosquito taking a blood meal

Culex mosquitoes seek out dirty water for oviposition, and lay their eggs on the water's surface in the form of an egg raft. With respect to ovipositioning, Culex mosquitoes are not dissimilar from the Aedes mosquitoes in that they both seek out water rich in decayed nutrients (i.e. dirty) for their developing larvae. However, Culex mosquitoes bite primarily at night, and also during the dusky times of dawn and sunset, whereas Aedes mosquitoes bite throughout the day. Both mosquito genera are well adapted to anthropogenic settings and therefore do well in urban areas in and around human habitation.


Culex life cycle

Another difference between Culex and Aedes mosquitoes is their host preference. While Aedes mosquitoes prefer humans, the Culex species are ornithophilic, meaning they prefer bird species. This has two important implications for their ability to act as a vector in human disease. First, if appropriate and sufficient bird populations are available, these mosquitoes will seek their blood meals primarily from the birds, meaning they will have less contact with humans and thus less opportunity to transmit the virus to this secondary host. Second, since birds serve as the reservoir for WNV, the virus is amplified in birds thus providing a consistent source of reinfection to mosquitoes, who can subsequently reinfect other birds, or humans.

Alameda County Vector Control Services

Ok, we've talked about the West Nile virus, the disease it can cause, and the mosquito vector that can transmit it. There is still a critical element that is essential to the etiology of this disease: the environment, and more specifically, the weather.

 As described so far, the virus needs two things to maintain itself in host populations: birds and mosquitoes. The reservoir and the vector, respectively. There are other arthropod vectors that can transmit WNV, but mosquitoes are the most important for maintaining the virus, so we limit this discussion to the mosquito vector. So, birds and mosquitoes. As you would, perhaps, expect, both the reservoir and the vector are highly attuned to particular ecologies. One universal necessity to both birds and mosquitoes is water. Water is essential for both to maintain healthy populations. However, birds and mosquitoes utilize water for different purposes, and when water and temperature interact, ecologies can change, resulting in a change in each's ability to maintain its populations. For example, during periods of increased heat combined with decreased precipitation, drought can ensue. This kind of interaction between temperature and precipitation can disturb bird ecology such that birds are forced to leave the area to find better water sources in different geographic landscapes. At the same time, as water sources are dried up, they become more concentrated with organic matter, i.e. more polluted, which is favorable to the culicine mosquitoes. In addition, these mosquitoes can often find suitable water conditions in and around human habitation even during times of drought. Therefore, drought conditions can reduce the the availability of preferred bird species, while at the same time maintain mosquito populations as long as the drought is not so extensive as to eliminate all water sources entirely. If mosquitoes are able to maintain their populations in a local geographic region while the birds are not, then the mosquitoes will be forced to find alternative hosts for their blood meals. Thus, humans and horses can become targets and subsequently acquire WNV infection.

There are a number of studies in the WNV literature that have shown that dry times in temperate regions lead to increased occurrence of WNV infection in humans. However, there are also studies showing positive associations between increased precipitation and the subsequent occurrence of infection. However this direct relationship is typically shown when precipitation is considered in combination with increased temperature. So the sensitivities of birds may be to precipitation, or they may be to temperature. Likely there is an important interaction between the two that is involved in bird ecology and which determines bird species' localized geography from one season to the next. Nevertheless, there is strong evidence to suggest the spatial movement of birds, and specifically the ability of bird species favored by Culex mosquitoes to exploit a particular localized ecological niche from season to season, plays an important role in human incidence of WNV infection from season to season. Accomplished parasitologist and medical ecologist, Dickson Despommier, has written extensively on the balance between birds and mosquitoes acting in the etiology and epidemiology of WNV. He has an excellent presentation at here.

The situation as it has evolved in the United States is interesting. From 1999 through 2006 the virus was able to move farther west with each season. This pattern of movement is due to a complete causal mechanism, all the components of which we do not yet understand. Nevertheless, there are some factors that are likely components in this causal mechanism. The movement of birds is a distinct phenomenon that has undoubtedly played a role. As described above, the changing distribution of bird species according to variation in weather can leave alternate host populations, such as humans, more available. In addition, migratory birds can play an important role in moving WNV across large distances simply by way of their natural seasonal trajectories across geographic space. There is also the possibility of westward mosquito migration. Culex mosquitoes are capable of traveling up to a kilometer per day making them additional potential disseminators of WNV across a shifting landscape.

While WNV was introduced into the northeastern US, the current annual epidemic peaks are occurring in the drier western climates of the US, which is consistent with what we discussed above about the scarcity of favored bird species in such conditions:



The arid west and southwest are more likely to experience regular periods of diminished precipitation, which could lead to a settled equilibrium of endemicity in this region of the country. 

WNV persistence in the southwest may also be affected by the mortgage crisis during the last few years, which has left very many homes foreclosed, and containers (and pools) abandoned to collect rainwater. Indeed, a study published by the CDC in Emerging Infectious Diseases examined this relationship in Kern County, California, finding that these pools naturally served as excellent egg sites for Culex mosquitoes leading to an increase in WNV cases nearly three times what would normally be expected. Nevertheless, while these localized conditions may be contributing to WNV incidence in a short period of time, it is unlikely that this will be a major factor in the long-term establishment of WNV in the US. Climate factors, and climate change, are much more likely to play a role as temperatures increase, and we continue to experience wild fluctuations in levels of precipitation. These will undoubtedly affect bird ecology, mosquito ecology, and human ecology, and will therefore affect the disease ecology of WNV. 


37 comments:

  1. This was a very informative post. I thought the section on the ecology relating to the spread of the virus was particularly strong. One thing I would have liked to have seen touched on are some historical outbreaks of WNV (in the U.S. or elsewhere) and what was done by local or national public health authorities to help control them and prevent future outbreaks. I would like to know what we as humans can do to help reduce the spread of WNV and lessen its impact. I was also curious to hear more about the severe neuroinvasive form of WNV. For example the mortality rate, clinical prognosis, treatment, etc. One final point is that I would have liked to have seen the point you made on the correlation between age and incidence of severe WNV expanded on. Is this due to a weakened immune system in elderly people or that fact that more elderly people live in climates conducive to the spread of WNV to humans or maybe something else.

    ReplyDelete
  2. In terms of the neuroinvasive form of the disease, I am wondering if there is an increased elderly population in Arizona that contributes to the high rate of incidence of this particular form of WNV. The figure provided by the CDC showed the incidence at over 100 per 100,000 people. This statistic doesn’t appear to take into account age in the denominator.

    Also, I perused the CDC site to learn more about the preventative measures of WNV. Beyond the normal recommendations of avoiding bites and personal health vigilance, it was recommended to promptly dispose of dead birds. Bird death can indicate the presence of WNV in this important vector and the carcass provides a ready meal for the mosquito.

    ReplyDelete
  3. Craig, where did you find this information on mosquitoes biting dead birds? I didn't see it on the CDC site. I would like to contact the relevant agency as this is not correct and should be addressed, particularly if it is the CDC. Culicine mosquitoes will NOT seek a blood meal from a dead bird. Most likely they will not even be able to identify the animal as mosquitoes rely on following a CO2 gradient to locate a prospective host, and then on specific chemo-attractants that are secreted in various pathways. Dead birds don't breath and they don't secrete, so they are not producing CO2 and other chemo-attractants and, thus, will likely go unnoticed by the mosquito. Even if the mosquito did notice the dead bird it not going to take blood from the animal.

    ReplyDelete
    Replies
    1. I guess this is nature’s way of protecting “the innocent.” The thought of mosquitoes feeding on dead animals and then having the ability to inject their acquisitions in us humans is quite disturbing. Certainly their method of interaction with us is annoying at best, and can be potentially life threatening if they pass on viruses such as WNV to us. It is comforting to know that since expired animals don’t respire or secrete, mosquitoes will not be interested in them or their decaying chemo-attractants.

      Delete
  4. It looks like the dead bird thing has more to do with evidence of WNV in the area. These birds may be contaminated. For example, one CDC article is titled "Dead Bird Clusters as an Early Warning System for West Nile Virus Activity." It doesn't seem to be that they are a concern in terms of sustaining/feeding mosquito populations.

    -Becca

    ReplyDelete
  5. I have heard that WNV was first found in Queens right near JFK. The implication is that it literally landed. Does anyone know if this is true and, if so, has this pattern of spread been found with any other infections?

    -Becca

    ReplyDelete
  6. Is there any hypothesis about whether climate change has had any affect on the prevalence of West Nile Virus in North America?

    ReplyDelete
  7. Jamal-

    You are absolutely correct to assume the warming climate has had an effect on the prevalence of West Nile in the US. There is an interesting article titled, "Infectious disease in a warming world: how weather influenced West Nile virus in the United States (2001-2005)" by Soverow et al, that was published in the Environmental Health Perspectives Journal that looked at that exact hypothesis. Anyway, here's the link: http://www.medscape.com/viewarticle/708625

    ReplyDelete
    Replies
    1. The question concerning the effect of climate shift on the prevalence of West Nile in North America is an important one, and many point to record heat levels in order to explain this year's record West Nile prevalence (cf. Wilcox, "Is Climate Change to Blame for This Year's West Nile Outbreak?" - August 2012 Scientific American). This year certainly provides strong evidence in favor of the strong effect of weather on the spread of West Nile (i.e. the relevant mosquitoes thrive in warmer weather). However, it's worth remembering that the original U.S. West Nile Outbreak in 1999 began in the New York metropolitan (tristate) area and spread from that location (not from the South, for example). I can't find detailed information on hypotheses on how the first West Nile infections appeared in the United States. Does anyone have information on this?

      Delete
  8. After reading the article, I have to agree with becca about her point that the dead birds provide evidence or indicate the possibility of WNV in the area. The dead birds in no way increase the chances of WNV nor do they act as a food reservoir for the mosquitoes.

    ReplyDelete
  9. In response to John's question, the West Nile virus first appeared in the United States (NYC) in 1999, from Africa. They believed that the virus entered the US in an infected bird or mosquito, but there is no clear evidence.

    ReplyDelete
  10. I wonder if the west nile has had some mutations already. It's been a year, so I guess it's already gained a new strand or something.

    ReplyDelete
  11. The other day I read about "mosquito fish" aka gambusia, which have become a popular mode of prevention against West Nile Virus. Health departments are installing hundreds of fish in bodies of water that may be breeding grounds for mosquitoes. One of the advantages of these fish is that they are less harmful to the environment than sprayed larvicides (which are best suited for stagnant bodies of water). For whatever reason, mosquito fish have a preference for eating mosquito larvae, making them an interesting piece of the prevention puzzle.

    ReplyDelete
    Replies
    1. That is a very interesting idea and certainly more environmentally friendly when it comes to prevention. I know that this technique is used to control malaria and has been quite successfully from what I read. I think this in combination with larvicides can make for great control measures for WNV and malaria.

      Delete
    2. There are a number of areas in California that use mosquitofish as vector control and prevention of West Nile virus, such as Los Angeles, San Diego, and Santa Clara. Mosquitofish may seem like a less toxic means of controlling the mosquito population. compared to larvicides. However, mosquitofish can be toxic to the ecosystem if introduced into an environment where they are invasive. For example, in NY State, mosquitofish are listed as an invasive species. They do not have natural predators in NY State, and they can be quite aggressive and eat the larvae and egg of other indigenous species. So, when it comes to finding less toxic means of controlling mosquito populations, there is not general way. Whatever method is used must be appropriate for that particular environment.

      Delete
    3. The idea of mosquito control is very challenging in that there is no one effective solution. With pesticides and invasive fish species being toxic to the environment. Even if mosquito fish were allowed in New York, they would still only work for large bodies of water. Puddles and other smaller pools of standing water also serve as problematic mosquito breeding grounds. I wonder if they will ever be able to find a non-toxic solution.

      Delete
  12. What is the correlation coefficient between west nile virus and sewage system in NYC.? In United States do the residents in suburb or rural areas and use septic system have a higher or lower chance of getting infected to west nile virus compared to residents in city?

    ReplyDelete
  13. How efficient is the mosquito control program in New York to prevent the outbreak of West Nile virus? What additional precautions should be added to the community to protect people from West Nile virus during high season?

    ReplyDelete
    Replies
    1. I found it difficult to obtain any numbers on how effective the spraying of aerial larvicides has been in New York. I also found that there are releases on the NYCDOHMH website regarding the intent to spray. In the past, the news would stress that people should stay indoors during spray times, yet the schedule of application stresses that the spraying does not occur in residential neighborhoods. Perhaps in the past this referred to pesticides using organophosates? It concerns me though that in such a metropolis, we can even make a distinction between residential and non-residential areas. People are everywhere! Likewise, non-residential doesn't mean that there aren't people around. It just means they don't live there. I am familiar with the location recently sprayed in Queens. It is surrounded by two shopping centers, a movie theater, and a public park.


      However, I am proud of how proactive the city has already been considering that the spraying has begun without any reported cases of WNV. I would be interested in learning more about the non-chemicals used in regards to human exposure and how they differ from what was used in the past. Do we know if the non-chemical spray would still be harmful since there is mention that the spraying does not occur in residential neighborhoods?

      The types used are listed in the release: http://www.nyc.gov/html/doh/downloads/pdf/wnv/wnv-notice-20150605.pdf

      http://www.nyc.gov/html/doh/html/environmental/wnv-spray.shtml

      The big question is, who is the release even intended for? Theres valuable information on how to report standing water and what preventive measures to take. I question who is actively seeking out the information.

      Delete
  14. It is interesting to note from the post how the economic crisis could relate to the persistence of an infectious disease in humans. While the climate factor could explain the annual epidemic peaks of WNV cases in the drier western U.S., does the climate factor play a role in other areas around the world where localized sporadic WNV cases are observed? I also wonder whether the rapid and wide spread of WNV across the U.S. is due to the mutated virus thus increasing its infectivity, as the WNV is a RNA virus that has a high rate of spontaneous mutation.

    ReplyDelete
  15. Nicole MastrogiovanniJuly 15, 2014 at 5:38 PM

    It is very interesting to learn that West Nile Virus actually is modified by the climate and bird migration patterns. I think climate change could possibly have a serious affect on the mosquito population as well as a rise in West Nile virus cases due to decreased water and higher temperatures. I noticed that there are more cases in the southwest states versus the northwest and east areas, could this be due to less populations of waterfowl such as geese? I know some birds migrate south for a few months during the winter but a majority of geese, “Canadian geese” are able to handle the winter and stay North, therefore creating less choices for the mosquitos who have ventured down south to choose for meals. I also find it interesting how states such as Alabama and North Carolina have a greater amount of animal cases when the states that share borders with those two states are heavier in human cases of West Nile virus. Either way, I think staying on top of bird migration patterns and bird infections as well as trying to tame climate change can help stop the spread of the virus.

    ReplyDelete
    Replies
    1. Passerine birds are the primary reservoir for WNV in North America. Waterfowl don't play much of a role.

      Delete
    2. Nicole,

      Great point, climate change has certainly effected the rate of many diseases. These past few years, the weather patterns have been different and drastic. It would be interesting to see if there is a difference in rate of West Nile cases during normal temperature years compared to the years where we have had drastic temperature changes.
      It would be interesting to see how recent outbreak have been handled and if they were effective, and if so how does the current weather change effect that?

      Delete
  16. I vividly remember the introduction of West Nile virus to the US. There was a lot of media coverage about WNV. Since 1999, the media attention has decreased and general concern about WNV seems low. Climate change may make these type of disease entries to the US more common. What will be interesting to see is the geographic characteristics of these diseases. As the WNV example shows, it is difficult to predict the ecology of a disease especially when it is introduced to a foreign environment.

    ReplyDelete
    Replies
    1. Hi Sofia,

      In response to your comment about the importance of Climate change and how it can effect the distribution of WNV especially in a place like the United States where its not naturally found. At the University of Arizona, they developed a climate-drive mosquito population model to simulate the abundance across southern US and found that the future climate conditions predicted by the models will see a lengthening of mosquito season but shrinking summer mosquito populations due to hotter dryer condition for larvae to survive. However since climate change is different for different regions, such as that in the north there might still be enough precipitation for the mosquito population to still thrive. Moreover, in SW US, hotter and drier summers delay onset of mosquito season but late summer and fall rains result in a longer season. Conversely, South central US will have fewer mosquitos due to less rain during summer and early fall. Public Health Efforts thus need to take more on a local level due to various climates

      C. W. Morin, A. C. Comrie. Regional and seasonal response of a West Nile virus vector to climate change. Proceedings of the National Academy of Sciences, 2013; DOI: 10.1073/pnas.1307135110

      Delete
  17. I am surprised to see how quickly WNV spread from NY across the country, when it took a much longer time for it travel across the African continent (and still not affect all the African nations, at least according to the graph above in the year 2000). It's unfortunate that the mosquito itself thrives in draught-like landscape since that appears to be the direction global warming is pushing us. While this virus pales in comparison to Dengue, it is still of concern. I wonder by what means the CDC will target this virus. Clearly eradication is impossible since birds are a reservoir, but elimination from human population is something to strive for.

    ReplyDelete
    Replies
    1. One of the reasons why I think WNV has spread so rapidly in the Americas is because it was a newly presented vector affecting a vulnerable population, and also I think the lineage of WNV might also contribute to this rapid spread. I agree more needs to be done by the CDC and maybe more programs should be developed that actually target the life cycle of these mosquitos.

      Delete
  18. In the US there should be more emphasis on being green, the drastic climate changes have contributed to the rapid change of the weather, making it more favorable for mosquitos. I was also unaware until last year that pools have an impact and influence mosquito reproduction, so more educational information should be provided to home owners.

    ReplyDelete
  19. If we think along the ecological lines pertaining to what happens when birds seek out new water sources during a drought, could the same pattern come to fruition in the context of the recent avian flu outbreak? Will the vectors carrying west nile seek out a greater number of humans and horses as hosts? Is this a pattern that we might be able to see occur in the short term since the summer hasn’t begun yet?

    Also, west nile is purely vector-borne correct? That is to say, it needs the mosquito in order to transmit from host to host. It is not possible for it to be food-borne, right? Are there particular bird species that are common reservoirs?

    ReplyDelete
    Replies
    1. WNV is primarily vector-borne, although some transmission through blood transfusions and transplantation has been documented. Passerine birds are the primary reservoir in the US.

      Delete
  20. This information about mosquitos is very interesting and to the point. I find mosquitos really disturbing and problematic. They are everywhere not just outside of our homes but they are hidden in our houses and they bite people during the night. To me all mosquitos are morphologically the same. I would like to know what is a morphological as well as genomic difference between Culex and Aedes species?

    ReplyDelete
    Replies
    1. Katarzyna, you posed some very interesting questions! I didn't realize that West Nile is transmitted by two different types of mosquitoes either. I think knowing the morphological differences are important, as the two types of mosquitoes can each transmit other types of arboviruses. I found a very nice (and extensive) document on the World Health Organization about these mosquitoes (link: http://www.who.int/water_sanitation_health/resources/vector007to28.pdf) First, there are differences between Aedes and Culex in the morphology of laid egg clusters. Aedes eggs are laid singly, while Culex eggs are laid in rafts. In terms of the adults, they can actually look pretty similar! The most important difference is in the the tip of the female abdomen. That of Aedes is pointed, while that of Culex is usually blunt. I wish I can post some pictures, but it looks like I can only post text. So, here's a link to a website run by the University of Sydney's NSW Arbovirus Surveillance and Vector Monitoring Program... Enjoy!

      Delete
  21. I thought this was a very good and informative post, and frankly I was unsure of what I could write about, so I googled WNV and found this as the first link, published yesterday: http://time.com/3941841/west-nile-virus-new-york-mosquito/
    The story is that the first WNV-carrying mosquitoes have arrived in a few neighborhoods in Staten Island, and further google searches for news related to WNV showed a number of newly reported cases in Northern Texas, Nebraska, California...kind of all over the place. Is there any research or talk discussing changing habitats for disease-carrying mosquitoes in the age of global climate change?

    ReplyDelete
    Replies
    1. Ben, you make a great point. Besides Staten Island there are more reports as of late the conversation of WNV in NYS altogether in which NYSDOH has certain counties in particular on alert.
      WNV is discussed every year time and time after again but still there is no vaccine. However, I was pleasantly surprised to see that safety trials in humans are to begin. Link for those interested: http://www.medicalnewstoday.com/articles/296460.php

      Delete
    2. I went to school in the Hudson Valley, and in 2013 someone died in Poughkeepsie from encephalitis which was later discovered to be caused by West Nile. We only knew because my professor got a call from the CDC. It was shocking that it had occurred so close to campus, and so many of us suffered daily from mosquito bites. For diseases like this, and infectious diseases in general, I wonder what the protocol is for communicating an outbreak to the public, especially one that doesn't have a vaccine or adequate treatment.

      Delete
  22. While reading this post on WNV two thoughts come to mind.
    Firstly, as stated in the beginning of the segment WNV is a Flavivirus, which is similar in nature to Yellow Fever, dengue virus and the Hep C virus. They all share the same common vector which is usually the bite from an arthropod. However, they are all transmitted via different types of mosquitos. I wonder why this is so and why each Flavivirus has its own method of transmission. Although this article addresses that there are different biological niches for each mosquito genus, one would assume that they would overlap in some form providing a common method of infection via mosquitoes in general.
    2. It was mentioned that age is a risk factor for WNV. Can it be that the older an individual, the weaker their immune system, and therefore a higher viral load is able to accumulate?

    ReplyDelete
    Replies
    1. Hi Eli,
      I think the reason why each Flavivirus has its own method of transmission is due to evolution. Each virus needs a host cell to replicate, so while the types of cells and modes of transmission may be similar, there is a need for an individualized combination so each virus can continue to exist without too much competition. Yellow fever has a high mortality rate, so it would be a disadvantage for other viruses to be transmitted at the same time as the host may die soon. It is interesting that Hep C has developed to not need mosquitos at all—it has flourished in the human population mainly due to blood transfusions and needle sharing.
      I think you are right that the elderly are most at risk of fatality from WNV, like with the flu, due to the increased risk of medical complications.

      Delete

Note: Only a member of this blog may post a comment.