Wednesday, 19 September 2012

Yet another reason to hate mosquitoes

Living in the UK I am often sucked into writing my blogs based only on news stories occurring here. This time I have decided to change that formula and focus on a story that has hit the headlines the other side of the pond in the United States. The headlines are surrounding the increasing spread of West Nile virus. As of 14th August there had been 696 cases and 26 deaths. Come the start of September these numbers had increased to 1993 cases and 87 deaths, then as of the 9th September (only a week later) the numbers had reached 2636 cases and 118 deaths. This outbreak is the largest in the US since 2003 in terms of the number infected, and also has the highest proportion of the most serious form of infection since the virus was first discovered in the US in 1999. West Nile virus is not necessarily something many people know about beyond the headlines, so as usual, this has spurred me into writing a blog giving some detail on the virus itself.

Culex mosquito
The first thing to point out with this virus is that it is classed as an arbovirus. This is the fancy word for a virus that is transmitted by an arthropod vector, much like malaria for example (though that is not caused by a virus). West Nile virus (WNV) is transmitted by a species of mosquito known as Culex, while malaria is carried by a different species known as Anopheles. WNV is the most widely distributed of all the arboviruses, seen in every continent, except Antarctica. It is important to know that the virus is spread by mosquitoes as this plays a major role in the epidemiology of the virus, as will hopefully become apparent in this blog.

Along with mosquitoes (and obviously humans) the virus is found in birds, and it is in fact these animals that are the natural reservoir for the virus, not humans. Evidence for this comes from the fact that the virus replicates best when in a species with a high body temperature. Most birds have a natural body temperature of around 41OC, which is ideal for the replication of WNV. Humans on the other hand have a temperature around 37OC, which has been shown to limit the replicative ability of the virus, indicating humans are merely an incidental host. The virus is best adapted to replicate in birds, but can use humans if needs be.

Usually Culex mosquitoes feed on birds that are carrying the virus and spread it amongst the bird population. However, in very hot and dry weather this changes. Under these climactic conditions it becomes harder for plants to survive. Plant death has an impact on the population levels of the insects that feed on them. Since many birds feed on insects, if there are less insects the population number of birds will also suffer. If there are less birds to feed on the mosquitoes will move on to a less favoured, but more accessible meal; humans. So when hot and dry weather is seen for prolonged periods of time there is an increase in the level of mosquitoes feeding on humans, raising the chance for spread of WNV in the human population. This is exactly what is happening in America right now, some of the worst affected areas (such as Texas, where roughly 40% of cases have occurred) are currently in drought which very high temperatures (even for summer).
West Nile virus particles
 WNV in humans is often an asymptomatic infection (partly down to the fact that we are only an incidental host), with roughly 80% of people who get the virus not developing any symptoms. Close to 20% of people infected will develop non-neuroinvasive disease that is characterised by fever, aches, vomiting etc. While West Nile fever is nasty for a few weeks, people with this form of infection will usually recover and be completely healthy. However, around 1 in 150 people infected by the virus will develop the most severe, neuroinvasive, form of the disease, which is characterised by virus entering the brain. This form of WNV infection is responsible for most fatalities.

The current outbreak in America has so far been characterised by a much higher percentage of patients showing neuroinvasive infection, though it is not clear why. These patients develop encephalitis (swelling of the brain) and can develop other complications such as meningitis. The fact that the virus causes swelling of the brain adds an interesting characteristic to the infection. Young children have a suppler skull than adults. If a child develops encephalitis from neuroinvasive WNV infection there is much more give in the skull meaning they are less likely to suffer severe complications. People over the age of 55 are at this highest risk of severe complications from neuroinvasive WNV due to their skull being firm (the brain has no room in which to swell).

Currently there is no vaccine and no specific antiviral drugs against WNV. Weirdly the main form of treatment that is given for WNV infection is steroids. This is weird in the sense that steroids suppress a patient’s immune system. While suppressing the immune system is clearly not good in terms of fighting off a viral infection, inflammation is also controlled by the immune system. Therefore, if the system is suppressed there will be less inflammation, meaning the swelling of the brain seen in neuroinvasive infection will not occur (or will at least be suppressed). Over time the patients on steroid treatment will be able to remove the virus due to the fact that it does not replicate particularly well in humans (as I mentioned previously). However, it is likely that this recovery will be a drawn out process due to the compromised immune system (the patients will suffer the fever, aches, vomiting etc for longer). This is clearly not a particularly good strategy to deal with the virus, and clearly more work needs to be done in the field. Hopefully the current outbreak in the US will help keep funding up in the WNV area and allow for the development of an effective vaccine, or at least specific antiviral drugs.

The current outbreak of WNV in the USA has the second highest number of cases seen since the virus was first introduced to the country in 1999. The outbreak is also the most severe in terms of a high percentage of patients developing neuroinvasive infection. There has recently been a huge jump in the number of confirmed cases; back in mid-August there were 693 cases, come the start of September there were 1993 cases and a week later there were 2636 cases. These figures imply that the virus is spreading rapidly, however this is slightly misleading, as there is a delay between patients being tested and the results being received. Another contributing factor to the slightly misleading figures is that more and more people are being tested due to the current hype around the outbreak. In a normal year it is likely that many people will develop non-neuroinvasive WNV but will pass it off as just a bout of flu (fever, aches, vomiting etc), so will not be tested (and therefore recorded). Since WNV is in the headlines, more people are being tested, meaning the numbers are slightly distorted. I’m not denying that this outbreak is serious; I’m just pointing out a small fact about these kinds of statistics that is often overlooked. Fortunately, in some respects, the virus is completely reliant on the mosquitoes, therefore when summer ends and the mosquito numbers drop the epidemic will end. There is an end point to this epidemic, hopefully sooner rather than later.

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