West Nile Virus Disease Cases in the United States
Introduction
West Nile fever is a disease caused by the West Nile virus, which is a single-stranded RNA virus spread by mosquitoes. The natural hosts of the West Nile Virus are birds and mosquitoes, however, WNV is also able to infect mammals, including humans. The virus was first isoloated from a human in Uganda in 1937.
Most West Nile virus infections in humans are asymptomatic, however, 1 out of 5 infected persons develop symptoms including fever, headache, myalgia etc.. About 1 out of 150 cases develop severe illness with neurologic complications. Patients with neurologic symptoms face a 10% risk of death.
In this article we will explore human disease cases caused by the West Nile virus within the United States, where the first case of the virus was reported in New York City in 1999.
Geographic Distribution
After the first 62 cases of the West Nile virus disease were reported in New York City in 1999, the virus slowly spread westward in the subsequent years until it was reported at least once in every US state. Predominantly though, cases were reported in states with a warmer climate, where there are more mosquitoes and therefore a higher possibility of transmission.
The states with top Wet Nile virus disease cases varied from 1999 to 2019, but it looks like eventually the virus established primarily in California, Nevada, Arizona, Texas, North and South Dakota, as well as Nebraska, Wyoming, Mississippi, Louisiana and Colorado.
If we look at the distribution of neuroinvasive cases by average incidence, we will get a more accurate information about where the disease has been reported most often. The top states are South Dakota, North Dakota, Nebraska, Wyoming, Colorado, Mississippi and Lousisiana. This is a pretty surprising insight, since you would expect that the virus should spread mostly in the states with a warm and humid climate. Could it be because of less mosquito control in those states or is there another reason? This interesting trend will require a close-up investigation.
Seasonality
If we look at the distribution of cases by month of illness onset, we can see that the virus spreads mostly in July, August and September. This insight is not very surprising, since those months are the warmest months of the year, when the chances of getting mosquito bites are the greatest.
Disease incidence and number of cases from 1999 to 2019
It looks like the most significant incidence peaks occurred in 2003 and 2012. There is no distinct upward trend, and there is a very low incidence overall, however, this could change in the upcoming years with all the consequences of climate change or due to other factors, such as mutations of the virus.
The graph below shows us that there is a relatively consistent proportionality between non-neuroinvasive and neuroinvasive disease cases. Since many non-neuroinvasive disease cases are not reported, it is safe to assume that the proportion of non-neuroinvasive cases is a lot larger than what the available data shows us.
Neuroinvasive Disease Cases by Age Group
The bar-chart below communicates very clearly that older people are more likely to develop neuroinvasive disease than younger people. This is the case with most of infectious diseases, and the reason for it is the weakening of the immune system as humans age.
Deaths from West Nile Virus Disease
The available data shows us that people are more likely to die if they develop neuroinvasive disease. The percentages of deaths from neuroinvasive and non-neuroinvasive disease remained relatively consistent since 1999, which is an indicator that the virulence of the virus has not changed a lot since then.
Conclusions
Although the current epidemiological data of the West Nile virus disease is not very alarming, we still need to carefully record and track the behaviour of the virus in order to spot any alarming changes as soon as possible. Careful analysis of all vector-borne diseases is especially important due to the tendency of these kinds of viruses to be generally more virulent than diseases spread by direct contagion. Although humans are not currently the primary hosts of the West Nile virus, this could potentially change, and it is extremely important to notice this change on time.
