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West Nile Virus: The Sequel

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There is no doubt that filmgoers love sequels. The adulation is so great that in 2011, nine of the top 10 grossing films were continuations of cinematic storylines. For studios, a sequel is usually easy money as success is almost guaranteed with only a small evolution in the characters and plotlines as well as an environment full of hype.

Germs are also very good at producing sequels although their return is significantly less thrilling to the general public. The reoccurrence of a pathogen can be a rather unpleasant experience to health officials and epidemiologists who have to take time and money to investigate yet another outbreak from a bacterium or virus that had been thought beaten and yet somehow returned. But this year has seen one of the most troublesome comebacks in years and there appears to be no end in sight.

The West Nile Virus (WNV) has been by far this year's sequel star. The virus is known as a cause of a neurological disease that can lead to meningitis and death. The virus' natural hosts are birds, particularly crows, and transmission can only occur through the bite of a mosquito.

Until the 1990s, cases were always sporadic and outbreaks were limited to Africa and the Middle East. That changed in 1999 when the virus was accidentally imported into New York City. While the exact route from Africa to America is still unknown, the virus somehow managed to enter the mosquito population and then into both birds and humans.

The outbreak caused 59 infections and seven deaths but that was just the beginning of the WNV epic. As the new millennium arrived, the virus spread through bird migration all over the United States and Canada causing at its height in 2006 over 4,000 cases and nearly 200 deaths. By 2008, however, the cases numbers receded and by 2010, WNV was barely on the radar.

But now WNV is back with a vengeance and has the entire United States populace on the edge of its seat. So far, the virus has already infected 1,500 people and killed 41. In Canada, the worry has yet to be fully realized as there have been less than 100 cases and no deaths. But the virus continues to spread and the number of cases is expected to rise substantially.

The origin of this comeback is a mystery as there has been little to no indication that the virus was even capable of returning with such force. The virus is highly stable, meaning that it rarely evolves; when it does, the mutations are small and usually inconsequential to infection. Scientists fully expected that both bird and human immune systems would develop a lasting protection against the virus and thus prevent future infections. As predicted, immunity grew, cases dropped and a future without WNV appeared to be close.

This year, however, the virus has done the unexpected and evolved; gaining the ability to evade the immune system and once again cause disease. This has been a complete shock that continues to confound researchers and health officials. Unfortunately, the actual mechanisms behind this change have yet to be uncovered as the majority of effort in the fight against the disease is focused on treating the sick and keeping the message of prevention alive. Eventually the data will emerge although the timing will be far too late to stop the current outbreak.

Evolution is not the only reason WNV is back. In Africa, the virus was cyclical and depended on the availability of water. When the water levels were low, birds and mosquitoes shared the same water source, allowing more infection in birds and more virus-carrying mosquitoes. The same situation occurred in New York City in 1999 as a drought afflicted the entire northeastern region. Mosquitoes and crows intermingled where water was available and the virus spread. In the same way, this year's crippling drought has given the virus the ability to spread by forcing the mixing of the two species.

While the WNV resurgence has gripped North America, it is not the only germ making a return. This year alone, there have been two other sequels that are grabbing headlines and leading to fear and worry. The Ebola virus reared its ugly head in Uganda causing an outbreak that left 24 infected and 17 dead. The virus has since made a comeback in the Democratic Republic of the Congo and has so far taken 11 lives. Also "Swine Flu 2" has emerged causing over 250 infections and one death. So far, the new flu appears have stalled in its spread but fears of a wintertime pandemic sequel continue to be raised.

The cyclical nature of infections is always on the mind of public health officials and epidemiologists worldwide. However, this year has been a bellwether indicating that Hollywood alone is not going to be increasing the number of sequels. While these returning health villains continue to represent only a minority of worldwide infections, the future is looking rather glum. With a combination of microbial evolution and environmental changes, there are certain to be more germs that will re-emerge and send us all into a frenzy that may surpass that of The Dark Knight films.