April 2005

West Nile Virus in 2004

Compared to 2003, the effects of West Nile Virus (WNV) were less severe in 2004, throughout the Country. The virus continued it’s march westward to the Pacific Ocean and has now been detected throughout the continental United States. The states hardest hit in 2004 were California, with 771 reported cases and 23 deaths and Arizona, with 391 reported cases and 14 deaths. In 2004 there were 2,470 reported cases with 88 deaths in the U.S., much improved over 2003, when there were 9,862 reported cases with 264 deaths. The numbers for Florida reflected a similar drop. In 2003 Florida had 94 reported cases and 6 deaths, while in 2004, last year, there were only 42 cases with 2 deaths, more than a 50% reduction of cases.

                        So the question is why was the 2004 season less severe that 2003? You would think that the more territory exposed to the WNV the greater number of cases there would be. Is the virus dying off? Is it because of mosquito control efforts? Are people getting the message and taking measures to protect themselves from mosquito bites? There are many theories being proposed for the behavior of the West Nile Virus. Last year, in Florida, one theory for the mild effect of WNV was that the numerous hurricanes disrupted the cycle of the disease. This may be so for Florida but why the mild WNV activity in the rest of the Country? The states of California and Arizona were the hardest hit by WNV, new areas, never before exposed to WNV. Which points out another aspect of viruses. Upon their introduction to a “virgin” area, viruses will spread rapidly and virulently through a newly exposed host population provided conditions are favorable. After this initial infective period the transmission rate slows or “burns out”. This burnout occurs because host organisms that survive infection are now immune. After a year or two, instead of the entire host population being susceptible like it is upon the introduction of the disease, only a percentage are susceptible. When this occurs the disease cycle is short circuited, transmission is slowed, outbreaks are less severe and the disease becomes more focal and sporadic, affecting smaller areas on a limited basis.

            Some authorities believe, that in Florida, we have not yet experienced the “big event” like Colorado’s epidemic in 2003 when they had 2,947 reported cases. If Florida experiences a WNV event like Colorado did, we will most likely have thousands and perhaps tens of thousands of cases because of Florida’s larger, more dense population.     Another theory concerning the behavior of WNV is that outbreaks of WNV will occur when conditions favorable for the disease cycle take place... simultaneously. There are 3 conditions necessary for the disease cycle. First, the West Nile Virus must be present. WNV is and will continue to be in our environment, reservoired in birds and mosquitoes. Second, a susceptible, amplifying host population is needed. In the springtime, newborn birds, hatchlings, are numerous and susceptible, never before having been exposed to the virus. The third condition required for the disease cycle is a vector, mosquitoes capable of incubating and transmitting the virus... lots of them. When these conditions coincide, WNV breaks out. The one variable factor in this cycle is the weather. The virus is here, birds have their broods in the spring but the weather (rainfall) can vary. If there is an excessively wet spring that creates standing water, (breeding habitat for mosquitoes) expect WNV to breakout. Otherwise, if it is a normal or dry spring the virus is merely maintained and occurrence of the disease is sporadic.

            Since 1999, the year WNV was discovered in the U.S., a great deal of research has been done and data collected concerning the effect and behavior of this virus. A lot of questions remain. The knowledge obtained over the years has given us the ability to plan and prepare for outbreaks of the disease. We can’t get rid of the virus, we can’t get rid of birds, we can’t control the weather, but we can affect one condition needed for the disease cycle and that is to control the mosquito populations that transmit the disease.