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Abstract & Introduction

Background

Materials & Methods

Results & Discussion

References

Appendices

BACKGROUND INFORMATION

Virology       Epidemiology       Trends in Human Cases        Future Patterns       Trends in Equine Populations
WNV Trends within Indiana        Research on WNV        Weather as a Factor in Spread

VIROLOGY

West Nile virus (WNV), often referred to as a singular infection, is actually a collection of several similar viruses from the family Flaviviridae (3). The genus Flavivirus is part of the Japanese Encephalitis Antigenic Complex that encompasses about sixty viruses including the closely related St. Louis encephalitis virus (SLE). Cross-reactivity with SLE resulted in some confusion with WNV in the initial New York outbreak in 1999. Subsequent genetic sequencing, however, allowed the virus to be identified as West Nile virus and not as SLE (4). WNV can be genetically divided into two lineages, only one of which has been directly linked to the human disease (5).
WNV is spherical in shape and is approximately 50 nm in diameter. It is a single-stranded RNA virus surrounded by an envelope (6). For an illustration of this virus, see Figure 1 (7). Recently, researchers have obtained their first high-resolution look at the West Nile virus; this image is presented in Figure 2 (8). The virus can cause a disease that ranges in symptoms from asymptomatic to fatal encephalitis (9). The percentages of symptoms are as follows: about 80% are asymptomatic, about 20% will develop a mild illness, and a very small percentage will develop severe, potentially fatal, encephalitis (6).

EPIDEMIOLOGY

The primary reservoirs of West Nile Virus are birds, and the most common mode of transmission to humans is by mosquitoes that have bitten infected birds (9). Humans, horses, various mammals and some reptiles are known to contract the virus (9). There is no evidence that West Nile virus can be transmitted from equines to other animals.
A recent study published in the New England Journal of Medicine discussed blood transfusions as a means of viral transmission (10). The study showed that 16 donors were responsible for the transmission of the virus to at least 23 mostly elderly or immunosupressed transfusion recipients (10). Other modes of transmission reported include: needle sticks, organ transplants, breast milk, and transmission across the placenta to the fetus (11,12).
In 2002, two laboratory workers involved in West Nile virus work were stuck with needles while performing an autopsy on an infected animal (12). Both workers contracted the disease, and it was determined that the source was the needle sticks because the workers had no other risk factors for the disease (12).
In 2002, four organ recipients contracted the virus, and all were thought to have received their transplanted organ from a single donor (12). The suspected donor presumably received the virus in a blood transfusion (12).
A new mother, in 2002, received West Nile virus from a blood transfusion (13). Her breast milk also tested positive for the virus, and since she had been breast-feeding her newborn baby, the baby was tested for the virus (13). The infant tested positive, and the mode of transmission was determined to be through the breast milk, as the baby had no other contact with any known causes of the disease (13).
West Nile virus can cross the placenta and infect the fetus (12). In 2002 a pregnant woman tested positive for West Nile virus, and when her baby was born, the baby had signs of the disease, including decreased brain development in the temporal and occipital lobes (12).

West Nile Virus, since its initial American outbreak in New York City in 1999, has continued to advance westward across the North American continent. Experts predict that the virus eventually will affect California in much the same way Colorado has been affected in 2003 (13). The following map presents a state-by-state listing of the total number of confirmed West Nile human cases presented to the Centers for Disease Control (Fig. 3).

In 2003, some 9100 cases of WNV were confirmed with 222 deaths. This is over twice the number reported in 2002 (4156 with 284 deaths). As expected, more cases were detected in states west of the Mississippi River.
Figure 4 shows the top five states with the highest number of West Nile Virus cases in 2002 and 2003, respectively. By comparing these numbers and the states that contained them, some interesting trends can be discerned. During 2002, high numbers of cases of the virus were reported in Illinois, Indiana, Michigan, Ohio, and Louisiana (14). In 2003, the top five states are all farther west. Most of these states are in their second year of exposure to the virus. Experts have postulated possible reasons for the dramatic drop in Illinois and Michigan including cooler temperatures and increased immunity in birds and humans. For example, Illinois has only reported 53 human cases as of January 2004 (14). Last year, they had the highest number of human cases at 884. Michigan also has the same pattern. In 2002, they reported 614 human cases, and as of January 2004, they have only reported 18 cases (14).

In this 2003 season, Colorado has been hardest hit. (13). During 2003, the state reported 2477 human cases of WNV, including 45 deaths, to the CDC (13). Looking back at 2002, Colorado only reported 14 human cases with no deaths. The reasons for these dramatically high numbers may revolve around avian immunity and weather conditions (13). The second year an avian is exposed to WNV is when the area may be the most vulnerable. Why? The locality has an avian population that has not yet built sufficient immunity to the virus. Thus, when the mosquitoes bite the infected birds, then they will be carriers of it to the human population. As time progresses, birds develop an immunity to the virus thus preventing the disease from spreading. WNV may be cyclical due to the fact the virus may resurge when the older immune birds die off (13). These factors could explain the possible trends in Colorado and other western states. In the eastern states, Pennsylvania reported a three-fold increase in human cases. Scientists in the state, where the virus is in its third year, attributed the increase to wet weather and additional counties being affected by the virus.

Because the virus appears correlated with avian immunity, states with lower numbers of West Nile Virus may eventually see an increase in numbers. California, which is basically in the first year of exposure, should expect higher numbers next year. This is, of course, dependent upon the amount of prevention and protection that people take in regard to WNV. States in their second year, such as Colorado and Nebraska, should observe a decrease in cases next year. States approaching the third and fourth years of WNV exposure may have the older immune birds dying off, prompting a resurgence of the virus.

Horses are affected by WNV more than any other domestic animal (15). It appears on inspection that the equine population follows the same pattern observed in the human populace. By examining maps from 2002 and 2003, one will notice an overall decline in the number of equine WNV cases. Horses prove to be susceptible to the virus on the basis of the level of avian immunity; so many of the states with an older immune bird population also have experienced less equine cases. The year 2002 was dramatically worse in relation to 2003 (15).

Figure 5 --Source: http://www.aphis.usda.gov/vs/nahps/equine/wnv/maps.html

By examining Figures 5 and 6, one can observe the difference in equine cases. For instance, those states in 2002 reporting 400 or more confirmed cases numbered 14 (15). In 2003, Colorado, New Mexico, and Pennsylvania reported cases above 400 (15). Total cases of WNV in horses were 15,257 in 2002 compared with just 4636 in 2003 (16). Reasons for this overall drop in equine cases may actually be linked to several factors. Horse owners have increased vigilance on adult mosquitoes in the area, thus limiting exposure to the insects (15). Possibly the most significant reason for the drop in WNV among horses is vaccination (15). In 2001, the USDA-APHIS’ Center for Veterinary Biologics released an equine WNV vaccine that has been proven to safe and effective. After initial doses of the vaccine, owners must give their horses a booster shot as the mosquito season starts (15). This has undoubtedly led to the decline in equine cases.

Figure 6 –Source: http://www.aphis.usda.gov/vs/nahps/equine/wnv/map2003.html

The state of Indiana has experienced a reversal in the occurrence of WNV within the state. In 2002, Indiana was in the top five of states with confirmed human positive WNV with 293 cases, including 11 deaths (16). In 2003, these numbers have declined to only 47 reported cases, with just 4 deaths (16). The following Figures 7 and 8, demonstrate this drop by highlighting the counties of Indiana with positive test results in red for the year 2002 and 2003 as of October 1, 2003.

Within southern Indiana, the WNV confirmed human cases have been rather low. Northwestern Indiana experienced a surge in the number of cases in 2002 in those counties near the Chicago metropolitan area. However, within the Louisville metropolitan area, only Floyd County in southern Indiana had a confirmed human case (16).
In 2003, confirmed or probable cases within southern Indiana have remained low. In Harrison County, Indiana one mosquito group, one dead bird and one horse have been confirmed to have WNV (17). In Clark County, Indiana, two mosquito groups, one horse and two dead birds have been identified to have WNV (17). In Floyd County, Indiana, three mosquito groups, one horse and one dead bird have been identified as WNV positive (17). This does not take into account observations not reported to the CDC. For instance, a dead crow was discovered on the campus of Indiana University Southeast, located within Floyd County, by IU police officer, Mr. Phil McBride. The crow was tested for West Nile by swabbing its mouth and testing the material removed. After performing the Vec-Test, the crow was determined to be positive for West Nile Virus. Not long after the crow had been tested, two human probable cases were discovered on campus. These individuals believe they contracted the disease away from the IUS campus.

Significant information is available on West Nile virus. However, research is continuing. As mentioned above, a vaccine is now in use for equines. This has led to the drop in equine cases noticed this year. Research also continues in the area of humans. Scientists are working to discover how West Nile works. It has already been discovered in mice that some antibodies, such as B cells, are critical in fighting the virus (18). The NIAID (National Institute of Allergy and Infectious Diseases) is also stepping up research on WN, and has created a new vaccine that has been successful with monkeys (19). Scientists substituted part of the West Nile virus proteins for a distant cousin, dengue type 4. This hybrid virus then protects the monkeys from a stronger, more potent WN type (19). All of these steps will be helpful in the fight against West Nile.

Scientists are now taking a closer look at the relationship between weather and the severity of outbreaks of West Nile virus and St. Louis encephalitis. Large outbreaks of West Nile virus and St. Louis encephalitis occurred in Midwestern states after prolonged periods of drought (20). Some 10 of 12 outbreaks of St. Louis encephalitis were associated with two or more months of consecutive drought. The increase in mosquito borne illness during lack of rainfall may be a result of drought-induced amplification (21). When a summer-fall drought occurs, water sources for birds are reduced significantly. Birds are eventually drawn to water supplies where mosquitoes are abundant. Many of these are organically enriched sites where Culex pipiens are present—drains, catch basins, areas of failing septic tanks, and stagnant ponds (20).
We compared the departures from average temperature and precipitation for 2002 and 2003 to determine if the patterns were significantly different (Figs. 9 and 10). In the first graph (2002), the mosquito season begins in March and April with significant rainfall. This is followed by a period of above average temperatures and very little rain in the summer months. This is the classic pattern for drought-induced amplification. In 2003, above average rainfall occurred in the spring, but the summer was much cooler with average to above average rainfall.
Based on these data, it is tempting to conclude that the number of cases of West Nile virus found in 2002 was amplified by weather conditions. Likewise, weather may have been a factor in the significant drop in cases in 2003.