The West Nile Virus Essay Example

Aviv, while the one in 1951 occurred 15 miles southeast of Tel Aviv.

In 1952-1954, cases were reported in both areas, showing the virus's ability to spread epidemically within a localized area. From 1962-1964, epidemics of WN occurred in the Rhone delta region of France. The largest epidemic was reported in South Africa in 1974, with thousands of clinical cases. South Africa also experienced epidemic activity in 1983 and 1984. Human cases were reported in southeast Romania in 1996 and 1997. The most recent introduction of WN virus(es) was in the northeastern region of the borough of Queens in NYC during the summer of 1999.

The West Nile virus shows greater mobility compared to similar viruses such as the SLE viruses in North America and the MVE viruses in Australia. West Nile viruses have successfully spread from Africa to Western Europe, the Middle East, Eastern Europe, and currently North America. In contrast, SLE and MVE, which are close relatives of WN, have remained confined to their respective regions. In humans, infection with WN can lead to clinical or subclinical symptoms. Clinical symptoms range from temporary fever to severe encephalitis. While older adults may experience severe forms of the disease, it tends to be mild in healthy adults and children.

The incubation period for WN is 3-6 days and the onset of disease symptoms is sudden. Symptoms include persistent high fever, severe headache, rash primarily on the trunk, swollen lymph nodes, eye pain, muscle pain, back pain, gastrointestinal problems. Severe cases may exhibit signs of encephalitis leading to neurological complications and potentially death. During the infection, humans experience a low-level viremia lasting approximately 6 days.

The mortality rates in humans

vary from 5 to 13%. Only horses have shown clinical signs of WN infection among domestic animals, although most horse infections are symptomless. However, during the WN epidemic in France from 1962-64, horses had a mortality rate of 25%. Similar to horses, WN can infect cattle, sheep, and camels, but these hosts have not shown clinical symptoms or viremia capable of spreading the infection to arthropod vectors. On the other hand, birds do experience viremias capable of infecting arthropod vectors.

Mosquitoes, including Culex univittatus and Culex pipiens, are natural vectors of WN. Most virus isolates have been found in mosquitoes, indicating that they are the primary vectors. In Africa, Culex univittatus is the main WN vector, while in South Africa it is a secondary vector and in Israel it may be the primary vector.

Members of the Cx. vishnui complex have been identified as the main carriers in India and Pakistan, whereas infected ticks from the Argas, Hyalomma, and Ornithodoros genera have been reported in northern Africa and eastern Europe. The specific vector(s) that caused the 1999 NYC outbreak remain unknown.

The most probable contenders for transmitting West Nile virus are members of the Cx. pipiens species complex. These mosquitoes have been connected to West Nile outbreaks in other parts of the world and are prevalent in NYC during the summer. Like other viruses spread by mosquitoes, many different vertebrate species have been found to have natural infections with WN.

Wild and domestic birds, including Hooded Crows and House Sparrows in Egypt, show consistent evidence of infection with West Nile (WN) virus. In Egypt, both Hooded Crows and House Sparrows exhibited a high prevalence of antibodies associated with the

virus. Additionally, naturally infected Hooded Crows have been found to carry the WN virus. Furthermore, horses with clinical encephalitis infection during the 1962-64 outbreak in France, as well as camels in Sudan, were found to be infected with West Nile. In Eastern Europe, a WN virus was isolated from a tick collected on a camel in Central Kara-Kum. Moreover, pigeons in Egypt, turtledoves in Turkey, and various wild bird species in Borneo, Cyprus, and Nigeria have been found to carry the West Nile virus. Experimental infection of crows with WN has resulted in a high mortality rate.

Although some crows in nature can resist infection despite being exposed to individuals with antibodies, a domestic pigeon in Egypt showed signs of illness and was infected with WN. Urban areas with large populations of domestic pigeons may contribute significantly to epidemics like the one observed in New York City in 1999. WN viruses have been detected in the blood of infected humans, particularly during the initial stages of illness. A study conducted in Israel involved daily collection and analysis of blood samples for the virus.

The rate of isolation for West Nile virus was 77% on day one, followed by 27% on day two, 18% on day three, and 6% on day four. In the early 1950s in Egypt, febrile children produced twenty-three strains of WN virus. The introduction of WN to NYC is a topic that will likely generate debate, with various potential mechanisms for its entry. One possibility is that WN has been circulating in low-level transmission cycles in northeastern USA for years and only became noticeable in 1999 due to environmental conditions favoring epidemic transmission of

a flavivirus like WN.

Urban epidemics of SLE in North America have often happened during very dry summers, similar to those in NYC in 1999. There is also a potential recent introduction of WN to the NYC area. It is feasible that a person newly infected with the virus may have come from Africa, Eastern Europe, or another location where WN transmission is ongoing, and arrived in NYC when their viremia (the amount of virus in their peripheral blood) was at its highest. If an individual's viremia is at a level where it can infect mosquitoes that act as vectors (specifically Cx.), then transmission may occur.

pipiens in NYC), some mosquitoes that feed on the infected blood will become infective. Newly infected mosquitoes need a temperature-dependent extrinsic incubation period (EIP) of at least 2 weeks to become infective. During the EIP, the virus multiplies in mosquito tissue and eventually infects most internal organs, including the salivary glands. Once the salivary glands are infected, the mosquito can transmit the virus to other hosts during blood feeding. In NYC, these hosts may have included local wild birds such as crows, pigeons, sparrows, gulls, or unidentified amplification hosts, establishing an amplification and transmission cycle that spreads the virus beyond the initial transmission focus.

The focus areas during the 1999 New York City outbreak were likely the Whitestone, Auburndale, and Flushing neighborhoods in Queens. It is also possible that a bird carrying the WN virus was brought into the city, either legally or illegally. Legally imported birds are quarantined for at least 30 days to prevent contact with arthropod vectors. However, illegally imported birds are not quarantined and could have introduced the

virus to local mosquitoes. Another possibility is infected ticks or mosquitoes hitching a ride on an international flight and being found at Kennedy International airport.

Infected nymphs or adult ticks may have also attached themselves to a human traveler or a wild or domestic animal and thus traveled. These ticks may have then fallen off and fed on a susceptible animal in New York, starting a transmission cycle in the city. It is also possible that the virus was accidentally released from a legitimate scientific experiment or intentionally released as an act of bio-terrorism. Releasing any mosquito-borne pathogen, whether accidental or intentional, would require timing that aligns with conditions that support virus amplification in vertebrate hosts and transmission by suitable vectors, before the virus can spread widely to humans. West Nile viruses have distinctive RNA profiles that can be used to identify different genetic forms, which in turn can be linked to their original geographic location.

Recent studies have shown that the RNA profiles of West Nile (WN) viruses, including those from the New York City (NYC) outbreak, are very similar. The NYC WN isolates are also closely related to a virus found in an Israeli goose in 1998, suggesting that there is only one genetic strain of WN in North America. This supports the idea that the virus was recently introduced and has not yet diversified into different strains.

THE FUTURE OF WEST NILE VIRUS IN NORTH AMERICA.

Regardless of its origin, it is highly probable that WN virus will continue to exist in North America. However, whether and when it will spread beyond the NYC metropolitan area remains uncertain.

The importance of whether this virus will persist during

the winter, either in a vertebrate reservoir or in an overwintering mosquito, is significant. Current observations indicate that both possibilities for persistence have occurred. In January and February 2000, three pools of Culex species were collected at Fort Totten, New York City (northeastern Queens). The highly sensitive assay called reverse transcriptase polymerase chain reaction (RT-PCR) was used to detect WN virus RNA in these pools. Further analysis showed that one out of the three RT-PCR positive pools contained live WN virus. This discovery was confirmed by staining infected cells with a WN-specific monoclonal antibody and sequencing the virus gene.

Additionally, in mid-February 2000, WN virus was isolated from a Red-tailed Hawk that was found moribund near Bronxville, New York (north of NYC and east of Yonkers, NY).

These data demonstrate that live WN virus was consistently found in the NYC metropolitan area in mid-winter 2000. The transmission of WN could be facilitated by different factors, including infected humans, infected vectors (ticks and mosquitoes), and infected amplification hosts (domestic birds, wild resident birds, and wild migratory birds). It is not difficult to envision how this virus could quickly disseminate across the nation. Many cities harbor a substantial population of vector mosquitoes capable of transmitting this virus. Cx. species are merely a few illustrations.

Multiple mosquito species in North America have a high likelihood of transmitting the West Nile (WN) virus to both birds and humans. These species include Culex pipiens in the northern region, its close relative Culex quinquefasciatus in the southern region, Culex tarsalis in the western region, and Culex nigripalpus in the deep south.

Monitoring the spread and introduction of West Nile Virus (WN) across North America is

crucial for surveillance purposes. It is imperative to remain vigilant nationwide due to the presence of various mosquito and tick species that can contribute to the transmission of WN within specific regions. Detection of WN in domestic avian populations will occur if it is introduced into another part of the USA.

In order to effectively monitor the spread of WN in new cities and areas, surveillance efforts involve tracking vectors, hosts that amplify the virus, weather patterns, and the virus itself. Throughout the USA, mosquito and vector control programs already possess extensive experience in monitoring St. Louis Encephalitis (SLE), which closely correlates with WN (refer to page on SLE).

Florida has developed a comprehensive integrated surveillance program for St. Louis encephalitis (SLE), which serves as a model for other regions at risk of arboviral transmission, including West Nile (WN), SLE, and dengue viruses. Severe outbreaks of SLE have occurred in areas like the upper midwest, Louisiana, Texas, and California. In the southern USA, dengue viruses pose a significant threat to humans. Therefore, it is crucial to establish vigilant surveillance systems to detect epidemics before they catch local, state, and federal health officials by surprise. The sudden appearance of a high number of infected individuals in cities such as NYC demonstrates this necessity.

The accurate assessment of a vector borne epidemic's risk will provide public health authorities with time to implement control strategies and public awareness campaigns that can lessen the impact of an epidemic. Another important concern is how to minimize the impact of WN or other vector borne pathogens when they establish themselves in a region. It is crucial to apply the most efficient and effective control

or risk management strategies. Authorities base their decisions on scientific information about the pathogen and vectors involved, as well as local or regional environmental conditions. It is widely acknowledged that vaccinating large human populations to prevent a vector borne epidemic would be extremely costly and challenging, even if vaccines for these viruses were available.

Vaccines are not currently accessible for most arthropod-borne pathogens, such as WN and SLE. One of the most crucial methods for preventing diseases is personal protection against infected biting arthropods. To avoid disease, it is important to steer clear of mosquitoes and ensure that house screens are intact.

If you need to go to areas with infected mosquitoes, wear protective clothing and use a personal insect repellent with a reasonable Complete Protection Time (CPT). The CPT is the duration after applying the repellent that the person will not get bitten. For instance, a 5% DEET formulation, which is currently the most effective repellent, provides approximately 2 hours of CPT under normal conditions.

The CPT for a 24% DEET formulation is over 4 hours. Thankfully, the USA has exceptional mosquito and arthropod control programs. The best way to prevent infection from vector-borne pathogens is through vector control and personal protection. For instance, effective strategies for WN in NYC include reducing mosquito breeding sites, applying insecticides to adult and immature mosquitoes, informing residents about the vector and disease, providing tips to prevent home invasion by infected vectors, and educating about personal protection. Recent outbreaks in Florida have shown that disseminating accurate information through the media is highly effective in reducing human infection rates of SLE. Knowledge and personal protection are an individual's primary defense during a

vector-borne disease emergency.

The US Centers for Disease Control and Prevention (CDC) has established the National WN Virus Surveillance System for 2000 in response to the introduction of the WN virus to North America. The purpose of this surveillance system is to monitor the potential spread of the WN virus in the eastern and southern regions of the US, develop national strategies for surveillance, gather information on the distribution and incidence of other important arboviruses in the area, provide information to public health officials and the public, and assess funding and resource needs. References to studies on West Nile Virus isolation from mosquitoes, crows, and a Cooper's Hawk in Connecticut, as well as arbovirus surveillance in Florida, are also included.

Journal of Wildlife Diseases 11:348-356.

Endemic eastern equine encephalomyelitis in Florida: A twenty-year analysis, 1955-1974. American Journal of Tropical Medicine and Hygiene 25:884-890.

CHAMBERLAIN, R. W. 1980. History of St.

Louis encephalitis is a topic discussed in the book "St. Louis encephalitis" edited by T. P. Monath for the American Public Health Association, Washington D.C., spanning pages 3-61.

2000. MMWR 49:178-179. West Nile virus surveillance in overwintering mosquitoes in New York, 2000. DAY, J. F.

AND G. A. CURTIS. 1993. Annual emergence patterns of Culex nigripalpus females before, during, and after a widespread St. Louis encephalitis epidemic in south Florida.

Journal of the American Mosquito Control Association. 9: 249-255.

CURTIS. 1994. When it rains, Culex nigripalpus, a dangerous mosquito, soars. This information is mentioned in an article

by American Entomologist 40: 162-167.