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Table 1: Examples of Surveillance and Laboratory Workload Experienced by Selected Involved Agencies During and Since the West Nile Outbreak

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Example of workload experienced during and since the outbreak

August to
December
1999

August to
December
1999

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Human case surveillance: The city health department investigated 622 suspect hospitalized cases and 88 suspect outpatient cases that were reported. Case investigation included contacting physicians and hospitals, interviewing patients, and coordinating specimen collection and transport.

Animal and mosquito surveillance: The city set up and maintained mosquito traps, established
a hotline to receive dead bird reports, and triaged the collection of specimens and submission
for testing to appropriate laboratories.

Human case surveillance: The state and local health departments (outside of NYC)
investigated 229 suspect cases, of which 196 were hospitalized.

Human sample testing workload: Polymerase chain reaction (PCR) tests for 13 viruses were
performed on 198 specimens from 190 patients. Serology tests were performed on almost
600 specimens. A laboratory official indicated that before the outbreak, the average number of
requests for arbovirus tests was 40 to 50 per year.

Animal and mosquito surveillance: As many as 17,000 dead birds were reported by local health departments to the state (one-third were crows). The state health department coordinated submissions for testing, with 130 dead birds confirmed positive (at federal laboratories). Twenty-five horse cases were reported. Over 25,000 mosquitoes (in more than 1,500 pools) were collected, with 15 pools testing positive (at CDC).

Avian sample preparation and pathology investigations: The department received, catalogued, and performed necropsies (postmortem examinations) on 880 dead birds submitted from various parties across the state; prepared and mailed samples to various laboratories for testing.

Human sample testing workload: In 1999, CDC-Fort Collins received over 1,200 samples from New York alone for testing, representing more than 700 patients. In past years, CDC has received 10 to 20 specimens per year from the state.

Avian sample testing workload: CDC tested approximately 1,000 avian tissue samples and
3,000 serum samples from all locations.

Avian sample testing workload: The Center received almost 1,200 specimens from 22 states
and local jurisdictions, including nearly 500 animal carcasses for necropsy and testing and
699 tissues for testing for West Nile virus from states such as New York and New Jersey.
Live bird surveillance: The Center received almost 1,500 serum samples for West Nile testing
from collaborative surveillance activities with USDA in 10 states and from zoos and
endangered species in captivity. Collected over 1,000 blood samples from wild birds in New
York City in October 1999 for expanded surveillance.

Animal sample testing workload: The laboratory performed almost 120 virus isolation attempts
and 640 serum sample tests.

Note: Several other state and private agencies experienced significant workloads. These examples
represent New York City and State as well as federal government activities.

*PCR is a laboratory process in which a particular ribonucleic acid (RNA) or deoxyribonucleic acid
(DNA) segment is rapidly replicated, producing a large, readily analyzed sample of a piece of RNA or
DNA.

"In addition to laboratory testing, a large number of staff are involved for surveillance and control and to respond to public inquiries, set up mosquito traps, collect birds and mosquitoes for shipment, collect blood samples from horses, establish computerized databases, complete specimen and summary reports, prepare public information and maps, organize prevention and response programs, and so forth.

Specimens from almost 20 crows stored in 1998 were sent for West Nile testing in the fall of 1999.

Many Federal, State, and
Local Activities Have
Continued Since the Initial
Outbreak

While the first frost of the season signaled the end to the initial outbreak in 1999, activities at the national, state, and local levels have continued. In the first week of October 1999 the New York City Department of Health and CDC conducted a random survey of Queens residents to assess the overall infection rate associated with the outbreak. The results of this serosurvey (in which a blood test for West Nile antibodies is performed) revealed that between 1.2 and 4.1 percent of the population in the area surveyed had been infected with West Nile virus.

The change in diagnosis from St. Louis encephalitis to West Nile also caused public health agencies to evaluate whether aspects of their intervention response should be changed. While the West Nile and St. Louis encephalitis viruses are closely related and mosquito-borne, the change in diagnosis had some implications for the intervention approach. For example, past research had shown that different types of mosquitoes might carry the viruses. Both West Nile and St. Louis encephalitis are carried by a certain species of mosquito, Culex pipiens. However, West Nile is also carried by other species, including Aedes vexans and Anopheles. Some of these species have different habitat and activity patterns. For example, Culex pipiens breeds in polluted water and is active at night, while A. vexans has been found in natural areas and is active during the day. Once the distinction between the viruses was made, the public health interventions were changed accordingly to reflect the other types of mosquitoes potentially carrying the West Nile. For example, local public health notices stated that the public should also avoid contact with mosquitoes active during the day. While these differences are not considered significant since the public health recommendations for mosquito control are appropriate in either case, they illustrate the potential significance of an accurate diagnosis in that even closely related viruses might require different responses.

"Due to increased surveillance since the initial outbreak, a new species of mosquito, Aedes japonicus, has been found to carry the virus.

Some of the activity since the initial outbreak has involved learning more about where the virus came from and when it arrived. Research into the origins of the virus found that it is most closely related to a strain isolated in a goose found in Israel in 1998. Testing of previously stored bird tissue samples at the Bronx Zoo was negative for West Nile virus, suggesting the virus was introduced in 1999.

Much of the ongoing effort has been applied to determining whether West Nile will be an ongoing threat to animal and human health. The West Nile outbreak represents a potential problem stretching well beyond New York City, because the virus can spread through bird migrations. In fiscal year 2000, HHS and CDC will provide approximately $10 million for West Nile virus activities. This amount includes grants totaling $4.5 million available to 19 state and local health departments along the Atlantic flyway of migratory birds for West Nile virus surveillance in humans, mosquitoes, and birds. An additional $2.7 million of the $10 million has been made available to 31 other state health departments to expand surveillance capabilities. As of August 2000, communities in at least seven eastern states' had undertaken active mosquito control programs, such as spraying, as well as public education campaigns and surveillance activities.

Surveillance activities have already produced evidence that West Nile has spread to other areas. In October 1999 a dead crow carrying the virus was found in Baltimore, Maryland. In 2000, as of August, West Nile virus had been detected in birds in nearly all New York counties as well as in Massachusetts, Connecticut, Rhode Island, and New Jersey and in mosquito pools in several states. If West Nile is carried further south along bird migratory routes (see fig. 3 for examples), it could become permanently established in the Western Hemisphere.

"Massachusetts, Rhode Island, Connecticut, New York State, New York City, Pennsylvania, New Jersey, Delaware, Maryland, the District of Columbia, Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas.

"Connecticut, Delaware, Maryland, Massachusetts, New York, New Jersey, and Rhode Island.

Figure 3: Examples of Migratory Patterns of Three Bird Species Susceptible to West Nile

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Source: New Scientist, http://www.newscientist.com/nl/0708/birds.html.

The spread of the virus by birds and mosquitoes has significant implications for animal health as well. Animal health officials are concerned about the potential effects on wildlife and other animals, particularly those birds that are susceptible to fatal illness from the virus. The USGS, which conducts surveillance of wildlife health, has helped develop and maintain national maps showing the current wildlife surveillance data now submitted by states.8 Economic concerns also have been raised. While wild birds were the primary carrier of West Nile in last year's outbreak, the disease was also detected in domestic livestock. Twenty-five cases were identified in horses on Long Island, nine of which died or were euthanized. Although there is no evidence that the virus can spread from infected horses to uninfected horses or other animals, countries from Argentina to the United Arab Emirates placed import restrictions on horses from affected areas. In addition, the role of commercial poultry in maintaining or transmitting the virus is not thoroughly understood. CDC research has found that chickens can develop a short-lived infection without clinical signs.10

Several organizations, including CDC, USDA, the Wildlife Conservation Society, and Flushing Hospital, have organized conferences and workshops to review the West Nile virus outbreak. In December 1999, CDC issued guidelines for West Nile virus surveillance, prevention, and control. In the spring of 2000, HHS and USDA appointed West Nile coordinators to oversee efforts against the virus. See appendix III for a list of some key publications about or related to the virus outbreak.

Developed in partnership with National Atlas of the United States, http://www.nationalatlas.gov.

"Horses are thought to be terminal or “dead-end" hosts, in that they can be infected with the virus, but the virus does not develop in their blood at sufficient quantities to reinfect mosquitoes.

10As of August 2000, no clinical signs of the virus had been reported in poultry in the United States, according to USDA.

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