West Nile Fever: Background and Epidemiology

1
West Nile Fever Background and Epidemiology

• First isolated in West Nile district, Uganda,
  1937.
• Commonly found in humans, birds and other
  vertebrates in Africa, Eastern Europe, West Asia,
  and the Middle East, but has not previously been
  documented in the Western Hemisphere.
• Basic transmission cycle involves mosquitoes
  feeding on birds infected with the West Nile
  virus.
• Infected mosquitoes then transmit West Nile
  virus to humans and animals when taking a blood
  meal.

2
(No Transcript)
3
West Nile Virus Transmission Cycle
Mosquito vector
Incidental infections
West Nile virus
West Nile virus
Incidental infections
Bird reservoir hosts
4
West Nile Virus in Domestic Animals

• Domestic rabbits, (swine, dogs)
• Others
• chipmunks
• gray squirrels
• bats
• eastern striped skunks.

5
West Nile Virus in the United States
National Surveillance Plan

• Monitor geographic and temporal spread
• Develop more effective strategies for
  surveillance, prevention and control
• Define regional distribution and incidence of
  other arbovirus diseases
• Provide current national and regional information

6
Partnerships for West Nile Virus Surveillance and
other activities in the United States

• Centers for Disease Control and Prevention
• U.S. Department of Agriculture
• U.S. Geological Survey
• Department of Defense
• Environmental Protection Agency
• State and Local Health Departments
• State and Local Veterinarians
• State and Local Wildlife Biologists

7
Partners with MDA

• CDC, USFW, USDA
• MSU, MDCH, MDNR
• Local Mosquito Districts
• Local Health Departments
• Physicians
• Veterinarians
• Nature Groups
• Others

8
West Nile in Birds

• West Nile infects and multiplies in birds
• Bird serve as reservoirs in nature
• At least 76 kinds of birds have the potential to
  carry the virus
• The disease effects bird in a variety of ways,
  including death
• American Crows often die from the infection

9
West Nile in Birds

• The first bird documented with WNV in the Western
  Hemisphere was identified in August, 1999
• Since this first incidence, WNV has spread
• One reason is sue to migration of birds

10
Birds spread the virus

• Bird migrating to different areas carry WNV with
  them in their blood
• This greatly effects the spread of West Nile
  Virus across North America

11
Clinical Signs in Birds

• Inability to fly
• Incoordination
• Abnormal movements
• Ataxia
• Weakness
• Tremors

12
WNV Surveillance Modalities for Birds

• Dead birds (especially crows)
• Mosquitoes
• Live wild bird sentinels
• Live captive sentinels (e.g., chickens)
• Veterinary surveillance
• Human surveillance

13
Chickens??

• There has been research into whether or not
  chickens would contract WNV
• Inoculation of 21 chickens with needle, oral, and
  mosquito bite
• None showed clinical signs
• All developed neutralizing antibodies
• Viremia was established in all but one chicken,
  but the level of virus was insufficient to infect
  vector mosquitoes

14
Cases in the US

• In 1999 292 avian cases (8 of which were crows)
• By 2000 4312 avian cases (still mostly crows)

15
Avian Cases from 2000

• Courtesy of Dr. Jon Patterson

16
Virus Cycle

• The virus circulates from the bird reservoir to
  mosquitoes
• Occasionally the virus can infect horses or
  humans, which are considered incidental hosts
  because they cannon continue the transmission
  cycle

17
Cycle of WNV cont.

• Enzootic cycle involves transmission from bird
  to bird
• Mosquitoes bite, or take a blood meal, from an
  infected bird
• The mosquito bites a second bird and transmits
  the virus

18
Testing of Dead Birds

• Necropsy crows and blue jays
• Histopathology of major organs
• Immunohistochem for WNV
• PCR for WNV
• Courtesy of Dr. Jon Patterson

19
(No Transcript)
20
Dead Birds in the U.S.,WNV Confirmed
21
Birds Collected for Testing

• American Crows, Blue Jays, and Fish Crows,
  members of the Corvid family, which was most
  affected by the WN virus outbreak in 1999, were a
  top priority for submission, followed by raptors
  and house sparrows
• As the outbreak progressed, birds from counties
  without documented WN virus were given higher
  priority, as well as migrating species of birds.

22
Necropsy of Birds

• Organs collected for laboratory testing included
  brain, kidney, heart, liver, and spleen.

23
Looking for WNV

• During gross pathologic exam, the pathologist is
  looking for the following signs of WNV
• emaciation
• Splenomegaly
• Hepatomegaly
• cardiac or pericardial lesions
• possible signs of encephalitis

24
Pathological Findings

• Some reports indicate that the connection between
  the bird testing positive for WNV and having
  suspicious pathological lesions was highest in
  American Crows (51.8)
• The positive predictive value (i.e. the number of
  birds having suspicious pathological lesions and
  also testing positive) was relatively low (27.9)
• WNV infection was confirmed with at least 2
  positive assays
• Birds tested for WNV in NY state in 2000

25
Gross Pathologic Lesion in Birds

• Pictures courtesy of Dr. Jon Patterson

26
WNV Surveillance Totals, 2000Reported to CDC
through January 25, 2001

• Dead birds 4312
• Mosquito pools 480
• Horses 63
• Other mammals 25
• Humans 21
• Sentinel chickens 13
• Confirmed Probable cases

27
West Nile Virus in Horses
28
Transmission Cycle
29
Clinical Disease

• Listlessness
• Stumbling
• Incoordination
• Weakness of limbs
• Ataxia
• Partial paralysis
• Possible death

30
Diseases With Similar Signs

• Equine Protozoal Myloencephalitis (EPM)
• Herpes
• Rabies
• Eastern Equine Encephalitis (EEE)
• Western Equine Encephalitis (WEE)
• Venezuelan Equine Encephalitis (VEE)

31
Distribution of EEE, WEE, VEE
32
Need for Surveillance

• Zoonotic Disease
• Year 2000
• EEE
• WEE
• WNV

33
National WNV Surveillance Veterinary Case Testing
2000
34
Michigan WNV Active Surveillance 2000
35
Michigan Equine Arbovirus Surveillance 2001 Goal
36
Current Veterinarian Participants
37
2001 Vector-borne Disease Surveillance Status
Report

• 32 counties participate in mosquito, bird
  surveillance.
• 20 veterinary practices covering 35 counties.
• Over 25,000 mosquitoes and 1,066 bird sera
  samples collected July-August.
• 4 labs MDA, MDCH, MSU-AHDL MSU-MEL test for
  EEE, SLE, and West Nile virus.

38
WNV Confirmed Equine Cases in the U.S.
39
Diagnosis of WNV

• Testing blood for antibodies
• Samples are sent to NVSL or MSU AHDL
• Diagnosing live animals is difficult

40
Treatment of WNV

• Currently no treatment
• Supportive therapy and prevention of secondary
  infection
• Approximately 65 recover
• Long term immunity likely

41
Equine WNV Vaccine

• Fort Dodge
• Provisional USDA License
• Killed, adjuvanted vaccine
• Two series vaccine
• Vaccinated animals will test positive for West
  Nile Virus
• Given annually

42
References

• Dr. Jon Patterson, MSU AHDL CVM
• Dr. Steve Halstead, MDA
• http//www.cdc.gov/ncidod/dvbid/westnile/misc/slid
  es/index.htm
• www.mda.state.mi.us
• www.cdc.gov
• http//www.cdc.gov/ncidod/eid/vol7no4/langevin.htm