Mosquito-Transmitted Disease in Minnesota

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Mosquito-Transmitted Disease in Minnesota
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Diseases Transmitted by Mosquitoes in Minnesota

• LaCrosse encephalitis
• Western equine encephalitis
• Eastern equine encephalitis
• West Nile virus disease

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LaCrosse Encephalitis

• Family Bunyaviridae genus Bunyavirus,
  California group
• Majority of cases are asymptomatic
• Headache, fever, nausea, vomiting, lethargy,
  seizures which may be frequent
• Disorientation, coma
• Median case age in MN 6 years old
• 1 mortality in symptomatic cases

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LaCrosse Encephalitis

• Vector Aedes triseriatus (Eastern tree hole
  mosquito)
• Forest-dwelling mosquito
• Viral amplification occurs during the summer from
  viremic chipmunks and squirrels
• Virus over-winters in mosquito eggs

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LaCrosse Encephalitis Tree Hole Mosquito
Breeding Sites

• Breeding site (stagnant water)
• any water collecting containers
• old tires
• treeholes
• Location shade, proximity to people

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Reported Cases of LaCrosse Encephalitis by County
of Residence, Minnesota, 1985-2007
n 123 cases
Sherburne
Anoka
1
71
Wash-ing-ton
Wright
Hennepin
6
Ram- sey
1
18
Carver
1
McLeod
8
Scott
Dakota
5
5
Sibley
1
2
Rice
Goodhue
Nicollet
Wabasha
Le Sueur
2
13
1
5
Brown
Dodge
Olmsted
Blue Earth
Winona
Steele
Waseca
2
14
2
7
Freeborn
Houston
Faribault
Mower
Fillmore
7
1
21
Cases include confirmed and probable cases
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Reported Cases of LaCrosse Encephalitis by Date
of Illness Onset, Minnesota, 1985-2007
n 123 cases
Number of Cases
Jun
Jul
Aug
Sep
Date of Onset
Cases include confirmed and probable cases.
Three cases had unknown onset dates.
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Western Equine Encephalitis (WEE)

• Family Togaviridae genus Alphavirus
• Majority of WEE infections are asymptomatic
  (10001 for adults 501 for children)
• Headache, high fever, chills, vomiting
• Progress to lethargy, disorientation, tremors,
  coma
• Mortality, in symptomatic cases 5-10

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Epidemiology of WEE

• Reported in Western Canada and states west of the
  Mississippi River
• Cycle involves birds, principal enzootic vector
  Culex tarsalis
• C. tarsalis breeds in sunlit grassy marshes
  feeds exclusively on birds in spring and early
  summer

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Epidemiology of WEE (cont.)

• A shift in feeding patterns to mammals occurs
  midsummer
• Few human cases reported in Minnesota last major
  outbreak was in 1975 (15 cases) single cases in
  1983 and 1999
• Humans and horses are dead-end hosts

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Eastern Equine Encephalitis (EEE)

• Family Togaviridae genus Alphavirus
• Majority of EEE infections are asymptomatic (401
  for adults 171 for children)
• Headache, high fever, chills, vomiting
• Progress to lethargy, disorientation, tremors,
  coma
• 50-75 mortality in symptomatic cases

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Epidemiology of EEE

• Occurs locally along the U.S. eastern and Gulf
  coasts
• Principal enzootic vector Culiseta melanura
• C. melanura breeds in freshwater swamps and feeds
  on birds
• Other mosquitoes transmit the virus to mammals
  while feeding

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West Nile Virus (WNV)
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WNV Background

• First isolated in 1937 from West Nile province in
  Uganda from blood of a febrile woman
• family Flaviviridae
• genus Flavivirus
• Japanese Encephalitis Antigenic Complex
• complex includes Alfuy, Japanese encephalitis,
  Kokobera, Koutango, Kunjin, Murray Valley
  encephalitis, St. Louis encephalitis, Stratford,
  Usutu, and West Nile viruses

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West Nile Virus Epizootology
Incidental infections
West Nile virus
West Nile virus
Incidental infections
Bird reservoir hosts
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WNV Encephalitis,New York City, 1999

• Physician awareness of two cases of encephalitis
  with muscle weakness in the same community
  (Queens)
• Initially suspected to be St. Louis encephalitis
• Concurrent mortality of crows noticed in the same
  area
• Deaths of several exotic birds from the Bronx Zoo
  were noted and sent to NVSL for testing

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Culex tarsalis, the mosquito primarily
responsible for transmitting WNV and WEE in
Minnesota
Image provided by Dale Parker
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WNV Reservoir Hosts

• Isolated from 308 wild bird species (1999-2005)
• Birds are primary amplifier hosts
• reservoir
• Migratory bird role in distribution and
  reintroduction of virus into northern latitudes
• Role of other vertebrates not known

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Human WNV Infection - West Nile Fever

• Majority of symptomatic WNV infections
• Sudden onset
• High fever
• Headache
• Fatigue
• Rash

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Human WNV Infection Encephalitis and Meningitis

• Less than 1 of infected people will have this
  severe form of WNV
• Stiff neck, lethargy, confusion, limb paralysis,
  weakness, tremors

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WNV Human Infection Iceberg
10 fatal (lt0.1 of total infections)
1 CNS disease case 150 total infections
lt1 CNS disease
Very crude estimates
20 West Nile Fever
80 Asymptomatic
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Human WNV Disease Incidence, by Age Group and
Clinical Category, United States, 2004
Incidence per 100,000
Reported as of 1/11/2005
Entire US population
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1999
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2002
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2005
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Human West Nile Virus Activity, 2007
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Human Cases of WNV in U.S.
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Human Cases of WNV in Minnesota
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Human WNV Cases in Minnesota, 2002-2007 by
Clinical Severity
No. of Cases
()
Diagnosis
270 89 82
Fever Aseptic meningitis Encephalitis
(61) (20) (19)
N 441 cases
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Human WNV Cases by Median Date of Illness Onset,
Minnesota, 2002-2007
2007 (n101)
Year and Number of Cases
5
10
15
20
25
5
10
15
20
25
5
10
15
20
25
5
10
15
20
25
5
10
15
20
25
Oct
Aug
Jul
Sep
Jun
Onset Date
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Minnesota West Nile Virus Monitoring Final
Summary - 2002
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WNV Monitoring Minnesota, 2003
Mille Lacs 1 human 1 bird 1 horse 3 asymp.
horses 5 mosq. pools
Anoka 1 human 1 horse 1 wolf 37 birds
Washington 2 humans 7 horses 19 birds 1 mosq. pool
Ramsey 5 humans 88 birds 1 dog 5 squirrels
Hennepin 11 humans 3 horses 145 birds 14 mosq.
pools
Carver 1 human blood donor 1 horse 4 birds
Scott 1 human 2 human blood donors 3 horses 3
sentinel chickens 14 birds
Dakota 5 humans 2 human blood donors 4 horses 28
birds
Olmsted 2 humans (1 death) 2 horses 2 birds
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Human WNV Cases in Minnesota, 2002-2007
Residence of locally-acquired cases
Rural Small town City/suburb
190 (52) 99 (27) 74 (20)
n363 (82) of 441 cases that may have been
exposed at home
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WNV Infection Prevention

• Apply insect repellent (DEET) sparingly to skin
• Spray clothing with insect repellent (DEET or
  permethrin)
• Long-sleeved shirts and long pants
• Stay indoors from dusk until dawn
• Install and repair door and window screens

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Prevention N,N-diethyl-m-toluamide (DEET)

• Long history of use very few confirmed cases of
  toxic reactions
• Higher of DEET protection not better just
  lasts longer
• 6.5 DEET 2 hours of protection
• 24 DEET 5 hours protection
• protection plateaus at 50 DEET
• Infants and children maximum concentration
  recommended by AAP is 30 (approx. 5 hours of
  protection)

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For More Information, Please Contact
Minnesota Department of Health Acute Disease
Investigation and Control 651-201-5414 1-877-676-5
414 www.health.state.mn.us