Rift Valley Fever: Epidemiology of Human Disease

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Rift Valley FeverEpidemiology of Human Disease

• Rebecca Shultz, MPH
• Bureau of Environmental Public Health Medicine
• Florida Department of Health

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Transmission routes

• Majority - tissue or body fluids of infected
  animals
• Aborted fetuses
• Slaughter
• Necropsy
• Veterinary procedures
• Carcass disposal
• Aerosol
• Slaughter
• Laboratory

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Vector transmission

• Arthropod vector
• Mosquitoes
• Aedes
• Anopheles
• Culex
• Others
• Mosquito species in the U.S. could serve as
  vectors
• Biting flies are possible vectors

Center for Food Security and Public Health Iowa
State University - 2003
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Additional transmission routes

• Some evidence of infection from consuming
  uncooked/unpasteurized milk from infected animals
  
• No direct person-to-person transmission
• No evidence of transmission in health care
  settings with infection control measures in place

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Human Disease

• Incubation period 2-6 days
• Infections range from asymptomatic to severe
• Overall fatality less than 1
• Mild disease
• Flu-like signs
• Fever, headache, joint and/or muscle pain
• Stiff neck, photophobia, anorexia, vomiting
• Recovery in 4-7 days

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Severe Disease

• Retinopathy (0.5 - 2 of cases)
• 1-3 weeks after onset of symptoms
• Blurred or decreased vision
• Photophobia
• Can resolve in 10-12 weeks without treatment
• Can lead to permanent vision loss
• 50 in those with macular lesions
• Death is uncommon

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Severe disease contd

• Encephalitis (less than 1)
• Onset 1-4 weeks after initial symptoms
• Memory loss
• Confusion
• Disorientation
• Lethargy
• Coma
• Neurologic complications gt60 days later
• Low mortality, lasting neurologic damage

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Severe disease contd

• Hemorrhagic fever (less than 1)
• Onset 2-4 days after initial symptoms
• Liver impairment jaundice
• Hemorrhage gums, skin, nose, blood in stool
• Case fatality ratio 50
• Death usually occurs 3-6 days after hemorrhagic
  symptoms appear

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Diagnosis and Treatment

• Diagnosis
• ELISA, human blood
• Demonstration of viral antigen
• Treatment and vaccine
• May not be needed
• Symptomatic and supportive therapy
• No commercially available vaccine

Center for Food Security and Public Health Iowa
State University - 2003
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Important Outbreaks

• Senegal, Africa, 1987
• Differed from other outbreaks
• Not associated with rainfall
• Kenya, 1997-1998
• Est. 89,000 humans cases
• 478 deaths
• Saudi Arabia, 2000
• First outbreak outside of Africa
• Egypt, 2003
• 45 cases, 17 deaths

Center for Food Security and Public Health Iowa
State University - 2003
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Kenya, 2006-2007

• Associated with heavy rainfall/flooding
• Spread to Tanzania and Somalia
• 1,000 cases with 300 deaths
• Case fatality 23-45

Courtesy of CDC
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Risk factors associated with human disease

• Studies done during different outbreaks
• Male gender
• Close contact with animals
• Drinking raw milk
• Housing animals indoors
• Living lt100m from a swamp

RVF distribution map, courtesy of CDC
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Vector information

• Dominant vector species varies between regions
• Female mosquitoes can transmit virus
  transovarially
• Outbreaks associated with heavy rainfall
• Humans develop enough viremia
• to infect mosquitoes

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Prevention and control

• Risk reduction!
• Avoid close contact with infected blood or
  tissues
• Wear appropriate PPE
• Thoroughly cook all animal products before
  consumption
• Vector control
• Protection from mosquito bites
• Personal insect repellent
• Avoid being outdoors during peak feeding
• Wear long shirts and pants
• Larvicide identified vector breeding sites

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Prediction by modeling

• Outbreaks associated with above-average rainfall
• Remote Sensing Satellite Imagery can measure
  response of vegetation to increased rainfall
• Heavy rainfall occurs during warm phase of El
  Nino/Southern Oscillation (ENSO) phenomenon
• Development of forecasting
• models and early warning
• systems
• Predictions can be made up
• to 5 months in advance in East
• Africa (Linthicum, 1999)

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In the United States

• Could this happen here?
• RVF as a bioterrorism agent
• Aerosol or droplets
• 50kg could cause 10,000 illnesses and 100 deaths
• International tourism and trade
• More than 1600 flights arrive in the U.S. each
  day from foreign countries
• Animals as sentinels

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Response in Florida

• DOH response
• Support DACS responder health
• Surveillance for human illness
• Diagnostics
• Investigate human cases
• Identify risk factors
• Communicate prevention messages
• Serosurvey

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References

• World Health Organization. Rift Valley fever Fact
  Sheet. Rev. 9/07. www.who.int/mediacentre/factshee
  ts/fs207/en/print.html. Accessed 10/3/08.
• CDC. Rift Valley fever outbreak Kenya, November
  2006-January 2007. MMWR 2007 5673-76.
• Madani TA, Al-Mazrou YY, Al-Jeffri MH, et al.
  Rift Valley fever epidemic in Saudi Arabia
  epidemiological, clinical, and laboratory
  characteristics. Clin Infect Dis
  2003371084--92.
• CDC. Outbreak of Rift Valley fever---Yemen,
  August--October 2000. MMWR 2000491065--6.
• Linthicum KJ, Anyamba A, Tucker CJ, Kelley PW,
  Myers PF, Peters CJ. Climate and satellite
  indicators to forecast Rift Valley fever
  epidemics in Kenya. Science 1999285397--400.
• CDC. Rift Valley fever webpage. Accessed 10/3/08.
  http//www.cdc.gov/ncidod/dvrd/spb/mnpages/dispage
  s/rvf.htm.
• Rift Valley fever. Center for Food Security and
  Public Health at Iowa State University.
  http//www.cfsph.iastate.edu/factsheets/pdfs/rift_
  valley_fever.pdf.

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Questions