Waterborne Pathogens: Viruses

February 16th-18th, 2010

Viral pathogens in water

• Enteric diseases• We don’t always know the distribution of

causes of diarrheal disease, but many diarrheal disease outbreaks of unidentified cause are probably viruses

• Diseases contracted by ingestion of contaminated water

• Viral diseases in developed and developing countries impose a heavy disease burden on children

Categories of waterborne disease

• Diseases contracted by ingestion of contaminated water

• Also fecal-oral transmission without water as an intermediate– norovirus– rotavirus– hepatitis A– poliovirus

Norovirus

• Family Calciviridae• Genus norovirus and saprovirus• Self-limiting diarrheal disease• Diagnosis by PCR• Food and waterborne• Also spreads in crowded conditions• Cruise ships• Institutional settings

– Schools, nursing homes, psychiatric units

Rotavirus

• Family Reoviridae• Genus rotavirus• Humans are only reservoir• Fecal-oral• Can spread via fomites• Largest burden of disease for young children• Infection is nearly universal in first few years of life• Is now vaccine preventable

Hepatitis A

• Family Picornaviridae• Genus hepatovirus• Diarrheal disease• Liver disease• Humans are only reservoir• Fecal-oral• Infection is nearly universal in first few years of life;

many people are immune by adulthood• Vaccine preventable

The Poliovirus

• Family Picornaviridae, genus enterovirus

• 3 serotypes

• Fecal-oral spread

• Paralysis: mostly Type 1 infection

Belnap et al., Proc Natl Acad Sci U S A. 2000 January 4;

97(1): 73–78.

Poliovirus

• Polio is an enteric infection with central nervous system complications

• Often asymptomatic

• Destruction of motor neurons

– skeletal muscles (spinal poliomyelitis)

– respiratory muscles (bulbar poliomyelitis)

– both (bulbo-spinal poliomyelitis)

Polio in the United States• 20th century: seasonal epidemics• Last wild-type case: 1979

The Salk vaccine

• Inactivated virus vaccine• Tested in one of the largest clinical

trials ever done at the time: over 1 million U.S. schoolchildren

• Only vaccine used in current U.S. immunization program

The Sabin vaccine

• Live attenuated virus vaccine

• Tested in the Soviet Union

• Easy to administer

• Spreads immunity

The role of sanitation

• In areas of poor sanitation, infection rates are high

• Many infected very early in life

• As improved sanitation reduces exposure, control by vaccination is crucial

• Improved sanitation reduces both primary exposure and secondary spread of vaccine strains

Objective: Eradication

• Only one other human infectious disease ever successfully eradicated

• WHO campaign to end polio

• Original objective: year 2000

• Large areas of the world are polio free

• Virus is still endemic in some countries

Polio: an ideal target• Person-to-person transmission• No animal reservoirs, insect vectors• Limited survival in the environment• No persistent carrier state• Effective vaccine

Steps in eradication

• Interrupt natural transmission

– National Immunization Days

– Mop-up campaigns

• Confirm global eradication

– No transmission for 3 years

• Phase out the oral polio vaccine

• Laboratory containment of polioviruses

Polio in the world today

Total cases Year-to-date 2008Total in

2007

Globally 1308 1315

- in endemic countries: 1228 1208

- in non-endemic

countries:80 107

Geographic distribution of polio

Country Year-to-date2008

Total in2007

Pakistan 67 32 

India 449 874 

Afghanistan

20 17 

Nigeria 692 285 

Chad 21 22 

Angola 25 8 

Niger 13 11 

Eradication: the last push

• Endemic countries: from 125 in 1988 to four today

• Spread in endemic areas still aided by poor sanitation

• Natural disasters

• Civil and political unrest

• Cultural factors

• The last few places are the hardest

Control of waterborne viruses

• Drinking water:

• Prevention of water contamination– ADEQUATE SANITATION

• Barrier methods– Treatment trains– Too small for filtration– Disinfection (more resistant to disinfectants than

bacteria)

Control of waterborne viruses

• Vaccination to prevent infection

• Poliovirus

• Hepatitis A

• Rotavirus