Transmission of VirusesTransmission of Viruses

IntroductionIntroduction VocabularyVocabulary

Routes of transmissionRoutes of transmission Some examplesSome examples

““Biology” of VirusesBiology” of Viruses

Introduction: Why is transmission Introduction: Why is transmission important to a virus?important to a virus?

Finite life spanFinite life span Host response/immunityHost response/immunity

Maintain propagationMaintain propagation Spread within populationSpread within population

A dead host is no good to a virusA dead host is no good to a virus

VocabularyVocabulary Route of infectionRoute of infection Portal of entryPortal of entry ReceptorsReceptors Susceptible/ non-susceptible cell (receptor)Susceptible/ non-susceptible cell (receptor) Permissive/non-permissive (replication)Permissive/non-permissive (replication) Abortive/incomplete vs productive/complete Abortive/incomplete vs productive/complete

infectioninfection TropismTropism Localized/systemic infectionLocalized/systemic infection Vector/fomite Vector/fomite

Some routes of infection for animal Some routes of infection for animal (human) viruses(human) viruses

Distinctions are somewhat artificial and Distinctions are somewhat artificial and arbitrary-they are for virologists not for arbitrary-they are for virologists not for

virusesviruses What is the “natural” route?-not the “lab” What is the “natural” route?-not the “lab”

routeroute Viruses may use more than oneViruses may use more than one

1. Direct contact transmission1. Direct contact transmission

Viruses persist in Viruses persist in environmentenvironment

Host comes in contactHost comes in contact Surfaces or fomitesSurfaces or fomites Human rhinovirus (the Human rhinovirus (the

“common cold virus”)“common cold virus”) Evidence from Evidence from

volunteer studiesvolunteer studies

2. Respiratory route2. Respiratory route

Adenoviruses eg Adenoviruses eg adenovirus 5adenovirus 5

5-10% of urt infections in 5-10% of urt infections in humanshumans

conjunctivitisconjunctivitis Localized infections due to Localized infections due to

tissue tropismtissue tropism Progeny virions released Progeny virions released

into secretionsinto secretions Inflammations/irritationsInflammations/irritations

Respiratory droplets contain active virus!

Other respiratory route virus Other respiratory route virus examplesexamples

Paramyxoviridae (measles, mumps, Paramyxoviridae (measles, mumps, respiratory syncytial virus RSV)respiratory syncytial virus RSV)

Orthomyxoviridae (influenza A, B, C)Orthomyxoviridae (influenza A, B, C) Poxviridae (variola virus-smallpox)Poxviridae (variola virus-smallpox)

laboratory outbreaks in Britain-1973laboratory outbreaks in Britain-1973 and 1979and 1979

clinical practice of variolation clinical practice of variolation illustratesillustrates

adaptibility of virusesadaptibility of viruses

Family Paramyxoviridae has Two Subfamilies

Paramyxovirinae Genus Rubulavirus Mumps VirusGenus Morbillivirus

Measles VirusRinderpest Virus

Pneumovirinae Genus Pneumovirus

Respiratory Syncytial Virus

MeaslesMeasles Japanese Measles = rubeola, 14-Japanese Measles = rubeola, 14-

day measles, “true” measles, red day measles, “true” measles, red measles, morbillimeasles, morbilli

a very serious diseasea very serious disease Historical effectsHistorical effects

Measles symptoms and Disease Syndrome

Contagious 1 to 2 days before symptoms appear.Continue to transmit virus for four days after rash appears.Transmission is by respiratory secretions: Aerosol.Incubation is 10 to 14 days.Cough, Fever, 2 to 4 days malaise - Pronounced rash.

Rash on Face and bodies of Children Pneumonia in rare cases

Measles Virus History

Humans are the only measles virus reservoir in nature. Measles infection results in solid, life-long immunity. A minimum population size of about 250,000 is required to

maintain measles. Measles probably could not have existed solely in humans before human populations reached this size.

“critical community size” This may have occurred as late as 1000 years ago well after

plant and animal domestication. When the virus was first introduced into the Americas during

European exploration 500 years ago, the death toll was enormous. Measles and smallpox caused massive depopulation of the native Americans .

Measles is also a leading cause of death in developing nations. http://www.who.int/topics/measles/en/

Unvaccinated people can spread Unvaccinated people can spread the virusthe virus

Measles death in the US-2015Measles death in the US-2015 http://www.forbes.com/sites/tarahaelle/http://www.forbes.com/sites/tarahaelle/

2015/07/02/first-u-s-measles-death-in-2015/07/02/first-u-s-measles-death-in-more-than-a-decade/more-than-a-decade/

http://thinkprogress.org/health/http://thinkprogress.org/health/2015/07/06/3677118/measles-death-2015/07/06/3677118/measles-death-united-states/united-states/

Measles-a candidate for Measles-a candidate for eradicationeradication

Measles is one of the leading causes of death among young Measles is one of the leading causes of death among young children even though a safe and cost-effective vaccine is available. children even though a safe and cost-effective vaccine is available.

In 2012, there were 122 000 measles deaths globally – about 330 In 2012, there were 122 000 measles deaths globally – about 330 deaths every day or 14 deaths every hour. deaths every day or 14 deaths every hour.

Measles vaccination resulted in a 78% drop in measles deaths Measles vaccination resulted in a 78% drop in measles deaths between 2000 and 2012 worldwide. between 2000 and 2012 worldwide.

In 2012, about 84% of the world's children received one dose of In 2012, about 84% of the world's children received one dose of measles vaccine by their first birthday through routine health measles vaccine by their first birthday through routine health services – up from 72% in 2000. services – up from 72% in 2000.

Since 2000, more than 1 billion children in high risk countries were Since 2000, more than 1 billion children in high risk countries were vaccinated against the disease through mass vaccination vaccinated against the disease through mass vaccination campaigns ― about 145 million of them in 2012.campaigns ― about 145 million of them in 2012.

Efforts to stem measles have been successful, but uneven political commitment, lack of funds and public fear threaten to

undermine the progress.

““German” measlesGerman” measles German measles = rubella, 3-day German measles = rubella, 3-day

measles measles caused by a togavirus, mild initial caused by a togavirus, mild initial

disease but may cause birth disease but may cause birth defects (CRS)defects (CRS)

Vaccine preventionVaccine prevention

Impact of vaccine in USImpact of vaccine in US