4 - Emerging Infectious Diseases Flashcards
Two types of emerging infectious disease
- Previously unknown disease suddenly appears in a population
- Known disease suddenly appears in a new population (diff age group, part of world)
What is a re-emerging infectious disease
Known disease, previously on a constant or declining level, that is becoming more common and will likely continue to do so
What is the amphibian pandemic? (cause, spread, effects)
Caused by fungus Batrachochytrium dendrobatidis. Causes difficulty breathing, survives in water.
Spread worldwide by amphibian trade
90 species extinction, 501 species decline (largest pandemic ever?)
Why is there a banana pandemic
99% of bananas in supermarkets are one variety (clone): Cavendish bananas
Became popular because of resistance to Panama disease, but not panama dz has changed (cavendish no longer resistant)
Three characteristics of emerging, re-emerging diseases
- increased quantity of disease (incidence/prevalence of new, endemic or foreign diseases)
- change in characteristics of a disease
- something new (diagnostic dilemma): previously unrecognized disease
How would the characteristics of a disease change
Change in the way a pathogen affects the host
Increased severity (morbidity and mortality)
Change in disease presentation
Reduced productivity, repro performance
What do all emerging infectious disease epidemics have in common?
They all start small
Pathogen existence requires… Why?
Transmission
Can’t exist for long periods within a single host. If it kills the host, the pathogen also dies. Or the hosts immune system attacks and destroys the pathogen
Route of pathogen from one reservoir to the next
Reservoir (pathogen infected host) -> escape (route of exit) -> transmission (movement to a new host) -> entry (route of entry) -> reservoir (pathogen infected host)
An integral part of transmission is often…
disease production
What are some strategies for sustainable existence of a pathogen?
- surviving in the environment (e.g. anthrax)
- co-evolution with the host
What is a spillover event?
Required for emergence
Pathogen “spills over” from the reservoir host to a new host (transmission from one host in reservoir pop to one host in new pop)
Example of spillover event in Bangladesh
Nipa virus (paramyxovirus)
Reservoir: fruit bats
Date sap is sweet, collected over night and fermented and sold as a drink
At night fruit bats drink sap, urinate, defecate in sap
Led to neurological disease in people with high mortality (50%) and no treatment
How often to spillover events occur?
Difficult to know: we only see successful emergences
How many pathogens only have one known host?
37%
But, do we just not know the other hosts they infect?
What is required once a pathogen is transmitted to a new host?
It must be transmitted among the new hosts, and disease must be produced in the new host population
What is a hit and run pathogen? e.g.
Pathogen already very well adapted to the new host at the time of spillover
e.g. PEDV present in Asia and EU since 1970s, spilled over into NA and caused disaster
What is PRRS? What kind of pathogen?
Porcine reproductive and respiratory syndrome
- 1980s in the USA (little disease caused)
- nothing done about it = worldwide spread
- most costly disease to swine industry
It is a hide and persist pathogen
Disease can enhance transmission, what can we expect in terms of virulence after a spillover event? Examples
- virulence increases (SIV from non-human primates to people HIV)
- no change in virulence (H3N8 horses to dogs (mild to mild))
- virulence decreases (infectious haematopoietic necrosis virus spread from sockeye salmon to rainbow trout)
Five levels of pathogen transmission
- Pathogen exclusive to an animal reservoir
- animal reservoir transmits to humans/other animals, but no transmission among them
- animal reservoir transmits to humans/other animals with a few cycles of transmission among them
- animal reservoir transmits to humans/other animals with sustained transm among them
- pathogen exclusive to humans/new animal reservoir
How did human intervention affect Marek’s disease
First vaccine developed in 1960s
Reduced disease symptoms, did not prevent virus replication/transmission
Extended infectious period allowed virulent genes that otherwise would be removed to propagate
An observed emergence is likely a signal that there…
was a previous change in the complex system where the pathogen, reservoir and new host exist
Describe the Nipah Virus outbreak in Malaysia
Sudden outbreak in pigs and people (1998-1999)
Reservoir host is bats
Sick pigss = respiratory, rapid spread, 1.1 million culled
Sick people = encephalitis, 40% mortality
Distal risk factors for nipah virus in Malaysia
- reduced global pork production due to classical swine fever and foot an mouth disease = opportunity
- asian pig producers moved into malaysia: bought and deforested land displacing fruit bats
- fruit bats moved into open sided pig barns
Proximal risk factors for nipah in malaysia
Working in a pig barn
Two types of risk factors that enhance disease emergence
Distal and proximal risk factors occur before a spillover event
Factors enhancing disease emergence
- pathogen adaptable to new host
- spillover opportunities (anything that increases opportunity for contact btw reservoir and new host)
- transmission among new hosts
When is spillover most successful
If reservoir and new host are closely related
What pathogens are more associated with emerging diseases?
Zoonotic pathogens twice as likely to be cause
Viruses are most common
Transmission of pathogen is associated with…
- low mortality
- chronic infections
- not transmitted by vectors
What can increase spillover events? (6)
- increase pathogen prevalence in reservoir
- increase size/density of reservoir pop
- deforestation, agricultural expansion
- climate change
- change in beh of reservoir (e.g. coyotes)
- vectors (increased prevalence in vectors, density/number of vectors)
Human related factors that increase spillover events
- increased pop density
- poverty, war, social change
- inadequate hygiene/sanitation
- global travel
- human beh increasing contact w animals
Possible spillover events related to humans and wildlife
Animal trade (live animal markets, exotic pet “swap meets,” international transport
How was monkey pox a spillover event in 2003?
African rodents were the reservoir for monkey pox
Imported into texas?
Spread through states in rodents (groundhog?)
Humans got it: 71 cases, 18 hospitalized
How do we deal with the wildlife trade?
Little direct evidence documenting spillover events from the wildlife trade
What is a zoonose?
Infection derived directly or indirectly from animal reservoirs that cause disease in humans
Where do zoonoses come from?
99% from domestic animals
1% from captive managed/farmed wildlife, free ranging
What are ‘emerged from animals’ diseases
Diseases that originated from animals but are now transmitted in humans independent of animals (72% from wildlife)
Most important factor enhancing emergence
New host susceptibility of infection with the pathogen
Greater the amount susceptible the greater the number that can become infected
What is disease X?
Placeholder name that was adopted by the world health organization in Feb 2018 on their shortlist of blueprint priority disease to represent a hypothetical, unknown pathogen that could cause a future epidemic
Characteristics of disease X
Rapidly spread in humans
Severe clinical symptoms
Air borne transmission
No vaccine
No treatment
Mutating
Unknown animal origin
Types of susceptibles
- intensive agriculture: genetically similar hosts managed under same conditions
- susceptible on purpose: countries free of FADs do not vaccinate, lack of antibodies in animals proves they are “disease free”
- weakened immune systems: more elderly, HIV/AIDS, cancer patients, transplant recipients
Factors enhancing transmission among new hosts
- biological similarity
- pathogen already adapted
