Emerging Diseases Flashcards
Emerging Diseases
– PREVIOUSLY UNKNOWN disease that suddenly appears (emerges) in a population
– KNOWN disease that suddenly appears (emerges) in a new population
Re-Emerging Diseases
– KNOWN disease, previously on the decline, that is becoming more common and will likely continue to do so
What are the 5 stages of cross-species disease emergence
- 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 transmission among them
- Pathogen exclusive to humans/new animal reservoir
Stage 1
Stage 1: Pathogen exclusive to a single species
Many pathogens have multiple host species
Overall, only 37.3% of pathogens have one known host
Stage 2
Stage 2. Animal reservoir transmits to humans/other animals, but no transmission among them (dead end hosts)
Rabies
– Reservoirs are carnivores and chiroptera (bats) – Cattle and horses are dead end hosts
WNV, EEEV, WEEV
– Reservoir: Birds
– Humans, horses, and dogs are dead end hosts
Influenza H5N1
– Reservoir: waterfowl and poultry – Humans are dead end hosts
Stage 3
Stage 3. Animal reservoir transmits to humans/other animals with a few cycles of transmission among them
Mycobacterium bovis
– Reservoir: cattle, bison, elk
– Transmitted to people via respiratory route – Poor human to human transmission
Nipah Virus
– Reservoir: fruit bats
– Human to human transmission
– Pig to pig transmission
H5N1
First outbreak: 1997, Hong Kong – A few human respiratory cases
– Acquired directly from chickens
– 1.2 million chickens culled
Southeast Asia, 2004…
– Widespread outbreak; not controlled, in spite of
massive culling of domestic poultry
End of 2012
– Outbreaks continue to occur in chickens
– 578 human cases have been reported since 2003, with a case fatality rate of 58%
STAGE 2
Nipah Virus
Suddenly emerged in Malaysia – Outbreak Sept 1998 – April 1999
Sick pigs
– Respiratory, rapid spread
– 1.1 million were culled to prevent spread (out of a national herd of 2.4 million!)
Sick people (n=265) – Encephalitis in many cases – Cases had 40% fatality rate (n=105) – Survivors often have neurologic sequelae
Stage 4
Stage 4. Animal reservoir transmits to humans/other animals with sustained transmission among them
SARS: Severe Acute Respiratory Syndrome – Reservoir: fruit bats
– Efficient transmission in humans
Schmallenberg virus
– Reservoir species: ?
– Efficient transmission in sheep, goats and cattle
2009 Influenza H1N1 “Swine Influenza” – Reservoir: swine
– Very efficient transmission in humans
Swine Influenza
H1N1, “swine flu”, “Mexican flu”…
A recombinant strain– The segments in the current strain have been “together”, as a pig virus in North America, since 1998
First human cases recognized in April, 2009, from Mexico
– Global spread within a few weeks
– June 11: officially declared pandemic by the WHO
First swine cases were recognized in Canada in late April, 2009
Stage 5
Stage 5. Pathogen exclusive to new reservoir species
Human examples
– HIV/AIDs- non-human primates
– Measles- cows
– Smallpox- maybe camels
– Dengue fever- Old world primates
– Yellow fever - African primates
Pathogen: Type of Agent
Of the >1400 species of pathogen known to infect humans, 61% are zoonotic
75% of emerging diseases of humans are zoonotic
Zoonotic pathogens are twice as likely to be associated with emerging diseases
Viruses are most likely to be emerging diseases
Reservoirs
Phylogenetic distance between reservoir and new host
– Best transmission: within a species
– Pathogens are more likely to cross between closely related species than distant ones
– Pathogens that somehow cross between distantly related species often cause very different, often more severe, disease
New host: susceptibility!!!
– Applies to all diseases, even those that emerge into new populations of the same species
– Intensive agriculture: a lot of genetically similar hosts, managed under the same conditions, will have the same susceptibility
– More populations with weakened immune systems: elderly, HIV/AIDS, cancer patients and survivors, organ transplant recipients
Factors Increasing Transmission
The probability of transmission from a reservoir to a new host increases with:
Increasing abundance of the reservoir
Increasing pathogen prevalence in the reservoir
Increasing contact between the reservoir and the new host
