Multi-host pathogens and host extinction Flashcards
Multi-host pathogen
A pathogen that infects multiple host species
Can have reservoir (sustained transmission) and dead-end hosts (pathogen dies out)
- They can exist in dead-end hosts due to repeated introduction from the reservoir host
Host range
The number of hosts that a pathogen can use to maintain itself in nature (does not include dead end hosts)
Generalist pathogen
- A large host range. Can persist indefinitely and independently in multiple hosts
- Always multi-host pathogens but not all multi-host pathogens are generalists
Specialist pathogen
A small host range. Can only persist in one host
Example of single-host pathogens
- HIV
- Measles
- Smallpox
- Treponema pallidum
**none of these have been able to be reproduced in an animal model to study. R0 only greater than 1 in humans
Examples of multi-host pathogens
- rinderpest
- Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis
Rinderpest- multi-host pathogen
- Host: even toed ungulates
- Varying levels of disease displayed in different hosts
Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis
- Fungal pathogen that infects hundreds of amphibian species
- Greatest documented loss of biodiversity attributed to a pathogen
- One of the most destructive invasive species comparable to rodents and cats
Why are there no cases of Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis in asia?
Thought to be because it is where the pathogen originated so populations there evolved to live with it. Other populations around the world were not as well adapted
Generalist pathogen and genetics
Single generalist pathogen species was found in different host species but they are actually a complex of specialist pathogens where each strain/variant is specific to one or only a few hosts
- Can’t be seen with morphology, need molecular biology
Rabies virus as a complex of variants
Considered a classical multi-host pathogen as it infects many different species of mammals in North America but genetics revealed that there are genetic variants that are associated with different host species and they are specialized to complete that life cycle
Ex. raccoon rabies, bat rabies, skunk rabies
Are multi-host pathogens common or rare?
- Study showed 61% of 1415 species of infectious organisms to humans were zoonotic
- Proved that many pathogens can infect multiple host species and that they are quite common
Zoonotic pathogens
Multi-host pathogens but not necessarily generalists
Can pathogens drive their host populations to extinction?
When looking at single host-single pathogen models, it was found that infectious disease alone cannot drive host populations extinct
- Roy Anderson and Robert may developed these models
De castro and Bolker summarized quote
Deterministic models of directly transmitted specialist parasites with density-dependent transmission predict that disease will always die out when the host population falls below a threshold density, before the host population can go extinct
- When K drops below KT, the pathogen will die out and the species can rebuild
Major threats to species
- Habitat destruction (biggest threat)
- Overexploitation (biggest threat)
- Invasive species
- Infectious disease
Is there any evidence that pathogens have driven their host populations to extinction?
Use IUCN Red List categories to determine role of infectious disease in global species loss
- Contributed to less than 4% of species extinctions, and less than 8% of critically endangered status
Infectious disease plays a minor role in global species loss and never act alone to cause extinction
What species are involved in most of the critically endangered by infectious disease?
Mostly amphibians which is due to fungal pathogen (Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis)
What animals are most involved with extinctions?
Birds. Associated with islands which have high extinction rates due to closed population. If something gets in, there is no escape.
Mechanisms of disease-induced extinction
- Small populations
- Non-density dependent transmission
- Reservoirs
Small populations predisposing factors to disease-induced extinction
- Allee effects
- Inbreeding effects
Non-density dependent transmission predisposing factors to disease-induced extinction
- Frequency-dependent transmission
- Sexually transmitted diseases
Reservoirs predisposing factors to disease-induced extinction
- Biotic: apparent competitors (multi-host pathogens)
- Abiotic: amplification in the environment
Allee effects
Occurs when the per capita growth rate declines at lower densities
- Occurs in social organisms with cooperative behaviour
Passenger pigeons
- Endemic to north America. Population of 3 billion birds
- Hunting and habitat loss caused extinction because they need a certain population size/flock to decide to reproduce
Inbreeding effects
Small population have reduced genetic variation compared to large populations. These small populations reproduce causing inbreeding and the expression of deleterious recessive alleles.
- Results in decrease of fitness
Inbred individuals will be more vulnerable to novel pathogens
Cheetahs
- Highly inbred due to small populations
- They are highly susceptible to feline coronavirus (FCoV)
Density-dependent vs. frequency dependent transmission
Density-dependent: per capita contact rate between susceptibles and infected individuals depends on the host population density. Transmission rates increase with density of infected and uninfected individuals
Frequency-dependent transmission: per capita contact rate between susceptibles and infected individuals does not depend on the population density but instead the transmission depends on the prevalence of the infection
Frequency-dependent transmission
- The frequency of the pathogen within the population
- Occurs in sexually transmitted disease because individuals are motivated to find each other so transmission depends on how many individuals are infected with the pathogen
Chlamydia in koalas
- STDs are a model of frequency-dependent transmission
- Some populations have 100% prevalence making it very easy to pass along
Grey squirrels vs. red squirrels in UK
Red squirrels are native, grey squirrels are invasive
- 140,000 red, 2.5 million grey
Grey squirrels have expanded their range from 1945-2010
- Greys are more competitive than reds (Weigh more, consume more energy, use 1.65x more food)
Squirrelpox was brought to UK by greys and passed to reds
- Mortality rate for reds= 100%; die within 5 days
- Mild symptoms or asymptomatic in greys. Allows them to recover or easily pass on to reds
What is considered the main cause of decline of red squirrels?
- Greys being more competitive than reds is considered the main reason
- Squirrelpox has a minor role in the reds decline (considered minor because diseased red squirrels are rarely seen outside highly localized epidemics
Why is competition not considered the only reason for the decline in red squirrels?
Because the model’s speed of replacement did not line up
- Model with only competition showed complete replacement of reds by greys in 15years whereas a model with competition and disease showed replacement in 6 years
(Looking at replacement at a local scale not for all of UK)
Abundance/prevalence of infected squirrels
In UK the number of infected is quite low. Displays how a virulent disease with low prevalence can have a big impact.
Invasive species use pathogens as bioweapons
Interspecific competition occurs between invasive and native species sharing an ecological niche
- Invasive species bring new pathogens to the area where the native species have zero immunity so become infected, providing the invasive species with a competitive advantage
Christmas Island Rat
- Went extinct over 100 years ago (Within 10 years of first contact with European sailors)
- Australian territory in Indian ocean
- Extinction due to the trypanosome parasite introduced by flea-infested black rats that jumped from ships
In 2008, scientists found trypanosome DNA in museum specimens of christmas island rats. Only found in them after the first contact
**First example of a mammal driven to extinction by a parasite
Ethiopian wolf, domestic dogs, and rabies
Ethiopian wolf is one of the worlds rarest canids living in mountain ranges of Ethiopia
- Small population so rabies and canine distemper unable to survive within the population. But due to large population of free-ranging dogs, disease is able to be constantly reintroduced (repeated spillovers)
Threats to the Ethiopian wolf
- Expanding human population
- Habitat degradation
- Disease from free-ranging dogs
Multi-host pathogens and competition
Multi-host pathogens can increase competitive advantage of the less vulnerable host as it infects and can drive vulnerable species to extinction
