Multi-host pathogens and host extinction

Question 1

Multi-host pathogen

Answer 1

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

Question 2

Host range

Answer 2

The number of hosts that a pathogen can use to maintain itself in nature (does not include dead end hosts)

Question 3

Generalist pathogen

Answer 3

• A large host range. Can persist indefinitely and independently in multiple hosts
• Always multi-host pathogens but not all multi-host pathogens are generalists

Question 4

Specialist pathogen

Answer 4

A small host range. Can only persist in one host

Question 5

Example of single-host pathogens

Answer 5

• 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

Question 6

Examples of multi-host pathogens

Answer 6

1. rinderpest
2. Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis

Question 7

Rinderpest- multi-host pathogen

Answer 7

• Host: even toed ungulates
• Varying levels of disease displayed in different hosts

Question 8

Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis

Answer 8

• 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

Question 9

Why are there no cases of Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis in asia?

Answer 9

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

Question 10

Generalist pathogen and genetics

Answer 10

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

Question 11

Rabies virus as a complex of variants

Answer 11

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

Question 12

Are multi-host pathogens common or rare?

Answer 12

• 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

Question 13

Zoonotic pathogens

Answer 13

Multi-host pathogens but not necessarily generalists

Question 14

Can pathogens drive their host populations to extinction?

Answer 14

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

Question 15

De castro and Bolker summarized quote

Answer 15

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

Question 16

Major threats to species

Answer 16

• Habitat destruction (biggest threat)
• Overexploitation (biggest threat)
• Invasive species
• Infectious disease

Question 17

Is there any evidence that pathogens have driven their host populations to extinction?

Answer 17

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

Question 18

What species are involved in most of the critically endangered by infectious disease?

Answer 18

Mostly amphibians which is due to fungal pathogen (Batrachochytrium dendrobatidis (Bd)- Chytridiomycosis)

Question 19

What animals are most involved with extinctions?

Answer 19

Birds. Associated with islands which have high extinction rates due to closed population. If something gets in, there is no escape.

Question 20

Mechanisms of disease-induced extinction

Answer 20

• Small populations
• Non-density dependent transmission
• Reservoirs

Question 21

Small populations predisposing factors to disease-induced extinction

Answer 21

• Allee effects
• Inbreeding effects

Question 22

Non-density dependent transmission predisposing factors to disease-induced extinction

Answer 22

• Frequency-dependent transmission
• Sexually transmitted diseases

Question 23

Reservoirs predisposing factors to disease-induced extinction

Answer 23

• Biotic: apparent competitors (multi-host pathogens)
• Abiotic: amplification in the environment

Question 24

Allee effects

Answer 24

Occurs when the per capita growth rate declines at lower densities
- Occurs in social organisms with cooperative behaviour

Question 25

Passenger pigeons

Answer 25

• 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

Question 26

Inbreeding effects

Answer 26

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

Question 27

Cheetahs

Answer 27

• Highly inbred due to small populations
• They are highly susceptible to feline coronavirus (FCoV)

Question 28

Density-dependent vs. frequency dependent transmission

Answer 28

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

Question 29

Frequency-dependent transmission

Answer 29

• 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

Question 30

Chlamydia in koalas

Answer 30

• STDs are a model of frequency-dependent transmission
• Some populations have 100% prevalence making it very easy to pass along

Question 31

Grey squirrels vs. red squirrels in UK

Answer 31

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

Question 32

What is considered the main cause of decline of red squirrels?

Answer 32

• 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

Question 33

Why is competition not considered the only reason for the decline in red squirrels?

Answer 33

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)

Question 34

Abundance/prevalence of infected squirrels

Answer 34

In UK the number of infected is quite low. Displays how a virulent disease with low prevalence can have a big impact.

Question 35

Invasive species use pathogens as bioweapons

Answer 35

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

Question 36

Christmas Island Rat

Answer 36

• 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

Question 37

Ethiopian wolf, domestic dogs, and rabies

Answer 37

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)

Question 38

Threats to the Ethiopian wolf

Answer 38

• Expanding human population
• Habitat degradation
• Disease from free-ranging dogs

Question 39

Multi-host pathogens and competition

Answer 39

Multi-host pathogens can increase competitive advantage of the less vulnerable host as it infects and can drive vulnerable species to extinction