ecology of disease# Flashcards
Key points
Disease is not a problem for an ecosystem but an integral part of it
Close relationship between hosts and pathogens
Humans have a large influence affecting disease spread etc.
Key definitions
Parasite – an agent that takes resources from a host
Pathogen – a parasite that causes disease
Disease – change from normal state to a lower level of function
Disease transmission
-direct contact
-vector-borne – by carrier
-fomites – inanimate objects carrying disease
-aerosol – usually respiratory ejection
-oral ingestion
What impacts the rate of transmission?
Population density
Conditions – coevolution
environmental factors – land use change
Behaviour can facilitate or reduce rate of transmission
Behaviour affecting transmission examples:
isolation of infected individuals benefits hosts and reduces transmission
Interspecies transmission can occur due to predation patterns in the food web
Mate choices trade offs: choice of healthier mates/ promiscuity spreads disease
Sickness behaviour: mycoplasma in finches makes the males seem less dominant so others forage near them and they transmit the disease
Group size – can allow transmission/ can allow others to care for the sick e.g. large wolf packs can support individual members with mange by letting them share in hunted kills
Impacts of disease
Depends on transmission and pathogen virulence
(ability to infect and how much it impacts host health)
Extinction unlikely due to disease <4% since 1500 – it is not in the parasites interest to kill host
Host-parasite co-evolution – hosts evolve to evade parasite
Introduction of non-native species can rapidly spread disease (enemy-release hypothesis) e.g. myxoma virus - mytsimatosis has very little impact in american rabbits and killed the majority of Uk rabbits on its introduction affecting grazing patterns and the food web
Not in the interest of the pathogen to kill hosts – they need to survive to pass it on
Red queen hypothesis –
thought to be the reason for evolution of sexual reproduction as hosts recombine genotypes to protect against infection of pathogens
Not all impacts are negative
Preventative of competitive exclusion (e.g. yellow rattle reducing grass monoculture allowing other plants to grow)
Provision of habitat and food sources e.g. mistletoe
Keystone species
Population regulation
Human influences on disease behaviour
Humans reduce biodiversity – increasing ability of disease to infect as species are less able to adapt (reduced genetic variation)
Habitat degredation and introduction of new species/diseases
Climatic conditions – can allow disease to spread faster (warmer temps)
Modern farm practices:
- Monocultures – lack of biodiversity e.g. cavendish banana
- Antibiotics and vaccines e.g. Marek’s disease vaccine resistance in chickens
- Lifestock high density enables disease spread e.g. salmon farming
- Pest management and pollution
Global transport
- Goods and people travelling regularly increasing epidemic risk
- Non-native species introduction e.g. European green crab (enemy release hypothesis)
- Illegal wildlife trade – 30% species at risk from this not due to capture but disease transfer e.g. monkeypox
- Lifestock transport – resulting in foot and mouth disease issues not a problem in wild stocks
Habitat destruction
- Malaria increase as a result of warmer ground temp after deforestation
Climate change and the ecology of disease
Rising temps allowing diseases to expand their geographic range e.g. malarial mosquitos will be able to reach higher altitudes – e.g. cholera cases peak with peak temps
Exacerbates weather events e.g. flooding due to heavy rains increasing waterborne disease proliferation
Pollution such as field runoff introduces agricultural pathogens and chemicals this contaminates riverways causing disease in birds and freshwater species
Can give rise to new diseases – creating conditions for new host/pathogen interactions
Can cause seasonal disruption increasing distribution of disease vectors e.g. longer mosquito season increases malaria risk
Studying disease ecology
Population estimates
Biological samples e.g. blood and tissue
Analysis methods
PCR to identify presence/ absence of pathogens
Seriology tests – blood antibodies can identify disease infection history
Genetic tracking
Mathematic modelling: micro and macroparasites
Microparasite: pathogenic bacteria/viruses, reproduce rapidly and usually inducing immunity after infection.
^ Models for microparasites measure number of host in each stage of infection.
Macro-parasite: parasitic protozoa and helminths, more complex lifecycle so infections may be more chronic.
^ Models for macro-parasites measure number of parasite in each host and their distribution
Mathematical modelling: Basic reproductive number
Basic Reproductive number (R0): the number of secondary infections caused by a single infection in a previously unexposed host population
SIR model (susceptible, infectious and recovered model)
SIR Model – susceptible, infectious, recovered.
- Static, parametric basic model.
- Adaptable to the complexity of a pathogen.
Adaptations of the SIR model
SEIR model – Exposed category for infections with latency period.- example: measles
SIS model - infections with low recovery rates where individuals are still susceptible to reinfection.- example: gonorrhea, strep throat
SI model – infected individuals do not leave the infectious stage.- example: HIV
The role of disease in an ecosystem
Disease is an integral part of any ecosystem
Deep relationships form between host and pathogen
Regulates populations; impacts specific to ecological conditions
Pathogens and hosts drive each other forward on their evolutionary journey
