L2 Host and parasite populations

Question 1

What determines a hosts chance of infection?

Answer 1

Exposure and susceptibility

Question 2

How are direct diagnoses made?

Answer 2

Helminthes:
- GI egg count and morphology. 
- Urinary schistosomiasis - eggs in urine. 
- Filarial nematodes - microfilaraemia
- Onchocerciasis - skin slips
Protozoa: 
- bloodsmears
- GI in feces

Question 3

How are indirect diagnoses made?

Answer 3

Look for disease/parasite indices pathology
eg v heavy hookworm infection can cause liver hardening.
Immunological assays - ELISA, but often cant distinguish btw current and prior infection

Question 4

mean intensity =
mean abundance =
why arent these used for microparasites?

Answer 4

mean intensity = no. parasites/no. infected hosts.
mean abundance = no.parasites/no. hosts
They change number rapidly.

Question 5

Incidence=

Prevalence =

Answer 5

incidence = change in parasite population per unit time
Prevalence = no. infected hosts/ no. total hosts (%)

Question 6

How are helminthe populations dispersed?

Answer 6

All are overdispersed. Have a negative Binomial distribution, variance>mean.
Due to variation in host resistance, parasite exposure, genetic, behavioural and occupational variance.

Question 7

What kinds of people have high exposure to parasites, causing aggregated helminth popuations?

Answer 7

• People with water contact - schistosomiasis
• Developing world field workers - GI nematodes.
• Children- due to close contact w others, age aquired immunity.

Question 8

Why is there negative correlation between peak prevalence and age of peak prevalence?

Answer 8

High transmission environments - hosts younger when getting infected and develop immunity.
Low transmission environments - older individuals infected

Question 9

eg of negative density dependent effects

Answer 9

No. worms affect no. eggs per female.

If there was no density depedent effect, would have a linear relationship between FEC of sheep and no. female worms.

Question 10

What are DALYS

Answer 10

Disability adjusted life years - used by the WHO
GI nematodes - 39 million
Malaria - 36 million
GI nematodes cause more global harm than malaria.

Question 11

How large can parasite biomass be in ecosystems?

Answer 11

in a study of 3 mexican estuaries, parasites make up 1% of total animal biomass.
Carpinteria salt marsh - parasite biomass estimated to be equivalent to 7-10 elephants
Reproductive rate equivalent to 1-2 baby elephants per day for 200 days
trematode biomass> bird and = fish.