• Cause persistent infections
• Long generation time
• Specialized infective stages
• Morbidity rather than ortality immunity complex
• Impact is a function of number of parasites per host
• No replication within hosts

80% of the population is concentrated in 20% of the hosts

• Examines relationship btw Power and Mean
• Aggregated distribution= variance increases faster than mean
• log(s^2)=b(log m)+ log(a)

(a is intercept, b is slope)

• K defines the neg binomial, it is an inverse measure of aggregation ( smaller k means higher aggregation)

k>20 poisson distribution, k=20-1 random, k< 0 aggregated

• K is defined by mean and variance

s^2/m-->0

k=?

• Population is distributed at a regular rate
• under dispersed
• uniform distribution

s^2/m=1

k=?

• k--> ∞
• No pattern among hosts
• Random distribution
• Poisson

s^2/x--> ∞

k=?

k-->0

Only one host infected

Aggregated

(Over dispersed)

k is less than zero

• Parasites arent regulated by hosts, but by their own density (mortality, fecundity, etc)-fecundity decreases w/ more parasites in 1 individual
• Parasite induced mortality
• Host immunity

Heterogenity in :

1. Susceptibilty to infection
2. Exposure
3. Immunity
4. Direct reproduction of parasites in host

More aggregated eggs means more aggregation in host.

Males tend to be more infected than females

(higher prevalence), mammals tend to have higher intensity (particularly nematodes).

Exposure:

Behaviour- stay in pools longer, while females leave th pools to lay eggs

Diet-

Body size- eat more w/ a bigger body size

Sex differences in steroid hormones:

Testosterone-

Progesterone-

Oestrogen-

They have an effect on parasite growth and development

• treatment in males causes a great drop in females infected.
• males arent the most likely to get infected, but also the most likelt to transmit infection.

Who are the 20% super-infected hosts?

• Type 1-linear increasein parasite intensity, constant infection, no parasite mortality
• Type 2-parasite birth & death process, 1st parasites increase in host, then there is constant infection and a mortality rate proportional to this infection.
• Type 3-parasite intensity increases, then peaks, and decreases, can be caused by 4 things

• Age-related change in exposure:older host is less exposed to infection
• Parasite induced host mortality (they kill heavily infected hosts)
• Frailty in data (few old hosts in sample, and aggregated parasites)
• Aquired immunity-develops as a cmulative exposure to parasites

Biological interest

• Parasites in the tail overall experience higher densities
• Mortality and morbidity occurs just in the tail
• impact on host and selection pressure higher in tail
• Where to focus chemotherapy!