Lecture_8_Macroparasites3

Lecture_8_Macroparasites3 - 2/5/2009 Heterogeneities in...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
2/5/2009 1 Heterogeneities in macroparasite infections What causes variation in parasitism among individuals? Prevalence: Proportion infected Intensity . Macroparasite Epidemiology How do we measure infection? 35 40 45 50 Number of parasites per host individual Avg number parasites per infected host 0 5 10 15 20 25 30 024681 0 Large proportion of worm population in small proportion of host population S 2 /X S 2 /X = 1 S 2 /X 0 All hosts same number REGULAR Very few hosts heavily RANDOM Poisson K K 0 K ~ 20 Macroparasite Epidemiology Measures of aggregation (under dispersed) Positive Binomial infected AGGREGATED (over dispersed) Negative Binomial What causes parasite aggregation? Intensity Determined by: Exposure Density of infective stages Density of hosts Behavior (encounters) Susceptibility Innate Resistance Acquired Resistance Behavior (defense) Variation in the density of infectious stages Could be caused by host Processes that generate variation Exposure to free living stages behavior (e.g., use of latrines) Habitat suitability for parasite (e.g., moisture, temperature, availability of intermediate hosts) raccoon roundworm ( Balyisascaris procyonis )
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2/5/2009 2 Processes that generate variation Exposure to free living stages Keymer & Anderson 1978: Hymenolepis diminuta and Tribolium confusum Even distribution Aggregated Eggs distributed at 15 per cm 2 6 s per beetle 3 8 - 95% CI) (a) (b) 20 tle More eggs = greater infection More aggregated eggs = more aggregation in host Processes that generate variation Exposure to free living stages 0 1000 2000 3000 Variance:mean ratio of eggs per unit area 0 2 4 Variance:mean ratio of number of parasite 0 1 2 k of number of parasites per host 0 0 2 4 6 Mean number of parasites per host (+/- 0 2000 4000 6000 Parasite eggs per unit area 0 5 10 15 Mean parasite intensity per bee Note aggregated even when an even distribution of infective stages Spadefoot toads and Pseudodiplorchis americanus
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 5

Lecture_8_Macroparasites3 - 2/5/2009 Heterogeneities in...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online