Characteristics of Parasite Populations A. General Definitions recall characteristics of ‘populations’ population biology of human parasites = epidemiology population biology of wildlife parasites = epizootiology Hierarchical nature of parasite populations: –infrapopulation vs. component population vs suprapopulation –e.g. definition of a typical trematode population Recall characteristics of ‘communities’ B. Descriptors Parasite quantity –prevalence, mean abundance, mean intensity, antibody titre –immuno-diagnostic methods

–e.g. horse cestode (Anoplocephala) Known positive horses –Fecal analysis (54%) –Western blot for hs antibodies to one antigen (100%), but 20% false positives –Coproantigen ELISA (74%), positive correlation between antigen concentration and worm intensity –molecular diagnostic methods (many nematodes)

Parasite quality –biomass, reproductive maturity Macro vs. Microparasites C. Factors affecting parasite populations Rate of recruitment –Passive (Giardia, Crypto, many cestode eggs, some trematode eggs, Ascaris) –Active (cestode coracidia, trematode miracidia and cercaria, many nematode larvae) Rate of parasite mortality –Natural host and parasite mortality (many annual parasites) –Host-induced mortality (immunity) –Intra- and interspecific competition for resources

External factors –What environmental factors limit the transmission of free- living stages e.g. miracidia e.g. cercariae e.g. nematode larvae –Host diet Recall metacercariae in minnows ’you are what you eat’ –Linkages to global change and the field of Landscape Epidemiology Internal factors –Age, sex, host genetics, behaviour, host physiology

Conclusion Many internal and external factors affect the size of parasite populations ’Genes’ vs ’ecology’ hypotheses for determination of parasite population sizes Interactive effects D. Distribution of parasites in host populations Terms –Recall parasite prevalence, intensity and abundance –sample mean and variance 3 general distribution patterns patterns in nature

Frequency distributions of metacercariae in minnows Prevalence = 62 % Mean intensity = 1.8 (1.9) Range = 0-8 Var-mean ratio = 1.8 N = 51 Prevalence = 98 % Mean intensity = 26.9 (24.0) Range = 0-77 Var-mean ratio = 21.6 N= 51

No. metacercariae vs. host size Brain Body cavity

Intensity ’brain’ vs. Intensity ’body cavity’ P =

Causes of aggregated distributions distribution of infective stages (e.g. Leuchochloridium, Echinococcus) distribution of vectors, larvae (e.g. clumping in Dermacentor) inherent variation in hosts (Ascaris in pigs) age, sex, behaviour, nutritional status host genetics/immunity –P. falciparum and MHC alleles (west Africa) –S. mansoni in Brazilian villages (genes, not exposure to water) Host genetics/not immunity –P. falciparum and sickle-cell gene

Consequences of aggregated distributions affects on ps population regulation –Density dependent regulation –Ps-induced host mortality diagnosis