The population dynamics of red grouse, Lagopus lagopus scoticus and the parasitic nematode, Trichostrongylus tenuis were explored to determine whether interactions between the parasite and host were sufficient to generate cycles in grouse abundance. Two alternative models were used that explicitly consider the dynamics of either the free-living, or arrested larval stages of the parasite. Providing that the life expectancy of the free-living larvae is more than 2-4 weeks, the parasite can readily establish in gousse populations

Regulation and stability of a free-living host - parasite system : Trichostrongylus tenuis in red grouse. II: Population models

A Handbook Jørgen Hansen, DVM, PhD Animal Production and Health Division Food and Agriculture Organization Rome, Italy Brian Perry, BVM&S, DTVM, MSc, DVM&S, MRCVS International Laboratory for Research on Animal Diseases Nairobi, Kenya © ILRAD 1994 Published by the International Laboratory for Research on Animal Diseases, P.O. Box 30709, Nairobi, Kenya Cover Design by Dynamic Advertising Ltd. Printed by the International Livestock Centre for Africa Addis Ababa, Ethiopia ISBN 92-9055-703-1 This electronic document has been scanned using optical character recognition (OCR) software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO declines all responsibility for any discrepancies that may exist between the present document and its original printed version.

The epidemiology, diagnosis and control of helminth parasites of ruminants

Summary 1. The population dynamics of red grouse, Lagopus lagopus scoticus and the parasitic nematode, Trichostrongylus tenuis were explored to determine whether interactions between the parasite and host were sufficient to generate cycles in grouse abundance. Two alternative models were used that explicitly consider the dynamics of either the free-living, or arrested larval stages of the parasite. 2. Providing that the life expectancy of the free-living larvae is more than 2-4 weeks, the parasite can readily establish in grouse populations. Larval arrestment tends to reduce the intrinsic growth rate of the parasite and thus increases the size of the host population required for the parasite to establish. 3. Grouse numbers will tend to cycle when the parasites exhibit low degrees of aggregation and parasite-induced reductions in host fecundity are greater than parasite-induced increases in host mortality. The population cycles produced in the model have the slow increase followed by a rapid decline characteristic of the grouse population studied at Gunnerside. 4. The period of the cycles is determined by the intrinsic growth rate of the grouse population and either larval life expectancy (Model I), or the duration of larval arrestment (Model II). Cycle periods decrease as host population growth rate increases, and lengthen with increases in either free-living larval life expectancy, or the duration of larval arrestment. If the duration of larval arrestment is sufficiently long (>6 months), the cycles die out and the dynamics of the grouse-nematode system are very stable. 5. Estimates of all of the model's parameters may be made from long-term records of grouse populations. Numerical analysis of the model's behaviour suggest that a model with limited arrested larval stages more closely corresponds to the grouse populations in the North of England. The 4-5 year cycles exhibited by these populations will be more sporadic, or absent on estates where the parasite is unable to establish. 6. The analysis shows that the empirical data collected on T. tenuis are consistent with it being the cause of the cycles observed in grouse populations in the North of England.

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system: Trichostrongylus tenuis in red grouse. II. Population models

Even though theoretical models show that parasites may regulate host population densities, few empirical studies have given support to this hypothesis. We present experimental and observational evidence for a host-parasite interaction where the parasite has sufficient impact on host population dynamics for regulation to occur. During a six year study of the Svalbard reindeer and its parasitic gastrointestinal nematode Ostertagia gruehneri we found that anthelminthic treatment in April-May increased the probability of a reindeer having a calf in the next year, compared with untreated controls. However, treatment did not influence the over-winter survival of the reindeer. The annual variation in the degree to which parasites depressed fecundity was positively related to the abundance of O. gruehneri infection the previous October, which in turn was related to host density two years earlier. In addition to the treatment effect, there was a strong negative effect of winter precipitation on the probability of female reindeer having a calf. A simple matrix model was parameterized using estimates from our experimental and observational data. This model shows that the parasite-mediated effect on fecundity was sufficient to regulate reindeer densities around observed host densities.

The role of parasites in the dynamics of a reindeer population

Sex biases in parasitism may be expected in mammals because estrogens stimulate immunity, whereas androgens depress immunity. Parasites should, therefore, become more readily established in male hosts, leading to higher levels of parasitism for males than for females. We tested this general hypothesis using 145 tests based on mammal hosts taken from 38 published studies. Male biases in parasitism existed overall and for tests restricted to arthropod (but not helminth) parasites. We then controlled for potential dependence by choosing single parasite species based on their likelihood of eliciting immune responses. For these 48 tests, male biases still existed for arthropod, but not for helminth, parasites. As predicted, the average degree of male bias was much higher for studies involving an experimental infection than for field studies. In experimental tests, researchers controlled for differences between the sexes in exposure to parasites. Any biases in parasitism, therefore, should have been due primarily to gender differences in immune response. There also were no male biases in parasitism among juveniles, presumably because juvenile males and females differ less in their hormone profiles or stress levels than do adult males and females. In , our results suggested that hormonally mediated sex differences in susceptibility to parasitism exist for mammals, even though our tests were extremely conservative. Our results also indicate that, on average, differences in parasitism between the sexes are small, and that statistically significant male biases in parasitism are not a general rule.

https://www.jstor.org/stable/info/3545801

Male biases in parasitism of mammals : effects of study type, host age, and parasite taxon

Publisher Summary This chapter reviews the role of hypobiosis in the biology of nematodes. Successful transfer from an infected host to a susceptible one represents a most important achievement in the parasitic way of life. Any developmental adaptation that serves to facilitate this process is, therefore, extremely important in the epidemiology of these infections. Hypobiosis or facultative arrested development represents such an adaptation which, by facilitating the persistence of the larval forms for prolonged periods in the host, enables the parasite to capitalize on optimal opportunities for transfer. The phenomenon appears to be widespread among parasitic organisms. The chapter considers several aspects of this phenomenon as the species involved, factors associated with its induction as well as those influencing a resumption of development, implications of the phenomenon for understanding the epidemiology of these conditions, and its adaptive and population regulatory significance. Host resistance causes the arrested development and that resumption of development is a result of the relaxation of immunity. It is considered to be a host-regulated phenomenon that served to limit the size and pathogenicity of the population of worms, which is accumulating during the season of increasing larval availability. The chapter describes the types of hypobiosis, which include immune-mediated arrest and seasonally induced arrest.

Hypobiosis in parasitic nematodes--an update.

Haemonchus contortus and other trichostrongylid infections in parturient, lactating and dry ewes.

Mechanisms of survival of nematode parasites with emphasis on hypobiosis.

Nematodiasis in cattle. Importance, species involved, immunity, and resistance.

Outbreaks of avian cholera (Pasteurella multocida) occur frequently in common eiders (Somateria mollissima dresseri) in Maine during early summer. Studies over a seven year period show that over 90% of the loss occurred in incubating females and might be associated with their weakened condition because females do not feed during the incubation period. High nesting densities also may contribute to the losses. The exact source of P. multocida is unknown although carrier birds were found.

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Harold C. Gibbs

Papers

Hypobiosis in parasitic nematodes--an update.

Haemonchus contortus and other trichostrongylid infections in parturient, lactating and dry ewes.

Mechanisms of survival of nematode parasites with emphasis on hypobiosis.

Nematodiasis in cattle. Importance, species involved, immunity, and resistance.

Avian cholera in eider ducks in Maine.