Survival Of Free-living Stages Research Articles

The role of sex in parasite dynamics: Model simulations on transmission of Heligmosomoides polygyrus in populations of yellow-necked mice, Apodemus flavicollis

We investigated possible mechanisms that could cause sex-biased parasite transmission of the helminth Heligmosomoides polygyrus in its rodent host, Apodemus flavicollis, using a modelling approach. Two, not mutually exclusive, hypotheses were examined: that sex-biased parasite transmission is caused by differences in immunity that influence the success of free-living stages and/or is caused by sex differences in host behaviour and the dissemination of infective stages. Model simulations were compared with results from a field manipulation experiment of H. polygyrus in replicated populations of A. flavicollis. Simulations predicted the experimental field results, and both hypotheses explained the pattern observed. Transmission is male-biased if a male immune response increases fertility, hatching or survival of free-living stages. Alternatively, transmission is male-biased if their behavioural characteristics allow them to spread infective larvae in areas more frequently used by females. These results highlight that host sex is not only responsible for differences in parasite susceptibility, but may profoundly influence host–parasite interactions, resulting in a sex bias in parasite transmission.

The feasibility of using Udonella sp. (Platyhelminthes: Udonellidae) as a biological control for the sea louse Caligus rogercresseyi, Boxshall and Bravo 2000, (Copepoda: Caligidae) in southern Chile

Caligus rogercresseyi is a sea louse of native and farmed fish in southern Chile. Due to the impact that this copepod has on the Chilean salmonid production, it is necessary to search for control mechanisms that can effectively reduce its population. This study focuses on determining the feasibility of using Udonella sp., a common flatworm found on copepod ectoparasites of the Chilean rock cod, Eleginops maclovinus, as a biological control for sea lice. The following aspects were studied: distribution and preference of Udonella sp. regarding both host sex and body location on copepods collected from both the Chilean rock cod and cultured salmonids; the effect of Udonella on both fecundity and survival of the free-living stages; and seasonal variations of population of both Udonella and the copepod. Results indicated that Udonella sp. is frequently found on copepods collected from the Chilean rock cod, but is absent on those from cultured salmonids. The flatworm showed a marked preference for female copepods and the main body locations used by Udonella are the genital complex and egg sacs. The three life stages of Udonella are distributed across the copepod body in different ways since different body locations represent feeding and nursery areas. Udonella has no significant effect either on fecundity or survival of free-living stages. Udonella may maintain its population relatively independent of the Caligus infra-population all year round. Seasonal patterns of variation were not evident for the population parameters of either Caligus or Udonella, probably due to the exceptional environmental conditions that occurred during 1997 that allowed Caligus populations to remain high during fall and winter, instead of decreasing as it has been reported previously. Results suggest that Udonella cannot be used as a biological control for sea lice since it is not found naturally on salmonid species and because the presence of Udonella did not reduce either fecundity or survival of free-living stages of copepods.

Ecology of the cattle tick (Boophilus microplus) in subtropical Australia. II. Resistance of different breeds of cattle

The resistance to the cattle tick, Boophilus mircoplus, of four herds of cattle with different Bos indicus (zebu) content, grazed at each of two locations in the subtropics of Queensland, Australia, was measured using artificial tick infestations at intervals of either 2 or 6 months over periods of up to 4 years. The factors affecting resistance of crossbred herds at each location in order of decreasing importance were the proportion of R. indicus genes, lactation, age interacting with nutritional conditions, a seasonal cycle which differed in timing at the two locations, and rainfall during the week of tick infestation. These variables accounted for 62% of the observed variation and the statistical model explained concurrent changes in resistance of other herds equally well. These findings are combined with data on the survival of freeliving stages of the ticks in a later paper to explain the observed variation in populations of cattle ticks on the untreated herds when grazing in separate pastures.

Parasitic gastro-enteritis in lambs — A model for estimating the timing of the larval emergence peak

The life history of the nematode parasites of domestic ruminants usually involves the development and survival of free-living stages on pasture. The pasture is, therefore, the site of deposition, development and transmission of nematode infection and meteorological factors affecting the pasture will affect the parasites. Recently Thomas and Starr (1978) discussed an empirical technique for forecasting the timing of the summer wave of gastro-intestinal parasitism in North-East England in the lamb crop using meteorological data and in particular estimates of the duration of “surface wetness”. This paper presents an attempt to model “surface wetness” and the temperature limitation to nematode development.