Benefits of Induced Host Responses against an Ectoparasite

As a consequence of the deleterious effects of parasites on host fitness, hosts have evolved responses to minimize the negative impact of parasite infection. Facultative parasite-induced responses are favoured when the risk of infection is unpredictable and host responses are costly. In vertebrates, induced responses are generally viewed as being adaptive, although evidence for fitness benefits arising from these responses in natural host populations is lacking. Here we provide experimental evidence for direct reproductive benefits in flea-infested great tit nests arising from exposure during egg production to fleas. In the experiment we exposed a group of birds to fleas during egg laying (the exposed group), thereby allowing for induced responses, and kept another group free of parasites (the unexposed group) over the same time period. At the start of incubation, we killed the parasites in both groups and all nests were reinfested with fleas. If induced responses occur and are adaptive, we expect that birds of the exposed group mount earlier responses and achieve higher current reproductive success than birds in the unexposed group. In agreement with this prediction, our results show that birds with nests infested during egg-laying have (i) fewer breeding failures and raise a higher proportion of hatchlings to hedging age; () offspring that reach greater body mass, grow longer feathers, and hedge earlier, and (iii) a higher number of recruits and first-year grandchildren than unexposed birds. Flea reproduction and survival did not differ significantly between the two treatments. These results provide the first evidence for the occurrence and the adaptiveness of induced responses against a common ectoparasite in a wild population of vertebrates. [References: 50]

Host condition and host immunity affect parasite fitness in a bird-ectoparasite system

1. Parasites might preferentially feed on hosts in good nutritional condition as such hosts provide better resources for the parasites' own growth, survival and reproduction. However, hosts in prime condition are also better able to develop costly immunological or physiological defence mechanisms, which in turn reduce the parasites' reproductive success. The interplay between host condition, host defence and parasite fitness will thus play an important part in the dynamics of host-parasite systems.;2. In a 2 x 2 design, we manipulated both the access to food in great tit Parus major broods and the exposure of the nestlings to hen fleas Ceratophyllus gallinae, a common ectoparasite of hole-breeding birds. We subsequently investigated the role of manipulated host condition, host immunocompetence, and experimentally induced host defence in nestlings on the reproductive success of individual hen flea females.;3. The food supplementation of the nestlings significantly influenced the parasites' reproductive success. Female fleas laid significantly more eggs when feeding on food-supplemented hosts.;4. Previous parasite exposure of the birds affected the reproductive success of fleas. However, the impact of this induced host response on flea reproduction depended on the birds' natural level of immunocompetence, assessed by the phytohaemagglutinin (PHA) skin test. Flea fecundity significantly decreased with increasing PHA response of the nestlings in previously parasite-exposed broods. No relationship between flea fitness and host immunocompetence was, however, found in previously unexposed broods. The PHA response thus correlates with the nestlings' ability to mount immunological or physiological defence mechanisms against hen fleas. No significant interaction effect between early flea exposure and food supplementation on the parasites' reproductive success was found.;5. Our study shows that the reproductive success of hen fleas is linked to the hosts' food supply early in life and their ability to mount induced immunological or physiological defence mechanisms. These interactions between host quality and parasite fitness are likely to influence host preference, host choice and parasite virulence and thus the evolutionary dynamics in host-parasite systems.

Immunocompetence of nestling great tits in relation to rearing environment and parentage

Theoretical models of host-parasite coevolution assume a partially genetic basis to the variability in susceptibility to parasites among hosts, for instance as a result of genetic variation in immune function. However, few empirical data exist for free-living vertebrate hosts to support this presumption. In a cross-fostering experiment with nestling great tits, by comparing nestlings of the same origin we investigated (i) the variance in host resistance against an ectoparasite due to a common genetic origin, (ii) the effect of ectoparasite infestation on cell-mediated immunity and (iii) the variance in cell-mediated immunity due to a common genetic origin. Ectoparasitic hen fleas can impair the growth of nestling great tits and nestling growth was therefore taken as a measure of host susceptibility. A common origin did not account for a significant part of the variation in host susceptibility to fleas. There was no significant overall effect of fleas on nestling growth or cell-mediated immunity, as assessed by a cutaneous hypersensitivity response. A common rearing environment explained a significant part of the variation in cell-mediated immunity among nestlings, mainly through its effect on nestling body mass. The variation in cell-mediated immunity was also related to a common origin. However, the origin-related variation in body mass did not account for the origin-related differences in cell-mediated immunity. The results of the present study thus suggest heritable variation in cell-mediated immunity among nestling great tits. [References: 49]

Parasite- and predator-induced maternal effects in the great tit (Parus major)

Both predators and parasites can elicit behavioral and physiological responses in prey and hosts, respectively. These responses may involve the reallocation of resources and may thus limit each other. We investigated the effects of concurrent pre-laying exposure of great tit females (Parus major) to both a simulated predation risk and a nest-based ectoparasite, the hen flea (Ceratophyllus gallinae), on nestling growth and development. We manipulated perceived predation risk using models and vocalizations of sparrowhawks (Accipiter nisus). At the start of incubation, we swapped whole clutches between treated and untreated nests to separate pre-laying maternal effects from posthatching effects. Since costs and benefits of maternal responses to parasites need to be assessed under parasite pressure, we infested half of the rearing nests with hen fleas. Parasites had negative effects on mass gain and wing growth, both via maternal effects and via direct exposure of nestlings, whereas maternal predation risk had no significant effect. The interaction between predator and parasite treatments was not significant and, thus, suggests the absence of a trade-off between the 2 stressors operating at the level of maternal effects. Alternatively, the complexity of the design, despite a relatively large sample size, may have limited the power for detection of this expected trade-off.

Molecular detection of Bartonella spp. in deer ked pupae, adult keds and moose blood in Finland

SUMMARY The deer ked (Lipoptena cervi) is a haematophagous ectoparasite of cervids that harbours haemotrophic Bartonella. A prerequisite for the vector competence of the deer ked is the vertical transmission of the pathogen from the mother to its progeny and transstadial transmission from pupa to winged adult. We screened 1154 pupae and 59 pools of winged adult deer keds from different areas in Finland for Bartonella DNA using PCR. Altogether 13 pupa samples and one winged adult deer ked were positive for the presence of Bartonella DNA. The amplified sequences were closely related to either B. schoenbuchensis or B. bovis. The same lineages were identified in eight blood samples collected from free-ranging moose. This is the first demonstration of Bartonella spp. DNA in a winged adult deer ked and, thus, evidence for potential transstadial transmission of Bartonella spp. in the species.

The ectoparasitic mite Tropilaelaps mercedesae reduces western honey bee, Apis mellifera, longevity and emergence weight, and promotes Deformed wing virus infections

Historically an ectoparasite of the native Giant honey bee Apis dorsata, the mite Tropilaelaps mercedesae has switched hosts to the introduced western honey bee Apis mellifera throughout much of Asia. Few data regarding lethal and sub-lethal effects of T. mercedesae on A. mellifera exist, despite its similarity to the devastating mite Varroa destructor. Here we artificially infested worker brood of A. mellifera with T. mercedesae to investigate lethal (longevity) and sub-lethal (emergence weight, Deformed wing virus (DWV) levels and clinical symptoms of DWV) effects of the mite on its new host. The data show that T. mercedesae infestation significantly reduced host longevity and emergence weight, and promoted both DWV levels and associated clinical symptoms. Our results suggest that T. mercedesae is a potentially important parasite to the economically important A. mellifera honey bee.