The distribution of Cryptocotyle spp. metacercariae in the skin of caged Atlantic cod (Gadus morhua)

In caged Atlantic cod (>40cm), Cryptocotyle spp. metacereariae were concentrated on the dorsal surface, but relatively scarce on the fins. Behaviour and mechanical forces probably affect this distribution.     

On the track of the Red Queen: bark beetles, their nematodes, local climate and geographic parthenogenesis

Geographic parthenogenesis has been explained as resulting from parasite pressure (Red Queen hypothesis): several studies have found high degrees of sexuals where the prevalence of parasites is high. However, it is important to address whether prevalence of parasites mirrors risk of infection. We explored geographic parthenogenesis of Ips acuminatus bark beetles and their nematodes. Local climate is crucial for nematode stages outside the host, in spring and summer, and prevalence should thus be associated with those temperatures if prevalence reliably reflects exposure risk across populations. This was the case; however, high prevalence of a virulent nematode species was not associated with many sexuals, whereas highly sexual populations were characterized by high infection risk of benign nematodes. Low virulence of the latter makes Red Queen dynamics unlikely. Geographical patterns of parthenogenesis were instead associated with winter temperature and variance in temperature.     

Ectoparasite intensities are correlated with endoparasite infection loads in willow ptarmigan

Most studies exploring the effect of parasites on host fitness traits deal with a small subset of the parasite community, or with a single parasite species. The results of such studies may be difficult to interpret, because the potential effects of other parasites are not controlled for. If intensities of different parasite species tend to covary, any demonstrated effect by one parasite species could be caused by another, covarying species. In the current study we found that intensities of two different feather lice on willow ptarmigan were positively correlated. Moreover, ectoparasite intensities could be reliably predicted by endoparasite loads. This is unexpected since feather lice are controlled by preening, while endoparasites are kept in check by the immune system. Our results suggest a link between these two aspects of parasite defense, possibly mediated by endoparasite infections reducing host energy available for preening.     

Evolution of virulence under intensive farming: salmon lice increase skin lesions and reduce host growth in salmon farms

Parasites rely on resources from a host and are selected to achieve an optimal combination of transmission and virulence. Human‐induced changes in parasite ecology, such as intensive farming of hosts, might not only favour increased parasite abundances, but also alter the selection acting on parasites and lead to life‐history evolution. The trade‐off between transmission and virulence could be affected by intensive farming practices such as high host density and the use of antiparasitic drugs, which might lead to increased virulence in some host–parasite systems. To test this, we therefore infected Atlantic salmon (Salmo salar) smolts with salmon lice (Lepeophtheirus salmonis) sampled either from wild or farmed hosts in a laboratory experiment. We compared growth and skin damage (i.e. proxies for virulence) of hosts infected with either wild or farmed lice and found that, compared to lice sampled from wild hosts in unfarmed areas, those originating from farmed fish were more harmful; they inflicted more skin damage to their hosts and reduced relative host weight gain to a greater extent. We advocate that more evolutionary studies should be carried out using farmed animals as study species, given the current increase in intensive food production practices that might be compared to a global experiment in parasite evolution.     

Life history and virulence are linked in the ectoparasitic salmon louse Lepeophtheirus salmonis

Models of virulence evolution for horizontally transmitted parasites often assume that transmission rate (the probability that an infected host infects a susceptible host) and virulence (the increase in host mortality due to infection) are positively correlated, because higher rates of production of propagules may cause more damages to the host. However, empirical support for this assumption is scant and limited to microparasites. To fill this gap, we explored the relationships between parasite life history and virulence in the salmon louse, Lepeophtheirus salmonis, a horizontally transmitted copepod ectoparasite on Atlantic salmon Salmo salar. In the laboratory, we infected juvenile salmon hosts with equal doses of infective L. salmonis larvae and monitored parasite age at first reproduction, parasite fecundity, area of damage caused on the skin of the host, and host weight and length gain. We found that earlier onset of parasite reproduction was associated with higher parasite fecundity. Moreover, higher parasite fecundity (a proxy for transmission rate, as infection probability increases with higher numbers of parasite larvae released to the water) was associated with lower host weight gain (correlated with lower survival in juvenile salmon), supporting the presence of a virulence–transmission trade‐off. Our results are relevant in the context of increasing intensive farming, where frequent anti‐parasite drug use and increased host density may have selected for faster production of parasite transmission stages, via earlier reproduction and increased early fecundity. Our study highlights that salmon lice, therefore, are a good model for studying how human activity may affect the evolution of parasite virulence.     

Vector-borne parasites decrease host mobility: a field test of freeze or flee behaviour of willow ptarmigan

Transmission mode has been suggested to be a strong predictor of virulence. According to theory, the transmission of vector-borne parasites should be less dependent on host mobility than directly transmitted parasites. This could select for increased exploitation of host resources in parasites transmitted by vectors, which may be manifested as higher virulence. Here, we test the prediction that there is an association between transmission mode and the effect on host mobility by comparing parasite infection levels and mobility in willow ptarmigan (Lagopus lagopus L.). We examined the endoparasite infracommunities of individual hosts to obtain annual, quantitative data on four vector-transmitted species (Leucocytozoon lovati, Trypanosoma avium, Haemoproteus mansoni and microfilaria), two directly transmitted species (Trichostrongylus tenuis and Eimeria sp.) and two species with indirect life cycles (Hymenolepis microps and Parionella urogalli). We then used observed variations in freeze-or-flee responses of individual willow ptarmigan to assess whether parasite intensities were related to scored freezing responses. From a field data set covering a period of 9 years from a single area, we found that stronger freezing responses were associated with higher intensities of vector-borne parasites, especially with higher intensities of the haemosporidian L. lovati. Freezing responses were not associated with parasites transmitted in other ways. Thus, high intensities of vector-borne parasites tended to reduce host movements, while parasites with other transmission modes did not.     

The influence of a parasite community on the dynamics of a host population: a longitudinal study on willow ptarmigan and their parasites

Despite the fact that most host populations are infected by a community of different parasite species, the majority of empirical studies have focused on the interaction between the host and a single parasite species. Here, we explore the hypothesis that host population dynamics are affected both by single parasite species and by the whole parasite community. We monitored population density and breeding productivity of two populations of willow ptarmigan ( Lagopus lagopus ) in northern Norway for 8 and 11 years, respectively, and sampled eukaryotic endoparasites. We found that increasing abundances of the cestode Hymenolepis microps was associated with increased breeding mortality and reduced annual growth rate of the host population in both areas, and reduced host body mass and body condition in the area where such data were available. In one of the areas, the abundance of the nematode Trichostrongylus tenuis was associated with reductions in host body mass, body condition and breeding mortality and the filaroid nematode Splendidofilaria papillocerca was negatively related to host population growth rates. The parasite community was also negatively related to host fitness parameters, suggesting an additional community effect on host body mass and breeding mortality, although none of the parasites had a significant impact on their own. The prevalence of parasites with very different taxonomical origins tended to covary within years, suggesting that variability in the parasite community was not random, but governed by changes in host susceptibility or environmental conditions that affected exposure to parasites in general. Other variables including climate, plant production and rodent densities were not associated with the recorded demographic changes in the host population.     

Disease dynamics: all caused by males?

Some host individuals tend to acquire parasites at a much faster rate than do others--a consequence of heterogeneities in susceptibility and/or exposure. This is termed 'overdispersion' and, as for many other statistical phenomena, the degree of overdispersion often conforms to a 20/80 rule, where 20% of the host population is responsible for approximately 80% of the parasite transmission. But which are the hosts driving so much of the dynamics of an infectious disease? If host individuals at the tail of the frequency distribution can be identified by some common label, controlling parasitic diseases would be much easier. In two recent papers, Perkins et al. and Ferrari et al. have shown that male hosts are much more important than female hosts in the transmission of parasites.     

Aspects of the life cycle and pathogenesis of Elaphostrongylus cervi in red deer (Cervus elaphus)

Aspects of the migratory life cycle and pathogenesis of Elaphostrongylus cervi were studied in red deer (Cervus elaphus) using 2 farmed calves experimentally infected with 450 third-stage larvae killed 40 and 45 days postinfection and using 3 wild calves and 3 wild yearlings with natural infections killed during autumn hunting. A full necropsy was carried out on the experimental calves, but only the head, eviscerated carcass, and lungs were examined from the naturally infected animals. Histological examination included extensive studies of the central nervous system (CNS), spinal nerve roots, and lungs. The experimental calves had prepatent infections, with many immature adult nematodes in the CNS, whereas the wild calves showed CNS lesions indicating a very recent E. cervi infection. The yearlings had patent infections, with many mature E. cervi in their skeletal muscles, reflecting acquisition of infection during the previous summer. Our findings showed that E. cervi develop to the adult stage in the CNS (subarachnoid spaces) and subsequently migrate into the skeletal muscles, where the mature nematodes live in reproductive pairs and groups. In the nervous system, the nematode caused encephalomyelitis, focal encephalomalacia and gliosis, meningitis, radiculitis, ganglionitis, and perineuritis.