Elaphostrongylus rangiferi: Influence of temperature,substrate, and larval age on the infection rate in the intermediate snail host, Aarianta arbustorum

The infection rate of the first stage larval nematodes, Elaphostrongylus rangiferi, was studied experimentally, using the juvenile snail Arianta arbustorum as intermediate host. The nematode showed a linear, fivefold increase in infection rate within the temperature range of 4 to 28 C. The snails were exposed to the larval nematodes on three different substrates. The highest infection rate was recorded when snails were exposed in tap water and significantly slower infection rates were obtained when either lettuce or soil was used as the substrate. First stage larvae of E. rangiferi were infective for at least 2 months when stored at 12 C. Throughout this period, the infection rate showed a significant decline, while the motility of the larvae remained unchanged.     

Population biology of the nematode Camallanus lacustris in perch, Perca fluviatilis L., from an oligotrophic lake in Norway

Samples of perch, Perca fluviatilis , were examined for the contents of their gastrointestinal tract and for the intestinal nematode Camallanus lacustris (Zoega). The seasonal dynamics, dispersal pattern and site preference of the nematode were studied. Both larvae and adult worms were present in the perch throughout the year. The abundance of C. lacustris was non-seasonal, but seasonal cycles were found in the numbers of larvae and of gravid females. C. lacustris showed a tendency to an increase in mean intensity with the size of perch. The frequency distribution of the nematode followed the negative binomial at different times of the year and in different length groups of perch. C. lacustris showed a seasonal site preference in the perch gut.     

Intensive Farming: Evolutionary Implications for Parasites and Pathogens

An increasing number of scientists have recently raised concerns about the threat posed by human intervention on the evolution of parasites and disease agents. New parasites (including pathogens) keep emerging and parasites which previously were considered to be 'under control' are re-emerging, sometimes in highly virulent forms. This re-emergence may be parasite evolution, driven by human activity, including ecological changes related to modern agricultural practices. Intensive farming creates conditions for parasite growth and transmission drastically different from what parasites experience in wild host populations and may therefore alter selection on various traits, such as life-history traits and virulence. Although recent epidemic outbreaks highlight the risks associated with intensive farming practices, most work has focused on reducing the short-term economic losses imposed by parasites, such as application of chemotherapy. Most of the research on parasite evolution has been conducted using laboratory model systems, often unrelated to economically important systems. Here, we review the possible evolutionary consequences of intensive farming by relating current knowledge of the evolution of parasite life-history and virulence with specific conditions experienced by parasites on farms. We show that intensive farming practices are likely to select for fast-growing, early-transmitted, and hence probably more virulent parasites. As an illustration, we consider the case of the fish farming industry, a branch of intensive farming which has dramatically expanded recently and present evidence that supports the idea that intensive farming conditions increase parasite virulence. We suggest that more studies should focus on the impact of intensive farming on parasite evolution in order to build currently lacking, but necessary bridges between academia and decision-makers.     

Is reduced body growth of cod exposed to the gill parasite Lernaeocera branchialis a cost of resistance?

Three hundred and sixty-nine cod Gadus morhua were individually marked and caged for 19 months. During this period, each cod was inspected several times for Lernaeocera branchialis . Growth in four groups of cod, identified by their infection history, were compared. During the caging, 79% of the cod remained uninfected, 8·5% were infected, but lost the parasite, 8% were infected with one parasite and 4·5% were infected with more than one parasite. The infected fish either harboured the parasite at caging or were infected during the study period. The highest rate of increase, both in body mass and in standard length ( L _S), was recorded in the group of male fish infected with one parasite throughout the experimental period. Conversely, those males free from infection showed significantly lower growth. The observed differences in growth could not be explained by changes in variables related to reproductive strategies. The alternative explanation for these results is that resistance to L. branchialis was associated with costs in terms of reduced growth of body mass and L _S.     

Standard sampling techniques underestimate prevalence of avian hematozoa in willow ptarmigan (Lagopus lagopus)

A total of 68 willow ptarmigan (Lagopus lagopus L.) was collected during September 1995 from two localities in Troms County, northern Norway. Thin blood smears were prepared and examined for blood parasites. Of the 68 willow ptarmigan examined, 94% harbored one or more species of hematozoa. There were four (6%), 44 (65%), 16 (24%), and four (6%) birds infected by zero, one, two, and three species of parasites, respectively. Prevalences at the coastal locality, Kattfjord (n = 43), were Leucocytozoon lovati 86%, Trypanosoma avium (26%), and microfilariae (30%). At the inland locality, Iselvdalen (n = 25), prevalences were L. lovati 96%, T. avium 12%, and microfilariae 0%. We also searched connective tissues for the filaroid nematode Splendidofilaria papillocerca; in Kattfjord this parasite only occurred in adult hosts where prevalence was 94%, but the parasite was not found in Iselvdalen. To estimate the efficiency of parasite detection by standard blood sampling techniques, we sampled peripheral blood from the brachial wing vein and blood from the pulmonary system from willow ptarmigan. Sampling peripheral blood from the brachial vein led to underestimates of the prevalence of microfilariae. There was no significant difference between L. lovati and T. avium prevalence in blood collected from the brachial vein or deep circulation. Age of host had a strong impact on prevalence, especially for S. papillocerca and microfilariae.     

Transmission of Cryptocotyle lingua cercariae in natural environments: a field experiment

Atlantic cod were kept at two different depths in three cages placed at increasing distance from a rocky shore where natural infections of the digenean Cryptocotyle lingua occurred in the snail Littorina littorea . During a 6-month period the fish kept at 0–2 m accumulated significantly more parasites than those kept at 2–4 m. Distance from the shore did not affect infection rates. We conclude that host location behaviour of cercariae influences the infection pattern under natural conditions, and that transmission from snail to fish can take place over large areas of coastal waters.     

Density-dependent effects in a parasitic nematode,Elaphostrongylus rangiferi,in the snnail intermediate host

Summary Density-dependent effects in Elaphostrongylus rangiferi, a parasitic nematode in the CNS and muscular system of reindeer, were studied in a laboratory population of the snail intermediate host, Arianta arbustorum. The rates in parasite growth, development and mortality were all affected by parasite density. The effects on growth and development were, however, much more marked, than the effect on mortality.All density-dependent rates were intensified by decreasing snail size, and by snail starvation. The snail host showed marked tissue reactions against infection, and the intensity of these reactions increased with increasing parasite density. The mechanism behind the observed density-dependent rates is discussed, and is tentatively concluded to be competition for nutritive substances in the host tissue.The importance of a density-dependent developmental rate in natural populations of this parasite is discussed, and it is hypothesized that this effect may counteract the strong temperature-dependent developmental rate of E. rangiferi In a more general context it is pointed out that density-dependent developmental rates, although common amongst animal populations, has been neglected in models of population dynamics. Developmental rates are usually represented by a constant time lag in such models, but should be treated as a density-dependent variable.     

Life history shifts in an exploited African fish following invasion by a castrating parasite

Evolutionary theory predicts that infection by a parasite that reduces future host survival or fecundity should select for increased investment in current reproduction. In this study, we use the cestode Ligula intestinalis and its intermediate fish host Engraulicypris sardella in Wissman Bay, Lake Nyasa (Tanzania), as a model system. Using data about infection of E. sardella fish hosts by L. intestinalis collected for a period of 10 years, we explored whether parasite infection affects the fecundity of the fish host E. sardella, and whether host reproductive investment has increased at the expense of somatic growth. We found that L. intestinalis had a strong negative effect on the fecundity of its intermediate fish host. For the noninfected fish, we observed an increase in relative gonadal weight at maturity over the study period, while size at maturity decreased. These findings suggest that the life history of E. sardella has been shifting toward earlier reproduction. Further studies are warranted to assess whether these changes reflect plastic or evolutionary responses. We also discuss the interaction between parasite and fishery‐mediated selection as a possible explanation for the decline of E. sardella stock in the lake.