Spatial and temporal distribution of Ligula intestinalis (Cestoda: Diphyllobothriidea) in usipa (Engraulicypris sardella) (Pisces: Cyprinidae) in Lake Nyasa.

Engraulicypris sardella is an endemic and economically important cyprinid species in Lake Nyasa/Malawi which has recently been infected by the tapeworm Ligula intestinalis. This parasite is known to induce severe pathological and behavioural effects on other cyprinids, including castration, followed by a collapse of infected populations. As a first step to understanding the dynamics between this parasite and E. sardella, we studied the spatial and temporal variation in prevalence over a period of 1 year. Overall prevalence was about 15%, but we observed a consistently higher prevalence in the littoral compared to the pelagic zone. Fish in the upper water levels showed the highest prevalence, with a marked decline with increasing water depth down to 150 m. The proportion of infected fish varied over time, with a significantly higher prevalence in the rainy season. In a huge lake like the Nyasa, with a surface area of 29,000 km2 and a maximum depth of 785 m, the transmission success of the parasite appears to show large variations in time and space. We suggest that these conditions could lead the parasite to become persistent within the lake, rather than the typical epidemic situation as observed in smaller bodies of water.

The role of chewing lice (Phthiraptera: Philopteridae) as intermediate hosts in the transmission of Hymenolepis microps (Cestoda: Cyclophyllidea) from the willow ptarmigan Lagopus lagopus (Aves: Tetraonidae).

The cestode Hymenolepis microps is an intestinal parasite of tetraonid birds, including the willow ptarmigan (Lagopus lagopus). This parasite is able to maintain a high prevalence and intensity throughout the year, even in a subarctic environment in bird populations with relatively low host densities, indicating effective transmission routes. Willow ptarmigan consume mainly vegetal material and active consumption of invertebrates is confined to the first two or three weeks of life. Ptarmigan are infected by different species of ectoparasites, of which two species of feather lice, Lagopoecus affinis and Goniodes lagopi, are the most abundant. In this study, we explored the hypothesis that feather lice may be suitable intermediate hosts for H. microps. We applied histological techniques and light microscopy to investigate lice for the presence of larval cestode stages (cysticercoids). We found 12 cysticercoid-like structures inside chewing lice collected on L. lagopus hosts harbouring H. microps. In addition, a polymerase chain reaction (PCR) screening of Ischnocera lice DNA, targeting the 18S rRNA gene of the cestode, showed positive results for two different short fragments of the 18S rRNA gene of H. microps which were sequenced from lice collected on birds. Both independent lines of evidence support the hypothesis that Ischnocera lice might be suitable intermediate hosts in the life cycle of H. microps in L. lagopus.

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.

Atlantic salmon infected with salmon lice are more susceptible to new lice infections.

Aggregation is commonly observed for macroparasites, but its adaptive value remains unclear. Heavy infestations intensities may lead to a decrease in some fitness-related traits of parasites (e.g. parasite fecundity or survival). However, to a dioecious parasite, increased aggregation could also increase the chance of finding individuals of the opposite sex. In a laboratory experiment, we tested if previous experience with salmon lice (Lepeophtheirus salmonis) affected susceptibility of Atlantic salmon (Salmo salar) to later exposure to the same parasite species. We found that currently infected fish got higher intensities of new lice than naive fish. This suggests that hosts already carrying parasites are more susceptible to new lice infections. For this dioecious parasite, such positive density dependence might be adaptive, ensuring successful reproduction under conditions of low lice densities by increasing the probability of both sexes infecting the same host.

Parasite fecundity decreases with increasing parasite load in the salmon louse Lepeophtheirus salmonis infecting Atlantic salmon Salmo salar.

Aggregation is common amongst parasites, where a small number of hosts carry a large proportion of parasites. This could result in density-dependent effects on parasite fitness. In a laboratory study, we explored whether parasite load affected parasite fecundity and survival, using ectoparasitic salmon lice (Lepeophtheirus salmonis Krøyer, 1837) infecting Atlantic salmon (Salmo salar) hosts. We found a significant reduction in fecundity with higher parasite load, but no significant effect on survival. Together with previous findings, this suggests that stronger competition amongst female lice under high parasite load is a more likely explanation than increased host immune response.

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.

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.

The parasite Lernaeocera branchialis on caged cod: infection pattern is caused by differences in host susceptibility.

Variation in host susceptibility causes significant differences in infection rates between hosts living in a semi-natural situation. Such knowledge has implications for population dynamics and evolutionary models of host-parasite interactions as well as for estimations of parasite abundance. Infection rates by Lernaeocera branchialis (L.) were measured through time and space on caged Atlantic cod (Gadus morhua L.). One group of hosts, identified by their infection history, developed significantly higher infection rates than the others. These were fish which had been infected previously, but had lost their infection. Differences between groups were consistent through both time and space. Two types of cod seem to have been present in the caged population; a small group of inherently susceptible fish, which were infected, and reinfected if the parasite was lost, and another group of resistant hosts with a small chance of becoming infected.

Experimental Elaphostrongylus alces infection in goats.

Five goats aged 4 months were each inoculated with approximately 300 third-stage larvae of Elaphostrongylus alces, and killed for post-mortem examination after 14-150 days. No clinical signs of disease were observed during the experiment. Pathological examination revealed that the larvae penetrated the walls of the abomasum and small intestine and migrated towards the caudal vertebral canal. However, the great majority of larvae were apparently destroyed along the migratory route, and development to adult parasites in the vertebral canal was not seen. During migration, the larvae caused focal inflammation and necrosis in the gastrointestinal wall, liver, mesentery and lungs. The study suggests that the only effect of E. alces infection on goats is the formation of focal visceral lesions during abdominal larval migration; it also confirms the infectivity of E. alces for domestic ruminants.

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.

Experimental Elaphostrongylus cervi infection in sheep and goats.

The pathogenesis and migratory life cycle of Elaphostrongylus cervi were studied in four sheep and six goats killed and examined 6 days to 5 months after inoculation with infective third-stage larvae (L3). Detailed histological studies demonstrated that the L3 followed a porto-hepatic, and probably also a secondary lymphatic, migratory route from the abomasum and small intestine to the lungs, with subsequent spread via the general circulation to the central nervous system (CNS) and other tissues. In addition, the results suggested that haematogenously spread L3, arrested in arterial vessels outside the spinal cord, migrated into the cord along the spinal nerves. During migration, the L3 caused focal inflammation and necrosis in the organs and along the spinal nerve roots, and infarcts occurred in the myocardium, kidneys and CNS. Nematode development took place in the CNS. During development, there was a gradual die-off of nematodes and patent infections were not observed. However, in one animal many mature nematodes were demonstrated in the CNS. In the nervous system, the nematodes caused encephalomyelitis, focal traumatic encephalomalacia, gliosis, meningitis, choroiditis, radiculitis and perineuritis. Two goats and one sheep displayed long-lasting paraparesis starting 6 weeks after inoculation. The signs apparently resulted from nematode-induced spinal nerve root lesions. From 19 weeks after inoculation the sheep also showed signs of severe brain disturbances due to traumatic and inflammatory lesions caused by adult E. cervi in the cerebral parenchyma. We conclude that E. cervi represents a potential cause of neurological disease in small ruminants grazing areas inhabited by red deer. This is the first report confirming the infectivity of E. cervi for domestic ruminants.

Parasite abundance, body size, life histories, and the energetic equivalence rule.

If common processes generate size-abundance relationships among all animals, then similar patterns should be observed across groups with different ecologies, such as parasites and free-living animals. We studied relationships among body size, life-history traits, and population intensity (density in infected hosts) among nematodes parasitizing mammals. Parasite size and intensity were negatively correlated independently of all other parasite and host factors considered and regardless of type of analyses (i.e., nonphylogenetic or phylogenetically based statistical analyses, and across or within communities). No other nematode life-history traits had independent effects on intensity. Slopes of size-intensity relationships were consistently shallow, around -0.20 on log-log scale, and thus inconsistent with the energetic equivalence rule. Within communities, slopes converged toward this global value as size range increased. A summary of published values suggests similar convergence toward a global value around -0.75 among free-living animals. Steeper slopes of size-abundance relationships among free-living animals could be related to fundamental differences in ecologies between parasites and free-living animals, although such generalizations require reexamination of size-abundance relationships among free-living animals with regard to confounding factors, in particular by use of phylogenetically based statistical methods. In any case, our analyses caution against simple generalizations about patterns of animal abundance.

Cross-infection of moose (Alces alces) and reindeer (Rangifer tarandus) with Elaphostrongylus alces and Elaphostrongylus rangiferi (Nematoda, Protostrongylidae): effects on parasite morphology and prepatent period.

Moose (Alces alces) and reindeer (Rangifer tarandus) were experimentally cross-infected with Elaphostrongylus rangiferi and Elaphostrongylus alces, respectively. Both Elaphostrongylus species completed their development in the alternate hosts but produced fewer larvae than in their usual host species. Reindeer infected with Elaphostrongylus alces developed patent infections after 39-130 days. In moose, the prepatent period of this parasite was 39-73 days. Elaphostrongylus rangiferi infections were patent in moose after 133 days. The male morphological characteristic of E. alces in moose and reindeer, and E. rangiferi in moose and their migration pattern retained regardless of the host species. These results provide further evidence that E. alces and E. rangiferi are two distinct species.

The evolution of tissue migration by parasitic nematode larvae.

Migration by nematode larvae through the tissues of their mammalian hosts can cause considerable pathology, and yet the evolutionary factors responsible for this migratory behaviour are poorly understood. The behaviour is particularly paradoxical in genera such as Ascaris and Strongylus in which larvae undergo extensive migrations which begin and end in the same location. The orthodox explanation for this apparently pointless behaviour is that a tissue phase is a developmental requirement following the evolutionary loss of skin penetration or intermediate hosts. Yet tissue migration is not always necessary for development, and navigation and survival in an array of different habitats must require costly biochemical and morphological adaptations. Migrating larvae also risk becoming lost or killed by the host. Natural selection should therefore remove such behaviour unless there are compensating benefits. Here we propose that migration is a selectively advantageous life-history strategy. We show that taxa exploiting tissue habitats during development are, on average, bigger than their closest relatives that develop wholly in the gastrointestinal tract. Time to reproduction is the same, indicating that worms with a tissue phase during development grow faster. This previously unsuspected association between juvenile habitat and size is independent of any effects of adult habitat, life-cycle, or host size, generation time or diet. Because fecundity is intimately linked with size in nematodes, this provides an explanation for the maintenance of tissue migration by natural selection, analogous to the pre-spawning migrations of salmon.

The temperature threshold for development of Elaphostrongylus rangiferi in the intermediate host: an adaptation to winter survival?

To test the hypothesis that the relatively high developmental temperature threshold of the parasitic nematode Elaphostrongylus rangiferi in the intermediate snail host is an adaptation to minimize larval mortality during winter, an experiment was set up in which snails of the species Arianta arbustorum were experimentally infected with the parasite. The snails were divided into 3 groups known to contain 1st, 2nd or 3rd-stage larvae, and incubated at 3 degrees C for an experimental period of 18 weeks. First-stage larvae showed a significantly higher survival rate within snails than 2nd or 3rd-stage larvae. We also found that snails carrying 1st-stage larvae survived better than snails with other larval stages. It is concluded that if the nematode has started development before the hibernation, this has a real and significant effect on the risk of dying. The high developmental threshold is therefore likely to be an adaptation to reduce the chance of hibernating as developing larvae during long periods of low temperatures.

Migration of adult Elaphostrongylus rangiferi (Nematoda: Protostrongylidae) from the spinal subdural space to the muscles of reindeer (Rangifer tarandus).

To trace the intrahost migration of adult (fifth-stage) Elaphostrongylus rangiferi 9 reindeer calves were fed infective larvae and examined for worms 48-250 days postinfection. The average length of worms recovered increased asymptotically with time. Gravid females and adult males were recovered from 52 days postinfection (d.p.i.), and the fraction made up by these categories increased with time. Immature females and subadult males were recovered as late as 161 d.p.i. Nematodes were recovered from the spinal subdural space 48-161 d.p.i., from the cranial subdural space 48-90 d.p.i., and from the musculature 90-250 d.p.i. Immature females were found in the spinal subdural space and the cranial subdural space, whereas gravid females were found also in the musculature with an increasing fraction with time. Subadult and adult males were found in all 3 sites but with an increasing fraction of adults from the spinal canal to the cranium to the musculature.

Experimental cerebrospinal elaphostrongylosis (Elaphostrongylus rangiferi) in sheep.

Seven lambs were inoculated with 150-3,000 infective larvae of Elaphostrongylus rangiferi and subsequently killed for autopsy. From one week post inoculation (p.i.), a marked eosinophilia was found, and one of the lambs showed signs of coughing, increased respiratory frequency and elevated body temperature. Pruritus was observed in three lambs during the period 5-10 weeks p.i. Gross and microscopic lesions were found in the liver, lungs, myocardium, kidneys, spinal nerve roots and in the central nervous system (CNS). The lesions resembled those previously reported in experimentally infected goat kids. The migratory route of the infective larvae seemed to be haematogenous. Developing nematodes were sectioned in the CNS of a lamb killed at day 30 p.i. The nematodes found in the brain of this lamb were significantly thicker than those observed in the spinal cord. Intact nematodes were not detected in the remaining lambs which were all killed 149-151 days p.i. None of the experimental lambs shed E. rangiferi first-stage larvae in the faeces. These findings indicate that E. rangiferi gradually dies out in sheep and will not complete its life cycle in this animal species.

Experimental cerebrospinal elaphostrongylosis (Elaphostrongylus rangiferi) in goats: I. Clinical observations.

Clinical observations of 17 goat kids inoculated with 100-1000 infective larvae of Elaphostrongylus rangiferi and autopsied 21-154 days post inoculation (p.i.) are reported. Pruritus was common in the period 4-10 weeks p.i. Six animals displayed neurological signs starting 35 to 94 days p.i. The most frequent sign was posterior paresis. Other signs included cranial nerve paresis, ataxia, lameness, scoliosis, reduced vision, abnormal behaviour, depression and mental confusion. Recoveries were recorded. An apparent dose-response related eosinophilia was observed at days 14 and 32 p.i. None of the experimentally infected kids shed E. rangiferi first-stage larvae in the faeces, indicating that the nematode will not complete its life-cycle in goats.

Experimental cerebrospinal elaphostrongylosis (Elaphostrongylus rangiferi) in goats: II. Pathological findings.

Pathological findings in 17 goat kids inoculated with 100-1,000 infective larvae of Elaphostrongylus rangiferi and autopsied 21-154 days post inoculation (p.i.) are reported. The lungs, heart, diaphragm, liver and kidneys contained small foci of necrosis or fibroblastic scarring and interstitial infiltrates of inflammatory cells. The lungs also contained many parasitic granulomas. Infarcts were observed in the myocardium and kidneys. Accumulations of inflammatory cells and granulomas were seen in the peri- and epineurium of the spinal nerve roots, connective tissue of the spinal ganglia, dura and epidural tissue of the cord, choroid plexus of the brain and leptomeninges of the entire central nervous system (CNS). Within nerve fascicles there were endoneural cell infiltrates, axon and myelin sheath degenerations and granulomas. The CNS parenchyma contained foci of traumatic encephalomyelomalacia, microgliosis, secondary axon degeneration, perivascular cuffs and granulomas. Sections of intact nematodes were found in the subarachnoid spaces, brain ventricles, central canal of the spinal cord and CNS parenchyma. Pathological findings from individual animals are compared to the clinical signs described in a separate paper. The development, migration and pathogenesis of E. rangiferi in goats are discussed.

The early migration of Elaphostrongylus rangiferi in goats.

The migration of nematodes of the genus Elaphostrongylus (Metastrongyloidea: Protostrongylidae) from the gut into the tissues of ruminants has not been described. Detailed histologic studies were performed on five goat kids that had received oral doses of 1,000-1,500 infective larvae of Elaphostrongylus rangiferi Mitskevich, 1960 and were subsequently autopsied at days 2, 4, 6, 10, and 14 post inoculation. The main migratory route of E. rangiferi larvae seemed to be haematogenous. The larvae penetrated the venules of the abomasum and followed the blood stream via the liver to the lungs. In the lungs, the larvae entered pulmonary venules and passed into the arterial circulation reaching the central nervous system (CNS) and other organs between days 6 and 10 post inoculation. Some of the larvae that had lodged in tissues outside the CNS probably migrated into it along the spinal nerves. A marked eosinophilia was present from day 8 post inoculation.