Dissociation of the protective immune response in the mouse to Strongyloides ratti

The generation of protective immunity by various stages in the life-cycle of Strongyloides ratti and the phases against which resistance is directed has been examined in murine strongyloidiasis. Mice were exposed to natural, complete infections, were treated with thiabendazole (which largely resembles the natural infection), were treated with cambendazole (which restricts infection to the larval stage), or infected directly by oral transfer of adult worms. Mice that were infected with infective larvae alone did not become resistant to infective larvae or the complete infection but were resistant to adult worms implanted directly into the gut. Mice exposed to adult worms alone were resistant to natural infections and adults worms implanted directly but were not resistant to infective larvae. On the other hand, mice that had received prior natural infections showed evidence of resistance to infective larvae, adult worms, and natural, complete infections. It is concluded that there is immunological cross-reactivity between infective larvae and adult worms but that under certain circumstances the infective larvae are able to evade the host's protective immune response.     

The oral route as a potential way of transmission of Schistosoma bovis in goats.

The infectivity of Schistosoma bovis cercariae administered orally was evaluated in Sahelian goats. Compared to the percutaneous route, a single massive oral dose resulted in a worm burden and in fecal egg excretion reduced by one-half. Surprisingly, tissue egg counts were increased by more than 4-fold. Fecundity of individual female schistosomes was, therefore, markedly increased. When infective doses were administered weekly for 20 wk, both worm and egg burdens were doubled without modification of the individual worm pair fecundity. Repeated oral infections seem to have induced an acquired tolerance toward parasite antigens. These results confirm the epidemiologic relevance of the oral route in a host species inclined to become infected through drinking water rather than percutaneous exposures.     

Capillaria hepatica infection of Mastomys natalensis: on the development, egg production and host reaction (author's transl)]

Studies on the development and egg production of Capillaria hepatica and on the macroscopically visible alterations of the liver and spleen of the host were carried out in experimentally infected Mastomys natalensis. Following the oral administration of infective eggs the first stage larvae hatched in the caecum of fasting and fed animals after about 8 and 15 hours respectively. The prepatency could be found with 20 days. The dynamics and duration of egg production of the parasite proved to be dependent on the infective dose. After the first week of patency increasing numbers of eggs/liver were found with increasing doses of infection up to 800 eggs per animal. This relation could not be observed 76 days post infection. After infections with 50, 300 und 800 eggs per animal maximum egg production was found between 60 and 72, 36 and 48 and about 30 days p.i., respectively. The egg production of the parasites correspondently continued for more than 85 days after infection or had ceased 72 or 48 days p.i. Intraperitoneal administration of infective eggs revealed a lower infection rate, evaluated by the number of eggs per liver, than oral infection.     

Experimental infection routes of Angiostrongylus cantonensis in mice.

Stomach intubation is the most common method used in the experimental infection of animals with Angiostrongylus cantonensis. In order to compare the effectiveness of other possible transmission methods, groups of BALB/c mice were given infective third-stage larvae of A. cantonensis by different routes including intraperitoneal or subcutaneous injections, and penetration of anal mucosa, vaginal mucosa, conjunctival mucosa, lacerated skin, unabraded skin, foot pad and tail skin, while stomach intubation was used as control. Recovery of fifth-stage larvae was higher in mice inoculated with third-stage larvae subcutaneously. Successful infections were established through all experimental transmission routes except tail skin penetration. This study suggests that oral infection may not be the only route for the transmission of human angiostrongyliasis, and subcutaneous infection may be a better method for experimental infection.     

The dynamics of infection of Tribolium confusum by Hymenolepis diminuta: the influence of infective-stage density and spatial distribution

The mean parasite burden of a population of Tribolium confusum is shown to rise to a plateau as the exposure density of infective eggs of Hymenolepis diminuta increases. The level of this plateau is shown to be dependent on the nutritional status of the host population, being depressed from approximately 18 cysticeroids/beetle in hosts which have been starved prior to experimentation, to approximately 2 cysticercoids/beetle in satiated hosts. A simple model is used to describe the shape of this infection functional response in terms of the predator-prey interaction between hosts (T. confusum) and parasite infective stages (H. diminuta eggs). The distribution of successful infections/host is shown to be over-dispersed, even when hosts are exposed to infective stages arranged in a uniform spatial pattern. The over-dispersion of parasite numbers/host is shown to become more severe as the spatial pattern of infective stages changes from under-dispersed, through random, to over-dispersed. Experimental results are discussed in relation to the dynamics of parasite-host interactions, in which infection takes place by host ingestion of a free-living infective stage.     

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.     

Experimental heteroxenous cycle of Lagochilascaris minor Leiper, 1909 (Nematoda: Ascarididae) in white mice and in cats.

Reports of natural infections of sylvatic carnivores by adult worms of species similar to Lagochilascaris minor in the Neotropical region led to attempts to establish experimental cycles in laboratory mice and in cats. Also, larval development was seen in the skeletal muscle of an agouti (Dasyprocta leporina) infected per os with incubated eggs of the parasite obtained from a human case. In cats, adult worms develop and fertile eggs are expelled in the feces; in mice, larval stages of the parasite develop, and are encapsulate in the skeletal muscle, and in the adipose and subcutaneous connective tissue. From our observations, we conclude that the larva infective for the mouse is the early 3rd stage, while for the final host the infective form is the later 3rd stage. A single moult was seen in the mouse, giving rise to a small population of 4th stage larvae, long after the initial infection.     

Plagiorchis elegans (Digenea: Plagiorchiidae) infections in Stagnicola elodes (Pulmonata: Lymnaeidae): host susceptibility, growth, reproduction, mortality, and cercarial production.

Eggs of Plagiorchis elegans were readily ingested by Stagnicola elodes of all ages, but were more infective to immature than mature snails. Infection enhanced the growth of the host in a dose-dependent manner. The number of cercariae released by immature snails increased with the age of the snail host; mature snails yielded fewer cercariae. Heavily infected snails tended to die prematurely, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails that acquired the infection as juveniles never produced eggs. Actively reproducing snails ceased egg laying within days of infection and never recovered. All parasite effects on the growth and reproduction of the snail host first manifested themselves during the early stages of infection, long before the development of daughter sporocysts and cercariae, and are therefore attributable to the effects of mother sporocysts. The study provides insight into how this natural enemy of mosquito larvae may be established in natural snail populations by means of strategically timed introductions of parasite eggs, with a goal of maximizing cercarial production for the biological control of sympatric mosquito larvae.     

Host Density and Competency Determine the Effects of Host Diversity on Trematode Parasite Infection

Variation in host species composition can dramatically alter parasite transmission in natural communities. Whether diverse host communities dilute or amplify parasite transmission is thought to depend critically on species traits, particularly on how hosts affect each other’s densities, and their relative competency as hosts. Here we studied a community of potential hosts and/or decoys (i.e. non-competent hosts) for two trematode parasite species, Echinostoma trivolvis and Ribeiroia ondatrae, which commonly infect wildlife across North America. We manipulated the density of a focal host (green frog tadpoles, Rana clamitans), in concert with manipulating the diversity of alternative species, to simulate communities where alternative species either (1) replace the focal host species so that the total number of individuals remains constant (substitution) or (2) add to total host density (addition). For E. trivolvis, we found that total parasite transmission remained roughly equal (or perhaps decreased slightly) when alternative species replaced focal host individuals, but parasite transmission was higher when alternative species were added to a community without replacing focal host individuals. Given the alternative species were roughly equal in competency, these results are consistent with current theory. Remarkably, both total tadpole and per-capita tadpole infection intensity by E. trivolvis increased with increasing intraspecific host density. For R. ondatrae, alternative species did not function as effective decoys or hosts for parasite infective stages, and the diversity and density treatments did not produce clear changes in parasite transmission, although high tank to tank variation in R. ondatrae infection could have obscured patterns.     

Infection dynamics of the lungworm Rhabdias pseudosphaerocephala in its natural host, the cane toad (Bufo marinus), and in novel hosts (native Australian frogs)

Host-parasite systems have often evolved over time, such that infection dynamics may become greatly modified from the time of initial contact of the host with the parasite. Biological invasions may be useful to clarify processes in the initial contact of hosts with parasites, and allow us to compare parasite uptake between the ancestral (coevolved) host and novel (noncoevolved) hosts. Cane toads (Bufo marinus) are spreading rapidly through tropical Australia, carrying with them a nematode lungworm (Rhabdias pseudosphaerocephala) congeneric with those found in Australian frogs. We investigated the dynamics of infections of the toad parasite by conducting histologic examinations of cane toads and three native Australian frogs (Litoria dahlii, Litoria nasuta, and Opisthodon ornatus) at 2, 6, and 10 days after experimental exposure to the toad lungworm. More worms were found in toads than in frogs, especially at longer periods postexposure. In toads, the infective larvae entered the skin and muscles within 2 days postexposure, passed into the coelom in 6 days, and reached the lungs at 10 days. In frogs, larvae were found in many organs rather than migrating to consistent target tissues; a few larvae reached the lungs of L. dahlii. Migratory larvae caused increasing inflammation (primarily granulomatous admixed with granulocytes then lymphocytes) through time, especially in frogs. Evolution has resulted in an enhanced ability of the lungworm to locate the target organ (the lungs) of the toad, and an increase in rates of parasite survival within this host.     

STUDIES ON SOME ASPECTS OF THE ECOLOGY OF MESOCOELIUM MONDODI, A TREMATODE PARASITE OF REPTILES AND AMPHIBIA

Some aspects of the autecology of Mesocoelium monodi Dollfus, 1929, a parasite of lizards and Amphibia in Africa have been studied because of the dearth of knowledge in this field of ecology. It was found that although the larvae were confined to one species of molluscan host, the adults occurred in a wide variety of taxonomically diverso hosts including reptiles and amphibia. The possible reasons for this this low order of host specificity are discussed. Although the parasite occurred in all parts of the alimentary canal of Agama with the exception of the rectum, its preferred microhabitat was the duodenum where it feeds on food detritus and mucus. M. monodi does not appear to produce any tissue reactions and the lizards do not acquire a durable immunity or premunition to it. There is no evidence of intra- specific and interspecific competition taking place within this microhabitat as most of theavailable gut space was not utilizedand Mesocoelium and Physaloptera , the other helminth occurring in the duodenum, did not appear to be mutually exclusive. The frequency distribution of the parasite in the lizards is not random and it is suggested that the main reason for this is that there are foci of infestation. The incidence and intensity of infestation which are controlled by the rate of ingestion of intermediate hosts containing metacorcariae vary seasonally and arc greater in females than in males. Tho most important climatic factor influencing the seasonal changc is rainfall, probably because it controls the activity of both the molluscan intermediate host and the infective larvae. The adult parasite has a short life span and the mortality rate was about 1.5 to 2.5 per cent per day.     

Infection of adult migratory river shrimps,Macrobrachium ohione,by the branchial bopyrid isopod Probopyrus pandalicola

Abstract. Macrobrachium ohione is a migratory (amphidromous) river shrimp (Decapoda, Caridea) that may be parasitized by the branchial parasite Probopyrus pandalicola (Isopoda, Bopyridae). The parasite disrupts gonadal maturation and spawning in female shrimps, resulting in the total loss of reproduction. Shrimps are usually infected by bopyrid parasites during the late zoeal or early postlarval stages; in this study, we investigated the apparent parasite infection of adult shrimps. We analyzed the relationships between parasite body size (total length) and host shrimp body size (carapace length) to test the hypothesis that parasite infection of adult shrimps occurs during the shrimps' reproductive migrations. The results presented here indicate that infection of adult shrimps is common in M. ohione in the Atchafalaya and Mississippi Rivers, Louisiana, USA. In the two upriver sites sampled, Butte La Rose (BLR) and River Bend (RB), parasite size was not associated with host body size. In these locations, many parasitized adult M. ohione were infected with immature P. pandalicola (40.3% in BLR and 51.2% in RB), indicating that the shrimps were adults at the time of infection. A possible explanation is that when female shrimps enter the estuary to hatch larvae, they molt and spawn another brood. The smaller male shrimps that accompany the females downstream are also assumed to molt and continue growth. The intermediate host of the parasite is an estuarine copepod, and thus the parasite cryptoniscus larva that infects the host shrimp is primarily estuarine as well. Newly molted shrimps have soft cuticles, which may facilitate their infection by parasite cryptonisci. Our conclusion is that most infections of adult shrimps occur during their migration into estuarine waters, the primary habitat of infective parasite larvae, and that host vulnerability is probably increased following host ecdysis.     

Alternative life cycle strategies of Megalodiscus temperatus in tadpoles and metamorphosed anurans

Megalodiscus temperatus (Stafford, 1905) is a common paramphistome trematode of North American amphibians with a two host life cycle and has been reported to infect frogs and rarely tadpoles. In this study we document the alternative life cycle strategy of M. temperatus in tadpoles and metamorphosed anurans. We show through field work and experimental infections that M. temperatus can establish in both anuran life stages and worms become gravid and release eggs in both tadpoles and metamorphosed frogs. However, worms exhibit differences in route of infection, development, egg production, and diet in tadpoles and metamorphosed anurans. These alternative life history strategies of M. temperatus suggest different selective pressures on the development and reproductive success of these worms in tadpoles and metamorphosed anurans, and we discuss the evolutionary avenues for and constraints on amphibian trematode life cycles presented by these two different anuran life stages.     

Infectivity of two nematode parasites, Camallanus lacustris and Anguillicola crassus, in a paratenic host, the three-spined stickleback Gasterosteus aculeatus

Three-spined sticklebacks Gasterosteus aculeatus are frequent paratenic hosts of the nematode parasites Anguillicola crassus and Camallanus lacustris. As paratenic hosts, sticklebacks could spread infection by carrying high numbers of infective stages. In contrast, low infective ability of either parasite for the paratenic host could hinder the spread of infection. In the present study, G. aculeatus was, for the first time, infected under controlled laboratory conditions with defined doses of the parasites. Sticklebacks were exposed to 6, 12, 18 and 24 parasite larvae to determine the infective ability of the 2 nematode species. There were significantly higher infection rates for C. lacustris (18 to 49%) than for A. crassus (4 to 14%) at each exposure dose. In C. lacustris-infected sticklebacks, infection rates tended to be highest after exposure to 12 C. lacustris larvae and lowest after exposure to 24 parasites. In A. crassus-infected sticklebacks, no effect of parasite exposure dose on infection rates was observed. Immunity parameters such as respiratory burst activity and lymphocyte proliferation of head kidney leukocytes recorded 18 wk post exposure were not significantly affected by either parasite or exposure dose. Granulocyte:lymphocyte ratios were elevated only within the stickleback group showing the highest infection intensity of C. lacustris, i.e. to those exposed 18 parasites.     

Experimental and natural host specificity of Loma salmonae (Microsporidia)

The microsporidian Loma salmonae (Putz, Hoffman & Dunbar, 1965) Morrison & Sprague, 1981 has caused significant gill disease in Pacific salmon Oncorhynchus spp. Host specificity of the parasite was examined experimentally by per os challenge of selected salmonids and non-salmonids with infective chinook salmon O. tshawytscha gill material. Pink Oncorhynchus gorbuscha and chum salmon O. keta, brown Salmo trutta and brook trout Salvelinus fontinalis, and chinook salmon (controls) were positive, whereas Atlantic salmon Salmo salar and Arctic char Salvelinus alpinus were negative. In addition, no non-salmonids were susceptible to experimental exposure. Wild Pacific salmon species in British Columbia, Canada, were examined for L. salmonae during their freshwater life history stages (smolts, prespawning, spawning). All stages were infected, although infections in smolts were only detectable using a L. salmonae-specific PCR test. Many previous Loma spp. described from Oncorhychus spp. are likely L. salmonae based on host, parasite morphology, and site of infection.     

The effects of parasite age and intensity on variability in acanthocephalan-induced behavioural manipulation

Numerous parasites with complex life cycles are able to manipulate the behaviour of their intermediate host in a way that increases their trophic transmission to the definitive host. Pomphorhynchus laevis, an acanthocephalan parasite, is known to reverse the phototactic behaviour of its amphipod intermediate host, Gammarus pulex, leading to an increased predation by fish hosts. However, levels of behavioural manipulation exhibited by naturally-infected gammarids are extremely variable, with some individuals being strongly manipulated whilst others are almost not affected by infection. To investigate parasite age and parasite intensity as potential sources of this variation, we carried out controlled experimental infections on gammarids using parasites from two different populations. We first determined that parasite intensity increased with exposure dose, but found no relationship between infection and host mortality. Repeated measures confirmed that the parasite alters host behaviour only when it reaches the cystacanth stage which is infective for the definitive host. They also revealed, we believe for the first time, that the older the cystacanth, the more it manipulates its host. The age of the parasite is therefore a major source of variation in parasite manipulation. The number of parasites within a host was also a source of variation. Manipulation was higher in hosts infected by two parasites than in singly infected ones, but above this intensity, manipulation did not increase. Since the development time of the parasite was also different according to parasite intensity (it was longer in doubly infected hosts than in singly infected ones, but did not increase more in multi-infected hosts), individual parasite fitness could depend on the compromise between development time and manipulation efficiency. Finally, the two parasite populations tested induced slightly different degrees of behavioural manipulation.     

Endemic infection reduces transmission potential of an epidemic parasite during co-infection

Endemic, low-virulence parasitic infections are common in nature. Such infections may deplete host resources, which in turn could affect the reproduction of other parasites during co-infection. We aimed to determine whether the reproduction, and therefore transmission potential, of an epidemic parasite was limited by energy costs imposed on the host by an endemic infection. Total lipids, triacylglycerols (TAG) and polar lipids were measured in cockroaches (Blattella germanica) that were fed ad libitum, starved or infected with an endemic parasite, Gregarina blattarum. Reproductive output of an epidemic parasite, Steinernema carpocapsae, was then assessed by counting the number of infective stages emerging from these three host groups. We found both starvation and gregarine infection reduced cockroach lipids, mainly through depletion of TAG. Further, both starvation and G. blattarum infection resulted in reduced emergence of nematode transmission stages. This is, to our knowledge, the first study to demonstrate directly that host resource depletion caused by endemic infection could affect epidemic disease transmission. In view of the ubiquity of endemic infections in nature, future studies of epidemic transmission should take greater account of endemic co-infections.     

CD4+-dependent immunity to Onchocerca volvulus third-stage larvae in humans and the mouse vaccination model: common ground and distinctions

Onchocerciasis is a major filarial disease and is the second most common cause of infectious blindness in the world. Disease development after infection with Onchocerca volvulus varies widely and is determined by the host's immune response to the parasite. Vector control and administration of ivermectin has reduced infection and disease rates significantly. However, limitations of these programmes, including ivermectin's selective activity on microfilariae, the need for 10-15 years of annual treatments, logistical obstacles and the potential emergence of drug-resistant strains demand alternative strategies. A vaccine that targets O. volvulus infective third-stage larvae (L3) could provide an additional tool to guarantee successful elimination of infection with O. volvulus. An essential step in the development of immunoprophylactic procedures and reagents is the identification of host immune responses toward antigens of O. volvulus L3 and L3 developing to the fourth-stage larvae that are associated with protection against these stages of the parasite. This review summarises the recent advancements in understanding the immune mechanisms in particular the CD4(+) responses to L3 stages in humans and in the mouse vaccination model. Comparison between the two uncovered common immunological elements in naturally exposed humans and mice vaccinated with radiation attenuated L3 or recombinant O. volvulus antigens, as well as significant differences. These studies promisingly suggest that the O. volvulus mouse model is a very useful adjunct to the studying of natural infection in humans and could provide us with the tools to identify the target molecules and the effector immune correlates of protection in humans responsible for attrition of L3 stages. Since some of these antigens may exist in other nematodes, any insight gained into the mechanisms of vaccine-induced anti-O. volvulus L3 protective immunity in both humans and mice could be applicable to the development of vaccines against other nematode infections.     

Effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni (Lepidoptera: Noctuidae) and the parasite, Hyposoter exiguae (hymenoptera: Ichneumonidae)

The effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni and the parasite, Hyposoter exiguae, was investigated to determine if the virus could invade and multiply in the tissues of the parasites, if parasites which emerged from virus-infected T. ni larvae had normal emergence, fecundity, and longevity, and if the parasite could serve as a vector for the virus. Light microscopy revealed particles which appeared to be polyhedra within the lumen of the midgut of parasite larvae from virus-infected hosts. Transmission electron microscopy confirmed the presence of polyhedra and free virions within the midgut of the larvae. Polyhedra or free virions were never found within any parasite tissues. Parasite larvae within hosts exposed to virus before parasitization perished when their hosts died of virus infection. Parasite larvae in hosts exposed to virus after parasitization completed their development before their hosts died of virus infection. The proportion of parasites which survived increased as the time between host parasitization and host virus exposure increased. Parasite larvae which developed in hosts exposed to the virus soon after parasitization spent significantly less time in their hosts than did parasites which developed in noninfected hosts. There was no significant difference in time spent in the pupal stage, percent adult emergence, adult longevity with and without food and water, and fecundity of parasites which developed in virus-infected hosts and those which developed in noninfected hosts. Female parasites laid as many eggs in virus-infected hosts as they did in noninfected hosts. Sixty percent of the female parasites which oviposited in virus-infected hosts vectored infective doses of virus to an average of 6% of the healthy hosts subsequently exposed to them. None of the healthy host larvae exposed to male parasites which had been exposed to virus-infected host larvae became infected with the virus. Forty percent of the female parasites which developed in virus-infected hosts transmitted infective doses of the virus to an average of 65% of the healthy host larvae exposed to them. Ninety percent of the male parasites which developed in virus-infected hosts transferred infective doses of the virus to an average of 21% of the healthy host larvae exposed to them.     

Effects of route of inoculation on the site of development of Caryospora bigenetica (Apicomplexa: Eimeriidae)

The sites of infection by Caryospora bigenetica in Swiss-Webster mice (Mus musculus) were demonstrated after 7 routes of inoculation: oral, intraperitoneal, intravenous, intramuscular, subcutaneous, dermal, and intraocular. All mice exhibited clinical signs of dermal coccidiosis 9 days after inoculation regardless of the inoculation route. Signs included swelling of the facial tissue, footpads, and scrota (male mice). Developmental stages of the parasite were found in the muzzle, tongue, footpad, lumbar subcutaneous tissue, biceps femoris muscle, conjunctiva, and eye; the latter 3 sites represent new sites of development. The site of development of the parasite in the host tissue was independent of experimental inoculation route.