The relationship between Pseudodiplorchis americanus (Monogenea) density and host resources under controlled environmental conditions

Increased virulence due to multiple infection in Daphnia leads to limited growth in 1 of 2 co-infecting microsporidian parasites

Recent outbreaks of various infectious diseases have highlighted the ever-present need to understand the drivers of the outbreak and spread of disease. Although much of the research investigating diseases focuses on single infections, natural systems are dominated by multiple infections. These infections may occur simultaneously, but are often acquired sequentially, which may alter the outcome of infection. Using waterfleas (Daphnia magna) as a model organism, we examined the outcome of sequential and simultaneous multiple infections with 2 microsporidian parasites (Ordospora colligata and Hamiltosporidium tvaerminnensis) in a fully factorial design with 9 treatments and 30 replicates. We found no differences between simultaneous and sequential infections. However, H. tvaerminnensis fitness was impeded by multiple infection due to increased host mortality, which gave H. tvaerminnensis less time to grow. Host fecundity was also reduced across all treatments, but animals infected with O. colligata at a younger age produced the fewest offspring. As H. tvaerminnensis is both horizontally and vertically transmitted, this reduction in offspring may have further reduced H. tvaerminnensis fitness in co-infected treatments. Our findings suggest that in natural populations where both species co-occur, H. tvaerminnensis may evolve to higher levels of virulence following frequent co-infection by O. colligata.

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.

Macroparasite infections of amphibians: what can they tell us?

Understanding linkages between environmental changes and disease emergence in human and wildlife populations represents one of the greatest challenges to ecologists and parasitologists. While there is considerable interest in drivers of amphibian microparasite infections and the resulting consequences, comparatively little research has addressed such questions for amphibian macroparasites. What work has been done in this area has largely focused on nematodes of the genus Rhabdias and on two genera of trematodes (Ribeiroia and Echinostoma). Here, we provide a synopsis of amphibian macroparasites, explore how macroparasites may affect amphibian hosts and populations, and evaluate the significance of these parasites in larger community and ecosystem contexts. In addition, we consider environmental influences on amphibian-macroparasite interactions by exploring contemporary ecological factors known or hypothesized to affect patterns of infection. While some macroparasites of amphibians have direct negative effects on individual hosts, no studies have explicitly examined whether such infections can affect amphibian populations. Moreover, due to their complex life cycles and varying degrees of host specificity, amphibian macroparasites have rich potential as bioindicators of environmental modifications, especially providing insights into changes in food webs. Because of their documented pathologies and value as bioindicators, we emphasize the need for broader investigation of this understudied group, noting that ecological drivers affecting these parasites may also influence disease patterns in other aquatic fauna.

Parasite adaptation to extreme conditions in a desert environment

Deserts represent universally recognized extreme environments for animal life. This paper documents the highly specialized adaptations of Pseudodiplorchis americanus, a monogenean parasite of the desert toad, Scaphiopus couchii. Building on a long-term record of parasite population ecology (continuing since the early 1980s), field studies focus on the effects of severe drought in the Sonoran Desert, Arizona, in the mid 1990s. This provides a test of the ability of the host-parasite system to tolerate exceptional perturbation. The analysis provides new insight into parasite infection dynamics in a natural wildlife system through integration of host and parasite population age structure. The environmental check interrupted host recruitment in 1993–95 and parasite recruitment in 1995–97. This produced an imprint in age structure and infection levels recognizable over several years: parasite recruitment failure reduced transmission 2–3 years later. The host (maximum life span 17 years) tolerated the disruption but the impact was more serious for the parasite (life span 3 years) leading to extinction of some previously stable populations. Despite this demonstration of a rare event exacerbating external environmental constraints, experimental studies suggest that the internal (host) environment normally creates the most severe conditions affecting P. americanus. Only about 3 % of parasites survive from invasion until first reproduction. Post-invasion factors including host immunity, characteristic of most parasite life cycles, constitute a greater constraint upon survival than external conditions, even in a desert environment.

Life history specializations of monogenean flatworms: a review of experimental and microscopical studies

This review illustrates the use of experimental approaches combined with microscopy to study the biology of monogenean parasites. Studies of feeding, development, reproduction, and systematics have been based on gyrodactylids, flatworms infecting teleost fishes. In a contrasting system involving an amphibian host in a desert environment, analysis of adaptations to extreme conditions has focused on Pseudodiplorchis americanus. The unusual reproductive strategies, particularly the interactions between mother and offspring, are highlighted for both monogeneans. Species of Gyrodactylus are viviparous, maintaining up to three generations of embryos simultaneously in utero, and many of their reproductive specializations are related to progenesis. Embryo nutrition takes place via a metabolically-active syncytial uterine lining that has close association with the parental gut. Microscopy has also proved an essential adjunct to molecular studies of speciation and host specificity. P. americanus is ovoviviparous and the adaptations for embryo maintenance are unique. The primary keratin-type eggshell is replaced by a flexible secondary elastin capsule produced by the uterus; parental nutrients are transferred through cytoplasmic connections to the developing embryo. TEM has demonstrated unique adaptations of P. americanus to its micro-environments, including secretion of tegumental vesicles that provide protection from digestive enzymes during migration through the host gut. This paper highlights the potential of monogeneans for studies of fundamental biological principles.

Parasitic disease in amphibians: control by the regulation of worm burdens

This review considers three case studies based on macroparasites of anurans: (a) natural infections in the permanently-aquatic Xenopus laevis which represent the worm burdens acquired, and the implications for pathology, when hosts are exposed to continuous, year-round, transmission; (b) the desert toad, Scaphiopus couchii, which experiences invasion very briefly each year and provides a simplified system involving only a single significant infection (Pseudodiplorchis americanus); (c) the mesic Bufo bufo which has been the subject of experimental laboratory studies designed to measure the effects of Rhabdias bufonis infection on host growth, physical performance and survival. Experimental manipulation of both Scaphiopus and Bufo provide quantitative data on disease effects of macroparasites, including precise measurements of parasite-induced host mortality. Field data for Xenopus and Scaphiopus show that, despite high initial worm burdens from efficient transmission, infection levels at parasite maturity are modulated below those leading to significant disease. Experimental data for Scaphiopus and Bufo have documented the time-course and magnitude of this decline in intensities, and there is circumstantial evidence for Scaphiopus that this regulation is host-mediated. Immunological studies on Xenopus show that disease effects of the pathogenic Pseudocapillaroides xenopodis are exacerbated in thymectomised hosts and reversed by implantation of thymuses from MHC-compatible donors. Thus, whilst factorial experiments can demonstrate the potential of helminths to cause significant disease and mortality in anuran host-macroparasite interactions, powerful post-invasion regulation of worm burdens appears to exert a strong control of parasite-induced disease in natural host populations.

Survival of Pseudodiplorchis americanus (Monogenea) under controlled environmental conditions

Populations of Pseudodiplorchis americanus infecting the desert toad, Scaphiopus couchii, have previously been shown to be remarkably stable from year to year, despite wide variation in conditions promoting invasion. The present study aimed to document the survival of both first-year and pre-existing adults under controlled laboratory temperatures. First-year worm survival in experimentally infected toads was shown to be very high for the first 5 months after migration to the definitive site, and there was no difference in survival at 25 degrees C or 15-20 degrees C. There was also no density-dependent survival during the same period: 20% of worms were recovered from the host urinary bladder, irrespective of initial intensities. After the first 5 months, there was a progressive loss of worms in toads maintained at 25 degrees C but not at 15-20 degrees C. Pre-existing adult populations were shown to be virtually identical to those under natural conditions for the first 4 months after toad capture. Following this, at 25 degrees C, populations declined and no pre-existing adults were recovered after 11-14 months. There was no such loss of pre-existing adults at 15-20 degrees C for up to 14 months. A diurnal temperature cycle of 20-34 degrees C (simulating temperatures during the desert summer) did not lead to a significant loss of worms. These observations suggest that during one annual cycle in the desert, when temperatures remain above 20 degrees C for less than 6 months, most adult worms established in the host urinary bladder will survive to the next transmission opportunity. However, the similar longevity of first-year and pre-existing adults in laboratory maintained toads shows that loss of worms cannot be due solely to parasite ageing. Temperature-dependent survival of P. americanus is suggestive of a host immune response. Low temperatures, which inhibit parasite growth and development, are essential for the survival of P. americanus.