Helminth Life Cycles Research Articles

To grow or not to grow? Intermediate and paratenic hosts as helminth life cycle strategies

Larval helminths in intermediate hosts often stop growing long before their growth is limited by host resources, and do not grow at all in paratenic hosts. We develop our model [Ball, M.A., Parker, G.A., Chubb, J.C., 2008. The evolution of complex life cycles when parasite mortality is size- or time-dependent. J. Theor. Biol. 253, 202–214] for optimal growth arrest at larval maturity (GALM) in trophically transmitted helminths. This model assumes that on entering an intermediate host, larval death rate initially has both time- (or size-) dependent and time-constant components, the former increasing as the larva grows. At GALM, mortality changes to a new and constant rate in which the size-dependent component is proportional to that immediately before GALM. Mortality then remains constant until death or transmission to the definitive host. We analyse linear increasing and accelerating forms for time-dependent mortality to deduce why there is sometimes growth (intermediate hosts) and sometimes no growth (paratenic hosts). Calling i the intermediate or paratenic host, and j the definitive host, conditions favouring paratenicity are: (i) high values in host i for size at establishment, size-related mortality, expected intensity, (ii) low values in host i for size-independent mortality rate, potential growth rate, transmission rate to j, and ratio of death rate in j/growth rate in j. Opposite conditions favour growth in the (intermediate) host, either to GALM or until death without GALM. We offer circumstantial evidence from the literature supporting some of these predictions. In certain conditions, two of the three possible growth strategies (no growth; growth to an optimal size then growth arrest (GALM); unlimited growth until larval death) can exist as local optima. The effect of the discontinuity in death rate after GALM is complex and depends on mortality and growth parameters in the two hosts, and on the mortality functions before and after GALM.

Proteomics Analysis of the Excretory/Secretory Component of the Blood-feeding Stage of the Hookworm, Ancylostoma caninum

Hookworms are blood-feeding intestinal parasites of mammalian hosts and are one of the major human ailments affecting approximately 600 million people worldwide. These parasites form an intimate association with the host and are able to avoid vigorous immune responses in many ways including skewing of the response phenotype to promote parasite survival and longevity. The primary interface between the parasite and the host is the excretory/secretory component, a complex mixture of proteins, carbohydrates, and lipids secreted from the surface or oral openings of the parasite. The composition of this complex mixture is for the most part unknown but is likely to contain proteins important for the parasitic lifestyle and hence suitable as drug or vaccine targets. Using a strategy combining the traditional technology of one-dimensional SDS-PAGE and the newer fractionation technology of OFFGEL electrophoresis we identified 105 proteins from the excretory/secretory products of the blood-feeding stage of the dog hookworm, Ancylostoma caninum. Highly represented among the identified proteins were lectins, including three C-type lectins and three beta-galactoside-specific S-type galectins, as well as a number of proteases belonging to the three major classes found in nematodes, aspartic, cysteine, and metalloproteases. Interestingly 28% of the identified proteins were homologous to activation-associated secreted proteins, a family of cysteine-rich secreted proteins belonging to the sterol carrier protein/Tpx-1/Ag5/PR-1/Sc-7 (TAPS) superfamily. Thirty-four of these proteins were identified suggesting an important role in host-parasite interactions. Other protein families identified included hyaluronidases, lysozyme-like proteins, and transthyretin-like proteins. This work identified a suite of proteins important for the parasitic lifestyle and provides new insight into the biology of hookworm infection.

Intestinal helminth infestation is associated with increased bronchial responsiveness in children

Non-atopic asthma is the predominant phenotype in non-affluent parts of Latin America. We recently reported that infestation with Ascaris lumbricoides increased the risk of non-atopic asthma in less affluent areas of Brazil but the mechanism is unclear. The present study was conducted to determine whether helminth infestation is associated with heightened bronchial responsiveness (BHR), a common finding in asthma. A random sample of 50 asthmatic and 50 non-asthmatic controls (mean age 10.1 years) were selected from a larger cohort (n = 1,011) without knowledge of their helminth infestation status. Three stool samples were collected from each child on different days and each sample was analyzed by the Kato-Katz method for quantitative determination of helminth eggs. Bronchial provocation tests were performed with inhaled 4.5% hypertonic saline using the ISAAC Phase II standardized protocol. There was no difference between the prevalence of positive BHR in the asthmatics (20.4%) compared with the controls (14.6%) (P = 1.0). Helminth infestation was detected in 24.0% of children, with A. lumbricoides being the most common. Children with high load infestation (>or=100 eggs/g) were five times more likely to have BHR than children with low load or no infestation. Despite the small sample size the results of the present study suggest that the link between high load helminth infestation and non-atopic asthma may be mediated via heightened bronchial responsiveness, possibly due to an inflammatory response to the pulmonary phase of the helminth life cycle.

The evolution of complex life cycles when parasite mortality is size- or time-dependent

In complex cycles, helminth larvae in their intermediate hosts typically grow to a fixed size. We define this cessation of growth before transmission to the next host as growth arrest at larval maturity (GALM). Where the larval parasite controls its own growth in the intermediate host, in order that growth eventually arrests, some form of size- or time-dependent increase in its death rate must apply. In contrast, the switch from growth to sexual reproduction in the definitive host can be regulated by constant (time-independent) mortality as in standard life history theory. We here develop a step-wise model for the evolution of complex helminth life cycles through trophic transmission, based on the approach of Parker et al. [2003a. Evolution of complex life cycles in helminth parasites. Nature London 425, 480–484], but which includes size- or time-dependent increase in mortality rate. We assume that the growing larval parasite has two components to its death rate: (i) a constant, size- or time-independent component, and (ii) a component that increases with size or time in the intermediate host. When growth stops at larval maturity, there is a discontinuous change in mortality to a constant (time-independent) rate. This model generates the same optimal size for the parasite larva at GALM in the intermediate host whether the evolutionary approach to the complex life cycle is by adding a new host above the original definitive host (upward incorporation), or below the original definitive host (downward incorporation). We discuss some unexplored problems for cases where complex life cycles evolve through trophic transmission.

Intestinal helminth parasites in flounder Platichthys flesus from the River Thames: an infracommunity analysis

An analysis was undertaken of intestinal helminth communities in flounder Platichthys flesus from two sites on the River Thames. A comparison was made between helminth community richness and diversity from these sites at the component and infracommunity levels. At the component community level, a richer and more diverse parasite community was found in flounder from the Tilbury location (marine influence) than that from the Lots Road location (freshwater influence). At the infracommunity level, more parasite species and parasite individuals per host were found at Lots Road and the percentage of similarity values were low at both locations. Helminth species with high prevalence values in the parasite communities of the flounder are the dominant species in any individual fish, harbouring multi-specific infections. The presence of more invertebrate species, which are intermediate hosts in the helminth life cycle in the Thames, fish vagility and the high prevalence and abundance values of Pomphorhynchus laevis in the flounder, may explain the differences between the two locations.

Experimental Infection of Corbicula fluminea (Bivalvia: Corbiculidae) with Echinostoma revolutum Cercariae

come established in the Delaware and Schuylkill Rivers in southeastern Pennsylvania and is being consumed by humans (Ettinger and Bye, 1984, Pennsylvania Wildlife and Outdoor Digest 5: 4445). Relatively little information is available on parasitic infections in C. fluminea. Sinclair (1971, Sterkiana 43: 11-18) noted that North American Corbicula were not involved in helminth life cycles of public health importance. Harrell et al. (1977, The Southwestern Naturalist 22: 280-281) could not find larval trematodes in more than 2,000 C. manilensis (a probable synonym of C. fluminea) examined from Lake Texoma, Oklahoma.

The Effects of Praziquantel on Various Larval and Adult Parasites from Freshwater and Marine Snails and Fish

In addition to human and veterinary medicine, the defaunation of helminths in hosts is important in studies of behavioral ecology, life cycles and invertebrate immunology. Ecological studies have investigated the effects of larval parasites on the behavior of marine and estuarine snails (James, 1968, Journal of Natural History 2: 21-37; Sousa, 1984, Journal of Experimental Biology and Ecology 73: 273-296) and crustacean hosts (Holmes and Bethel, 1972. In Behavioral aspects of parasite transmission, E. U. Canning and C. A. Wright (eds.). Linnean Society, London, pp. 123-150). Researchers often want to experimentally complete helminth life cycles or make comparisons between infected and uninfected hosts but are unable to do so because of the lack of uninfected hosts. It would be useful to challenge previously infected snails with subsequent infections to study invertebrate memory (Manning, 1980. Phylogeny of immunological memory. Elsevier/NorthHolland Biomedical Press, New York, 318 p.). Praziquantel (Droncit?, Bayvert Div., Cutter Lab) has been successfully used in veterinary and medical experiments against cestodes and trematodes in mammals and larval Digenea in fish (for review see Heckmann, 1983, Final Report, Idaho Department Fish and Game, 83 p.; Heckmann, 1984, American Fisheries Society, Fish Health Section 12: 7). The purpose of this study was to test the effectiveness of this drug on larval digenetic trematodes in freshwater and estuarine snails, larval cestodes in marine copepods and adult cestodes in elasmobranchs. The freshwater snails Physa sp. and Lymnaea sp., infected with larval Diplostomum spathaceum (Strigeoidae), were collected from the upper Salmon River Hatchery, Stanley, Idaho, in August 1983. All snails were exposed to artificial light to test for cercarial release. Physa sp. and Lymnaea sp. were 100% and 25% infected, respectively. The test and control groups had an equal number of each species of snails. There were 2 trails. The trails had 10 and 12 specimens of each species, respectively. The test groups were placed in aquaria containing 76.5 L of fresh water to which had been added 0.1 ml of injectable praziquantel (56.8 mg/ml active ingredient; final concentration 5.68 mg/ml). Approximately onethird of the snails were opened at 12, 24, and 48 hr to determine the condition of the sporocysts in the viscera. The viscera were examined histologically by standard light microscopic techniques. All tested snails contained degenerating sporocysts and dead furcocercous cercariae. The untreated snails had normal appearing sporocysts and larvae. To determine the effects of praziquantel at test concentrations on free living cercariae, a drop of praziquantel from the test aquarium was placed on a microscope slide containing cercariae from the control snails. All cercariae died upon exposure to the test solution. This experiment was repeated twice. The estuarine snails used in this experiment were Batillaria attramentaria and Cerithidea california. Batillaria attramentaria were collected from Elkhorn Slough in Monterey Bay, California. All specimens greater than 30 mm were infected with 1 species of larval trematode which had a sporocyst and non-furcocercous cercaria. Cerithidea california were collected in June 1984 in Goleta Slough, Santa Barbara, California. Half the specimens greater than 30 mm were infected by 1 or more species of larvae. Eleven species of larval digeneans have been identified from C. california in Goleta Slough (Yoshino, 1975, Veliger 18: 156-161). All snails used had shed cercariae when exposed to artificial light for 30 min. Fifty snails of each species were placed in each of 3 75.6-L aquaria. The salinity was 20-22 ppt. Test tanks (0.44 mg/L and 0.89 mg/L total concentration) contained either 1 or 2 praziquantel tablets (34 mg/tablet active ingredient) and 1 tank served as a control. The tablet was powdered in a mortar and pestle in which an aliquot sample of aquarium water had been added. This was repeated until all the powder went into solution. Ten snails were opened to check the condition

Helminth parasitism in raccoons, Procyon lotor hirtus Nelson and Goldman, in Saskatchewan

Thirty-one raccoons, Procyon lotor hirtus Nelson and Goldman, from southern Saskatchewan, Canada, were examined for helminths. Six species were recovered: Plagiorchis cf. cf. elegans (Rudolphi, 1802); Alaria marcianae (LaRue, 1917); Pharyngostomoides adenocephala Beckerdite, Harkema and Miller, 1971; Microphalus sp.; Atriotaenia procyonis (Chandler, 1942); and Physaloptera sp. Comparisons were made with findings in a series of 25 raccoons from the midwestern United States and with published results and surveys in other regions. Notable variation exists in species diversity and intensity of infection of parasites in raccoons from different geographic regions. This is a consequence of diverse ecological conditions in different geographic regions. Since P. lotor is extending its range in Saskatchewan, it is suggested that the low diversity of its helminthic fauna may be attributable to dispersal of animals into regions where they persist in low numbers and where conditions are not suitable for completion of life cycles of helminths that elsewhere occur commonly in raccoons. The trematodes P. elegans and A. marcianae have not been previously reported from raccoons.

Biological Activities of Various Lipid Fractions from Echinococcus granulosus Scolices on In vitro Cultures of Hymenolepis diminuta

Thin-layer chromatography of lipid extracts of Echinococcus revealed substances with stimulatory and depressive effects on growth of excysted cysticercoids of Hymenolepis diminuta in in vitro cultures. Cholesterol depressed growth and increased mortality, unidentified steroids increased mortality and a substance with Rfs similar to farnesal stimulated growth of these forms by as much as 100%. Similar substances, untested as yet for biological activity, were found in lipid extracts of infective larvae of Trichinella spiralis and miracidia of Fasciola hepatica. The complexity of the life cycles of parasitic helminths dictates reasonably sophisticated control mechanisms. The molts of larvae and maturation to adulthood are not unlike those of insects. The bulk of literature during the last 10 years on in vitro cultivation of parasitic helminths illustrates the rather primitive state of our knowledge regarding these phenomena. A finer understanding of the involved mechanisms might well open up new avenues for control of these parasites. A chance observation that lipoidal materials were being leached from scolices of Echinococcus granulosus during preparation of ES (excretion and secretion) antigens, and the fact that scolices of this organism live in hydatid cysts in an excellent culture medium, yet do not mature, prompted these studies. MATERIALS AND METHODS The procedure followed for the collection of scolices was essentially that of Agosin et al. (1957). Livers collected from sheep and cattle at the Beirut abattoir were brought to the laboratory within 2 hr after slaughter of the host animals. The fluid was withdrawn aseptically from hydatid cysts with a syringe and needle. The settling scolices were transferred to centrifuge tubes with a Pasteur pipette, washed 3 or 4 times with distilled water Received for publication 16 February 1968. * These studies were supported in part by research grants from the Medical Research Committee, American University of Beirut and from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Public Health Service (AI-02205-08 and AI-07218-01). t Present address: Department of Biology, University of Notre Dame, Notre Dame, Indiana. + Present address: School of Pharmacy, University of Kentucky, Lexington, Kentucky. and separated by low speed centrifugation. The thoroughly washed scolices were used alive for one experiment and lyophilized to a dry powder for the remainder. Total lipids from scolices were extracted by a procedure similar to that of Folch et al. (1957). The lyophilized scolices (0.495 g) were mixed with 100 volumes (w/v) of a mixture of chloroform: methanol (2:1, v/v) and agitated on a shaker for 6 hr. The insoluble residue was then separated from the solvent by low speed centrifugation and the supernatant fluid filtered through paper. This extraction was repeated 3 more times. The combined extracts were reduced to 10 ml under reduced pressure at 40 C and were subsequently washed with an equal volume of distilled water (Beames, 1965). After standing 1 hr, the separated chloroform layer was washed with 1/2 volume of 1% KC1 solution. To the aqueous phase, an equal volume of a mixture of chloroform: methanol (2:1) was added, and in a few minutes the chloroform layer (lower phase) was easily separated. The combined chloroform extracts were kept overnight at -20 C and quickly filtered in the cold. The filtrate was dried over Na2SO4 and evaporated to dryness under reduced pressure at 40 C. The residue was weighed (0.0672 g; 13.6% of dry weight) and immediately picked up in a minimum volume of chloroform and spotted on thin layer chromatography (TLC) plates for the fractionation of its lipid content. Preliminary qualitative extractions of infective larvae of Trichinella spiralis and miracidia of Fasciola hepatica were done in a similar way. Unless otherwise specified, thin layer plates 0.25 mm thick were prepared from Silica Gel G (Merck according to Stahl). The plates were oven-dried at 110 C for 30 min and stored in a dessicator. They were reactivated at 110 C for 10 min immediately before use. Eastman Chromagrams (0.1 mm thick) were also used. Solvent development was allowed to proceed to 12 to 13 cm from the point of sample application (1.5 cm from the lower edge of the plate) in preequilibrated chambers away from light.

HELMINTH LIFE CYCLES.

Although classical work continues, particularly on the trematodes, for the most part the basic life cycle patterns must be regarded as worked out. Helminthologists are increasingly turning attention to the mechanics of the life cycle with the result that parasitology is becoming an experimental science. Worms are now under study from the biochemical, physiological, and behavioral points of view. As a result, ideas of specificity have been altered, new physiological races of parasites discovered, and a number of new human helminthiases recognized, chiefly zoonotic and involving larval forms. The number of parasitic worms successfully adapted to laboratory propagation in animal hosts steadily increases, providing new and fruitful models for investigation. Even more striking is the great breakthrough in the last decade involving in vitro culture. Several cestodes and a variety of nematodes can now be reared through various stages, if not their entire life cycle, in partially defined media, and progress is being made with trematodes and acanthocephalans. Behavioral studies are beginning to shed light on how parasites find their hosts and how they locate each other inside the host. And in spite of long-held views to the contrary, evidence increases that certain activities of helminths may be under hormonal control.

ECOLOGICAL ASPECTS OF HELMINTH TRANSMISSION IN DOMESTICATED ANIMALS.

Some of the accumulated knowledge on the complex problems involved in the transmission of helminths of domestic animals in the USA is summarized. The lollowing conditions are usually necessary for the development outside the final hosts of infective stages of these helminths: moderate temperatures, adequate moisture, oxygen, and protection from freezing, desiccation and direct sunlight. Once the infective stages of helminths are attained, either free-living or parasitic in intermediate hosts, those of most species are more resistant to adverse conditions than pre-infective stages. Because temperature and moisture influence the development and survival of helminth infective stages, variable climatic and weather conditions drrectly or indirectly affect helminth transmission. Climate and weather maintain or alter the microclimates of the microhabitats in which helminth eggs and larvae normally live until infection of the final hosts is accomplished. We are only beginning to learn how long-term climatic trends and short-term weather influence helminth transmission. Recent attempts, however, to relate variations in climate and weather to degree and kind of helminth transmission by use of bioclimatographs, the interrelationship of evapotranspiration and precipitation, estimates of potential transmission periods, etc., have increased our understanding of helminth life cycles and helminth diseases of domestic animals, and have aided in developing increasingly effective methods of helminth control.

Observations on the Natural History of Pomatiopsis lapidaria Say

The successful pursuit of the life cycles of helminths usually is dependent upon a knowledge of the ecological relationships of the various intermediate and final hosts. These are often maintained in the laboratory for experimental purposes, and for their well-beinlg it is essential to provide conditions closely resembling those in which they were found in nature. Frequently, an investigator is confronted by the lack of detailed information on the natural history of invertebrate and vertebrate hosts and, therefore, valuable observations of an ecological nature often of necessity accompany the solution of a parasitological problem.