Host-parasite relationships of Zygocotyle lunata (Trematoda) in the domestic chick

How parasite exposure and time interact to determine Australapatemon burti (Trematoda: Digenea) infections in second intermediate hosts (Erpobdella microstoma) (Hirudinea: Erpodellidae)

Australapatemon spp. are cosmopolitan trematodes that infect freshwater snails, aquatic leeches, and birds. Despite their broad geographic distribution, relatively little is known about interactions between Australapatemon spp. and their leech hosts, particularly under experimental conditions and in natural settings. We used experimental exposures to determine how Australapatemon burti cercariae dosage (number administered to leech hosts, Erpobdella microstoma) affected infection success (fraction to encyst as metacercariae), infection abundance, host survival, and host size over the 100 days following exposure. Interestingly, infection success was strongly density-dependent, such that there were no differences in metacercariae load even among hosts exposed to a 30-fold difference in cercariae. This relationship suggests that local processes (e.g., resource availability, interference competition, or host defenses) may play a strong role in parasite transmission. Our results also indicated that metacercariae did not become evident until ~4 weeks post exposure, with average load climbing until approximately 13 weeks. There was no evidence of metacercariae death or clearance over the census period. Parasite exposure had no detectable effects on leech size or survival, even with nearly 1,000 cercariae. Complementary surveys of leeches in California revealed that 11 of 14 ponds supported infection by A. burti (based on morphology and molecular sequencing), with an average prevalence of 32% and similar metacercariae intensity as in our experimental exposures. The extended development time and extreme density dependence of A. burti has implications for studying naturally occurring host populations, for which detected infections may represent only a fraction of cercariae to which animals have been exposed. Future investigation of these underlying mechanisms would be benefical in understanding host-parasite relationships.

The crowding effect and morphometric variability in Echinostoma caproni (Digenea: Echinostomatidae) from ICR mice

Population density, or crowding, was examined to determine its effect on the morphometric variability of Echinostoma caproni (Digenea) in ICR mice. Six mice were infected with 25 and 100 metacercariae, and a single mouse was infected with 300 metacercariae. All mice were infected at necropsy 22 days postinfection with recoveries of 77%, 69%, and 7.3%, respectively. Whole mounts were prepared, and 31 characters were evaluated (25 direct measurements and 6 ratios). Univariate and multivariate statistical analysis revealed significant differences between adult worms from all 3 groups. Twenty-seven of 31 characters showed significant within-group differences, with the primary differences between worms from 25/100 versus 300 metacercariae infections. Discriminant function analysis yielded a 100% correct classification based on infection size, which is consistent with studies on distinct species of Echinostoma. The low recovery from the mouse infected with 300 metacercariae suggests inflammatory expulsion of juvenile worms and the possibility of immunity as a factor in the crowding effect. These results suggest that external factors may affect morphometric variability of digenetic trematodes to a larger degree than previously recognized.

Immunology and pathology of intestinal trematodes in their definitive hosts

This review examines the significant literature on the immunology and pathology of intestinal trematodes in their definitive hosts. We emphasize information on selected species in six families for which the literature on these topics is extensive. The families are Brachylaimidae, Diplostomidae, Echinostomatidae, Gymnophallidae, Heterophyidae, and Paramphistomidae. For most of these families, coverage is considered under the following headings: (i) background; (ii) pathology of the infection; (iii) immunology of the infection; (iv) immunodiagnosis; and (v) human infection. Some of these heading have been subdivided further, based on the literature available on a particular topic. Following this coverage, we include a final section on the important topical literature on selected trematodes in families other than the six mentioned above.

Crowding effect on adult growth, pre-patent period and egg shedding of Fasciola hepatica

Fascioliasis pathogenesis depends on fluke burden. In human hyperendemic zones, individual infection intensities reach very high levels and the majority of infected subjects should be in the advanced chronic phase. The rat model offers a useful approach for pathological research in the advanced chronic period. The influence of infection intensity per rat on fluke development, pre-patent period and egg shedding (eggs/g faeces/worm) was analysed in 3 groups (I: 1–3 worms/rat; II: 4–6; III: 7–9). Ontogenetic trajectories of fluke body measures followed a logistic model. Results showed that when the burden increases, the maximum values of fluke measures decrease. The crowding effect is manifest when fluke measures approximate their maximums in the advanced chronic stage. The pre-patent period and egg production decrease when the burden increases. This means that measurements of eggs per gramme of faeces tend to underestimate the fluke burden. The present study demonstrates how to quantify the fascioliasis experimental rat model crowding effect on adult growth, pre-patent period and egg production. This quantification may be of great interest in epidemiological studies and in experimental research on the in vivo actions of different anthelminthic drugs and vaccines, pathology, immunology and resistance studies.

Effect of temperature on the infectivity of metacercariae of Zygocotyle lunata (Digenea: Paramphistomidae)

As a test of the energy limitation hypothesis (ELH), we predicted that temperature would have a significant influence on the infectivity of metacercariae of the digenetic trematode Zygocotyle lunata. Snails infected with Z. lunata were collected from ponds near Crawfordsville, Indiana, isolated at room temperature, and examined for the release of cercariae. Newly encysted metacercariae were collected and incubated 1-30 days at 1 of 5 temperatures (0, 3, 25, 31, 37 C). Twenty-five cysts were fed to each of 5 or 10 mice per treatment group (temperature). At 17 days postinfection, mice were killed and worms were recovered; data were collected on levels of infection in each group and the total body area of each worm. No worms were found in mice fed cysts that had been held at 0 C or 37 C (after 30 days). There were no differences in prevalence, infectivity, or mean intensity among the 3, 25, and 31 C treatments. Infectivity of metacercariae incubated at 37 C for 1 day was significantly greater than in all other treatments, while infectivity of metacercariae in the 37 C/15-day treatment was significantly lower than in all others. Mean body area of worms at 37 C/15 days was significantly greater than at other temperatures, suggesting density-dependent increases in growth. These results, particularly those from the 37 C treatments, are consistent with the ELH; infectivity was lower at high temperatures or when incubated for more time at 1 temperature (fewer energy reserves). It is suggested that microhabitat conditions experienced by metacercariae of Z. lunata could contribute to longer larval life, thus influencing this parasite's temporal dispersal.

The effects of crowding on adults of Philophthalmus nocturnus grown in domestic chicks

The effects of crowding on growth, reproduction, rate of recovery and site of infection of Philophthalmus nocturnus were investigated by infecting various groups of day-old chicks with 25, 50, 75, or 100 excysted metacercariae and examining them at 10 or 25 days post-infection. Considerable reduction in the body length and the size of the gonads was noticed in flukes grown under crowded conditions. Crowding also caused some change in the habitat of flukes, but had no effect on their rate of recovery. Very few flukes reached the ovigerous stage under overcrowded conditions and none reached the embryonated larval stage.

In ovo cultivation of Microphallus primas (Trematoda: Microphallidae) metacercariae to ovigerous adults and the establishment of the life-cycle in the laboratory

This investigation reports for the first time the establishment of a trematode life-cycle in the laboratory using in ovo cultivation of the adult stage. Microphallus primas metacercarial cysts were removed from Carcinus maenas crabs and chemically excysted. Using a modification of the technique developed by Fried (1962) the released metacercariae were grown in fertile hen's eggs to ovigerous adult flukes. These were removed from the under surface of the chorio-allantoic membrane of chick embryos 5 days post-infection and the fluke eggs recovered were used to infect laboratory-reared Hydrobia ulvae snails. When challenged against parasite-free crabs the cercariae released from the experimentally infected snails gave rise to metacercarial cysts that were again identified as M. primas.

The effects of crowding on adults of Echinostoma caproni in experimentally infected golden hamsters

Fifty-nine of 60 (98%), 6-month-old male golden hamsters, Mesocricetus auratus, fed 15 (group A), 50 (group B), or 200 (group C) metacercarial cysts of Echinostoma caproni (Digenea: Echinostomatidae) were infected 7-34 days postexposure. The mean number of worms recovered in groups A, B and C were 9, 10, and 50, respectively. The percentage recovery was significantly different between group A (63%) and groups B (21%) and C (23%). The intestine was divided into three equal regions (I, II, III). Worms from group A were located in segments II and III of the small intestine whereas worms from groups B and C were distributed in all three segments. The body area, ovarian and testicular areas of worms from group A were greatest, followed in decreasing order by body and gonadal areas of worms from groups B and C. Echinostoma caproni eggs were found in the faeces of all the hamsters examined from groups A, B and C by days 9, 10 and 11, respectively. Physical damage occurred at the site of attachment of the echinostome. Pathological observations indicated the presence of enlarged lymphatic nodules with lymphocytes being the primary cellular infiltrate at the site of parasite attachment.

The effects of age, sex and breed of chicken on susceptibility to Clinostomum complanatum (Digenea: Clinostomatidae)

Susceptibility and development of Clinostomum complanatum in the chicken (Gallus gallus domesticus) have been studied. The progenetic metacercariae of C. complanatum developed into ovigerous worms in 3 days in the buccal cavity of the chicken and survived only 6-7 days in this experimental host. The susceptibility is influenced by the age, sex and breed/type of the host. Both percent susceptibility and recovery of the worms decreased with increase in the age of the host. The egg laying chickens were found to be least susceptible to the infection. Mature male chickens were more susceptible than the females. The single comb white Leghorn (SCWL) was found to be susceptible while the local type (LT) and the cross breed (SCWLxLT) were not susceptible to C. complanatum. The possible reasons for these variations have been discussed.

Pathological effects of Echinostoma caproni (Trematoda) in the domestic chick

Domestic chicks experimentally infected with Echinostoma caproni for 2 weeks showed a dilated ileum, unkempt feathers, watery diarrhoea, and weight loss. The ileum from infected and control chicks was fixed in 10% neutral buffered formalin, and prepared as 10 microns paraffin sections stained in haematoxylin and eosin, Papanicolau, periodic acid-Schiff, picro-ponceau, and alcian blue. The infected ileum showed atrophic villi, hypertrophied circular musculature with collagen-like fibres and haemorrhagic zones. The brush borders of epithelial cells and goblet cells were absent in the mucosa of the infected ileum. Worms in contact with host mucosa showed tissue plugs in the suckers. The cephalic spines of worms abraded the host mucosa.