Host factors limiting monogenean infections: a case study

Comprehensive field data on polystomatid monogeneans record low prevalence and intensity of infection and suggest that worm burdens in this group are strongly regulated: thus, in the majority of Polystoma species infecting anuran amphibians mean abundance is typically less than one parasite/host. There is circumstantial evidence that the dominant control is attributable to host factors which over-ride variations in transmission success. This review provides a brief summary of information on Pseudodiplorchis americanus, a parasite of the desert toad, Scaphiopus couchii, and then focuses in detail on the spectrum of factors regulating infrapopulations of Protopolystoma xenopodis, a parasite of the aquatic Xenopus laevis. Infection levels of adult worms and their contribution to transmission are regulated by external environmental factors (especially temperature), by host factors (including behaviour and population density), and by a range of parasite factors including intra- and inter-specific competitive interactions and variations in intrinsic characters, especially survivorship and reproductive output. In addition to these factors whose primary effect is to modulate transmission rates, there is a major attrition in parasite numbers between invasion and maturity (3 months post-infection). Long-term laboratory experiments on the Xenopus laevis/Protopolystoma xenopodis interaction demonstrate a powerful acquired immune response. Primary infection is characterised by a high prevalence of established adult worms but the success of subsequent challenge infection is greatly reduced, leading to low prevalence and extended pre-patent period. In the small proportion of hosts supporting a second infection of adult parasites, surviving burdens are small (one to two worms/host) and show reduced egg production. These results provide an explanation for the low burdens encountered in field studies: a majority of adult X. laevis in natural populations are likely to exhibit strong, relatively long-term, post-infection immunity after the loss of a previous infection.     

Size of inoculum dose regulates in part worm burdens, fecundity, and lengths in ovine Haemonchus contortus infections

Density-dependent factors frequently have been shown to regulate population parameters of free-living and parasitic helminths. To test the effects of various infection levels of Haemonchus contortus on fecundity and worm size, lambs were inoculated with 3,000, 10,000, and 30,000 infective larvae. Daily eggs per gram (epg) and daily total fecal production per lamb were monitored continuously. Worms were collected from abomasa on 6, 15, 22, and 30 days postinfection (PI). Female worms were smaller on each day in the high-dose group when compared to female worms from the low-dose group; males in the high-dose group were smaller from days 15 through 30 PI. The high- and medium-dose groups had higher mortality rates on day 30 PI, and fecundity (eggs/female/day) was 78% lower. Daily epg and daily total eggs/lamb/day were lower in the high-dose group. Fecundity and worm size were correlated with the log-transformed dose level but not with adult worm number. Early parasite and/or host responses apparently exert long-term negative effects on growth and reproduction relative to the size of the establishing population of H. contortus.     

Effects of albendazole against larval and adult Angiostrongylus cantonensis in rats

Effects of albendazole against larval and adult stages of Angiostrongylus cantonensis in rats were examined. (1) A single oral dose of albendazole of 10, 50 or 100 mg/kg at 7 or 14 days post-infection (PI) showed that the use of drug at 7 days PI was more effective in the larval infection. Rats treated 7 days PI showed significant reductions in the relative wet weight of heart and lungs (g/100 g body wt.), the mean total number of recovered worms and the mean body length of worms, as compared to those in the non-treated control. Similarly, visual morbidity assessment of the lung-tissues revealed a marked reduction in pathological changes in the rats treated 7 days PI. (2) Following two or three successive oral doses of 100 mg/kg at weekly intervals from 6 weeks PI, the first-stage larvae in rat feces completely disappeared 2 weeks post-treatment (PT) and this disappearance lasted during the experiment. In rats treated once, however, larval output reappeared from 3 weeks PT. Both histological sections of the rat lung-tissues and the recovered female worms showed degenerative changes in the female reproductive systems.     

The responses of a tropical breed of domestic rabbit, Oryctolagus cuniculus, to experimental infection with Trichostrongylus colubriformis

Clinical, parasitological and pathological responses of a tropical out-bred domestic rabbit to experimental Trichostrongylus colubriformis infection were used to evaluate its suitability as a laboratory host and model for studying the host-parasite relationships of T. colubriformis. In the first experiment, three groups each of 16, predominantly juvenile male, 8- to 10-week-old rabbits were given a single pulse infection with 500, 5000 or 25000 infective larvae (L3) of T. colubriformis, to represent low, medium and high levels of infection, respectively. A fourth group of 16 rabbits of similar age formed the uninfected controls. In the second experiment, two groups of 10 juvenile (8- to 10-week-old) and 10 adult (8- to 10-month-old) rabbits were similarly infected with 20000 L3, with appropriate naive controls. Prepatency was 14 and 16 days and peak faecal egg counts occurred on days 24 and 20 after infection in young and adult rabbits respectively. Peak worm counts occurred on day 14 in both age groups and at all levels of infection. Subsequently, parasite burdens declined in a highly significantly dose- and age-dependent manner. At low and moderate levels of infection, approximately 83-98% of worms were recovered from the first 60 cm of the small intestine. Worm fecundity was also significantly influenced by host age and larval dose. Host age also had a significant effect on worm length. Infections with T. colubriformis were associated with a highly significant loss of body weight, accompanied by anorexia, diarrhoea and 25% mortality at high dose levels during the patent period of infection. There were no significant changes in packed cell volume and eosinophil counts at all ages and levels of infection but significant lymphocytosis occurred at the high dose level between days 7 and 21. Parasite-specific serum IgG responses were not related to worm burden. Overall, data showed that this miniature, docile and relatively inexpensive breed of rabbit is a potentially valuable laboratory host for studying T. colubriformis infections. The larval dose, duration of infection and host age were major determinants of host responsiveness to primary infections in this rabbit genotype.     

Trichinella spiralis infections of inbred mice: genetics of the host response following infection with different Trichinella isolates

The immune response of inbred strains of mice was studied following infection with isolates of Trichinella from a pig (P1), an arctic fox (AF1), and T. spiralis var. pseudospiralis (TP). Strains of mice previously characterized as highly resistant to a separate pig isolate of T. spiralis responded to the P1 and AF1 isolates by expelling over 80% of the worms by day 10 postinfection (PI), and by suppressing the in vitro release of newborn larvae by female worms. However, the response induced by AF1 worms was expressed more quickly when compared to responses induced by the P1 and TP isolates. The host response to TP was less as recovery was always higher at day 10 PI and antifecundity effects were not induced in TP worms even in highly resistant strains of mice. Strains of mice previously characterized as susceptible to T. spiralis infection were slow to develop resistance when compared to the resistant mouse strains, but even among the susceptible strains, infection with AF1 induced a more rapid response. The mouse strains used in these experiments allowed us to assess the role of the major histocompatibility complex (MHC) and/or non-MHC genes in influencing the responses observed. As previously reported for a pig isolate of T. spiralis, both MHC and non-MHC genes influenced the rate at which worms were expelled from the gut and the host response that limits the fecundity of adult female worms.     

Effects of PGE1 or PGE2 and/or acetazolamide on expulsion of Nippostrongylus brasiliensis from rats

PGE1 and PGE2 have been reported to enhance natural expulsion of Nippostrongylus brasiliensis, a nematode parasite, from the intestine of the rat. Mucus production may also be a key element of worm rejection. Our study attempts to determine if 1) PGE1 or PGE2 alter the normal course of infection with N. brasiliensis in rats, 2) a known mucous enhancing drug, acetazolamide, can augment the rate of worm expulsion, and 3) combinations of prostaglandins and acetazolamide affect N. brasiliensis in the rat. Rats were inoculated with approximately 1,000 infective larvae of N. brasiliensis. Animals were administered, intraduodenally, one of the following: 0.2 ml 0.9% NaCl; 0.2 ml 100% ethanol; 250 micrograms PGE1/0.2 ml 100% ethanol; 250 micrograms PGE2/0.2 ml 100% ethanol; 250 micrograms acetazolamide/0.2 ml 100% ethanol; 250 micrograms PGE1 or PGE2 + 250 micrograms acetazolamide/0.2 ml 100% ethanol. These solutions were given in a single bolus on day 6 postinoculation (PI) or twice daily on days 6-9 PI. Following these treatments the number of parasite ova per gram feces per day for days 6-10 PI and numbers of worms present at necropsy on day 10 PI were determined. Treatment with prostaglandins or acetazolamide or both failed to adversely affect egg deposition by adult female worms or the number of worms in the small intestine. These results do not support the involvement of prostaglandins in the expulsion of N. brasiliensis from the host intestine.     

Efficacy of mebendazole and albendazole against Trichinella spiralis in mice

Changes in the sensitivity of Trichinella spiralis to anthelmintic treatment during the first 3 days of infection in mice were studied. Oral administration of either mebendazole or albendazole at 6.25 mh/kg 2 hr after exposure to infection eliminated 95-100% of the worms as determined at necropsy on day 7 postinoculation. Beyond the first day of infection the sensitivity of the parasite to benzimidazole therapy was much reduced and an oral dose of 50 mg/kg was only partially but significantly active against the adult worms. Despite decline in drug sensitivity during the enteral phase, gavage administration of either mebendazole or albendazole at 50 mg/kg for 5 consecutive days during the invasive phase of infection significantly reduced (96 and 67%, respectively) the number of larvae subsequently recovered from host musculature on day 45 postinoculation.     

Infections of forest rat filaria, Breinlia booliati, in neonate and juvenile laboratory white rats

The kinetics of Breinlia booliati infection in 3 inbred rat strains (Lewis, Wistar and Sprague Dawley) were investigated. One group of rats was infected as neonates (less than 24 hours of age) with third-stage larvae of B. booliati and the other group was infected as juveniles (4 weeks of age). The results showed that infection in the neonates were significantly different from the infection in the juveniles. The 60 rats infected as neonates, when necropsied between 8 to 10 months postinfection, yielded adult worms. The 2 neonatal infection groups of Lewis and Wistar strains showed highest susceptibility to the infections. The mean prepatent period was 85 days. Ninety to 95% of the infected rats were patent with microfilaraemia and a large percentage (33 to 47%) of them had high microfilaraemia counts exceeding 3000 mff/20 mm3 of blood and larger sizes (mean 157.11 mm for female adult worms and 61.88 mm for male adult worms. The adult worms were distributed equally in both the pleural (57%) and peritoneal cavity (43%). In most aspects, the neonatal infection group of the Sprague-Dawley strain was intermediate in susceptibility between the 2 neonatal infection groups of the Lewis and Wistar strains and the 3 juvenile infection groups. In contrast to neonatal infection groups, the 3 juvenile infection groups exhibited low infection rates (37%, 58% and 47% for the Lewis, Wistar and Sprague Dawley strains respectively), longer prepatent periods (mean 101 days), lower recovery rates (2 to 4%), lower adult worm loads (mean 0.4 to 0.8 female worms, and 0.2 to 0.8 male worms per rat), and smaller sizes (mean 141.24 mm for female adult worms and 53.75 mm for male adult worms). Forty-four to 57% of these infected rats harboured either single male or single female adult worms in the body cavity. Most (92%) of the adult worms recovered from the juvenile infection groups resided in the pleural cavity and the remaining 8% were recovered from the peritoneal cavity. Microfilaraemia could be detected in only 3/20 Lewis rats, 5/20 Wistar rats and 5/20 Sprague Dawley rats. The mean peak microfilaraemia of the 3 pooled juvenile infection groups was 632 mff/20 mm3 of blood, ranging from 7 mff/20 mm3 to 1856 mmf/20 mm3. Our results indicate that the susceptibility to B. booliati infection in white rats is both genetic and age-associated. The responses of the 2 distinct infection groups to B. booliati infections are discussed.     

Alterations in the longevity and fecundity of adult Trichinella pseudospiralis related to method of isolation of infective larvae

The infectivity of Trichinella pseudospiralis infective larvae was reduced significantly following exposure to low pH or a combination of 1% pepsin at low pH compared to that for larvae isolated in phosphate-buffered saline (PBS) at pH 7.0. Reduction of host gastric pH by administration to mice of sodium bicarbonate solution in PBS was accompanied by an increase in the infectivity of larvae isolated in 1% pepsin/HCl (P/HCl) compared to that for worms inoculated into hosts given PBS alone. Fewer adult worms developing from larvae isolated in P/HCl became established in the host small bowel than was seen with larvae isolated in PBS; moreover, the fecundity in vitro of adult worms developing from P/HCl-isolated larvae was reduced below that for adults developing from larvae isolated from host muscle in PBS. More adult worms were recovered following infection of immune hosts with PBS-isolated larvae than were recovered from immune mice challenged with larvae isolated in P/HCl. Similar findings were observed in mice immunized by infection with Trichinella spiralis and challenged with T. pseudospiralis larvae isolated in either P/HCl or PBS. Immunization of mice with T. pseudospiralis larvae isolated by either method and challenged with larvae of T. spiralis resulted in recovery of similar percentages of the challenge inoculum.     

Influence of immunizing dose and presence or absence of adult worms on the development of resistance to Nematospiroides dubius challenge infections of mice

H-2 congenic strains expressing resistant (H-2q, H-2f) or susceptible (H-2k) haplotypes were compared for their ability to resist challenge infection with N. dubius following a 6- or 14-day ivermectin-abbreviated immunizing infection. B10.BR mice (H-2k) were considerably more resistant to infection when the priming interval was shortened from 14 to 6 days. B10.Q (H-2q) and B10.M (H-2f) mice resisted challenge regardless of which immunization regimen was used. The influence of parasite numbers on the response to challenge was studied by comparing infections in resistant DBA/1 (H-2q) and susceptible CBA/J (H-2k) mice that differ at both H-2 and non-H-2 genes. DBA/1 mice, immunized with 50 or 150 L3 of N. dubius for 14 days, resisted challenge, whereas mice receiving 300 worms did not. In contrast, CBA/J mice failed to resist challenge at all priming doses tested. When the immunizing infection was shortened from 14 to 6 days, DBA/1 mice resisted challenge regardless of priming dose and CBA/J mice resisted challenge only when the highest dose of 300 worms was used for priming. The data suggest that susceptible strains of mice may be preferentially immunosuppressed, particularly at low infective doses, and that suppression is associated with adult worms present in the lumen of the small intestine.     

Recovery,growth, and development of Acanthoparyphium tyosenense (Digenea: Echinostomatidae) in experimental chicks

Chicks were experimentally infected with Acanthoparyphium tyosenense (Digenea: Echinostomatidae) metacercariae per os, and the growth and development of worms in this host were observed from days I to 38 postinfection (PI). The worms grew rapidly and matured sexually in the small intestine (chiefly in the jejunum) of chicks by day 5 PI. and survived at least up to day 38 Pi, although worm recovery decreased after day 5 PI. Both parenchymal and reproductive organs increased greatly in size from day 2 to day 10 PI and then continued to increase gradually in size up to day 38 PI. The number of uterine eggs reached a peak on days 10 and 15 PI and then decreased gradually. The results suggest that chicks are a fairly suitable definitive host for experimental infection with A. tyosenense.     

The effects of histamine and an antihistamine on Trichinella spiralis and on trichinous enteritis in the host

The effects of histamine and an antihistamine on the number and fecundity of adult Trichinella spiralis and on trichinous enteritis measured by determining myeloperoxidase activity in the small intestine of the host on days 7, 9 and 11 postinfection (PI) were examined during primary infections of the CD-1 Swiss white mouse. In mice receiving oral doses of histamine the fecundity of adult worms decreased, the number of adult worms was unaffected and enteritis was elevated above that seen with untreated mice or mice receiving oral doses of saline alone. In mice injected intramuscularly with antihistamine after day 7 PI fecundity of adult worms increased, the number of adult worms remained the same and enteritis decreased compared to untreated mice receiving intramuscular injections of saline. As the concentration of histamine present in the incubation medium (0.01, 0.1, 10, 25, 50 or 100 mg%) was increased above 0.01 mg% the fecundity of adult worms during culture in vitro decreased. As the concentration of antihistamine present in incubation medium (0.01, 0.1, 10 or 100 mg%) was increased above 0.1 mg% the fecundity of adult worms during culture in vitro decreased.     

Efficacy of ivermectin in hookworms as examined in Ancylostoma caninum infections

Single oral doses of ivermectin were given to dogs with moderate or heavy infections of Ancylostoma caninum (egg counts ranging from 7,100 to 41,700 eggs/g feces) at 100, 50, 30, or 10 micrograms/kg body weight. Each of these dosages was effective in clearing the infection completely, so that numerous worms were passed in the feces on days 1-3, but no worm was recovered from the intestinal tract at necropsy on day 4 after treatment. In contrast, an average of 178 worms per dog was recovered at necropsy from the vehicle-treated control and the untreated animals. Albendazole, a known anti-hookworm agent, even in a dose of 400 mg, eliminated only 21-65% of the worms harbored by the infected animals. No untoward reaction to ivermectin or significant pathological change was noted in the experimental animals. In vitro experiments demonstrated that ivermectin: (1) was highly detrimental to actively motile adult worms in concentrations greater than 5.60 micrograms/ml; (2) was detrimental to eggs inside the uterine tissue of female worms in dosages at or greater than 10 micrograms/kg body weight; and (3) killed infected larvae in concentrations as low as 0.0025 micrograms/ml.     

Recovery rate, growth and development of Heterophyopsis continua in experimental chicks

The growth and developmental pattern of H. continua was observed after experimental infection of their metacercariae to chicks. The recovery rate of worms from the chicks at 1 to 28 days post-infection (PI) was 12.8% in average. The rate remained fairly high for early 4 days of infection but decreased thereafter rapidly till 28 days PI. Most of the flukes, 91.9%, were recovered from the ileum of the chicks. In metacercariae, genital organs such as the ovary, testes, seminal vesicle, seminal receptacle and genital sucker were recognizable. At one day PI Mehlis' gland appeared, and at 2 days follicular vitellaria were observed. At 3 days PI, eggs were formed in the uterine tubule and increased in number as the worm grew old. The worms reached 2,990 microns in length and 525 microns in width at 28 days PI. Genital organs developed rapidly in early stages of infection but slowly thereafter to 28 days PI, whereas non-genital organs developed steadily through the infection period. It was proved by this experiment that chicks should be a moderately suitable final host of H. continua.     

Nematospiroides dubius: arrested development of larvae in immune mice.

When immune NIH mice were killed 10 days after a challenge infection with Nematospiroides dubius, approximately 10% of the inoculated larvae were recovered from the intestinal lumen, irrespective of the dose administered. When such mice were treated with cortisone from Day 10 for a period of 8 to 14 days and were subsequently killed for worm counts, it was found that they had significantly more worms than the immune control mice killed on Day 10. During the week following the beginning of treatment with cortisone there was little change in the low worm burdens in immune mice. However, 9 to 11 days after this treatment worm counts indicated that worms were accumulating in the intestinal lumen, and concurrently eggs were recorded in the feces of the mice. These observations indicated that a period of 9 to 11 days was required after the initiation of cortisone treatment on Day 10 for the worms in immune mice to complete their development to the adult lumen-dwelling stage. It is suggested that the larvae in the challenge infection became arrested early in their development in the intestinal wall and that growth resumed only after cortisone treatment. When treatment with cortisone was initiated later after challenge, it was still effective in reactivating arrested worms, but the lower worm recoveries in these mice indicated that the arrested larvae were being slowly rejected by the host. In subsequent experiments it was established that the arrested larvae of N. dubius were insusceptible to the activity of pyrantel embonate, an anthelmintic which is 99% effective against adult worms in the intestinal lumen. The mechanism whereby the larvae of N. dubius became arrested in immune mice and subsequently resumed their development after cortisone treatment is discussed.     

Concomitant and protective immunity in mice exposed to repeated infections with Echinostoma malayanum

Concomitant immunity and its consequence against infection play roles in regulating worm burdens in helminthiasis. Under natural conditions, this immunity is generated by exposure to repeated low dose or trickle infection. In this study, concomitant immunity was induced in mice exposed repeatedly to infection with Echinostoma malayanum and its protective effect on a challenge infection evaluated. A profile of worm burden from exposure to 10 metacercariae/mouse/week rose rapidly during the first 2 weeks reaching a plateau from week 3 to 8 post infection. Based on a cumulative dose of infection, worm recoveries were around 75% in the first 2 weeks, dropped to 50% at week 3 and 19% at week 8. After week 2, adult worm burden was constant and no juvenile worms were found after week 3 of the experiment. To examine the effect of resistance against reinfection, mice in the experimental group were primarily infected with 10 metacercariae/week for 5 weeks, treated with praziquantel and were challenged with 75 metacercariae/animal. The number of worms recovered from the experimental groups was significantly lower than that from naïve control groups beginning from 24 h to 28 days post challenge. The worms in the experimental group showed growth retardation and the proportion of adult worms was lower than that in the control animals especially during the first 3 weeks of the experiment. Parasite fecundity was also suppressed compared with that in the control group. The selective effects of protective immunity on establishment, growth, and fecundity of challenged worms affected the population dynamics of E. malayanum which is a similar phenomenon to concomitant immunity in schistosomiasis.     

Schistosoma japonicum is less sensitive to cyclosporin A in vivo than Schistosoma mansoni.

We compared the toxic effects of cyclosporin A (CsA) on postmigratory immature Schistosoma mansoni and Schistosoma japonicum in mice. For each species, CsA was administered either relatively early or late during development. Exposure of 20-day-old S. mansoni to 1 subcutaneous dose of CsA (50 mg/kg) reduced the worm burden by 45% and induced herniae and/or boli of the gut in 32% of perfusable worms. These results agree with previous reports. In addition, CsA induced a marked liver shift (37% of total worm number). For S. japonicum, CsA was administered at 11 days postinfection (PI) because this species migrates more quickly. Killing of worms and damage to the gut were not observed, and only a slight liver shift occurred. Similarly, these effects were not recorded when CsA was administered at the later times of 34 days PI for S. mansoni and 17 days PI for S. japonicum. For both species, CsA stunted worms, affecting both sexes early PI but only females late PI. In conclusion, immature worms of S. japonicum are less sensitive than S. mansoni to CsA. Also, S. mansoni displays marked age-dependent differences in its sensitivity to CsA.     

Anthelmintic effects of bithionol, paromomycin sulphate, flubendazole and mebendazole on mature and immature Hymenolepis nana in mice

The anthelmintic effects of anti-tapeworm drugs, bithionol, paromomycin sulphate, flubendazole and mebendazole on immature and mature Hymenolepis nana in mice were compared. Immature worms were not affected by paromomycin sulphate or flubendazole administered for 12 consecutive days (days one to 12 after infection) at 100 mg/kg/day but 48% and 100% of H. nana were eliminated from mice by bithionol and mebendazole respectively, at the same dosage regimen. Bithionol, paromomycin sulphate, flubendazole and mebendazole given at 100 mg/kg/day for five consecutive days (days 12 to 16 after infection) eliminated 32%, 29%, 36% and 100% of mature worms respectively. 10 and 20 mg of mebendazole/kg/day for five consecutive days (days 12 to 16 after infection) had little effect on mature worms whereas 50 and 100 mg/kg/day for the same period eliminated 99% and 100% of mature worms, respectively. ED50 of mebendazole in the elimination of mature H. nana was 14 or 15 mg/kg/day for five days from the reduction in dry weight or in number of worms recovered respectively. The effects of mebendazole given 2 to 4 days, 8 to 10 days or 13 to 15 days after infection at 100 mg/kg/day were compared. Very low, if any, activity of the drug given 2 to 4 days after infection was seen, whereas the drug given 8 to 10 days or 13 to 15 days after infection eliminated 84% and 86% of H. nana respectively.     

Humoral immune response and antigenemia in sheep experimentally infected with Schistosoma bovis. Cross-reactivity with Fasciola hepatica antigens.

Circulating antigen level, IgG antibody response to worm antigens and to excretory/secretory products (ES), and specificity to Fasciola hepatica antigens were determined in 6 Schistosoma bovis-infected sheep at weekly intervals for 15 wk. A noninfected control group was included. An enzyme-linked immunosorbent assay (ELISA) sandwich and a double-antibody ELISA test was used for antibody and antigen detection, respectively. The infection induced an early and relatively low IgG response to adult worm extract. This response was significantly elevated by 3 wk postinfection (PI), reached its maximum level at 9 wk PI, and was followed by a subsequent decrease. The response to ES antigens was slightly higher than that to adult worms, although the response started later, at 8 wk PI, and remained at its maximum level until 15 wk. A remarkable level of cross-reactivity was observed when adult F. hepatica extract was used. However, a low degree of cross-reactivity was found with ES antigen. The ELISA for circulating antigens was performed at weekly intervals for 8 wk. Antigens were detected as early as the first week of infection, although differences were statistically significant from week 5 onward. The highest values were observed at 7 week PI.