Studies with Brugia pahangi. I. Parasitological observations on primary infections of cats (Felis catus)

The large majority of cats given a single inoculation of third stage larvae of Brugia pahangi became microfilaraemic. Some cats had microfilariae in their blood 53 or 54 days after infection and most had become positive before 72 days after infection. In the majority of cats microfilarial counts remained very steady between 2 and 10 microfilariae per mm³ for long periods. At autopsy 10·7% of the infective larvae injected were recovered as adult worms. The recovery of adult worms was directly related to the number of larvae injected. The microfilarial level did not increase significantly with an increase in the number of adult worms.     

Resistance of dogs to Echinococcus granulosus

Further studies on the immunization of dogs against E. granulosus indicated that certain dogs have a natural resistance, which is not mediated by specific antibodies or sensitized lymphocytes to tapeworm secretory antigens. In a pilot experiment with two pups, E. granulosus recovered from an immunized pup were arrested in their development and exhibited 100% inhibition of egg production compared to 100% egg production in mature worms recovered from a control pup. When the experiment was repeated with 10 pups, three of five control pups given Freud's adjuvant and Bordetella pertussis carried E. granulosus which were arrested in their development and showed 100% inhibition of egg production. Two of five immunized pups also exhibited signs of resistance. Arrested worms recovered 40 days after infection appeared to be at a stage equivalent to about 18 days of optimal development. Although immunized pups exhibiting resistance showed strong positive in vitro and in vivo immune responses, all control pups gave negative reactions to tapeworm secretory antigens.     

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.     

Dipetalonema viteae: primary, secondary and tertiary infections in hamsters.

Hamsters were given primary infections of 100, 200, and 300 D. viteae larvae and groups killed at various intervals after infection. In addition, hamsters were sequentially infected with 100, 200, and 300 larvae and groups killed at 100 or 75 days after the secondary and tertiary infection, respectively. Blood microfilariae were detected on Day 60 following a primary infection, reached a maximum on Day 75, declined to low levels by Day 105, and were negative on Day 120. No microfilariae reappeared in the blood of hamsters given secondary or tertiary infections.Between 20–30% of the infecting larval dose had reached the adult stage by Days 75 or 100 postinfection in hamsters given primary, secondary, or tertiary infections. There was no evidence of arrested larval development in hamsters receiving a second or third challenge infection. Almost half of the tertiary infection hamsters developed subcutaneous nodules and their numbers varied greatly among individual animals. The nodules variously contained living worms, pus, and fragmented worms, or pus only. Hamsters given primary infections of 100, 200, or 300 larvae and killed 375 days after infection had no subcutaneous nodules; however, hamsters given the 200 and 300 larval infections were seen to have dead worms in the subcutaneous tissues. No stunting of adult worms was noted and all female worms had uteri packed with microfilariae.     

Ancylostoma duodenale: maintenance through six generations in helminth-native pups.

An Indian strain of Ancylostoma duodenale with a known propensity for arrested development in man was passaged serially in 2-month-old, helminth-naive, male beagles. The sixth passage was initiated about 500 days after laboratory maintenance began. Experimental animals were infected by stomach tube with 1500 larvae each. These infections became patent in 22–31 days. In a line passaged in dogs without the use of a corticosteroid (prednisolone), adaptation, as judged by fecal egg count, increased through the first three generations and then declined sharply. This line appears destined for extinction. In another line passaged in prednisolone-treated pups, the strain's original viability is being maintained. Infections were long lived, frequently surviving for 90 days or more. Maximum egg output in terms of eggs per gram of feces (EPG) was highly variable between generations and between individuals within generations, but commonly exceeded 1500 EPG. In pups necropsied between 44 and 55 days, harboring fifth- and sixth-generation infections, adult worm burdens ranged from 47–146 and 34–128 in the nonprednisolone and prednisolone lines, respectively. Arrested larvae have been found in the same pups and apparently contribute to a turnover in the population of adult worms. Although A. duodenale has yet to be fully adapted to dogs in the sense that indefinite survival without prednisolone is assured, the present system yields a variety of life history stages in quantities suitable for many experimental purposes. Thus it provides a utilitarian laboratory model for the investigation of one of man's major hookworms.     

Effects on plasma pepsinogen, gastrin and pancreatic polypeptide of Ostertagia spp. transferred directly into the abomasum of sheep

Ostertagia spp. obtained at slaughter from the abomasa. of previously infected sheep and transferred via cannulae into the abomasa of previously worm-free sheep produced severe effects. This was shown with populations of mainly adult parasites given to three sheep and also when mixed populations of 4th stage larvae and adults were given to three other sheep. Although more severe effects were produced when both larval and adult worms were transferred, there were clearly effects when isolates composed predominantly of adults were transferred. Sheep of each group had reductions in food intake and elevations in plasma pepsinogen and gastrin within 24 h of transfer of the parasites. Further increase in plasma pepsinogen and gastrin occurred when abomasal pH rose 5–7 days after infection. Plasma pancreatic polypeptide varied but showed a general tendency to be markedly lower after infection. Ostertagia spp. eggs were detected in the faeces of infected sheep 48 h after transfer. The proportions of adults and inhibited larvae recovered from the abomasa were similar to those in the donor sheep, showing that inhibition, or arrested development of larvae, continued after their transfer to worm free sheep.     

Philomena oncorhynchi: Effect of hormones on maturation in anadromous sockeye, Oncorhynchus nerka

Adult sockeye salmon, Oncorhynchus nerka (Walbaum), treated with salmon pituitary extract in July survived for up to 47 days and developed nuptial colours in both male and female fish during this period, whereas no such characteristics appeared in untreated control fish. Furthermore, results showed that 20 of 50 adult female Philomena oncorhynchi recovered from the 15 treated fish contained fully developed tailed larvae in the uterus as compared with only one of 44 female P. oncorhynchi from 31 untreated control fish. None of 51 worms from 13 stilboestrol treated adult sockeye showed development in utero beyond an elongated embryo, nor did the host fish develop nuptial colours. P. oncorhynchi and its immature sockeye host (approx. 1 year prior to spawning) remained unaffected by any of three separate hormone treatment experiments using injection of salmon pituitary extract, injection of 17-β-estradiol, and stilboestrol mixed in the food.     

Influence of primary infection on the population dynamics of Nematospiroides dubius after challenge infections in mice

Dobson C., Sitepu P. and Brindley P. J. 1985. Influence of primary infection on the population dynamics of Nematospiroides dubius after challenge infections in mice. International Journal for Parasitology15: 353–359. Similar proportions of the inoculum of Nematospiroides dubius larvae reached sexual maturity by 14 days after administration of 50–400 larvae but more adult worms had been expelled by day 63 after infection from those mice infected with 50 vs 400 larvae. There was a significant correlation between time and worm expulsion for all inoculum size groups except for mice given 400 larvae.In mice reinfected with 100 larvae, after termination of primary infections derived from 10 through 400 larvae, more worms from the challenging dose were recovered from mice given greater compared with those given smaller numbers of larvae at primary infection. The N. dubius population size after challenge infection was correlated positively both with number of larvae administered as the primary infection and with the resultant population size during that infection. The serum anti-N. dubius antibody titres after reinfection were higher in mice given 400 compared with those given fewer larvae at primary infection, and the fecundity and female to male sex ratio of the N. dubius populations decreased in proportion to these antibody titres.Protective immunity against challenge N. dubius infection, in mice which had been drenched free of adult worms established from 400 larvae for 5 down to 1 weeks before reinfection, increased from 45% (1 week) to 80% (5 weeks). There was a negative correlation between the population size of N. dubius during challenge infection and the duration between anthelmintic treatment and challenge infection.     

Dirofilaria immitis: fate and immunogenicity of irradiated infective stage larvae in beagles

Experiments were conducted on the fate of irradiated infective larvae of Dirofilaria immitis in dogs, and on the effect of these infections on a challenge dose of nonirradiated larvae administered at a later date. Six dogs were inoculated with 200 to 296 irradiated larvae; in no case was a patent infection established. No living worm was recovered beyond 66 days. Eight dogs inoculated with 200 to 2401 irradiated larvae over varying periods of time were exposed 57 to 190 days after the final inoculation of irradiated larvae, to a challenge infection of 200 to 250 nonirradiated (normal) larvae. The results showed that the number of worms which developed to maturity in these dogs was sharply reduced compared to that in the 5 controls (dogs inoculated with normal larvae only). The most striking effect was seen in “vaccinated” dogs which were challenged 3 months or more after the final administration of irradiated larvae.     

Enhanced resistance to infection with Haemonchus contortus in sheep treated with a corticosteroid

Adams D. B. and Davies H. I. 1982. Enhanced resistance to infection with Haemonchus contortus in sheep treated with a corticosteroid. International Journal for Parasitology12: 523–529. Fewer worms established from experimental infections with Haemonchus contortus in either immune or naive sheep or in sheep of undefined immune status given the corticosteroid, dexamethasone, around the time of challenge. In four experiments, the number of adult worms present in sheep treated with dexamethasone ranged from 32 to 36% of that in untreated animals. That the phenomenon did not stem from direct action of dexamethasone on the worms themselves was demonstrated by comparing continuous treatment with the drug from infection until patency with treatment given at infection and four days later. Sheep continuously treated with dexamethasone harboured similar numbers of worms as the infection controls whereas fewer parasites were found in sheep given dexamethasone around the time of infection. The results imply that active regulation of immunological unresponsiveness operates in sheep during infection with H. contortus and that disruption of immunoregulation by dexamethasone released protective responses thereby decreasing worm burdens. Because suppressor lymphocytes are implicated, cellular perturbations following treatment with dexamethasone were investigated. Dexamethasone did not cause marked lymphopenia. It, however, reduced blastogenic responses by lymphocytes to con A but not PHA. Comparison of responses to these mitogens in cells from blood and lymph demonstrates that con A and PHA-reactivity resides in identifiably different cell populations in sheep.     

Strongyloides stercoralis: Hyperinfection in immunosuppressed dogs

Hyperinfective strongyloidiasis involving the threadworm, Strongyloides stercoralis, is well known in humans and primates. Although this nematode also frequently parasitizes dogs, canine hyperinfective strongyloidiasis has not been reported. To determine whether a fulminant pattern of nematode development can occur in dogs, and to test the S. stercoralis/dog system for suitability as a model for human hyperinfective and disseminated strongyloidiasis, five canine infections with a dog-derived strain of S. stercoralis were monitored by the quantitative recovery of larvae from feces. Even 3-month-old pups controlled their initial infections successfully, the number of larvae excreted declining to near zero in 90 days. Immunosuppressive treatment with prednisolone, prednisolone and azathiaprine, or niridazole resulted in a rapid return to former or greater intensities of infection, as judged by larval output. Only first stage (rhabditiform) larvae were passed in the feces, although third stage (filariform) larvae occurred in the intestinal contents of dogs when they were examined at necropsy. In 3 of the 5 dogs, the adult worm recovery exceeded the inoculated dose greatly and, in one of these, adults and rhabditiform larvae were found in distant, extraintestinal sites. In the remaining 2 of the 5 dogs, the adult worm population was less than the inoculated dose, but, in both, the infection was terminated by the host's death before hyperinfection could have developed. The observations demonstrate that autoinfection occurs in dogs infected with S. stercoralis and that, if it is allowed to continue for a sufficiently long time in immunosuppressed hosts, massive hyperinfection, and even disseminated infection, may occur. This spectrum of increasingly invasive parasitism closely resembles strongyloidiasis in humans. Therefore, the S. stercoralis/dog system has excellent potential as a model for human hyperinfective and disseminated strongyloidiasis.     

Trichinella spiralis: role of different life cycle phases in induction, maintenance, and expression of rapid expulsion in rats.

The capacity of different phases of the life cycle of Trichinella spiralis to induce rapid expulsion was examined. The phases examined included enteral preadults, enteral adults, and parenteral larvae. All had the ability to induce rapid expulsion although there were significant quantitative differences in their inductive capacity and in the kinetics of expression. Immunization with preadults required only a 48-hr enteral exposure to 2000 worms to induce strong rapid expulsion. In contrast rats required a 14-day exposure to adult worms to elicit a comparable response. After immunization with adults the reaction was demonstrable for only 2 weeks. Parenteral larvae produced only a weak rapid expulsion reaction by themselves and this response did not develop until some 8 weeks after challenge. When immunization with the enteral phases (preadult and adult) was combined with exposure to parenteral larvae a strong and enduring rapid expulsion reaction was observed. Phase specificity was also observed in the susceptibility of worms to the rapid expulsion response. The preadult phases, from infectious larvae to worms of up to 2 days of age were highly susceptible. Older worms, from 3 to 4 days old were not susceptible to rapid expulsion and could invade and establish themselves in the primed intestine for at least a 48-hr period without apparent adverse effects.     

Kinetics of primary and secondary infections with Strongyloides ratti in mice

Dawkins H. J. S. and Grove D. I. 1981 Kinetics of primary and secondary infections with Strongyloides ratti in mice. International journal for Parasitology11: 89–96. The kinetics of infection with S. ratti were quantitated in normal and previously exposed C57B1 /6 mice. In primary infections, larvae penetrated the skin rapidly and were seen in peak numbers 12 h after infection. By 24 h after infection, larval numbers had declined appreciably and there was a slow decrease in numbers thereafter. Larvae were first observed in the lungs at 24 h and maximal recovery occurred at 48 h. It is thought that larval migration through the lungs is rapid. Worms were first seen in the intestines two days after infection. Maximum numbers were seen on the fifth day and worm expulsion was complete by day 10. Two moults took place in the small intestine during days 3 and 4 after infection. Rhabditiform larvae were first noted on the fourth day after infection. Mice exposed to S. ratti four weeks previously had significantly less larvae in the skin 4 and 12 h after infection but by 24 h there was no difference when compared with mice with primary infections. Peak recovery of larvae from the lungs occurred 24 h after infection; significantly less larvae were recovered on days 2 and 3 when compared with normal mice. There was a marked reduction in the adult worm burden in the gut; the number of worms recovered was less than one fifth of that seen in primary infections. Those worms which did mature were less fecund and were expelled from the intestines within 7 days of infection. It is suggested that in previously exposed animals, the migration of larvae from the skin is hastened, many of these larvae are destroyed in the lungs and that expulsion of worms which do mature in the intestines is accelerated.     

Trichinella spiralis: mediation of the intestinal component of protective immunity in the rat by multiple, phase-specific, antiparasitic responses.

Rats infected orally with Trichinella spiralis developed an immunity that was induced by and expressed against separate phases of the parasite's enteral life cycle. Infectious muscle larvae generated an immune response (rapid expulsion) that was directed against the very early intestinal infection and resulted in the expulsion of worms within 24 hr. This response eliminated more than 95% of worms in an oral challenge inoculum. Developing larvae (preadults) also induced an immune response that was expressed against adult worms. The effect on adults was dependent upon continuous exposure of worms to the immune environment throughout their enteral larval development. Immunity induced by preadult T. spiralis was not expressed against adult worms transferred from nonimmune rats. While adult worms were resistant to the immunity engendered by preadults they induced an efficient immunity that was autospecific. Both “preadult” and “adult” immunities were expressed in depression of worm fecundity as well as in the expulsion of adults from the gut. However, the two reactions differed in respect to their kinetics and their efficiency against various worm burdens. Preadult immunity was directed mainly against fecundity whereas adult immunity favored worm expulsion. All responses (rapid expulsion, preadult and adult immunity, and antifecundity) acted synergistically to produce sterile immunity against challenge infections of up to 5000 muscle larvae. These findings indicate that the host protective response to T. spiralis is a complex, multifactorial process that operates sequentially and synergistically to protect the host against reinfection.     

Schistogram changes after administration of antischistosomal drugs in mice at the early phase of Schistosoma mansoni infection

Mice infected with Schistosoma mansoni were treated with oxamniquine, praziquantel, artesunate at the pre-patent phase, aiming at observing schistogram alterations. Half of the animals were perfused five days post-treatment for counting and classification of immature worms, based on pre-established morphological criteria (schistogram); the remaining animals were evaluated 42 or 100 days after infection and perfusion of the portal-system was performed for collection and counting of adult worms and oogram. It was observed that oxamniquine and artesunate treatment administered at the pre-postural phase causes significant reduction in the number of immature and adult worms. However, there was little reduction with praziquantel when used at the dose of 400 mg/kg for treatments administered 14, 15, 21 or 23 days post-infection. Artesunate was responsible for significant alterations in development of young worms, as well as for a higher number of worms presenting intestinal damages. Immature adult worms were detected in mice treated with artesunate or oxamniquine at the pre-patent phase of infection and recovered by perfusion 100 days after infection. Schistogram proved to be a very useful tool for experimental evaluation of the activity of antischistosomal drugs and a good model to identify the most sensitive stages to drugs.     

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.     

Immunity to Litomosoides carinii in Mastomys natalensis. II. Effects of chemotherapeutically abbreviated and postpatent primary infections on challenges with various stages of the parasite

Naive Mastomys natalensis, Litomosoides carinii-infected M. natalensis at a postpatent stage of the infection and L. carinii-infected M. natalensis treated chemotherapeutically with furazolidone (FUR), FUR and diethylcarbamazine (FUR/DEC) or amoscanate (AMOS) were challenged by either injection or implantation of 40 third stage larvae (L3, s.c.), 40 fourth stage larvae (L4, 16 days old, i.p.), 20 male and 20 female preadult worms (36 days old, i.p.), 12 adult female worms (i.p.) or 6 X 10(6) microfilariae/kg (i.v.). Microfilaraemia in animals challenged at a postpatent stage (independent of the kind of challenge), was either totally suppressed or at least greatly reduced. Necropsy of L3-challenged animals showed that neither the length of the worms nor their content of morphologically intact, intrauterine stages was affected. Infected, treated animals challenged with developing stages (L3, L4 and preadult worms) showed reduced levels of microfilaraemia (by up to 75%). Dissection of AMOS-treated, L3-challenged animals showed that both the developmental rate and the fertility of the worms were affected. Microfilaraemia was also reduced after implantation of adult worms into treated animals. This was independent of the interval between treatment and challenge (44-150 days) except in animals challenged 10 days after AMOS-treatment, which showed no difference from naive controls. However, infected, treated M. natalensis, cotton rats and gerbils did not develop immunity against intravenously injected blood microfilariae.     

Response to ivermectin treatment of parasitic stages of Haemonchus contortus resistant or susceptible to ivermectin.

Two groups of 33 helminth-naive lambs were infected with 5,000 L3 of an ivermectin-resistant or -susceptible strain of Haemonchus contortus (groups R and S). On days 6, 10, 16, and 21 postinfection, 5 animals from each group were chosen at random and orally treated with 0.2 mg/kg of ivermectin. On each occasion, 2 randomly selected lambs from each group were also killed to determine the number and stage of development of the worms present at the time of treatment. These necropsies revealed that by day 6 early and late fourth-stage larvae were present, whereas on day 10 the early fifth stage had been reached; by days 16 and 21 all worms had reached the adult stage. Necropsies on day 28 postinfection revealed that although animals treated at day 6 had 26.3% fewer worms than the controls, there was no significant difference (P greater than 0.05) between worm burdens from any of the animals infected with the R strain and treated at different times after infection when compared with the untreated controls. With ivermectin significant reductions were obtained in the worm burdens of the animals infected with the susceptible strain; these were reduced by 96% when treatment was given on day 6 against fourth-stage larvae and 98.9% when the drug was given on day 21 against adult stages. From these results it is clear that resistance to ivermectin in this strain of H. contortus is present as early as the fourth larval stage.     

The immune response of the mouse to larvae and adults of Nematospiroides dubius

In mice, repeatedly infected orally with larvae of Nematospiroides dubius, resistance caused delay in the maturation of larvae, their investment in inflammatory nodules and, in sufficiently resistant animals, their death. The fecundity of adult worms was not affected by host resistance.Previously uninfected mice which had received adult worms by transplantation at operation produced only very low titres of reaginic antibody in comparison with mice infected with larvae by mouth.The migration of leucocytes from resistant mice was inhibited by a crude antigen derived from adult worms.In mice made passively immune by transfer of serum the entry of larvae into the wall of the intestine was delayed; there was no inflammatory response and the larvae did not die.In mice selectively depleted of thymus-derived lymphocytes no inflammatory response occurred and the maturation of larvae was not delayed in response to repeated infection.     

Effects of Toxoplasma gondii (Gleadle strain) on the host-parasite relationship in trichinosis.

The effects of concurrent infection with Toxoplasma gondii on the host-parasite relationship in trichinosis were studied. Infected mice showed a delay in expulsion of Trichinella spiralis adults from the gut. Persisting adult female worms were fecund but the numbers of larvae recovered from the muscles were not increased. Increased resistance to the systemic phase of trichinosis was shown by reduced numbers of muscle larvae after intravenous injection of newborn larvae in animals with toxoplasmosis as compared with control mice. There were no differences in small bowel pathology of trichinous mice with and without toxoplasmosis but inflammation around muscle cysts of T. spiralis was reduced in mice with toxoplasmosis. The eosinophilia which normally develops in mice with trichinosis was suppressed by concurrent toxoplasmosis. Trichinella infection did not alter the numbers of T. gondii cysts recovered from the brain 4 weeks after infection. It is suggested that the delay in expulsion of adult worms, decrease in muscle inflammation around T. spiralis cysts, and inhibition of eosinophilia result from immune suppression, while the reduction in numbers of muscle larvae after intravenous injection of newborn larvae reflects enhanced nonspecific resistance to infection in toxoplasmosis.