A mechanistic model of developing immunity to Teladorsagia circumcincta infection in lambs

Acquired immunity influences the severity of parasitic disease, but modelling the effects of acquired immunity in helminth infections has proved challenging. This may be due to a lack of suitable immunological data, or to the perceived complexity of modelling the immune response. We have developed a model of T. circumcincta infection in domestic sheep that incorporates the effects of acquired immunity on parasite establishment and fecundity. A large data set from commercially managed populations of Scottish Blackface sheep was used, which included relationships between IgA activity and worm length, and between worm length and fecundity. Use was also made of a recently published meta-analysis of parasite establishment rates. This realistic but simple model of nematode infection emulates observed patterns of faecal egg counts. The end-of-season faecal egg counts are remarkably robust to perturbations in the majority of the parameters, possibly because of priming of the immune system early in the season, reducing parasite establishment and growth and, therefore, faecal egg counts. Lowering the amount of early infection leads to higher end-of-season egg counts. The periparturient rise in egg counts in ewes appears to have an important role in supplying infection for the priming of the immune response. This feedback in the immune priming suggests that nematode infections may be difficult to eliminate.     

Trichinella spiralis: antifecundity and antinewborn larvae immunity in swine

The development of antifecundity and antinewborn larvae immunity in swine infected with Trichinella spiralis was investigated. In primary infections, adult female worm fecundity dropped sharply after 3 weeks, although adults could be recovered from the small intestine for at least 7 weeks after infection. In challenge infections of pigs infected previously, adult female worm fecundity was depressed up to 51% and the adults were expelled within 3 weeks. Since immune pigs are almost completely resistant to the secondary establishment of muscle larvae, this suggested the existence of immune effector mechanisms also acting on the newborn larvae. This was supported by observations, using an indirect fluorescent antibody assay, that pig antibody bound to the surface of the newborn larvae. Passive transfer of immune pig serum resulted in a large reduction in muscle larvae burden in both infected pig and rat recipients. Adult female worm fecundity in such immune serum recipients was reduced only by 20% and worm survival in the intestine was unaffected. These results indicate that immunity to the newborn larvae, in addition to antifecundity effects, are responsible for the high levels of acquired resistance to T. spiralis in swine.     

Persistence of acquired immunity to Trichostrongylus colubriformis in sheep after termination of infection

Lambs were infected with 6000 Trichostrongylus colubriformis L3 per week for 18, 12 or 6 weeks, beginning at ages 14, 20 and 26 weeks, respectively. At the end of the primary infection subgroups had geometric mean adult worm burdens of 5000, 15,000 and 18,000, respectively. In the remaining sheep the worm population was removed with anthelmintic, and sheep had no further larval intake until challenged with T. colubriformis 1,6, 12, 18 or 24 weeks later. In the groups given 18 or 12 weeks primary infection, establishment of challenge doses was low (less than 25% of establishment in helminth naive controls) in most animals at all challenge times. However, for the groups given 6 weeks primary infection, establishment was low only at the first two challenge times. Thereafter it had similar mean to control groups, but much greater variance. Other subgroups were challenged with T. colubriformis, Ostertagia circumcincta and Haemonchus contortus 1 week after worm removal. In these animals T. colubriformis establishment was not different to animals challenged at the same time with T. colubriformis alone, however immunity to T. colubriformis afforded little protection against the other species. The results of this experiment were incorporated into a simulation model of the population dynamics of T. colubriformis.     

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.     

Rapid improvement of immunity to Teladorsagia circumcincta is achieved through a reduction in the demand for protein in lactating ewes

Protein supplementation can improve the resistance to parasites of periparturient ewes, as indicated by reduced nematode egg excretion and worm burdens. However, the rate at which this improvement can occur is largely unknown. We investigated the rate of improvement by assessing temporal changes in faecal egg counts after we experimentally reduced nutrient demand. Three groups of nine pregnant ewes each were trickle infected with Teladorsagia circumcincta from day(-70) to day(16) (parturition is day0). Two groups of twin-rearing ewes were fed at 0.8 (L22) or 1.2 (H22) times their assumed metabolizable protein requirements, and a third group was fed the same daily food allowances as L22 ewes, but one of their lambs was removed on day10 (L21). Ewes were slaughtered on day21 to assess worm burdens, in vitro larval establishment on abomasal explants, and mucosal inflammatory cells. Faecal egg counts of L22 ewes were higher than H22 ewes throughout lactation. After the removal of one lamb, faecal egg counts of L21 ewes decreased within 5 days to levels similar to H22 ewes. Relative to L22 ewes, L21 and H22 ewes had lower worm burdens, parasite per capita fecundity and in vitro establishment rates of both T. circumcincta and Haemonchus contortus. Mucosal mast cell and eosinophil counts were similar for all ewes, but H22 ewes had higher globule leukocyte counts than L22 and L21 ewes. The data suggest that a reduction in protein demand can rapidly improve periparturient immunity to T. circumcincta. This may be associated with increased parasite expulsion, reduced fecundity and non-parasite specific reduction of in vitro larval establishment.     

Effect of BCG on the resistance of rats to infection with Fasciola hepatica

The effects of three different doses of BCG, given at various periods before infection, on the subsequent establishment of Fasciola hepatica metacercariae were studied. Although evidence was found to suggest that rats which received BCG had mounted a cell mediated immune response, there were no significant differences in worm recovery between BCG-treated rats and controls. The significance of these results in the light of current knowledge on immunity to F. hepatica in rats is discussed.     

Population dynamics of the parasitic stages of Oesophagostomum dentatum in pigs in single and trickle infections

A mathematical model was developed to describe the dynamics of the parasitic stages of Oesophagostomum dentatam in pigs. An immigration-death model with constant establishment, development and death rates was fitted to L3, L4 and adult worm burdens observed in a single-infection experiment. Female worm length was modelled by a function of worm age and total worm burden, while worm egg production (eggs per gram faeces per female worm) was modelled by a function of worm age and worm length. The model was then used to predict worm burdens observed in a trickle-infection experiment. The predicted worm burdens were much higher than those observed, suggesting that worm death rates were higher during the trickle infection. After increusing worm death rates to fit the observed worm burdens, female worm lengths and egg production in the trickle infection were predicted. At the medium- and high-dose rates, predicted worm lengths and, thus, egg preduction were lower than observed, while at the low-dose rate predicted egg production was too high. It appeared that in the trickle infections, total worm burden had less influence on observed female worm length and egg production than in the single infections. The results suggest that the demography of O. dentatum in pigs differs between single and trickle infections.     

Genetic resistance to helminths. The influence of breed and haemoglobin type on the response of sheep to re-infection with Haemonchus contortus.

The influence of genetic factors on acquired resistance to Haemonchus contortus infections in sheep was investigated. Animals whose primary infections were terminated with an anthelmintic failed to develop any immunity against subsequent challenge as judged by worm numbers. Nevertheless, all were better able to retard the development and reduce the fecundity and haematophagic activities of their parasite populations than animals undergoing primary infections. High levels of resistance, as judged by all these parameters, were observed in most animals when the challenge larvae were superimposed on existing worm populations. The patterns of worm establishment and disease indicated that genetic factors operated in determining resistance, since fewer worms became established and less severe clinical and pathophysiological changes were observed in Scottish Blackface than in Finn Dorset sheep with the same haemoglobin type. Similar advantages were displayed by animals with haemoglobin AA and to a lesser extent those with haemoglobin AB over haemoglobin BB types. The importance of breed was further indicated by the occurrence of 'self-cure' in the majority of the Scottish Blackfaces but in only one Finn Dorset. There was no evidence that this reaction was associated with haemoglobin type.     

Population dynamics of Trichostrongylus colubriformis in sheep: the effect of infection rate on the establishment of infective larvae and parasite fecundity

The establishment of Trichostrongylus colubriformis was estimated in helminthologically naive 20-week-old Merino sheep given third stage infective larvae (L3) at rates of 2000, 632 or 200 L3 per day, 5 days per week. After varying periods of continuous L3 intake, a levamisole-susceptible strain of T. colubriformis was replaced with a highly resistant strain for 1 week. The animals were then treated with levamisole to remove the susceptible population, and establishment of the cohort of resistant worms was estimated. In previously uninfected sheep, approximately 65% of the L3 given in the first week became established as adults. This fell to low levels (less than 5%) after 7, 10 and 14 weeks of continuous L3 intake for the high, medium and low infection rates, respectively. At the low infection rate, establishment remained at maximum levels for the first 4 weeks, but then fell at a rate similar to that observed for the higher infection rates. This implied that a threshold of worm exposure was required before resistance to establishment developed. Parasite egg production, expressed as eggs per gram of faeces, was proportional to infection rate and is explained by higher worm burdens occurring at high infection rates. However, estimates of fecundity in eggs per female per day showed the opposite relationship with rate of infection. Fecundity stayed high (approximately 600) for 5 weeks at the low infection rate but only maintained this level for 3 weeks and 1 week at the medium and high rates, respectively. This suggests that fecundity, like establishment, was similarly affected at threshold levels of immunological recognition.     

Efficacy of UV-irradiated larval vaccine of Ancylostoma ceylanicum (Looss, 1911) in golden hamsters (Mesocricetus auratus)

A vaccination trial in golden hamsters with UV-irradiated infective larvae of Ancylostoma ceylanicum was attempted. One oral vaccination of hamsters with 100 infective larvae irradiated by means of UV-tube (390 nm) at different time intervals induced the development of resistance. As the time exposure of irradiation was increased, there was a corresponding decrease in the subsequent worm establishment. A high level of protection afforded by larvae irradiated for 15 min UV-exposure was recorded giving 99.0% and 95.0% worm reduction against the challenge doses of 100 and 1000 normal larvae respectively. There was no marked difference in worm establishment in hamsters vaccinated either orally or subcutaneously, followed by oral challenge. In the vaccinated hamsters, the manifestations of resistance at 15 min UV-exposure were shown by marked reduction in worm establishment and highly reduced epg in pellets with significantly higher blood haemoglobin levels compared with those given normal larvae as vaccine and challenge controls.     

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.     

The curvilinear relationship between worm length and fecundity of Teladorsagia circumcincta.

The variation among sheep in fecundity of Teladorsagia (Ostertagia) circumcincta was estimated by dividing the faecal egg count by the worm number following deliberate infection of mature Scottish Blackface lambs. Fecundity was skewed and ranged from 0 to 350 eggs per worm per day. Most animals had relatively low worm fecundities, but a small number of individuals had relatively high worm fecundities. However, as fecundity is a ratio of two imprecise estimates, extreme values may be statistical artefacts. Following both deliberate and natural infection, differences in worm fecundity were associated with differences in adult female worm length. In both infections, fecundity varied with worm length to the power 0.4. This relationship should assist the measurement of fecundity in studies of host immunity, in epidemiological modelling and in estimating the influence of density-dependent relationships.     

日本血吸虫新基因Sj-MA的克隆、表达及保护性免疫

为发现新基因 ,寻找日本血吸虫病新疫苗候选分子 ,采用Sj雄虫免疫血清筛选Sj成虫cDNA文库。经测序发现新基因Sj MA含有一个完整的阅读框 ,推测其由 2 4 9个氨基酸组成 ,编码分子量为 2 8.8kD的可溶性蛋白质 ,并带有多个能被磷酸化激活的位点 ,提示其可能为一重要的信息传递分子。将Sj MA的cDNA亚克隆至原核表达载体pGEX 5X ,获得Sj MA原核表达的重组体rSj MA/GST ,并在E .coli中高效表达为谷胱甘肽S 转移酶 (GST)融合蛋白 ,分子量为 5 4 .8kD ,Western印迹显示融合蛋白质能被抗雄虫和抗GST血清识别。融合蛋白质免疫小鼠可诱导 34.2 9%的减虫率 ,与对照组有显著性差异 (P <0 .0 0 1 )。表明新基因Sj MA表达的蛋白质能诱导小鼠的抗日本血吸虫的保护性免疫 ,提示其作为日本血吸虫疫苗候选分子的潜在价值     

Failure of low level gamma-irradiation of infective larvae to influence establishment or expulsion of Trichostrongylus colubriformis in guinea pigs

Trichostrongylus colubriformis infective larvae were subjected to various levels of γ-irradiation and administered to guinea pigs. The worms surviving were subsequently counted. Irradiation with 12.9 C kg⁻ inhibited worm establishment but lower doses neither influenced worm establishment nor survival.     

Trichinella spiralis: selective intestinal immune deviation in the rat

In rats, infections with 100-2000 Trichinella spiralis muscle larvae lead to a prompt immunity that is expressed in parasite expulsion within 14 days. Rats infected with more than 2000 larvae display impaired immunity with rejection delayed by 50% (7 days) or more. Suppression is selective for expulsive immunity as the antifecundity response of rats is directly proportional to dose and is expressed sooner in heavily infected subjects. Suppression of intestinal expulsive immunity was suggested by the fact that, with low doses (2000 larvae or less), worm rejection was inhibited by cortisone, whereas cortisone inhibited antifecundity but had no discernable effect on worm rejection in high-dose infections. Evidence for local immune deviation as opposed to systemic immunosuppression was obtained in experiments using parabiotic rats. When one partner was infected with 6000 worms and the other with 200, the rat infected with 200 parasites showed earlier rejection than was seen in single controls infected with 200 worms. The prolonged survival of high-dose adults was not accompanied by a change in the site of worm residence in the gut. Immunological parameters such as serum antibody levels, the number of activated cells or specific anti-T. spiralis lymphocytes in thoracic duct lymph were all increased in a dose-dependent manner. These experiments therefore demonstrate a novel autoprotective mechanism by which adult T. spiralis selectively reduce the expression of expulsive immunity in the gut.     

Ivermectin against larval stages of Dirofilaria repens in dogs

The prophylactic efficacy of ivermectin against Dirofilaria repens infections in dogs was investigated. A first trial was carried out on 15 dogs exposed to four inoculations of L3 larvae at 15-day intervals and treated, in groups of five, with 0, 6 or 12 micrograms/kg body weight of ivermectin given per os 30 and 60 days after the first inoculation. Necropsy, performed about 9 months later, revealed that worm burdens were reduced by 86.6 and 92.8% for the 6 and 12 micrograms/kg dose levels, respectively. In a second trial with an otherwise identical protocol, a dose rate of 24 micrograms/kg of ivermectin was tested in 12 dogs. Only one of the six treated dogs was found worm free at necropsy. The worm burden was reduced by 87.9% in treated animals as opposed to controls. A lengthening of the prepatent periods, which might be considered dose related, was apparent in all treated groups. Ivermectin was not completely effective in preventing establishment of experimental infections with D. repens in dogs.     

Immunization of rainbow trout Oncorhynchus mykiss against Discocotyle sagiffata (Monogenea)

Rainbow trout Oncorhynchus mykiss were injected intraperitoneally with 2 different Discocotyle sagittata extracts dissolved in PBS and subsequently exposed to controlled infection. Immunization resulted in significantly reduced (p < 0.0001) worm intensities in > 50% of vaccinated fish (response arbitrarily defined as parasite burdens < mean control intensity - 1 SD), irrespective of the immunization regime (different parasite extracts, dosing and application schedules) employed. The protective effect of worm extract applied in Freund's complete adjuvant (FCA) did not differ significantly from extract given in PBS. Vaccination with embryonated parasite eggs extract and with FCA alone did not result in partial immunity, suggesting the observed protective effect is specific. Immunized fish had significantly higher specific antibody titres at the time of dissection (as determined by ELISA) than both naive and control fish. Overall, a significant negative correlation was found between antibody titres and worm burdens, suggesting immunoglobulins are implicated in mediating partial immunity. Western blot tests indicated the 2 different worm extracts used to immunize fish share antigens, but each one primarily induced recognition of a distinct band (30 and 38 kDa). Immunization seems to promote a shift between 2 equilibria, rather than progressively increasing protection. This would explain why boosting did not increase immunity, and why 2 different extracts primarily inducing recognition of 2 distinct antigens provide similar degrees of protection. Although several other non-specific and cellular factors are likely to be involved in controlling parasite numbers, it cannot be excluded that antibodies could be involved in mediating the observed partial immunity.     

Schistosoma mansoni: the effect of adrenalectomy on the murine model

Adrenal steroid hormones have been implicated, among others, as one of the most important host factors controlling the onset, establishment, and pathogenesis of schistosomiasis. They appear to inhibit oviposition by Schistosoma mansoni both in vitro and in vivo, and their effect is greatly enhanced when administered in combination with a schistosomicidal drug. Therefore, we hypothesized that adrenalectomy would greatly affect the course of the murine schistosomiasis infection. To test this hypothesis, adrenalectomized mice (Adx) infected with S. mansoni were compared with intact infected and sham-infected controls concerning their mortality rate, numbers of male and female worms, number of eggs, and liver pathology. Compared with controls, Adx infected mice showed an increase of 50% in the mortality rate, as well as 1.7-3 times as many adult worms and twice as many ova per worm pair in their liver. Thus, for the first time, there is evidence that lack of adrenal steroids mediate an increment of the adult worm burden and promote worm fecundity in vivo. The present work was done to test the hypothesis that lack of adrenal steroids enhances adult worm attrition, possibly by their direct effect on the parasite, and by upregulating or downregulating innate and adaptive immune responses.     

Failure of low level γ-irradiation of infective larvae to influence establishment or expulsion of Trichostrongylus colubriformis in guinea pigs

Trichostrongylus colubriformis infective larvae were subjected to various levels of γ-irradiation and administered to guinea pigs. The worms surviving were subsequently counted. Irradiation with 12.9 C kg⁻ inhibited worm establishment but lower doses neither influenced worm establishment nor survival.     

Amelioration of the periparturient relaxation of immunity to parasites through a reduction in mammalian reproductive effort

The degree of periparturient relaxation of immunity to gastrointestinal parasites has a nutritional basis, as overcoming protein scarcity through increased protein supply improves lactational performance, enhances local immune responses and reduces worm burdens. Herein lactating rats, re-infected with Nippostrongylus brasiliensis, are used to test the hypothesis that a similar and rapid improvement of immunity can be achieved through reducing nutrient demand at times of dietary protein scarcity. Reducing litter size from 12 to three pups during lactation resulted, as expected, in cessation of maternal body weight loss and increased pup body weight gain compared with dams which continued to nurse 12 pups. This increase in performance concurred with a rapid decrease in parasitism; within 3 days post nutrient reduction, a 87% reduction in the number of worm eggs found in the colon and 83% reduction in worm burdens was observed, which concurred with increased local immune responses, i.e. 70% more mast cells and 44% more eosinophils in the small intestinal mucosa, to levels similar to those in dams nursing three pups throughout. However, there were no concurrent changes in goblet cell hyperplasia, serum anti-N. brasiliensis-specific antibody levels or mRNA expression of IL-4, IL-10 or IL-13 in the mesenteric lymph nodes. To our knowledge the current study is the first to employ a litter reduction strategy to assess the rate of immune improvement upon overcoming nutrient scarcity in a non-ruminant host. These data support the hypothesis that periparturient relaxation of immunity to gastrointestinal nematodes can be reduced by restoring nutrient adequacy and, importantly, that this improvement can occur very rapidly.