The effect of γ-radiation on the development of infective larvae of Trichostrongylus colubriformis in guinea pigs and sheep

Infection of guinea pigs with normal Trichostrongylus colubriformis larvae was characterized by an initial logarithmic decline of total worms recovered over the first few days of infection (“loss phase”) followed by a “plateau phase” when worm numbers remained stationary and a third “expulsion phase” when worm numbers again declined logarithmically with time. Irradiation of infective larvae at 50 and 100 kilorads (krad) affected the subsequent pattern of infection by prolonging the initial “loss phase” and contracting the “plateau phase”. The rate of decline of total worms during either the “loss phase” or the “expulsion phase” was not affected by larval irradiation.Few males were recovered at any stage of infection with larvae irradiated at 50 or 100 krad. However, the irradiated larvae (females) which survived continued development at a rate which was similar to that of normal larvae and it was apparent that these parasites were not able to maintain their position in the small intestine, but continued development during migration towards the posterior.The radiosensitivity of male larvae and worm egg production (eggs in utero) over a range of radiation doses (25 to 100 krad) were remarkably similar. On the other hand, female larvae were clearly less sensitive to γ-radiation. These observations suggested that the depressed egg production in females may be due to the absence of males.     

Resistance dynamics of calves to infection with Trichostrongylus axei

Calves were immunized with 10 000 infective larvae each of Haemonchus contortus, Ostertagia ostertagi, and Trichostrongylus axei and challenged with 100 000 of each species. When compared with nonimmunized controls, the immunized calves had 95% fewer H. contortus, 10% fewer O. ostertagi, and 20% fewer T. axei; the latter two reductions were not significant. In four other experiments single or multiple immunizing inoculations of T. axei followed by challenge at various intervals after immunization failed to result in significant resistance measured by the number of worms of the challenge inoculation that became established, by post-challenge fecal egg count, and by gross pathologic alterations of abomasa at necropsy.     

Susceptibility of fourth-stage Ascaris suum larvae to fenbendazole and to host response in the pig

Fenbendazole given at the rate of 2.5 g/kg of feed for 3 days had 100% efficacy against 4th-stage Ascaris suum larvae in 8 pigs. Eight control pigs had a total of 108 A suum. In 6 pigs infected 3 times with 3rd-stage A suum larvae and treated with fenbendazole after the larvae molted to the 4th stage, the challenge exposure-derived population was decreased by 64%. Similar sequential infections in 6 pigs similarly infected, but not treated with fenbendazole, decreased the challenge exposure-derived population by 98%; however, developing and/or adult worms from the vaccinating infections were present.     

Development of immunity to incoming radiolabelled larvae in lambs continuously infected with Ostertagia circumcincta

Infective larvae of Ostertagia circumcincta were radiolabelled with 75selenium by a method which did not affect their viability. Three groups of five-month-old lambs were infected daily with 1000 unlabelled infective larvae for four, eight and 12 weeks, respectively. After each period one of these groups and a group of worm-free controls were challenged with three consecutive daily doses of 1000 radiolabelled third stage larvae. The lambs were killed 13 days after the first dose of challenge larvae and their worm burdens examined. The first indication of immunity was retardation of developing worms observed at four weeks. Resistance to the establishment of incoming worms developed between four and eight weeks and a brief period of population turnover probably took place at this time. Simultaneously a greater inhibition of worm development occurred resulting in an increase in the number of parasites recovered as early fourth stage larvae. By 12 weeks the animals were almost completely immune to incoming worms. The development of resistance to incoming worms correlated with a rise in serum antibody titre and an increase in the number of intraepithelial globule leucocytes in the gastric mucosa.     

A study of some factors influencing the immunization of sheep against Haemonchus contortus using attenuated larvae

The respective roles of age of lamb, size of immunizing dose, previous exposure to infection and anthelmintic therapy on the immune response of Scottish Blackface sheep to gamma-irradiated H. contortus larvae were studied.As previously shown, young lambs immunised at 10 and 14 weeks of age failed to develop a significant resistance to subsequent challenge.In contrast, parasite-naive lambs immunised at 9 and 10 months of age with 10 000 H. contortus larvae irradiated at 60 kiloröntgens developed a highly significant immunity to subsequent challenge. This degree of immunity was not significantly altered by the administration of spaced doses of normal larvae from 10 weeks of age until the point of vaccination.Worm-free lambs vaccinated at 9 and 10 months of age failed to develop any immunity to subsequent challenge when the anthelmintic thiabendazole was administered 3 weeks after each immunizing infection.     

Strategic anthelmintic treatment of young cattle during summer in a Mediterranean-type climatic environment

A worm-control program utilising treatment of young grazing cattle with fenbendazole on two occasions during summer was tested in the Mediterranean-type climatic environment of south-west Western Australia. The grazing system aimed to produce steers by introducing three-month-old weaned calves to pasture in mid-winter until they were sold in early summer. Comparisons were made of the numbers of worm eggs passed on to plots by treated and untreated animals during autumn, the performances of treated and untreated cattle and the performances of calves introduced to the plots in mid-winter. The “tracer” calf technique was used to determine the availability of infective larvae on one untreated and one treated plot for each of the two years of the experiment.Treated animals deposited less Ostertagia spp. eggs on to pasture during autumn than did untreated animals in one of the two years. In both years they deposited less eggs of worm species other than Ostertagia spp. Less intestinal worms were acquired by “tracer” calves grazing treatment plots than those grazing no-treatment plots in both years, but there were no differences in the number of abomasal worms acquired.The reduction in availability of infective larvae of intestinal worms was insufficient to prevent the occurrence of parasitic gastroenteritis in calves introduced to the plots in mid-winter. The fenbendazole treatments did not confer any immediate body-weight advantage on treated animals.On both treatment and no-treatment plots, there were few infective larvae available to grazing cattle during early autumn, there was a rapid attainment of peak availability in winter and then a decline to low availability by mid-spring. In one year, infective larvae of intestinal worms (almost exclusively Cooperia spp.) increased in availability again in late spring and early summer. A high proportion of retarded worms was never a feature of the worm counts of “tracer” calves.It was concluded that the treatments may have been more effective had they been given during autumn.     

Feasibility of herbage sampling in large extensive pastures and availability of cattle nematode infective larvae in mountain pastures

Herbage sampling has been used to ascertain the contamination and epidemiology of cattle nematode infective in large extensive pastures situated in the centre of France, where heifers graze for four months on a total area of one ha/heifer in mountains at 1200 or 1400 m above sea level. The sampling was done for each paddock in four virtual “hectares”, tufts of grass being picked both close to faecal pats or “refusals”, and far from these pats.Ostertagia was the predominant parasite and the occurrence of Dictyocaulus resulted from silent-carrier heifers. Just after the thawing of the snow, when the heifers arrived in the mountains, the contamination was very high: 8000–9000 L3 and 45.00 to 63.00 L3 kg^?1 dry herbage, respectively, far from and close to faecal pats, but this contamination decreases regularly during the season.The sampling of four areas (four “hectares”) in each paddock was found to be a very valuable method. The variation of the mean was low and found mainly when the number of larvae was high (6–19% only for the spring sampling).This technique could have some merit in parallel or concurrently with tracer calves which are always difficult and expensive to use.     

Distribution and pathogenicity during development of Camelostrongylus mentulatus in the abomasum of sheep

Fifteen lambs were each inoculated orally with 150 000 infective Camelostrongylus mentalutus larvae. Lambs were killed 3, 4, 8, 12, 16, 21 and 28 days after infection (DAI). Feed consumption and plasma total protein levels of the lambs killed 21 and 28 DAI did not differ significantly from those of uninfected controls. Three and 8 DAI most larvae were recovered from the fundic mocusa, but the intensity of establishment of larvae per gram of tissue was greatest at the cardia and diminished gradually toward the pylorus. Emergence of worms from the cardia and fundus occurred between 8 and 12 DAI, and a proportion of the lume-dwelling population shifted into the antral area, while the number of worms in the antral mucosa remained stable. Nodularity of the mucosa, mucous metaplasia of fundic epithelium, and vascular permeability as indicated by colloidal carbon label, were evident 3–4 DAI, and were most severe 8–12 DAI, when erosion of and inflammatory cell effusion from heavily infected mucosa was marked. pH and Na⁺ concentration in abomasal content were elevated at all times after infection, with peak values 12 DAI. Plasma pepsinogen levels in infected lambs were elevated beginning 3–12 DAI.     

Experimental Ostertagia spp. infection of sheep: development of worm populations and lesions resulting from different dose-levels of larvae

Worm-free sheep were dosed with 8 000, 64 000 or 512 000 infective Ostertagia spp. larvae and killed 4, 7, 10, 13, 18, 24 or 66 days later. Up to day 10 the worm counts ranged from 52 to 86% of the infecting dose and results suggested no substantial reduction either in rate of development or in percentage establishment with increasing dose level. However, a reduction of worm counts, which was substantial at the higher dose-rates but comparatively small at the low rate, occurred after day 10.Gross lesions in the abomasum ranged from little more than the development of a few nodules in the low-dose sheep, to a roughened congested surface with diptheritic membranes and severe oedema in the high-dose animals at days 10–24. These effects had subsided by day 66. Histopathological changes are described.The severity and extent of the reaction was related to the parasite dose-level, but not necessarily to the numbers of worms recovered post-mortem. The rate of return to normality was quicker at the lower dose rates.     

The importance of arrested larvae in the maintenance of patent infections of strongyloides papillosus in rabbits and sheep

Following subcutaneous injection, a proportion of the larvae of Strongyloides papillosus became arrested in the musculature of rabbits and sheep. By the anthelmintic removal of the adult worms from the small intestine, it has been shown in both host species that arrested larvae are capable of continuing their migration and replacing the adult population. While arrested larvae seldom remained longer than 4–5 weeks in the muscles of rabbits, and most migrated earlier, they remained in excess of 12 weeks in sheep. It is suggested that these arrested larvae have the function of maintaining the intestinal population of adult worms.     

The effect of phenothiazine treatment on arrested Haemonchus contortus larvae in sheep

An experimental infection of freshly cultured Haemonchus contortus larvae was given to housed, non-pregnant sheep in October. Phenothiazine was administered to all animals shortly after patency, but the egg output remained negative until mid-April indicating that removal of adult worms from the abomasum did not stimulate the resumption of development of arrested larvae. Worm burdens at post mortem suggest that adults were not being expelled to any great degree during the winter from the host, but this information is clouded by the difference in numbers of both adult worms and arrested larvae in sheep of different haemoglobin type. The results suggest that haemoglobin type may also be a factor in arrest of larvae as it is in the resistance of sheep to adult H. contortus.     

Efficacy of milbemycin oxime against fourth-stage larvae and adults of Ancylostoma tubaeforme in experimentally infected cats

The efficacy of milbemycin oxime against fourth-stage (L4) larvae and adults of Ancylostoma tubaeforme was investigated in a trial involving 24 young domestic shorthair cats. The animals were inoculated with approximately 300 infective stage three (L3) larvae and divided into three groups. After 12 days, eight cats (group 1) were treated with medicated tablets containing 4 mg milbemycin and 10 mg praziquantel to test the efficacy against L4 larvae; eight cats in group 2 were treated with the same tablets after 33 days to test the efficacy against adult worms; and eight cats in group 3 were treated with a placebo tablet. Faecal egg counts were determined periodically in each cat and after 40 or 41 days the number of worms in each animal was determined postmortem. The egg count reduction was determined by comparing the geometric mean numbers of eggs per gram of faeces in the placebo and medicated groups, and the worm reduction by comparing the geometric mean numbers of worms. The egg count reduction was more than 99 per cent in both treated groups, while the number of worms in groups 1 and 2 were reduced by 94.7 per cent and 99.2 per cent, respectively.     

Immunodiagnosis of experimental Parelaphostrongylus tenuis infection in elk

Elk infected with the meningeal worm, Parelaphostrongylus tenuis (Protostrongylidae), do not consistently excrete larvae in feces, making the current method of diagnosing live animals using the Baermann fecal technique unreliable. Serological diagnosis could prove more useful in diagnosing field-infected animals but depends on the identification and availability of good quality antigen. To mimic field infections, 2 elk were inoculated with 6 infective L3 larvae of P. tenuis, and another 2 with 20 L3 larvae. Fecal samples were examined for nematode larvae using the Baermann technique and serum samples taken were tested for anti-P. tenuis antibody with ELISAs by using the excretory-secretory (ES) products of L3, and sonicated adult worms as antigens. One animal passed first-stage larvae in its feces 202 days postinoculation, but passed none thereafter. The remaining 3 inoculated animals did not pass larvae. In contrast to parasite detection, antibodies against larval ES products were detected in all animals starting from 14 to 28 days postinoculation and persisted until the termination of the experiment on day 243 in 2 animals that harbored adult worms. Antibodies against somatic antigens of the adult worm were not detected until day 56 but also persisted until the end of the experiment in the animals with adult worms. In 2 elk that had no adult worms at necropsy, anti-ES antibodies were detected transiently in both, while anti-adult worm antibodies were present transiently in one. These findings confirm the superiority of P. tenuis larval ES products over somatic adult worm antigens as serodiagnostic antigens, as previously observed in studies of infected white-tailed deer, and extend the application of the newly developed ELISA test in diagnosing and monitoring cervids experimentally infected with P. tenuis.     

Characterization of the protective response against a homologous challenge infection with Strongyloides venezuelensis in rats

The protective response in rats against a homologous challenge infection with Strongyloides venezuelensis was characterized. In an initial infection with 1000 filariform larvae and migrating larvae (L(3)) of S. venezuelensis, the population of L(3) in the lungs on day 3 postinfection (PI), and that of adult worms in the small intestine on day 7 PI, were 180.8+/-14.5 and 336.8+/-70.7, respectively. The latter were gradually expelled towards day 42 PI. After the initial infection, the rats developed strong immunity against a homologous challenge infection as manifested by a marked reduction in worm populations, stunted body length and width, damage to reproductive organs, impaired egg production and rapid expulsion of the worms by day 14 after challenge. Expulsion of the worms was preceded by a significantly elevated (P<0.05) peripheral blood eosinophil (PBE) count, both in the initial (200.0+/-26.5 x 10(3)ml) and the challenge infection (400.9+/-165.4 x 10(3)ml). These findings suggest that rats acquire strong homologous immunity following initial exposure to S. venezuelensis. It is suggested that PBEs are involved in worm expulsion. A major target of these effector mechanisms is the reproductive system of S. venezuelensis.     

Differential activation of mast cells by antigens from Trichinella spiralis muscle larvae,adults, and newborn larvae

Mast cell (MC) hyperplasia and activation are prominent features in Trichinella spiralis infection. Indeed a temporal correlation has been shown between the kinetics of intestinal mastocytosis, release of inflammatory mediators from MC, and adult worm loss, which constitutes a major component of the defense against T. spiralis infection. It is well known that during the intestinal phase of trichinellosis, muscle larvae (ML) and adult worms (AD) enter into contact with the host; however, interaction with MC may also occur during migration of newborn larvae (NBL). Therefore, it is plausible that antigens from these developmental stages could activate MC. We have previously demonstrated by in vitro assays that T. spiralis muscle larval (TSL-1) antigens activate MC through an Ig-independent mechanism leading to the release of histamine, MC protease 5, IL-4 and TNF alpha. In this work we evaluated whether total antigens from AD or NBL could activate unsensitized MC and we compared this activation with the activation seen when MC are stimulated with TSL-1 antigens. MC activation was also tested with affinity chromatography purified antigens from NBL using the monoclonal antibody CE-4 that recognizes NBL surface components. The results obtained in this study showed that AD total extracts and TSL-1 antigens induced the release of histamine but not β-hexosaminidase from unsensitized MC, suggesting a selective secretion of MC mediators. In contrast, NBL total extracts or purified NBL antigens did not induce the release of either histamine or β-hexosaminidase from MC. Interestingly, AD and ML are the stages that interact with the host during the intestinal phase of infection. The mechanisms involved in TSL-1 and AD activation of unsensitized MC may function together with other mechanisms of MC activation in host protection against T. spiralis.     

Inhibited development of Obeliscoides cuniculi in rabbits: the effects of active and passive immunization and resumption of larval development

Rabbits were either actively or passively immunized against Obeliscoides cuniculi and challenged with 5 000 infective larvae (IL). Worm development was inhibited in actively immunized animals and worm egg production was impaired by administration of immune serum. Inhibited fourth stage larvae (L₄) from high-level infections (200 000 IL) and normal L₄ from low-level infections (4 000 IL) were transferred to actively immunized and normal rabbits. In both instances, large numbers of L₄ developed to maturity; however, a significant number of inhibited L₄ failed to resume development. Inhibition of larvae resulted in the development of fewer adult male than female worms, and the change in sex ratio was further accentuated in actively immunized recipients. Worm egg production was also reduced in actively immunized rabbits. These data indicate that host immune responses contribute to inhibition of O. cuniculi in rabbits and that worm egg production is also responsive to immunologic control.     

Impeded establishment of the infective stage of Trichinella in the intestinal mucosa of mice by passive transfer of an IgA monoclonal antibody

Our previous study showed that the IgA monoclonal antibody (mAb) HUSM-Tb1 forms immunoprecipitates on the cuticular surface of infective larvae of Trichinella britovi, and that intraperitoneal injection of this mAb to mice 5 hr before challenge infection confers a high level of protection against intestinal T. britovi. The same treatment produced a similar effect in BALB/c mice inoculated orally with Trichinella pseudospiralis larvae, indicating that the effects may be seen upon most members of the genus Trichinella. Worms recovered from the intestinal mucosa at 1 hr after challenge infection with T. pseudospiralis was few in mice passively immunized with the mAb, whereas a substantial number of worms were recovered from the mucosa of control groups. These results suggest that the IgA mAb impedes establishment of infective Trichinella worms in the intestinal mucosa. Trichinella worms inoculated orally into BALB/c mice vaccinated with ultraviolet-irradiated muscle larvae 3 weeks earlier were expelled between days 4 and 7 after challenge infection. Although the mAb HUSM-Tb1 originated from the mesenteric lymph node cells of mice vaccinated repeatedly with such irradiated larvae, IgA-mediated expulsion does not seem to play an important role in this vaccination model.     

Attempts to produce protection against Ostertagia ostertagi in cattle.

Various single or multiple doses of Ostertagia ostertagi were administered to young calves, and the production of protection phenomena elicited by single challenge inoculations ranging from 50,000 to 300,000 larvae or multiple challenge inoculations totaling 98,000 and 300,000 larvae was investigated. With some regimens, the vaccinations apparently resulted in protection against challenge exposure, as reflected by 36 to 56% fewer worms becoming established in challenge-exposed vaccinated calves than in challenge-exposed nonvaccinated, control calves. Other protection phenomena were elicited by some vaccinated calves of significantly more female worms lacking the distinctive vulval flap of O ostertagi and harboring significantly fewer eggs per female. Challenge exposure with a pathogenetic dose of 300,000 larvae produced the same degree of retarded weight gain in vaccinated as in nonvaccinated calves, and at necropsy, visceral lesions and pathologic alterations were equally severe in both groups of calves.     

Immunization of neonatal lambs against the larvae of Taenia hydatigena, using viable eggs followed by chemotherapy

The subcutaneous injection of viable eggs of Taenia hydatigena into neonatal lambs induced 100% protection against the development of viable T. hydatigena larvae from an oral challenge. However, contrary to some published results, no protection was induced against a simultaneous infection with eggs of T. ovis and Echinococcus granulosus.The short-acting partial resistance to oral infection with T. hydatigena eggs, transferred from infected ewes to their lambs, was not enhanced by hyperimmunization of the ewe. In the lamb, this resistance did not interfere with the development of subcutaneously injected T. hydatigena eggs into immature larvae, or with the consequent induction of resistance to an oral challenge with T. hydatigena eggs. The immunizing lesion regressed rapidly after treatment of lambs with Mebendazole, and after 6 months the lesions were negligible in both treated and untreated lambs.     

Efficacy of ivermectin against inhibited larvae of Ostertagia ostertagi

The efficacy of ivermectin against inhibited early 4th-stage larvae of ostertagia ostertagi and other nematodes of the abomasum and intestinal tract was determined in naturally infected yearling beef cattle. The time when large numbers of inhibited larvae were acquired was determined by monthly slaughter of monitor cattle, beginning in January. In April, 12 animals were removed from pasture and maintained free of further helminth exposure until slaughter (21 days). At 9 days after the cattle were removed from pasture, ivermectin was administered to the principals by subcutaneous injection (200 micrograms/kg); the other 6 animals were given subcutaneous injections of the ivermectin vehicle. both groups were klled and necropsied at 12 days after treatment. Mean numbers of O ostertagi in the 6 controls were: adults, 41,906; developing 4th stage, 73,813; and early 4th stage, 334,965. The mean proportion of early 4th-stage larvae was 73.7%. In the 6 principals (treated with ivermectin), the following reductions were observed: O ostertagi adults, 100%; developing 4th stage, 99.8%; and early 4th stage, 99,9%. Small numbers of dead and degenerated O ostertagi of all developmental stages were recovered from abomasal washings before fixation; few viable worms were recovered.