Eosinophil responses in sheep selected for high and low responsiveness to Trichostrongylus colubriformis

A breeding programme, based on selection for faecal egg counts, has produced lines of sheep which demonstrate either increased resistance (high responder) or susceptibility (low responder) to challenge infection with T. colubriformis after vaccination with irradiated larvae. Circulating blood eosinophilia, a hallmark of helminth infections, was examined in third generation lambs from two separate selective matings and random bred control lambs. Numbers of eosinophils were higher in high responder lambs when compared to low responders after vaccination and challenge infections. Analysis of eosinophil counts confirmed a strong line effect and there was no evidence of a sex effect. Random bred lambs showed wide individual variations in eosinophil numbers and their response to infection. It was concluded that peripheral eosinophilia was more a measure of host responsiveness to infection than an indicator of helminthiasis. As such the eosinophil may serve as an indicator of the hosts ability to respond to T. colubriformis vaccination and infection.

The effects of immunization of sheep with Trichostrongylus colubriformis larvae on worm burdens acquired during grazing

The effects of immunization of sheep with Trichostrongylus colubriformis larvae on worm burdens acquired during grazing. International Journal for Parasitology 19: 177-181. Romney sheep, reared helminth-free in pens to 5 months of age, were immunized against Trichostrongylus colubriformis by giving two doses of 200,000 T. colubriformis infective larvae at 15 day intervals to assess protection from natural challenge during grazing. Five immunized sheep and five unimmunized sheep were grazed on infested pasture for 4 weeks, and were then returned to the pens for 4 weeks before slaughter. Worm burdens, gastrointestinal histology and mucus antiparasite activity were examined at slaughter. Faecal egg counts and haematological examinations were carried out at regular intervals throughout the trial. Significant protection (P less than 0.05) was afforded immunized sheep against adult T. colubriformis (87%), T. axei (67%), Nematodirus spathiger (91%) and Ostertagia spp. (42%). Greater numbers of immature Nematodirus spp. and Ostertagia spp. were present in immunized sheep Overall, a significant (P less than 0.05) 42% reduction in total nematode burdens was afforded by immunization of the sheep with T. colubriformis larvae. Immunized sheep had significantly (P less than 0.05) more globule leukocytes, mast cells and eosinophils in gastrointestinal tissue and significantly (P less than 0.05) higher levels of mucous antiparasite activity than unimmunized sheep. Haematological observations showed some sheep had transient eosinophilia during immunization or grazing. Both immunized and unimmunized sheep showed depressed (P less than 0.05) total leukocyte counts during grazing which returned to pre-grazing levels within 1 week of return of the sheep to the pens. Overall, haematological parameters reflected parasite challenge and were unrelated to worm burdens acquired.

Overberg Research Projects. IV. The development of resistance to Nematodirus in lambs and spontaneous cure of Teladorsagia in weaners

Soon after 12-week old suckling lambs reached a peak infestation of Nematodirus resistance developed, worm burdens falling by 43.1-97.5% in 18 week and 11-month old sheep respectively. Both Teladorsagia (syn. Ostertagia) and Trichostrongylus reached a peak geometric mean (G) of 18,099 and 3,278 worms on 02 December 1987 in weaners 5-6 months old. Within 6 weeks-6 months of sheep grazing on safe pastures, spontaneous cure of Teladorsagia occurred, worm burdens falling by 77.3-98.7% but not of Trichostrongylus which was only reduced by 34-40%.

Development of immunity to incoming radiolabelled larvae in lambs continuously infected with Trichostrongylus vitrinus

Infective third stage larvae (L3) of Trichostrongylus vitrinus were radiolabelled with 75 selenium by a method which did not affect their viability. Three groups of six-month-old lambs were infected daily with 1000 L3 for four, eight and 12 weeks, respectively. After each period, one of those groups (n = 5) and a group (n = 4) of worm-free controls were challenged with three consecutive daily doses of 1000 radiolabelled L3, killed 10 days after the first dose, and their worm burdens examined. After four weeks of continuous infection partial immunity to the establishment of challenge L3 was apparent, and by eight and 12 weeks, with the exception of one sheep, there was almost total resistance to incoming worms. Immunity was also expressed as an inhibition of the development of established worms.

High concentration of serum gastrin immunoreactivity and abomasal mucosal hyperplasia in calves infected with Ostertagia ostertagi and/or Trichostrongylus axei

Parasite-free, 4-month-old-calves were inoculated with Ostertagia ostertagi and/or Trichostrongylus axei, followed 6 weeks later by inoculation with increasing doses of O ostertagi for 8 weeks in the 2 groups (n = 4) of calves that had been given O ostertagi. Gastrin immunoreactivity concentration in serum was measured before and after infection and was correlated with changes in mucosal thickness. Gastrin immunoreactivity concentration in preinoculation control sera ranged from 95.2 to 287.1 pg/ml, and increased values were measured in all parasitized calves after 15 weeks. Significantly (P less than 0.05) increased serum gastrin immunoreactivity concentration compared with the preinfection value, was found in calves infected with O ostertagi or T axei, and highly significant (P less than 0.01) values were observed in calves infected with both parasites. Abomasal mucosal hyperplasia was observed in all parasitized calves; increased mucosal thickness and mucosal cross-sectional area were most prominent in calves infected with O ostertagi and T axei.

Anthelmintic resistance in the field: changes in resistance status of parasitic populations in response to anthelmintic treatment

Changes in anthelmintic resistance in nematode parasites were monitored in sheep grazing on 2 separate farms, but with the same anthelmintic treatment program, over 16 years. High levels of benzimidazole resistance emerged in Ostertagia and Trichostrongylus spp populations on both farms following 9 years of continuous use of this class of drug. Subsequently, variations in the levels of resistance occurred for the same species between farms and between species on the same farm. A change to levamisole for 2 years resulted in a significant reversion towards benzimidazole susceptibility, but a concomitant rise in levamisole resistance, in Ostertagia on one farm. However, benzimidazole resistance increased rapidly following the re-introduction of oxfendazole into the anthelmintic treatment program. Results from both farms illustrate the pitfalls of using one anthelmintic class for an extended period and provide indirect support for the alternation of anthelmintic classes at approximately yearly intervals.

Selection of ivermectin-resistant Trichostrongylus colubriformis in lambs

A mixed population of Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis was passed for four generations through lambs. In successive passages and on Day 6 (D6), lambs were dosed subcutaneously with ivermectin at 100, 200, 250 and 225 micrograms kg-1 body weight, respectively. No larvae of H. contortus were recovered after the first passage and following the third drug exposure only T. colubriformis was present. Larvae of the selected strain of T. colubriformis developed to third stage, sheathed larvae in vitro in concentrations of ivermectin that inhibited development of larvae from the original strain of T. colubriformis. A definitive control test was then run with 45 lambs, to compare this selected strain to the original strain. The treatments were 150 micrograms kg-1 subcutaneous (SC) D6, 200 micrograms kg-1 SC D6, 200 micrograms kg-1 intraruminal (IR) D6 and 200 micrograms kg-1 SC D21; all treatments were compared to non-treated controls, selected or original strain as applicable. The D6 IR and D6 SC 150 micrograms kg-1 treatments were greater than 99% effective in both strains, although the D6 SC 200 micrograms kg-1 treatment was 85 and 48% effective in the original and selected strain, respectively. The D21 200 micrograms kg-1 was only 58% effective against the selected strain. There was significant (P less than 0.001) differences between the original and selected strains of T. colubriformis for both fecal egg output and worm counts. The exposure of four generations of immature T. colubriformis to ivermectin selected for partial resistance.

Influence of two levels of concurrent infection with Ostertagia circumcincta and Trichostrongylus vitrinus on the growth performance of lambs

Groups of seven lambs were infected on five days each week for 11 weeks with either 1000 or 2000 Trichostrongylus vitrinus larvae alone or concurrently with 1500 or 2500 Ostertagia circumcincta larvae and their growth performance compared to that of worm-free controls. The cumulative liveweight gain of the infected groups was significantly lower than that of the controls but the gain of the concurrently infected lambs did not differ significantly from the respective T vitrinus group. There were no significant differences in the numbers of T vitrinus or O circumcincta recovered from the single and concurrently infected groups, although there was a tendency for lower mean T vitrinus burdens at the higher level of ostertagia/trichostrongylus infection. The distribution of T vitrinus along the small intestine was similar in single and concurrently infected lambs. The reason for the lack of a marked additive effect on growth rate with concurrent infection is discussed in relation to changes in the concentration of two plasma constituents and lesions in the gastrointestinal tract.

Do intestinal helminths affect condition and fecundity of adult mountain hares?

The abundance of the intestinal helminth Trichostrongylus retortaeformis in adult mountain hares (Lepus timidus) from a moor in northeastern Scotland was assessed monthly. Weight and fatness of each hare was measured and the reproductive output of females was estimated by sectioning ovaries. Abundance of the parasite was lower in December and January than at other times of year, and there was no difference in abundance between male and female hares. There was no correlation between intensity of infection with T. retortaeformis and weight or fatness of hares either at the onset of the reproductive period (February and March) or at its termination (August and September). We found no relationship between fecundity of female hares and parasite abundance.

Field observations on the control of ovine parasitic gastroenteritis in south-east England

Three worm control strategies, on a commercial farm in south east England, were monitored parasitologically for a grazing season. Three peaks of larval infection in June, late August and late October/early November occurred on paddocks grazed by dosed ewes and lambs on a new ley (preventive strategy). Lambs dosed and moved on to a hay aftermath in July (evasive strategy), had a mean egg count of 2090 by late August, attributable to infection with Haemonchus contortus. Three weekly drenching of lambs (suppressive strategy), suppressed faecal egg counts until September when the treatment interval was extended due to other on-farm commitments. In addition to these observations, autoinfection, with Haemonchus contortus, was seen to occur in lambs grazing summer turnips. An in vitro egg hatch assay, and critical efficacy trial, confirmed the presence of a benzimidazole resistant strain of Haemonchus contortus on this farm. The implications of these findings are discussed.

Arrested development of Trichostrongylus tenuis as third stage larvae in red grouse

Worms were counted in red grouse (Lagopus lagopus scoticus) in north east Scotland in March, April, August, September, November and December 1986. Immature worms occurred as third (L3) and fourth (L4) stage larvae throughout. Two kinds of evidence suggest that L3 larvae spent much of the winter in a state of arrested development. First, recently acquired L3 larvae could be distinguished by the fact that they retained outer L2 cuticles for a few days; these were present only in the August and September samples. Secondly, the mean proportion of larvae (L3 and L4) present as L3 was less than 15 per cent in August and September, but increased to 92 per cent by December and then decreased to 71 per cent by March and April. During March and April the proportion of L4 increased as overwintering L3 resumed their development.

Susceptibility of the pre-parturient ewe to infection with Trichostrongylus vitrinus and Ostertagia circumcincta

During two successive years, groups of pregnant ewes and non-pregnant worm-free sheep were given anthelmintic, challenged one week later with 10,000 infective larvae of either Trichostrongylus vitrinus or Ostertagia circumcincta and killed two, four and six weeks after challenge. All of the ewes were killed within three days of their predicted lambing date. There was no annual variation in the response of the ewes to infection with either species. Compared to adult naive animals, ewes expressed resistance to infection with T vitrinus at all times, with significantly lower worm burdens composed largely of inhibited third stage larvae. Ewes challenged with O circumcincta were fully as susceptible as naive animals with regard to the size and stage of development of their worm populations. Mechanisms regulating the numbers of adult O circumcincta were operative in the adult naive animals but not in the ewes.

Primary and secondary infection of the domestic chicken with Trichostrongylus tenuis (Nematoda), a parasite of red grouse, with observations on the effect on the caecal mucosa

The course of primary and secondary infections with Trichostrongylus tenuis in the domestic chicken was investigated. Primary infections were established after the administration of single and trickle doses of infective-stage larvae. The worm burden in the caeca was highest after a single dose of 500 infective-stage larvae; this gave a mean of 87 nematodes per bird on days 89 of infection, 20 nematodes on day 14 and 0 on day 28 of infection. Following trickle doses of 60, 100, 200, 300, 400 or 500 infective-stage larvae, there was a rise and then a fall in nematode egg output in all groups. In chickens given a primary dose of 500 infective-stage larvae followed 30 days later by a single secondary dose of 500 infective-stage larvae, the mean worm burden during the secondary infection rose to 57 nematodes on day 9 of infection and then fell rapidly to 18 nematodes on day 15 and to 2 on day 30. Scanning electron microscopy showed changes in the caeca of infected birds, with the caecal surface being covered in a layer of mucus from 12 days after infection. Ball of blood-stained mucus containing nematodes were observed in the caecal droppings from day 9 of infection onwards. It is concluded that chickens rapidly expel an established infection of T. tenuis, unlike the normal host, the red grouse.

A field evaluation of benzimidazole and nonbenzimidazole drugs in a herd of dairy goats

A study was conducted to investigate the efficacy of six anthelmintics in a herd of dairy goats. Pretreatment larval cultures indicated that the goats were infected with Haemonchus contortus and Trichostrongylus colubriformis. Three separate treatment regimens were administered. In each trial, mature nonlactating goats were allocated into two treatment groups and a control group. Treatment groups received thiabendazole (TBZ) or levamisole (LEV), mebendazole (MBZ) or fenbendazole (FBZ), and morantel tartrate (MOR) or ivermectin (IVR). LEV, MOR, and IVR reduced fecal strongyle egg counts by 99% to 100% of pretreatment values. The benzimidazole (BZD) drugs changed pretreatment fecal egg counts by +2% to -32%. Results of posttreatment larval culture demonstrated the presence of H contortus larvae following the administration of BZD drugs.

Effect of prolonged subclinical infection with Trichostrongylus colubriformis on the performance and nitrogen metabolism of growing lambs

Ten 5-month-old lambs, reared free of parasites and prepared with abomasal and ileal cannulas, were paired and given rations of pelleted dried grass by means of continuous feeders. From 7 months of age one of each pair was dosed daily with 2500 Trichostrongylus colubriformis larvae for 34 weeks. An untreated animal served as a pair-fed control to each infected lamb. During 5 experimental periods (pre-infection, and Weeks 6-8, 12-14, 18-20 and 29-34 of dosing) measurements were made of the amounts of N leaving the small intestine, the digestibility of 35[S]-labelled microbial protein in the small intestine, and the amounts of plasma protein leaking into the digestive tract. The infection caused a growth check (P less than 0.05) from Week 6 to approximately Week 13 of dosing. This resulted from inappetence, but also from a disruption in the N digestion and metabolism of the lambs. There was a marked eosinophilia at this time. Measurements of ileal flow rates during Weeks 6 and 12 indicated that significantly more N flowed to the terminal ileum of infected lambs during this period (Week 6, 3.1 g N day-1 P less than 0.05; Week 12, 5.0 g N day-1 P less than 0.001). Plasma N leakage into the intestine during Weeks 8 and 14 was higher in infected lambs (1.2 g N day-1 P less than 0.01 and 0.9 g N day-1 P less than 0.05, respectively) but the digestibility of 35[S]-labelled microbial protein was depressed below that of the controls during the later part of this period, i.e. during Week 13. Continued larval challenge beyond this point caused a less severe nutritional penalty. Voluntary feed intake increased and the animals started to gain weight. The amount of extra N flowing to the terminal ileum of infected lambs was less at Week 18 of dosing (3.5 g N day-1 P less than 0.01) than it had been at Week 12. Similarly, extra plasma N leakage into the tract of infected lambs was less during Week 20 (0.26 g N day-1) than during Week 14. There was however, still a detectable reduction in the coefficient of digestibility of 35S microbial protein in infected lambs during Week 19. By Weeks 29-34 the only detectable difference between the infected and control lambs was in their eosinophil counts.

The effect of daily challenge with Trichostrongylus colubriformis larvae on the nutrition and performance of immunologically-resistant sheep

Immunological resistance to Trichostrongylus colubriformis was developed in sheep by challenging them with 2500 larvae per day for 34 weeks. They were then rested for 24 weeks before being re-challenged with the same dose rate of the same larvae for 10 weeks. Nutritional, haematological and parasitological parameters were measured during the first 8 weeks of re-challenge infection. There were no faecal worm eggs excreted during the re-challenge infection. There was a small but significant increase in plasma-N leakage from Days 4 to 9 of dosing but this then declined to initial levels. There was no other detectable nutritional disturbance associated with re-infection. Re-challenge caused a rapid development of eosinophilia which peaked during Week 6 before declining to levels of the control animals by Week 8. Sheep resistant to T. colubriformis appear to be able to re-activate their immunological mechanisms when re-challenged with the parasite with little associated nutritional penalty.

Measurement in vitro of a larval migration inhibitory factor in gastrointestinal mucus of sheep made resistant to the roundworm Trichostrongylus colubriformis

Lambs were challenged by dosing with 2500 T. colubriformis larvae per day for 34 weeks, rested for 4 weeks and then re-challenged with the infective larvae for a further 10 weeks. A technique for the measurement of inhibition of larval migration from agar gels was used to investigate the antiparasitic activity of mucus, obtained from the small intestine and abomasum of the lambs, at the end of the re-challenge period. Measurements were also made on ileal digesta samples collected at different times during the development of the initial resistance, the rest period and the re-challenge period, and on faeces collected during the re-challenge infection. The mucus from the small intestine and abomasum paralysed and inhibited larval migration from agar gels significantly more (P less than 0.01 and P less than 0.05, respectively) than corresponding mucus from parasite-free control animals. This inhibitory activity was also detected (P less than 0.01) in the ileal digesta of infected animals from Week 6 of primary dosing. The magnitude of the inhibition in the ileal digesta increased with time during the infection period and was again detected during re-infection (P less than 0.01). It was not detectable in resistant sheep towards the end of the rest period. Slight inhibitory activity was detected in faeces after 2 weeks of re-infection. These observations suggest that substances secreted into the lumen of the small intestine of infected animals are responsible for resistance against ingested larvae.

Arrested development of gastrointestinal trichostrongylids in goats in Nigeria

Arrested development of Haemonchus, Cooperia and Trichostrongylus spp. was studied in (1) 14 naturally infected and eight experimentally infected West African Dwarf (WAD) goats reared in the derived savanna zone of eastern Nigeria and (2) 55 naturally infected slaughter goats obtained from the savanna and sahel regions of northern Nigeria. Six of the WAD goats carried natural infections of H. contortus and T. colubriformis and eight other (tracer) goats acquired their infections from a grass paddock artificially contaminated with H. placei, C. pectinata and C. punctata, during May to October. Another three WAD goats were artificially infected with mixed cultures of L3 of the latter three nematodes, while five goats were inoculated with 1500-2000 L3 of H. contortus harvested from cultures incubated at 25-30 degrees C for 8 days either in the dark or under normal laboratory conditions. Approximately 41% (9/22) of the infected WAD goats harboured arrested larvae of Haemonchus and/or Cooperia. No arrested larvae of Trichostrongylus were found in the six animals that were infected with this nematode. The level of inhibition varied from 0.4 to 20% and only three animals showed greater than 10% inhibition. This very low level of inhibition occurred in naturally and experimentally acquired infections, irrespective of the time of year. In the case of Haemonchus, the species and strain of the parasite and infection with L3 cultured in the dark also appeared not to influence the level of inhibition. By contrast, 65.5% (36/55) and 5.5% (3/55) of the northern savanna and sahel goats harboured arrested larvae of H. contortus and T. colubriformis, respectively. The mean percentage inhibition of the former was low (2-25%) during most of the rainy season (June-August) and high (75-90%) during the late rains and the dry season (October-April). The lowest and highest mean percentage inhibitions occurred during July and November, respectively.

Relationship of nematode cholinesterase activity and nematode burdens to the development of resistance to trichostrongyle infections in sheep

The changes in nematode cholinesterase (ChE) activities were examined in relation to the development of resistance in (1) a flock of young grazing sheep, (2) grazing and penned sheep treated with dexamethasone and (3) penned sheep receiving a single mixed infection. Nematodes from grazing sheep with high faecal egg counts (FECs) had higher ChE activities than those from sheep with low FECs. Female nematodes tended to have higher ChE activities than males, and ChE activities in both tended to decline with increasing age of the sheep. The decline in female Trichostrongylus colubriformis ChE activity was associated with a decline in both worm length and in utero egg count. No decline in nematode ChE activity was observed when grazing sheep were treated with dexamethasone. ChE activity of T. colubriformis established in immunosuppressed penned sheep declined 10-20 fold 8 weeks after cessation of treatment. Nematode burdens in the small intestine and abomasum of grazing sheep were significantly correlated, and in individual species they were also correlated with ChE activities. The development of resistance in sheep and the elimination of adult nematode burdens is discussed in relation to gastrointestinal mucosal globule leucocyte numbers, mucus antiparasite activity and the impairment of nematode metabolic function.

Epidemiology of ovine helminthiasis in Haryana, India

Tracer lambs were used to study the pasture contamination with infective stages of helminth parasites during one annual cycle in a subtropical climate. Post-mortem worm counts indicated that low infections with Haemonchus contortus occurred throughout the year except in June. However, twenty five or more H. contortus per lamb were recorded in January, April, May and August. Trichostrongylus colubriformis infection was detected throughout the year and 150 or more worms per lamb were recorded during January to May and in August. Anoplocephalids were recorded from the lambs throughout the year but had no seasonal pattern. Low infections with Oesophagostomum columbianum and Trichuris ovis were observed. The faecal egg counts from the permanent flock with whom the tracer lambs were grazed revealed heavy to mild worm burdens throughout the year. Coproculture indicated that H. contortus predominated from the second fortnight of May to December except in the second fortnight of July. Infection with T. colubriformis was more severe from January to the first fortnight of May and in the second fortnight of July. Negligible infections with O. columbianum, Bunostomum trigonocephalum, Gaigeria pachyscelis and Dictyocaulus filaria were also observed. Biohythergraphs prepared for H. contortus and T. colubriformis showed differences between observed and expected results. It is suggested that for realistic biohythergraphs related parameters in addition to rainfall and temperature should also be considered.