Development and survival of Haemonchus contortus and Trichostrongylus sp. on pasture in Cameroon

The development and survival of Haemonchus contortus and Trichostrongylus sp. on pasture at Bamenda, Cameroon were studied by spreading faeces containing strongyle eggs from mixed infections on 12 experimental grass plots from April 1988 to March 1989. Development of eggs to infective larvae during the rainy season (April to October) took place within one week. The infective larvae of H. contortus survived for 11 +/- 2 weeks (with a range of 7 to 13 weeks) during this period while those of Trichostrongylus sp. survived for 9 +/- 2 weeks (with a range of 6 to 12 weeks). No larvae were recovered from eggs spread on pasture during the dry season, except in December when unseasonal rains fell within 9 days of spread of the faeces. Rainfall was identified as the most important climatic condition affecting the development and survival of infective larvae on pastures at Bamenda.

Cryopreservation of the first-stage larvae of trichostrongylid nematode parasites

First stage (L1) larvae of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta can be cryopreserved in the presence of DMSO using a two-step freezing protocol involving an initial period at -80 degrees C prior to transfer to liquid nitrogen. Thawed L1 larvae continue development in vitro producing third stage (L3) larvae that are infective to sheep when dosed per os. Establishment rates for L3 larvae grown from thawed L1 larvae were 40 and 80% for H. contortus and T. colubriformis, respectively. There was no difference in survival or infectivity between benzimidazole (BZ)-susceptible and BZ-resistant H. contortus isolates and cryopreservation caused no shift in their BZ-resistance status as indicated in an in vitro larval development assay. Cryopreservation also had no effect on the sensitivity of these isolates to the avermectins or levamisole in vitro. High survival rates (60-70%), good levels of establishment and the stability of anthelmintic resistance status of isolates indicate that little if any selection occurs during the cryopreservation process. L1 larvae of all 3 species have been successfully recovered after 16 months storage in liquid nitrogen, cultured to the L3 stage and established in sheep.

The persistence of the efficacy of injectable or oral moxidectin against Teladorsagia, Haemonchus and Trichostrongylus species in experimentally infected sheep

The persistence of the efficacy of moxidectin was evaluated against experimental gastrointestinal nematode infections in 55 lambs randomly allocated to 11 equal groups and infected on day 0. Moxidectin 1 per cent injectable solution was administered at a dose rate of 0.2 mg moxidectin/kg bodyweight to five of the groups on days -42, -35, -28, -21 and -14; five other groups were treated with moxidectin 0.1 per cent oral drench at the same dose rate on days -35, -28, -21, -14 and -7, and the 11th group remained untreated as a control. The lambs were infected experimentally with 8000 Teladorsagia circumcincta, 2000 Haemonchus contortus and 10,000 Trichostrongylus colubriformis infective larvae and killed three weeks later. Both formulations of moxidectin showed excellent activity against T circumcincta and H contortus with almost 100 per cent efficacy against the abomasal parasites for up to 35 days after treatment. The efficacy of moxidectin 1 per cent injectable against T colubriformis was much higher (> 99 per cent) than that of the oral drench and it was highly effective up to 21 days after treatment, and gave a moderate reduction in worm burden for up to 35 days after treatment. No adverse reactions to moxidectin were observed in any of the animals.

Further studies on the development and survival at low temperatures of the free living stages of Trichostrongylus tenuis

To assess the ability of the free living stages of Trichostrongylus tenuis, a pathogen of red grouse, to survive the winter in significant numbers, the temperatures prevailing during the autumn, winter and spring of 1991-92 on the North Yorkshire moors were simulated in an incubator into which replicate cultures of T tenuis eggs were placed at intervals. September and early October eggs developed into larvae which survived over winter and were infective the following spring. Few November to February eggs survived to become larvae but early March eggs were reasonably successful. Continuous temperatures of -15 degrees C were lethal to infective larvae within 12 days, but significant numbers of larvae survived temperatures of -10 degrees C or higher for up to three weeks and remained infective.

Acetylcholinesterase in infective-stage larvae of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis resistant and susceptible to benzimidazole anthelmintics

The amounts of acetylcholinesterase (AChE) in homogenates of infective-stage larvae of strains, from different countries, of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis resistant or susceptible to benzimidazole (BZ) anthelmintics, were compared using a rapid colorimetric assay. The levels of AChE were much greater in those strains which were BZ-resistant than in BZ-susceptible populations. The larvae of susceptible and resistant H. contortus contained less AChE than did the larvae of the other two species. One-way analysis of variance showed that the observed differences between susceptible and resistant strains were significant in each of the species studied. It is suggested that the extra enzyme may act as an anthelmintic-detoxifying enzyme, may inactivate the active site of the anthelmintic or may prevent binding of the anthelmintic to its receptor.

Comparative kinetic disposition of oxfendazole in sheep and goats before and during infection with Haemonchus contortus and Trichostrongylus colubriformis

The kinetic disposition of [14C]-oxfendazole (OFZ) and its metabolites, fenbendazole (FBZ) and fenbendazole sulphone (FBZ.SO2), in plasma and abomasal fluid were determined in Merino sheep and Angora goats before and during infection with Trichostrongylus colubriformis and Haemonchus contortus. The systemic availability (area under the plasma curve, AUC) of OFZ was significantly lower in goats (13.5 micrograms.h/ml) than in sheep (22.2 micrograms.h/ml) and was reduced with infection in goats (5.6 micrograms.h/ml) and sheep (15.1 micrograms.h/ml). The elimination of plasma [14C] was faster in goats than in sheep. The responses observed for [14C] were a reflection of the behaviour of OFZ. The concentration of OFZ and metabolites in abomasal fluid were similar in both species in the absence or presence of infection. However, as the mean flow rate of abomasal fluid was slower in goats (240 ml/h) than in sheep (488 ml/h), only 7% of the dose passed the pylorus in abomasal fluid of goats compared with 14% in sheep. The presence of gastrointestinal nematodes generally increased abomasal fluid flow rate but neither species nor infection had any effect on the rate or extent of [14C] excretion in urine or faeces. It is suggested that goats possess a faster hepatic metabolism than sheep resulting in more rapid elimination of OFZ.

Caecal threadworms Trichostrongylus tenuis in red grouse Lagopus lagopus scoticus: effects of weather and host density upon estimated worm burdens

Trichostrongylus tenuis eggs were counted in faeces from individually marked wild red grouse for 8 years. Egg counts varied seasonally and annually. In some years, a sudden increase in mid-April was consistent with delayed maturation of larvae which had overwintered in the birds in a hypobiotic state. A more gradual increase in summer was probably due to uninterrupted maturation of larvae ingested then. Despite 30-fold year-to-year variation in mean egg counts, relative differences in egg counts among known individuals within years tended to persist across years. Rainfall in previous summers explained much of the year-to-year variation in egg counts, probably because parasite recruitment was greatest during wet summers. Grouse density was only weakly related to worm egg counts. The data were not consistent with the hypothesis that the cyclic-type population fluctuation in red grouse numbers observed at the time of this study was caused by the parasites.

Hypobiosis of Trichostrongylus tenuis in experimentally infected grey partridges

Storage of infective larvae (L3) of Trichostrongylus tenuis at +4 degrees C for six weeks before being given as single infections to grey partridges (Perdix perdix) did not increase the proportion of worms subsequently found to be hypobiotic. However, after repeated infections, hypobiotic larvae were numerous. Three infections given over three weeks resulted not only in the presence of many hypobiotic larvae, but also in the coincident loss of most adult worms. This indication of the role of immunity in the production of hypobiosis in grey partridges suggests that this species is not a good model for trichostrongylosis in red grouse (Lagopus lagopus scoticus).

The survival of Ostertagia circumcincta and Trichostrongylus colubriformis in faecal culture as a source of bias in apportioning egg counts to worm species

When cultured alone or concurrently with Trichostrongylus colubriformis in sheep faeces, Ostertagia circumcincta produced fewer infective larvae per 100 eggs than did T. colubriformis. Averaged over five trials 60% of T. colubriformis eggs were recovered as infective larvae while for O. circumcincta the figure was only 39%. This result was observed for two strains of O. circumcincta and was independent of when larvae were harvested from culture (days 6-10 at 25 degrees C). The mortalities of both species occurred at the first and second larval stages. These observations are of concern when using larval differentiation from faecal culture to make quantitative estimates of worm egg numbers for each species present. Species such as T. colubriformis which have a low mortality during culture are likely to have their egg numbers overestimated when cultured with a species, like O. circumcincta, that suffers high mortality in culture.

Summer-arrested development of abomasal trichostrongylids in Awassi sheep in semi-arid areas of north-west Syria

Inhibition in trichostrongylids was studied in Awassi sheep in North-west Syria, which has cold winters and hot summers. On six occasions during a 1-year period, five ewes with natural helminth infections were slaughtered after being held in pens for 3 weeks. The percentage of inhibited larvae was lowest in January (6%); it increased during the spring to 76.7% in April and to 84.6% in June and then decreased during the autumn (57.1% in September, 26.1% in December). Inhibition occurred mainly during the early fourth stage in mucosa but was also observed during the late fourth stage in digesta. Teladorsagia circumcincta was the main species involved, but it seemed that this phenomenon also occurred in Marshallagia marshalli.

Population dynamics of Trichostrongylus colubriformis in sheep: computer model to simulate grazing systems and the evolution of anthelmintic resistance

A computer model was developed to simulate Trichostrongylus colubriformis populations, their level of resistance to the common anthelmintics, host mortalities and acquired immunity. Predictions were based on sheep management practices such as lambing, weaning, sheep/paddock rotation, anthelmintic treatment, the use of controlled release devices (capsules) for anthelmintic delivery and daily meteorological records to determine the development and survival of infective larvae (L3) on pasture. Evolution of drug resistance was determined by a simple genetic system which allowed for up to three genes, each with two alleles, to give a maximum of 27 genotypes associated with one drug or three genotypes for each of three drugs. The model was validated against egg counts, L3 counts on pasture and host mortalities observed in a grazing trial, however, aspects of the model such as the development of drug resistance and use of the model in a variety of climatic zones have yet to be tested against field observations. The model was used to examine the impact of grazing management and capsule use on anthelmintic resistance and sheep production over 20 years using historical weather data. Predictions indicated that grazing management can play a dominant role in parasite control and that capsule use will reduce sheep mortalities and production losses, and in some circumstances will not cause a substantial increase in anthelmintic resistance for up to 5 years.

Seasonal variations in the populations of infective larvae on pasture and the numbers of nematode eggs in the faeces of farmed goats

The levels of nematode egg production in goats and the availability of infective larvae (L3) on pasture were investigated on a dairy unit in New South Wales, Australia. The output of eggs by adult goats was always above 300 epg. The profile of the graph of larval availability in herbage paralleled those for temperature and rainfall, suggesting that larval peaks occurred when the temperature and availability of moisture were optimal. The dominant genus was Trichostrongylus, followed by Haemonchus, then Ostertagia. A larger proportion of Haemonchus larvae in the cultures of faeces were collected during the summer months.

Population dynamics of Trichostrongylus colubriformis in sheep: the effect of infection rate on loss of adult parasites

Rejection of adult T. colubriformis was examined in lambs given 2000, 1124, 632 or 200 L3 day-1, 5 days per week for up to 20 weeks. Rejection of adults began at approximately the same time for the first three infection rates, and took about 9 weeks to complete. Approximately 20% of adults were rejected by week 10 of infection at the higher infection rates and it was estimated that rejection commenced at about week 7. This coincided with a decline in establishment of larvae to about 1%. For the low infection rate, there was a delay of about 5 weeks before adult worm rejection commenced and reached equivalent levels to those observed at the higher rates. This delay also coincided with an estimated 5-week delay in larval establishment declining to 1% at the low rate.

Population dynamics of Trichostrongylus colubriformis in sheep: model to predict the worm population over time as a function of infection rate and host age

The developing immunity of sheep to Trichostrongylus colubriformis infections was described by a mathematical function. The rate of adult establishment was assumed to be a measure of the host's acquired immunity to this parasite. Prediction of establishment from infection rate and host age was used to estimate worm burden, worm rejection and arrested development.

Population dynamics of Trichostrongylus colubriformis in sheep: the effect of host age on the establishment of infective larvae

Sheep, reared worm-free in pens and aged 12-36 weeks, were infected each weekday with 2000 infective T. colubriformis larvae (L3). Establishment was measured at various times during the course of infection and the rate of development of resistance to new infection was found to be faster in older than in younger hosts. In addition arrested development at the exsheathed L3 stage was found to be less marked in older hosts. Sheep which had experienced natural infection on pasture up to 20 weeks of age before exposure, in pens, to the same experimental infections as their worm-free counterparts showed similar immune responses. However, at 36 weeks of age, pasture-reared sheep had acquired a high level of resistance to infection with T. colubriformis.

Development and survival of infective larvae of Haemonchus contortus and Trichostrongylus colubriformis on pasture in a tropical environment

A trial to determine the seasonal pattern of egg hatching and larval survival on pasture was carried out in representative wet and dry zones of Fiji. Fourteen plots were established on parasite-free pasture at each of two sites. One plot at each site was contaminated every month with faeces from naturally infected goats containing a known proportion of Haemonchus contortus and Trichostrongylus colubriformis eggs. Pasture was sampled at regular intervals after contamination and infective larvae identified and counted. Larvae of both species developed throughout the year in the wet zone but development was more sporadic in the dry zone. Larval counts generally declined to below detectable levels within 9 weeks of contamination between September and March but longevity increased during the cooler weather from April to August. The comparatively short larval survival times noted in this experiment may present opportunities for manipulation of parasite population dynamics in the wet tropics.

Anhydrobiosis in the infective juveniles of Trichostrongylus colubriformis (Nematoda: Trichostrongylidae)

The infective juveniles of Trichostrongylus colubriformis can survive exposure to 0% relative humidity after desiccation at higher relative humidities. During rehydration there is a lag phase before recovery. The infective juveniles are thus capable of anhydrobiosis. Removal of the sheath does not affect desiccation survival but does affect the duration of the lag phase. Morphological changes during the lag phase are described. The appearance of birefringence in the muscle cells during the anomalous shrinkage which occurs during the lag phase is considered to reflect physiological changes necessary for the recovery of normal muscle function.

The establishment and development of Trichostrongylus colubriformis in goats

Twelve goats were inoculated with 20,000 infective larvae of Trichostrongylus colubriformis and two were killed on each of Days 4, 7, 11, 14, 18 and 21 after inoculation (DAI). The number of worms that established, and the site of development were recorded. Most of the worms established within the first 3 m of the small intestine. There was little relocation or loss of nematodes after establishment. The worms started to migrate from the mucosa to the lumen 11 days after infection. At 4 DAI, 63% of the worms were still at the late L3 stage; the remainder of the worm population had completed the third moult to the L4 stage. The population at 11 DAI comprised largely young adults. When 21 DAI was reached, about 57% of the female worms had eggs in their uteri.

Faecal water content and egg survival of goat gastro-intestinal strongyles under dry tropical conditions in Guadeloupe

Faeces from naturally infected goats were deposited on a natural grassland during the dry season in Guadeloupe (French West Indies) at different times throughout the day. The grass was either 7 or 20-30 cm tall. After a period of between several hours and 7 days, the number of viable strongyle eggs and the faecal water content were measured. Faecal temperature was recorded continuously. Faecal temperature was greater than 40 degrees-45 degrees C at midday and dehydration was rapid between 8 a.m. and 2 p.m. Egg mortality was greater on short than on tall grass and higher in morning than in evening deposits. Minimal faecal water content during the first 36 h explained the 74%, 55% and 38% mortality rate for eggs of Oesophagostomum columbianum (OC), Haemonchus contortus (HC) and Trichostrongylus colubriformis (TC), respectively. In all, 5%-22% of the eggs of the latter species remained viable in a state of anhydrobiosis after 7 days on the ground. A delay of only 2 days between goat departure and irrigation would be sufficient to ensure that greater than 95% of O. columbianum and H. contortus eggs and 70% of T. colubriformis eggs are destroyed.

The role of parasite fecundity and longevity in the success of Trichostrongylus tenuis in low density red grouse populations

The prevalence of the caecal threadworm Trichostrongylus tenuis in red grouse in the north of Scotland was high despite low grouse densities. Prevalence, intensity and aggregation of threadworms was higher in old than in young grouse. Infections were long-lasting: populations of adult worms could survive for over 2 years in grouse, with little mortality. Parasite egg output decreased with the age of a worm population, largely as a result of a decrease in the fecundity of ageing female worms. Seasonal variations in worm fecundity were also evident. However, there was no evidence of an intensity-dependent decrease of worm fecundity with increasing worm numbers in either captive or wild grouse. The long life and high reproductive capacity of T. tenuis probably contribute to its effective transmission and high prevalence.

Factors affecting the establishment of the caecal threadworm Trichostrongylus tenuis in red grouse (Lagopus lagopus scoticus)

The resistance of captive reared red grouse to Trichostrongylus tenuis was measured as the proportion of ingested infective 3rd-stage larvae which failed to develop to adult worms. Individual grouse showed wide, repeatable variations in resistance. Seasonal variations may also have occurred. Red grouse acquired little or no effective immunity to reinfection after challenge with a single dose of infective larvae. In trickle-dosed grouse, however, adult worms weakly inhibited the establishment of incoming larvae in an intensity-dependent fashion. Despite this, the proportion of larvae established in trickle infections was similar to that in single-dose challenges.

Development of resistance to Trichostrongylus colubriformis in goats

A high level of host resistance to Trichostrongylus colubriformis was established in a group of Saanen goats infected weekly with 10,000 infective larvae for 10 weeks, treated with an anthelmintic and then challenged twice with 50,000 larvae, with a further anthelmintic treatment 28 days after the first challenge. A second group of goats exposed only to the two challenge infections developed worm burdens intermediate between, and significantly different from, the first group and a third control group given only the second challenge infection. Nematode fecundity (eggs per female) and male:female ratios were also significantly decreased in the first two groups. Faecal egg counts and worm counts were highly correlated and this relationship was dominated by the influence of the number of eggs/female nematode. The goats were approximately 9 months old at the start of the experiment.

Effects of temperature and relative humidity on development and survival of the free-living stages of Trichostrongylus colubriformis, T. rugatus and T. vitrinus

Temperatures of 4 and 10 degrees C reduced the rate of hatching of eggs and the development of the pre-infective stages of T. colubriformis and T. rugatus, but not T. vitrinus. Eggs of T. rugatus hatched at 4 degrees C, but did not develop to the infective stage. At 20 and 30 degrees C, no differences were detected between species in egg hatching or the development of infective larvae. Third-stage larvae of T. rugatus survived for significantly longer periods of time at 33 and 56% relative humidities at a temperature of 20 degrees C than did either T. colubriformis or T. vitrinus. At 30 degrees C, differences were less marked, but T. rugatus and T. colubriformis survived longer than T. vitrinus. The results are discussed in relationship to the known distribution of these species in eastern Australia.

Development and survival of the free-living stages of Trichostrongylus tenuis, a caecal parasite of red grouse Lagopus lagopus scoticus

In the laboratory, yields of infective 3rd-stage larvae of Trichostrongylus tenuis were determined by temperature when moisture was adequate. On the moor, eggs in caecal faeces did not develop to infective larvae during the winter. In summer, development did occur and yields of infective larvae were related to temperature: greatest yields were obtained when mean monthly soil surface temperatures ranged between 7.6 and 10.3 degrees C. Rainfall was unimportant, as was desiccation. During summer the mean monthly mortality of infective larvae was 44% but over winter survival was negligible.

Trichostrongylus colubriformis: epithelial cell migration in the proximal and distal small intestine of infected rabbits

In Trichostrongylus colubriformis-infected rabbits, epithelial cell migration rates and cell transit times along the villi were compared by radioautography on histological slides to normal values from noninfected small intestine. Regions of gut with high (upper jejunum) and low (ileum) burdens of worms were both examined. In the control rabbits, the estimated values for the cell migration rates in the proximal and distal parts of gut were respectively 5.8 and 2.8 microns/hr. Seventy-two hours after the thymidine injection, the labeled epithelial cells were near the tip of the villi in the jejunum whereas only 60% of the villous length was labeled in the ileum. In the infected rabbits, the presence of T. colubriformis was associated with a two-fold increase of the cell velocity, in the main site of infection. Although less prominent than in the proximal region, a significant acceleration in the cell migration was also noticed in the ileum. The cell transit time was markedly reduced in the parasitized jejunum, but no variation of this parameter was found in the distal part of gut. These changes in the dynamics of epithelial cells in both regions of the gut appeared to underlie the morphological and enzymological changes of the parasitized mucosa. They particularly contribute to create an adaptive region in the small intestine beyond the main site of infection.

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.

The formation of the first-stage cuticle within the egg of Trichostrongylus colubriformis

The ultrastructure of the formation of the first-stage cuticle within the egg of Trichostrongylus colubriformis is described. The first indication of cuticle formation was the formation of the epicuticle external to the epidermal plasma membrane. This becomes thickened to form the epicuticle of the fully formed cuticle. Microtubules and microfilaments associated with the epidermal plasma membrane were observed during the early phases of cuticle formation. The basal zone was secreted into the space between the epicuticle and the epidermal plasma membrane, followed by the formation of the cortical zone. The basal zone had a striated appearance during the earliest stages of its formation.

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.

Larval development test for detection of anthelmintic resistant nematodes

The growth, using freshly cultured Escherichia coli with ampicillin or heat-treated lyophilised E coli as a food source, of the larvae of the mouse nematode Nematospiroides dubius and the infectivity of resulting third stage larvae were determined. Concentrations of E coli between 0.5 and 1 mg dry weight ml-1 permitted optimal larval development for both N dubius and Trichostrongylus colubriformis. Development of larvae of susceptible and cambendazole-resistant strains of Haemonchus contortus in thiabendazole solutions showed clear differences between the strains and the larval development test was more sensitive than the egg hatch test. The test also detected a levamisole resistant strain of H contortus, although the degree of resistance could not be adequately measured. It is concluded that the test can be run with any anthelmintic to which resistance is suspected.

Further studies on the development and availability of infective larvae of bovine gastrointestinal trichostrongylids on pasture in eastern Nigeria

The dynamics of pasture populations of infective larvae (L3) of Cooperia, Haemonchus and Trichostrongylus species were studied at Nsukka, eastern Nigeria, during April to November 1984. Six paddocks were contaminated artificially and one was contaminated naturally. Five of the paddocks, P1-P5, were sequentially contaminated with faeces of naturally infected cattle at approximately 4-6-weekly intervals. Paddocks P6 and P7 were repeatedly contaminated every 4-6 weeks artificially and by the naturally infected cattle, respectively. Larval development and survival occurred very readily during the wet season (April-October) but apparently ceased in November at the start of the dry season. Larval migration, however, occurred not only during the rains but also during the first 4 weeks of the dry season. Single contaminations during the rains quickly gave rise to single waves of infestation which also declined rapidly, in spite of the continuously favourable conditions for larval development and survival. The repeated contaminations produced three and four distinct and relatively short-lived larval peaks, respectively, with the first three peaks on both paddocks, namely the May, July and September/October peaks, being coincident. The four waves of herbage infestation on P7, which occurred at approximately 4-5 weekly intervals, were considered to have originated from four separate generations of the three trichostrongylids. However, Trichostrongylus sp. predominated in the first (May) peak while Cooperia and Haemonchus dominated the later peaks.

Feed intake of grazing calves exposed to trichostrongyle infection and treated with the morantel sustained release bolus

Herbage intake was measured in two groups of 20 first-year grazing cattle. The animals in one group each received a morantel sustained release bolus at turnout to control nematode parasitism and the animals in the other group remained untreated. The latter group showed a mean peak faecal egg count of 655 eggs per gram (e.p.g.) in October associated with high serum pepsinogen concentration and clinical signs of ostertagiasis, compared with a peak of 119 e.p.g. in the treated group which remained in good health. In September the daily voluntary feed intake of the untreated animals was significantly depressed (94 g kg-1 body weight vs. 77 g kg-1 P less than 0.001), but no difference in digestive efficiency was recorded between the two groups. This difference in feed intake was associated with a 47 kg mean live weight advantage of the treated animals at housing.

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.

Relative contributions of late dry-season and early rains pasture contaminations with trichostrongyle eggs to the wet-season herbage infestation in eastern Nigeria

Two worm-free grass paddocks, P1 and P2, were artificially contaminated in March and April-May, respectively, with bovine faeces containing known numbers of trichostrongyle (mainly Cooperia, Haemonchus and Trichostrongylus spp.) eggs in order to determine the relative contributions of late dry-season and early rains pasture contaminations to the wet-season herbage larval infestation in Nsukka, eastern Nigeria. The resulting herbage infestation was assessed by means of larval counts and tracer studies. A sudden rise in herbage infestation occurred simultaneously in both paddocks in late April, this apparently being determined by the onset of the first substantial rainfall of the wet season. Peak infestations in both paddocks also occurred simultaneously in May. The infestation in P1 was much larger, and the larval population persisted longer, than that in P2 and later gave rise to a second smaller peak in June. No L3 were recovered in herbage samples from either of the paddocks after the third week of July. Both paddocks were infective to goats in May-June, while P2 was also infective in July-August. The results suggest that in the Nigerian derived savanna the initial wet-season herbage infestation in pastures grazed by infected cattle during the dry and wet seasons will consist of L3 from late dry-season and early rains pasture contaminations, the former being the major contributor to the infestation. Consequently, pastures contaminated during the late dry season may not be safe for susceptible animals to graze at the start of the succeeding rainy season.

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.

Anthelmintic activity of continuous low doses of fenbendazole into the rumen of sheep

Fenbendazole (FBZ) was continuously infused for 30 days into the rumen of 103 lambs which had mature or developing benzimidazole-susceptible or thiabendazole-resistant Haemonchus contortus and susceptible Trichostrongylus colubriformis infections. Ovicidal, larvicidal and adulticidal activities were exhibited against benzimidazole-susceptible and benzimidazole-resistant H. contortus worms by FBZ at a dose level of greater than or equal to 0.2 mg kg-1 body weight day-1. Reasonably consistent high level efficacy against H. contortus was obtained with dose levels greater than 0.4 mg kg-1. Excellent control of susceptible T. colubriformis worms was achieved with the lowest dose tested of 0.4 mg kg-1 day-1. The intraruminal infusion critical study method is a tool to determine the feasibility of incorporating a candidate anthelmintic compound in a continuous sustained-release rumen device formulation. The anthelmintic profile of FBZ obtained by low-level intraruminal administration suggests that it would be a potential candidate.

Studies on the pathogenesis of Trichostrongylus colubriformis in Mongolian gerbils (Meriones unguiculatus)

Sixty Mongolian gerbils were infected with 1500 T. colubriformis larvae. During the experiment, the animals were weighed regularly and faeces were examined for parasite eggs. Groups of 5 gerbils were necropsied at intervals throughout the experiment (60 days) and worm burdens, serum protein concentrations and packed cell volumes were ascertained. It was found that gerbils infected with T. colubriformis showed inappetence and loss in body weight. A modest hypoalbuminaemia also developed but there was no evidence of alterations in packed cell volumes in the infected animals. The results indicated that the pathogenic effects of T. colubriformis in the gerbil were similar to those observed in sheep, one of the natural hosts for this parasite. The conclusion was drawn that the Mongolian gerbil is a suitable laboratory host for T. colubriformis and is superior to other small animal hosts previously described.

Regulation of Trichostrongylus vitrinus and T colubriformis populations in naturally infected sheep in the Netherlands

During a study on the epidemiology of gastrointestinal helminth infections in sheep at the University of Utrecht (1974 to 1977) information was obtained on the population regulation of small intestinal Trichostrongylus species. Comparison of the cumulative burdens in tracer lambs with those in flock lambs showed an accumulation of Trichostrongylus species in the latter until autumn followed by an expulsion of worms. The dominating species, T vitrinus, was expelled earlier than T colubriformis. The proportion of inhibited early third stage Trichostrongylus species larvae in tracer lambs was low, but it tended to increase in autumn. In most flock lambs the levels of inhibition were also low. In non-lactating ewes, populations consisted predominantly of inhibited early third stage larvae but when adults were present they tended to be mainly T colubriformis. Similar populations were seen in some lactating ewes. In other lactating ewes Trichostrongylus species populations with low proportions of inhibited early third stage larvae which were dominated by T vitrinus, indicated a relaxation of resistance around parturition.

Monthly and seasonal fluctuations in abomasal nematode worm burden of naturally infected cattle in Austria

From October 1980 until September 1981, 191 abomasa of naturally infected cattle were obtained from the slaughter-houses of Mürzhofen and Graz in the province of Styria, and of Wiener Neustadt in the province of Lower Austria and examined to study the monthly and seasonal occurrence of hypobiosis in Ostertagia ostertagi and Trichostrongylus axei. Infections in pastured animals (cows, steers and heifers) were compared with those in stabled animals (bulls fed on green forage and silage). Except for 10 bulls fed on silage all animals were parasitized. The results indicate that in Austria inhibited early fourth stage larvae of Ostertagia accumulate during the winter. Pastured animals showed a higher intensity of infection than stabled animals. Cows less than 3 and greater than 5 years of age had higher worm counts than those between these ages. There was a simultaneous accumulation of third stage larvae and adult worms of Trichostrongylus axei in autumn in steers and heifers. In bulls fed on green forage a higher adult worm burden was seen in winter. No significant correlation existed between the number of worm nodules in the abomasal mucosa and the various developmental stages of the worms. The count of in-utero immature eggs was higher in stabled than in pastured animals. No significant correlation between the number of in-utero eggs and that of female worms was noticeable. Numbers of mature Ostertagia eggs rose in spring, while in fall and winter the immature eggs predominated. An increase in the uterine egg count was observed in Trichostrongylus in autumn. Statistical analyses of our data show that the most decisive factor influencing the numbers of inhibited and adult Ostertagia as well as in-utero eggs is the system of animal management.

Haemonchus contortus and other trichostrongylid infections in parturient, lactating and dry ewes

From autumn (April) to spring (November), groups of pregnant and dry Merino ewes grazed pasture contaminated with infective larvae of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis. A periparturient rise in faecal egg counts occurred in the pregnant ewes, following the maturation in early spring of arrested fourth-stage larvae of H. contortus and failure of pregnant ewes to expel the resulting adult worms. Peak egg counts were seen in September, just before lambing. In dry ewes, egg counts were slightly elevated at the time of the rise in the pregnant ewes, but fell to very low levels as adult worms developing from previously arrested larvae were expelled. Lactating ewes acquired greater burdens of O. circumcincta and T. colubriformis than did dry ewes, but were equally refractory to new infections with H. contortus. It is therefore suggested that the impairment of immunity to helminth infection seen in reproductive ewes may be more specific than was previously envisaged.