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.     

Arrested development of Ostertagia ostertagi: effect of the exposure of infective larvae to natural spring conditions of the humid pampa (Argentina)

The aim of this study was to determine the effect of environmental conditions and the time of exposure to the conditions required for Ostertagia ostertagi to become inhibited in development at the early fourth larval stage in the host. Two comparable experiments were conducted from September to January, experiment I in 1997-1998 and experiment II in 1999-2000. Twenty-thousand third-stage larvae (L3), freshly obtained from coprocultures, were spread in different parasite-free grass plots at the beginning of September, October and November in each experiment and exposed to environmental conditions throughout spring and early summer. Duplicate plots for each exposure period were grazed for 3 days by two dewormed tracer calves after 1, 2, 3, 4 weeks of exposure during the corresponding month, and the remaining plots were grazed for 3 days at monthly intervals until the end of the experimental period. For each month in both experiments, control animals were inoculated orally with 20,000 L3 newly recovered from coprocultures (week 0 animals; infection controls). The control and tracer calves were sacrificed and their parasite burdens analysed. The time required to obtain greater than 50% inhibited larvae (IeL4) in the tracer animals during September and October was 3 weeks, whereas during November around 60% of the parasites were inhibited after one week of exposure. During the period tested, greater than 50% inhibition was found in concurrence with a photoperiod ranging between 13 and 14 h. The highest proportion of IeL4 (75% average) in the animals was found concomitant with a 14 h 43 min photoperiod. A high correlation between the percentage of inhibition and day length was established (0.870 p < 0.001 and 0.815 p < 0.001 for experiment I and II, respectively). In both years, the capacity for developmental arrest was lost by the end of December, when the photoperiod begins to decrease, suggesting either a disappearance of the induction stimulus, or that an excess of the stimulus could block the mechanism of inhibition. The induction time was extended 2 weeks in all months tested when the coprocultures were maintained in the dark (experiment II), suggesting that accumulation of the light stimulus contributes to shortening of the induction time. The data presented here would suggest that photoperiod is a key environmental factor for the induction of hypobiosis.     

The grazing behaviour of cattle in relation to the sampling of infective nematode larvae on pasture

A method of sampling pasture to estimate the numbers of infective nematode larvae to which grazing cattle were exposed was based on the grazing patterns and behavioural activities of two groups of cattle and was compared with other sampling techniques. Each group of cattle consisted of six permanent members, two members fistulated at the oesophagus and one worm-free tracer calf. Grazing time and the area where grazing occurred was not significantly different for tracer calves, fistulated cattle and permanent group members, and there was no relationship between grazing time and the live weight of cattle. Grazing time, the percentage of paddock area grazed intensively and the percentage of the paddock not grazed varied with season. The most intensively grazed areas were always visited between first light and the first rest period during mid-morning, and the plant parts and pasture species eaten could easily be identified by visual examination of these areas of the paddock. Larval recoveries per 100 g pasture ingested were estimated for comparison with the grazing area method using two other manual pasture sampling methods, a sampling method using tracer calves and one using fistulated calves. Correlations between these methods were not consistent but indicated that, given the small number of data sets, all methods were sensitive enough to estimate larval availability on pasture with the exception of the tracer calf method in the overstocked 3.4-ha paddock.     

Pathogenesis of simulated natural infections with Ostertagia ostertagi in calves

The possibility of a mucosal hypersensitivity reaction and its relationship to the pathogenesis of simulated natural infections with Ostertagia ostertagi were studied in calves. Four groups of 4 calves each were used. One group was used as noninfected control; a 2nd group was given increasing doses of infective larvae; a 3rd group was given increasing doses of larvae and these were removed by succeeding treatment with an anthelmintic; and a 4th group was given an initial dose of larvae which was then eliminated with an anthelmintic. All calves given larvae became sensitized, as shown by an intradermal skin test. The continuously infected calves had significantly (P less than 0.05) higher fecal egg counts, eosinophil counts, plasma pepsinogen values, and worm burdens and significantly (P less than 0.05) lower lymphocyte counts than did the other groups of calves. These animals also had the most extensive mucosal pathologic changes. The group given intermittent larval challenge exposures followed by an anthelmintic showed decreased lymphocyte values, but these were not significant. Plasma pepsinogen values of this group increased between every challenge exposure and treatment, a 3-day period. This indicated that a mucosal hypersensitivity reaction had occurred in these calves at these times, because they were shown to have been sensitized, and challenge-exposure infections were not present for sufficient time to have produced direct pathologic effects. It therefore seems that a part of the pathologic changes in O ostertagi infections may be the result of the continuous challenge exposure experienced by the animals through a constant intake of larvae from pasture and the intestinal reaction to this challenge exposure.     

Activity of fenbendazole against inhibited early fourth-stage larvae of Ostertagia ostertagi

The efficacy of fenbendazole (Panacur, Hoechst-Roussel) against inhibited early fourth-stage larvae of Ostertagia ostertagi and other nematodes of the abomasum and intestinal tract was investigated in naturally infected, yearling cattle in April 1978. The time when peak levels of inhibited larvae occurred was determined by epizootiologic study which began in November 1977. All animals were removed from pasture and maintained free from further helminth infection until slaughter (19 to 21 days). The fenbendazole liquid suspension was administered as an oral drench at dose level of 10 mg/kg to 10 animals and then at dose level of 15 mg/kg to an additional 10 animals at 10 days after removal from pasture. Eleven animals were maintained as untreated controls. In cattle given the dose of 10 mg/kg, the following reductions were observed: O ostertagi adults--100%, developing stages--80%, and inhibited larvae--97%; other worm genera in the abomasum and nematodes of the intestinal tract--100%. In the cattle given the larger dose, the following reductions were observed: O ostertagi adults--100%, developing stages--98%, and inhibited larvae--99%; other worm genera in the abomasum and nematodes of the intestinal tract--100%.     

Exsheathment of Ostertagia ostertagi infective larvae following exposure to bovine rumen contents derived from low and high roughage diets

The objective of this study was to characterize the exsheathment kinetics of Ostertagia ostertagi infective larvae (L3) following in vivo exposure to bovine rumen contents derived from low and high roughage diets. O. ostertagi L3 were placed in disposable dialysis bags and incubated for various time points between 0 and 360 min in the rumen of a fistulated steer maintained on a 71% grain diet or a 100% grass diet. The maximum percentage of exsheathed L3 was observed 120 min post-exposure to grass-derived rumen contents, while maximum exsheathment for L3 exposed to grain-derived rumen contents did not occur until 360 min. This work provides the first report of the in vivo exsheathment kinetics for O. ostertagi in its bovine host. Results of this study also support earlier reports that rumen pH may affect the exsheathment efficiency of abomasal trichostrongylids.     

Factors influencing rain splash dispersal of infective larvae of Ostertagia ostertagi (Trichostrongylidae) from cow pats to the surroundings

Laboratory experiments were conducted to investigate rain splash dispersal of infective Ostertagia ostertagi larvae (L3) from cow pats to the surroundings during simulated rainfall. Simulated rainfall on dry cow pats resulted in dispersal of only very few L3. When cow pats were watered prior to the experiments, many infective larvae were stimulated to migrate up onto the surface of the pats, where they might be hit by water drops. When pre-watered pats were hit by falling drops, a considerable proportion of L3 was translocated. More than 90% of the translocated larvae were transported passively by splash droplets and only a minor part of the larvae migrated actively in water films or were transported passively by water run-off to the soil surrounding the pats. A reduction of the fall height of the simulated raindrops resulted in a significant reduction of the number of translocated L3. When cow pats were hit by drops of diameters between 2 and 5 mm, it was found that small raindrops were just as effective as large drops in splash dispersal of L3, provided the amount of rain was the same; but individual small drops were less effective in this respect than individual large drops. When splash droplets could travel freely, most of the splash-dispersed L3 were found within a horizontal distance of 90 cm from the pats; this applied for all sizes of drops. Most splash droplets carrying L3 were travelling within a vertical distance of 30 cm from the ground.     

Efficacy of fenbendazole against inhibited larvae of Ostertagia ostertagi in yearling cattle

Efficacy of fenbendazole, at doses of 7.5 and 10.0 mg/kg of body weight, against inhibited early 4th-stage larvae of Ostertagia ostertagi and other nematodes of the abomasum and intestinal tract, was investigated in naturally infected yearling heifers in late May 1982. In Louisiana, this is near the end of the period (March to May) in which maximal numbers of inhibition-prone larvae are acquired. The mean numbers of O ostertagi in 10 untreated control cattle were: adults, 4,880; developing 4th-stage larvae, 12,546; and inhibited early 4th-stage larvae, 167,931. At the 7.5 mg/kg dose level (10% liquid suspension) in 10 cattle, percentage reduction of O ostertagi in comparison with controls was: adults, 95.7%; developing 4th stages, 91.1%; and inhibited 4th stage, 55.0%. Percentage reductions of other genera were as follows: abomasum--Trichostrongylus axei, 99.6%; Haemonchus sp, 95.1%; intestinal tract--Cooperia spp, 97.8%; Trichostrongylus colubriformis, 100.0%; and Oesophagostomum radiatum 4th stage and adults, 100.0%. At the 10.0 mg/kg dose (10% liquid suspension) in 11 cattle, the percentage reduction of O ostertagi in comparison with controls was: adults, 98.6%; developing 4th stages, 92.9%; and inhibited 4th stage, 80.0%. Percentage reductions of other genera were: abomasum--T axei, 99.9%; Haemonchus sp, 98.8%; intestinal tract--Cooperia spp, 99.3%; T colubriformis, 100.0%; and Oes radiatum 4th stage and adults, 100.0%. Variability of efficacy against inhibited larvae was observed, particularly at the 7.5 mg/kg dose; at this dose, 7 of the 10 heifers in the group yielded in excess of 54,000 surviving larvae.(ABSTRACT TRUNCATED AT 250 WORDS)     

Investigation of maturation requirements for Ostertagia ostertagi and Cooperia oncophora larvae.

Infection of parasite-free six to eight week old calves with doses of 50,000 mixed Ostertagia ostertagi and Cooperia oncophora larvae varying in age from seven to 42 days did not reveal a significant larval maturation requirement.     

Induction of nematode-trapping organs in the predacious fungus Arthrobotrys oligospora (Hyphomycetales) by infective larvae of Ostertagia ostertagi (Trichostrongylidae)

Laboratory experiments were designed to study the influence of temperature, concentrations of nematodes, oxygen tension, light, and nutrient levels, on the induction of nematode-trapping hyphal nets in the predacious fungus Arthrobotrys oligospora. When induced by infective Ostertagia ostertagi larvae, a maximum number of nets was produced at 20 degrees C, at which temperature nets in surplus were produced at larval concentrations up to 1,000 larvae per cm2. A. oligospora did not produce nets in an anaerobic atmosphere containing 21% CO2 (v/v), and net induction was suppressed to a certain degree by exposure to light. The composition of the medium had an important influence on the saprophytic growth and the net-forming capability of A. oligospora as a maximum number of nets was induced at a relatively low concentration of corn meal supporting the relatively sparse mycelium. It was shown that a proportion of trapping nets in A. oligospora maintained their trapping potential for more than 7 weeks when the temperature was below 25 degrees C. Induction of nematode-trapping organs in A. oligospora is discussed in relation to control of infective nematode parasite larvae in cow pats.     

Synergistic influence of Ostertagia ostertagi and Trichostrongylus axei on Ostertagia ostertagi larval inhibition and abomasal lesions in cattle

Parasite-free 4-month-old calves were inoculated with Ostertagia ostertagi and/or Trichostrongylus axei followed 6 weeks later by increasing doses of O ostertagi for 8 weeks. Clinical signs of parasitism, fecal egg counts, and plasma pepsinogen concentrations were monitored, and gross lesions and parasite burdens were determined postmortem. Clinical signs of parasitism were not observed and weight gains were not affected in experimentally infected calves. In calves infected with O ostertagi, mean plasma pepsinogen concentrations were greater than for control calves and were diagnostically significant 4 weeks after inoculation and during the last 4 weeks of serial inoculations with O ostertagi. In calves that were given O ostertagi and T axei, abomasal pH was significantly increased, and abomasal lesions were more pronounced than in control calves or in calves inoculated with only O ostertagi or T axei. Abomasal lymph nodes were enlarged in all parasitized calves; other lymph nodes in the calves inoculated with both O ostertagi and T axei were usually smaller than in calves inoculated with only O ostertagi or T axei. Numbers of O ostertagi-inhibited larvae were small in all inoculated calves, but the percentage inhibition was significantly greater in calves inoculated with both O ostertagi and T axei. The percentage inhibition was 3.53% for the O ostertagi-inoculated calves and 7.07% for calves inoculated with both O ostertagi and T axei. These percentages indicated a synergistic effect of concurrent abomasal parasitism, whereas a synergistic effect on T axei worm burden was not observed. The low percentage of larval inhibition indicated that factors other than host resistance are involved in naturally occurring pretype II ostertagiosis.