Anthelmintic activities of fenbendazole against Nematodirus helvetianus in cattle: effect on egg production, embryogeny, and development of larval stages

Calves experimentally infected with Nematodirus helvetianus were treated with a single dose of fenbendazole at the rate of 5 mg/kg of body weight. Fenbendazole was found to have an efficacy of 97.4% against adult N helvetianus. Additionally, eggs recovered 12 hours after calves were given the anthelmintic had fewer and frequently abnormal blastomeres. Fewer eggs were evident at 36 hours after treatment, and none was found at 72 hours or later. On the culturing of eggs from manure passed at 12 hours and later, almost no larval stages developed.

Instability of benzimidazole resistance in nematode eggs

The resistance of the eggs of benzimidazole resistant Haemonchus contortus and Trichostrongylus colubriformis is not stable and will decrease in the absence of drug challenge. Dosing animals harbouring resistant H contortus with either thiabendazole or mebendazole selects for nematodes producing thiabendazole resistant eggs. The eggs of a thiabendazole selected strain of H contortus were more resistant to thiabendazole, parbendazole and oxibendazole but less resistant to cambendazole and mebendazole.

A field trial with a new anthelmintic oxfendazole in naturally infected calves

In a field trial on a series of 10 farms involving 116 calves the activity of oxfendazole at a dose rate of 2.5 mg/kg against gastro-intestinal helminths was assessed. On the basis of egg counts and larval differentiation before and after treatment it was shown that the compound had excellent activity against the most common nematodes. Although not proven in this trial, there were indications of some activity against inhibited stages of Ostertagia. Activity against Strongyloides was lower, while for Trichuris and Moniezia the infection rates before treatment were too low to draw any definitie conclusions about the activity of oxfendazole against these parasites.

Resistance of nematode eggs to the ovicidal activity of benzimidazoles

Eggs of benzimidazole-resistant Haemonchus contortus and Trichostrongylus colubriformis are resistant to the ovicidal activity of benzimidazoles. The possiblity is raised of using this as a simple screen for detection of resistance in nematodes of domesticated animals.

Detection of activity for various anthelmintics against in vitro-produced Cooperia punctata

The in vitro-grown parasitic stages of Cooperia punctata were used to evaluate 28 compounds with different kinds and degrees of in vivo activity. Using presumptive and confirmatory tests, it was possible to establish a group order of in vitro potency that compared favorably with an order based on established in vivo use of these compounds. The procedure lends itself to evaluating activity against a given parasitic growth stage and gives a quantitative estimate (range) of the concentration that produces 50% nematode kill. The system was most successful in detecting compounds with in vivo activity for C punctata, followed in order by compounds active against Cooperia spp, other gastrointestinal nematodes of ruminants, and other nematodes of non-bovine hosts. The procedure showed some differentiation between activity against nematodes versus that against cestodes, trematodes, and arthropods. The system permits considerable flexibility in experimental design, thus making possible the acquisition of the particular information desired. In addition to establishing lethal effects on the nematode, the procedure detected compounds with nematode-anesthetizing effects. The results indicate this in vitro system can be used with some expediency as a preliminary screening method in the search for new anthelmintic compounds.

Studies on the role of histamine and 5-hydroxytryptamine in immunity against the nematode Trichostrongylus colubriformis. I. In vivo and in vitro effects of the amines

Infusion of solutions of histamine and 5-hydroxytryptamine into the small intestine of guinea pigs during the fourth larval stage of a primary infection with the nematode <i>Trichostrongylus colubriformis </i>led to significant worm expulsion. Incubation of fourth stage larvae with 5-hydroxytryptamine significantly reduced their excretion of the metabolic end product propionic acid. These results support the hypothesis that histamine and 5-hydroxytryptamine are involved in the expulsion of <i>T.</i><i>colubriformis </i>by immune guinea pigs.

The effect of levamisole on the development of Ostertagia circumcincta in lambs

The majority of third-stage Ostertagia circumcincta larvae exposed to 0·5% levamisole failed to infect lambs even though the worms regained some activity after repeated washings in balanced saline. A low level of efficiency was recorded against 4·5-day-old O. circumcincta when lambs were treated with levamisole at 7·5 mg per kg. Nine-day-old worms from hosts which received anthelmintic on day 4·5 were at the same stage of development as those from untreated control lambs.

The influence of pCO2 and the rate of shaking on the ecdysis of infective Haemonchus contortus cayugensis larvae

Infective Haemonchus contortus cayugensis larvae were stimulated to exsheath in 0·02 m-Na2B4O7. 10H2O saturated with varying concentrations of CO2. Larvae stimulated to exsheath with 40% CO2 gave a higher exsheathment rate than larvae stimulated with 99·95 % CO2. There was a linear relationship between the exsheathment rate and the shaking rate over the range from 40 to 100 c/min when the larvae were stimulated to exsheath with 99·95 % CO2.

The inhibitory effect of CO2 on the ecdysis of infective Haemonchus contortus cayugensis larvae

Infective Haemonchus contortus cayugensis larvae were stimulated to exsheath in 0·02 m-Na2B4O7. 10H2O, into which was bubbled CO2. The larvae under the exsheathing stimulus were found to be coiled and sluggish. The larvae became very active after most of the CO2 was removed by compressed air, oxygen or nitrogen. The exsheathment rate in each of the latter instances was higher than when larvae were exsheathed completely in an atmosphere of CO2. Carbon dioxide is an essential component of the exsheathing stimulus, but the continued presence of a high pCO2 or carbonic reaction products appeared to inhibit larval motility and caused lower exsheathment rates.

The development of Haemonchus contortus in vitro. II. The effect of disulphide-reducing and sulphydryl-blocking reagents on the rate of development to the fourth-stage larvae

Cysteine (20–100 mM) and reduced glutathione (20–80 mM) potentiated the development of the exsheathed third-stage larva of Haemonchus contortus to the fourth stage. The potentiating effect of cysteine was greatest during the short period before and after the exsheathment of the third-stage larva. Two hundred mM cysteine and 1 mM p–chloro-benzoate inhibited development. This inhibition was reversible. Ten mM iodoacetate and 0·5 and 5 mM n–ethyl-maleimide inhibited development of the larvae. The larvae appeared to be most susceptible to the inhibiting effects of these reagents during the period of development approaching the third moult. Development was either little affected or potentiated, when larvae were incubated with these reagents for short periods immediately after exsheathment.