Efficacy of different anticoccidials against experimental coccidiosis in large white turkeys.

Two trials were conducted to compare the efficacy of currently approved anticoccidials for turkeys against challenge using a field isolate of mixed Eimeria species; E. adenoides, E. gallopavonis, and E. meleagrimitis. Poults in wire-floored cages were fed unmedicated diets from day-old to 3 wk of age. Diets were supplemented with either amprolium (AMP, 125 mg/kg), butynorate (BUT, 375 mg/kg), monensin (MON-60, 60 mg/kg; MON-100, 100 mg/kg), halofuginone (HAL; 3 mg/kg), zoalene (ZOA; 125 mg/kg), or sulfadimethoxine plus ormetoprim (SUL + ORM, 62.5 mg/kg and 37.5 mg/kg, respectively). After 2 days on the test diets, poults were individually weighed and inoculated with sporulated coccidial oocysts from the field isolate. Total fecal collections were obtained for Days 0 to 5 and 6 to 10 to estimate oocyst output. At 10 days postinoculation, the birds were individually weighed and killed to determine severity of intestinal lesions. The HAL and MON were most effective and AMP, ZOA, and SUL + ORM were least effective in maintaining weight and in reducing the severity of intestinal lesions. All the coccidiostats tested reduced oocyst passage, but poults fed HAL produced fewer oocysts. The results demonstrated differences in efficacy among anticoccidials with the more recently approved drugs providing the best protection against coccidiosis.

Ultrastructural studies of the effects of amprolium and dinitolmide on Eimeria acervulina macrogametes.

Medication of chicks with 125 ppm amprolium or dinitolmide adversely affected oocyst sporulation of Eimeria acervulina (Weybridge strain). Dinitolmide delayed oocyst production and no oocyst wall formation was seen up to 168 h post infection. Both drugs caused large numbers of abnormally small wall-forming bodies to be produced in the macrogametes. In amprolium-fed chicks, abnormal oocyst wall formation was seen. It was concluded that the main drug action was against wall forming bodies of type 2.

Effect of amprolium and dinitolmide on sporulation of oocysts of field isolates of Eimeria acervulina.

A laboratory strain of Eimeria acervulina and 9 field isolates consisting principally of E. acervulina were tested for sensitivity to amprolium (125 p.p.m.) or dinitolmide (125 p.p.m.) in the food and for effects of the drugs on sporulation of oocysts. Judged by weight gains and lesion scores, medicaments were only partially effective against the 9 field isolates, but were highly effective against the laboratory strain. Oocysts were produced in all the infections but the percentage sporulation of oocysts from field isolates was much higher than sporulation of oocysts of the "drug sensitive' laboratory strain. These results show that coccidia that are resistant to either amprolium or dinitolmide are able to cause lesions in the presence of the drugs and the oocysts that are produced will sporulate normally.

Ultrastructural changes in the macrogamete and early oocyst of Eimeria maxima resulting from drug treatment.

The ultrastructure of the macrogamete and developing oocyst of Eimeria maxima (Weybridge strain) was examined in the intestinal cells of chicks fed 3 different anticoccidial drugs. Amprolium at 125 p.p.m., arprinocid at 35 p.p.m. and dinitolmide at 250 p.p.m. caused considerable morphological abnormality and incomplete development of the wall-forming bodies of Type 2 (WFB II), which did not appear able to participate in oocyst wall formation. The wall-forming bodies of Type 1 (WFB I) were able in each case to participate in oocyst wall formation although amprolium and dinitolmide produced morphological abnormalities in them. In birds medicated with dinitolmide, the outer layer of the oocyst wall was formed initially at opposite poles of the macrogametes rather than as a uniform layer. Other abnormalities resulting from drug treatment are reported and some evidence that intravacuolar tubules may be formed by the parasite pellicle is presented.

The anticoccidial effects of amprolium, dinitolmide and monensin against Eimeria maxima, E, brunetti and E. acervulina with particular reference to oocyst sporulation.

Chicks infected with the Weybridge strains of Eimeria maxima and E. acervulina were not protected by the normal levels of amprolium, but the sporulation of the oocysts was inhibited. With higher drug concentrations, fewer oocysts were produced. E brunetti was not so markedly affected, although oocyst sporulation was reduced by the higher dosage levels. The effects were not enhanced by the inclusion of ethopabate. With dinitolmide the phenomenon was not so marked, although oocysts of E. maxima and E. acervulina were reduced in numbers by the normal drug concentration and sporulation was reduced when this level was increased. Much higher drug levels were required to obtain these effects with E. brunetti. Monensin at 120 ppm affected neither oocyst numbers nor their sporulation in any of the species tested. The significance of the effects of anticoccidial drugs on gametogony and sporogony is discussed.

Laboratory studies with some older anticoccidials.

Features of the anticoccidial activity of nicarbazin, amprolium, zoalene, sulphadimidine, diaveridine, Darvisul, spiramycin, chloramphenicol and oxytetracycline have been re-investigated both in vivo and in cell culture using Eimeria tenella. Of the drugs studied, only spiramycin was appreciably coccidiocidal, although nicarbazin and amprolium showed possibly slower coccidiocidal activity. In order to show activity against a particular stage in the life-cycle, higher concentrations of drug than those usually recommended for field usage had in most cases to be used. Under these conditions, parasites were usually inhibited as multinucleate 1st generation schizonts. With delayed medication, effects against 2nd generation parasites were in most cases found, and in many cases, although the parasites never matured to give viable merozoites, the large degenerating forms produced were able to cause extensive tissue destruction and haemorrhage. Methodology in this type of study is discussed in relation to more active and more recent anticoccidials, and some further experiments with robenidine reported.

Relative value of oocyst counts in evaluating anticoccidial activity.

Birds medicated with roxarsone and in another experiment with zoalene in the feed produced higher oocysts counts than unmedicated control birds receiving the same oocyst dose of Eimeria tenella or a mixture of six species (E. tenella, E. necatrix, E. brunetti, E. maxima, E. acervulina, and E. mitvati). These experiments confirm the conclusion that oocyst counts constitute an unsatisfactory and unreliable parameter for judging effectiveness of an anticoccidial even though such increases are a relatively rare occurrence in anticoccidial evaluation experiments.

Sequential use of coccidiostats: effect on development by Eimeria tenella of resistance to amprolium, nicarbazin, Unistat, and zoalene.

A strain of the cecal coccidian of chickens, Eimeria tenella, was propagated serially in chickens fed mash containing amprolium, nicarbazin, Unistat, or zoalene. Each group of chickens received a different coccidiostat on a rotating basis. The strain was propagated through 40 groups of chickens; thus, the strain was intermittently exposed 10 times to each coccidiostat. The end product of this simulated shuttle program of prophylactic anticoccidial medication was a strain resistant to three of the four coccidiostats involved. Resistance to nicarbazin was not evident.