EXPERIMENTAL CYATHOSTOME CHALLENGE OF PONIES MAINTAINED WITH OR WITHOUT BENEFIT OF DAILY PYRANTEL TARTRATE FEED ADDITIVE - COMPARISON OF PARASITE BURDENS, IMMUNITY AND COLONIC PATHOLOGY

Eighteen mixed-breed, naturally infected ponies ranging in age from 1 to 16 yr and four cyathostome-naive ponies reared and maintained under parasite-free conditions ranging in age from 1 to 4 yr were used in t his study. Naturally-infected ponies were treated with 1 dose of iverm ectin (IVM) at 200 mu g kg(-1), followed by a 5-day regimen of oxibend azole (OBZ) at 20 mg kg(-1) to remove existing cyathostome burdens: cy athostome-naive control ponies were treated with IVM alone. The natura lly infected ponies were matched on age and gender, then randomly assi gned to one of three treatment groups of six animals per group; the fo ur cyathostome-naive ponies constituted a fourth group. Following OBZ treatment, Group I Denies were treated with pyrantel tartrate (PT) in their pelleted ration; the remaining ponies received only the pelleted ration. Beginning on experiment Day 3, a daily challenge infection of 10(4) mixed cyathostome larvae was administered orally to ponies of G roup 1, Group 2 and the cyathostome-naive controls. Group 3 ponies ser ved as unchallenged controls to determine residual parasite burdens fo llowing IVM/OBZ treatment. Necropsy examinations were performed on thr ee Group 3 ponies on Day 1; the remainder of the necropsy examinations began on Day 41. Cyathostome burdens were evaluated by recovery of la rvae and adults from the luminal contents. by digestions of the intest inal mucosa, and by mural transillumination of full-thickness intestin al sections. Differences in postchallenge clinical responses were also compared. Necropsy examinations included comparisons of grossly visib le inflammation of the large bowel, weights of biopsy specimens from e ach region, and histologic evaluations of these biopsies. Parasite rec overies at necropsy indicated a strong protective effect derived from daily PT treatment. Mean weights of intestinal biopsies corresponded w ith worm burdens, bur histological evaluation did not reveal architect ural or cellular changes to account for the increase in weight; theref ore, edema was suspected. A strong age-related resistance to challenge infection was apparent in both the PT-treated and control groups by v irtue of the lower mean worm burdens found in older ponies compared to younger ponies of the same treatment group: however, daily PT treatme nt of older ponies reduced the variability of their worm burdens to a uniformly low level. Comparisons of luminal and mucosal parasite burde ns of age stratified nontreated controls further suggest that the age related resistance, which is acquired. targets increasing numbers of p arasite stages as this resistance matures. Further, there is no eviden ce for an immune mediated acquisition of hypobiotic L-3. (C) 1998 Else vier Science B.V.