Biomedical subjects
Publications and source records attributed to C R Reinemeyer.
A 3-mo-old male, feral opossum (Didelphis virginiana) was presented to the University of Tennessee College of Veterinary Medicine (USA) for evaluation of prolonged dyspnea. The animal died shortly after examination and a necropsy was performed. The cause of death was attributed to a verminous pneumonia, associated with numerous intrabronchial and intrabronchiolar nematodes, subsequently identified as Didelphostrongylus hayesi.
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Dairy replacement heifers given two strategic treatments with ivermectin (0.2 mg/kg) three and eight weeks after turnout to spring pastures in northern USA had mean adjusted weight gains (62.3 kg) that were 62 per cent (P less than 0.02) greater than those of control heifers (38.5 kg) at the time of winter housing. Pasture infectivity was consistently higher in control pastures and there was a sixfold difference between treatment (56 L3/kg) and control (358 L3/kg) pastures by the time of winter housing on October 18. This study confirmed earlier studies in northern USA demonstrating the value of two strategic treatments in achieving significant increases in weight gains and hastening the time of first breeding by one to three months at a saving of $40 to $140 per heifer. It was concluded that this dosing strategy is a satisfactory method of parasite control for dairy replacement heifers in northern USA without undue selection for drug resistance.
The recent increased interest in cyathostomes can be traced to simplification of their taxonomy, improved knowledge of pathogenicity, and failures of practical control due to anthelmintic resistance. Cyathostome ova develop to infective third-stage larvae (L3) at a rate that is directly proportional to environmental temperature. Equine feces serve as a reservoir for L3, which are liberated by moderate amounts of rainfall. Third-stage larvae persist for longer periods at low temperatures, easily surviving over-winter on pastures to provide a source of infection during the following grazing season. Third-stage larvae exsheath within the host and enter the mucosa and submucosa of the cecum and large colon. Larvae develop within mucosal cysts, molt to the fourth stage, and may persist within the tissues for up to 2 1/2 years. Larvae ultimately emerge from the mucosa to become adults in the lumen. Adult populations are replenished by recently ingested larvae and by immature worms newly emerged from arrested development. The magnitude of larval and adult populations within the host displays seasonal variations, with peak numbers occurring in early spring and autumn in the United States. In typical natural infections, a small number of species comprise the majority of the cyathostome populations. Cyathostome infection may result in anorexia, weight loss, diarrhea, colic, and death. Cyathostome ova are easily detected in feces, but ova may not be present during larval cyathostomiasis. Increased concentrations of beta-globulins, hypoalbuminemia, anemia, and leukocytosis occur inconsistently. Two major problems in the treatment of cyathostome infections are anthelmintic resistance and the insusceptibility of encysted larvae to recommended dosages of most anthelmintics. The major goal of cyathostome control is prevention of environmental contamination with nematode ova. Host resistance appears to protect against cyathostome disease rather than cyathostome infection, and one manifestation of this resistance appears to be prolongation of the prepatent period.
Monthly variations in the magnitude of adult and larval cyathostome burdens were observed in 55 horses necropsied over a 15-month period in the northern USA. Peak numbers of adult cyathostomes occurred in late winter (March) and late summer (September). Larval cyathostomes demonstrated peak numbers from February to April and again in October, beginning one month earlier than the spring adult peak and one month after the autumn adult peak, respectively. The reproductive status of individual female Cyathostomum catinatum, Cyath coronatum, Cylicocyclus nassatus, Cylicostephanus goldi and Cylicostephanus longibursatus was classified as immature, gravid or spent. Seasonal changes in these classifications were monitored as a marker for the age structure of these populations. Each reproductive category of female small strongyle was dominant during only one period per year and these periods were similarly distributed for all five species examined. Immature cyathostomes were most common from late winter to spring (March to May); gravid worms were predominant beginning in spring (April/May) and continuing into autumn (October to December). Spent females prevailed from autumn through winter (October to March/April).
Haustral portions of intestine of 6 horses were isolated by excising the taeniae coli from the cecum and the ventral colon. Uniform 5-cm X 5-cm sections were cut from the haustra and were illuminated from the serosal side with a strong light source (mural transillumination). Cyathostome larvae encysted in the mucosa and submucosa were observed at 15 X magnification and counted. Two separate counts of the larvae in 80 replicates of tissue by the mural transillumination technique (MTT) revealed no significant (P less than 0.05) difference between sample means. Larvae in tissue sections were counted in situ by MTT, and the mucosal scrapings of the tissue sections were digested in pepsin and HCl to determine larval yields for comparison with the MTT counts. Numbers of larvae recovered by pepsin and HCl digestion for 3 and 6 hours were significantly (P less than 0.01) lower than were numbers originally determined by MTT. Larvae recovered by tissue digestion for 3 or 6 hours were examined individually and given objective scores for morphologic damage. Distribution of scores was time-dependent; increased damage to larvae was associated with a longer time of digestion. Individual 4th-stage cyathostome larvae were dissected from cysts in the large intestinal wall and were incubated in water, 0.9% saline solution, 1.1% HCl, or pepsin (7,000 U of activity/ml). Significantly fewer (P less than 0.05) larvae were recovered from all solutions after 3 and 6 hours. The proportion of dissected larvae that were given high scores after exposure to pepsin was significantly (P less than 0.01) greater than were those held in HCl, saline solution, or water for both periods.
The large intestines of 6 horses were divided by length into 12 segments, and each segment was washed and weighed. At least 5% by weight of each segment was examined by mural transillumination, and encysted cyathostome larvae were counted. Total numbers of larvae in each segment were calculated. Encysted larvae (98%) were present in the proximal 7 segments of the large intestine (cecum and proximal 75% of the ventral colon), and 2% were present in the distal 25% of the ventral colon and entire dorsal colon. Encysted larvae (6%) were located in the dorsal colon of 1 heavily infected horse. Larval density was greatest in the cecum, which harbored 57% of encysted worms yet contributed only 27% to the total weight of the large intestine. Larvae dissected from tissue cysts consisted of species that were highly prevalent as adult worms. However, there were few Cylicostephanus longibursatus, probably because of the small size and fragility of its 4th-stage larvae.
Fifty-five adult horses were necropsied over a 15-month period, and their worm burdens counted and speciated. Twenty-one species of Cyathostominae were recovered. Ten species: Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus, Cyathostomum coronatum, Cylicostephanus goldi, C. calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateratum, comprised 98.9% of the total cyathostome burdens. These same 10 species also demonstrated high prevalences in 4 previous surveys. Eight of these 10 species have been shown to be resistant to benzimidazole anthelmintics. Anthelmintic resistance apparently had little effect on the prevalence of a species, but appeared to increase its relative abundance within a mixed population. The prevalence of other internal parasites was similar to previous reports, but Strongylus vulgaris adults and arterial lesions were less common.
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