Effects of Gastrointestinal Parasites on Cottontails

Abstract

Twenty pairs of cottontail rabbits (Sylvilagus floridanus) were kept in large, outdoor pens under seminatural conditions. One-half of these animals were treated with anthelmintic and coccidiostatic drugs in an attempt to reduce gastrointestinal parasites. The remaining animals served as untreated controls. Numbers of gastrointestinal coccidia and round worms were reduced 51 percent and 46 percent in the treated group. The treated group produced significantly more young (48 percent) than untreated controls. There was no significant increase in body weight in the treated animals. When data for both treated and control groups were combined, there was a significant association of low worm burdens with greater weight gains. Fluctuations in nematode parasitism of treated animals were similar to those observed in control animals but occurred at lower values. This observation suggests that worm populations are controlled by environmental factors. This study constitutes one phase of a project on the effects of certain parasites and diseases on cottontail rabbits. This paper describes certain effects of subclinical infection by gastrointestinal helminths and coccidia on reproduction and changes of body weight. The degree to which parasites in natural infections reduce the vitality of hosts living in the wild is a matter of conjecture. Most reports indicate that subclinical infections are the rule and clinical infections are the exception. Clancy et al. (1940), in a survey of Connecticut cottontail (Sylvilagus floridanus and S. transitionalis) parasites, found that half the rabbits showing signs of malnutrition had more species of parasites than did animals. Lechleitner (1959) concluded that Eimeria sp. under normal environmental conditions had little effect upon black-tailed jackrabbits (Lepus californicus). When the rabbit population was concentrated by a flood, clinical coccidiosis with deaths resulted. Controlled experiments with sheep (Herlich 1958), chickens (Reid and Carmon 1958), and cattle (Shelton 1956; Davis et al. 1959, 1960) have established that heavy parasitic burdens resulted in decreased host-weight gains. In unpublished data, A. C. Todd (Personal communication 1961) found that reduced helminthiasis in heifers during the first lactation resulted in significantly higher daily averages of total pounds of milk produced and in higher total pounds of butterfat. I attempted to carry out cottontail experiments of a design similar to that often used in parasitological experiments of domestic animals. Orrin Rongstad is acknowledged for assistance in the field. This study was conducted during the tenure of a Wisconsin Alumni Research Foundation Fellowship. Funds and facilities for this project were furnished by the Department of Veterinary Science and the Department of Forestry and Wildlife Management of the University of Wisconsin. MATERIALS AND METHODS Twenty pairs of cottontails from southern Wisconsin were placed in 20 outdoor pens measuring 26 x 60 feet each. The pens were arranged in two rows of 10 each, separated by a dirt roadway. While the pens in each row had common sides, the sandy soil and dense vegetational cover prevented nearly all drainage of surface water from one pen to another. The vegetation in the pens was mainly mixed