Eimeria tenella in Bacteria-Free Chicks of Relatively Susceptible Strains

Attempts to determine the importance of strain differences in the response of bacteria-free chicks to Eimeria tenella indicated that Coxy S and D168sm chicks remained highly susceptible to cecal coccidiosis in the absence of the normal intestinal flora, whereas previous experiments have shown that bacteria-free Honegger Layer chicks were more resistant than conventional controls. Host strain variation might explain the difference between the results of Clark and his colleagues and those obtained in our previous experiments with bacteria-free and gnotobiotic chicks. Long (1970) has suggested that the different results obtained by various investigators concering the pathogenicity of Eimeria might be a result of using experimental animals differing in their genetic resistance to coccidiosis. Rosenberg (1941) first demonstrated that there were breed differences among chickens in their resistance to Eimeria tenella and found that Barred Plymouth Rock and Jersey White Giants had significantly higher mortality than White Leghorn, New Hampshire, and Rhode Island Red chicks. Long (1968) confirmed the importance of breed differences but also noted that certain breeds need not always be more susceptible than others. Unselected lines within a breed may vary from highly resistant to highly susceptible to E. tenella and Jeffers (1968) found that line RPL-15 White Leghorns had 78% survival whereas D168sm chicks showed 20.4% survival upon infection with 100,000 oocysts at 11 days of age. After Rosenberg (1941) demonstrated that resistance to E. tenella was inherited, various investigators have been able to genetically select for resistance or susceptibility to coccidiosis and others have shown that these are stable traits which remain after intentional selection has ceased. Moultrie, Edgar, and King (1954) found that the mortality in Auburn Strain White Leghorns selected for resistance or susceptibility was 15 and 72%, respectively, compared to 34% for unselected controls. By Received for publication 19 August 1971. *This study was supported by research grant AI-04101 from the NIAID, U. S. Public Health Service, and by a University of Wisconsin Graduate School Predoctoral Research Fellowship. t Present address: Biology Department, University of North Carolina, Charlotte. 586 selection for two generations, Rosenberg (1948) produced two lines (Univ. Wisconsin Coxy S and Coxy R) which differed in survival by 37.6% and, with two further generations of selection, Champion (1951) found the difference to be almost 50%-61.9% survival of Coxy R chicks and 12.2% of Coxy S. Resistance and susceptibility have been shown to be controlled by multiple genetic factors. Sex linkage, maternal factors, and cytoplasmic inheritance are not significantly involved (Palafox, Alicata, and Kartman, 1949; Rosenberg, Alicata, and Palafox, 1954; Champion, 1954). In order to determine if the host genetic makeup might be involved in the parasiteintestinal flora interaction previously demonstrated with Honegger Layer Strain White Leghorn chicks (Visco and Burs, 1972b), Coxy S and D168sm White Leghor chicks were obtained from the Poultry Science Department of the University of Wisconsin. Jeffers (1968) showed that, since initial selection, the Coxy S line has retained its high susceptibility (10.5 ? 3.9% survival) to cecal coccidiosis and the unselected D168sm line was the next most susceptible (20.4 ? 4.9%) of 14 lines tested. Bacteria-free chicks from these two lines were infected with E. tenella and the mortality, cecal lesion scores, and clinical picture were compared with conventional controls. MATERIALS AND METHODS The Coxy S and D168sm chicks were provided as 18-day embryos by Professor W. H. McGibbon of the Department of Poultry Science, University of Wisconsin, and they were maintained bacteriafree in gnotobiotic isolators. The procedures for rearing bacteria-free chicks, inoculating them with E. tenella oocysts, and assessing the severity of the This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:20:54 UTC All use subject to http://about.jstor.org/terms VISCO AND BURNS-E. TENELLA IN GERM-FREE SUSCEPTIBLE STRAIN CHICKS 587 TABLE I. Mortality of bacteria-free (BfI) and conventional (CvI) chicks of relatively susceptible inbred lines after oral inoculation with Eimeria tenella oocysts. Group Oocyst Experiment dose BfI CvI Coxy Stl 100,000 6/6* 50/50 Coxy S 2 20,000 8/11 14/17 Coxy S 3 20,000 1/3 29/31 D168sm* 1 100,000 0/4 14/24 D168sm 2 100,000 4/10 7/21 D168sm 3 100,000 3/12 23/51 * Number dead/total. t Coxy S-Inbred line selected for susceptibility to Eimeria tenella. : D168sm-Unselected inbred line. Any two results not underscored by the same line are significantly different, i.e., P < 0.05. infection have been reported previously (Visco and Burns, 1972a). Conventional chicks were hatched in conventional incubators and reared in commercial brooders with no attempt to limit or control their bacterial flora whereas bacteria-free chicks were hatched and maintained free from demonstrable flora in flexible-film gnotobiotic isolators. The expressions bacteria-free infected (Bfl) and conventional infected (CvI) have been used to designate chicks of the above groups which were orally infected at 14 days of age with E. tenella oocysts. Mortality data were analyzed with a chi-square test and lesion scores were compared by analysis of variance using the least significant difference method (Steel and Torrie, 1960). In all comparisons, P < 0.05 has been taken as significant. RESULTS AND DISCUSSION The bacteria-free chicks in each experiment appeared normal but, in general, birds of both inbred lines were less sturdy and did not survive after hatching as well as those obtained from a commercial hatchery and used in previous experiments (Honegger Layer). The cecal material in bacteria-free chicks was watery and dark greenish-brown compared to the viscid light brown cecal contents in conventional animals. Bacteria-free Coxy S chicks were highly susceptible to coccidiosis and displayed clinical signs of the disease equivalent to conventional controls. The onset of the coccidiosis syndrome, however, was 20 to 24 hr later in bacteria-free fowl. Within experiments Coxy S 1 and 2, the mortality of E. tenella-infected bacteriafree and conventional chicks was not significantly different (Table I). In experiment Coxy TABLE II. Mean cecal lesion score in bacteria-free (BfI) and conventional (CvI) relatively susceptible D168sm chicks 7 days after oral inoculation with 100,000 Eimeria tenella oocysts.