Pathogenic Effects of Single or Mixed Infections of Eimeria mitis, Eimeria necatrix, and Eimeria tenella in Chickens

SIMPLE SUMMARY: The objective of this study was to investigate the effects of the presence of Eimeria mitis on the outcome of Eimeria tenella or Eimeria necatrix experimental challenge infection and to demonstrate synergistic or antagonistic effects occurring among different species in mixed Eimeria infections. Co-infection of E. mitis and E. tenella led to lower body weight gain, severer lesions, and higher mortality of challenged birds compared to a single E. tenella infection. Moreover, although not statistically significant, there appears to be a reduction in body weight gain and an increase in oocyst shedding and mortality in E. mitis/E. necatrix-coinfected group compared to E. necatrix-infected group. These observations suggest that E. mitis could enhance disease mediated by E. tenella, whereas E. mitis does not seem to affect the virulence of E. necatrix but might also have a synergistic interaction with E. necatrix in infection. In conclusion, a synergistic relationship between E. mitis and E. tenella/E. necatrix was demonstrated using experimental co-infection models, thus suggesting that the common natural mixed infection of chicken coccidia in the field was probably the result of a synergistic effect of Eimeria spp. rather than an antagonistic one. ABSTRACT: Avian Eimeria species vary in their replication location, fecundity, and pathogenicity. They are required to complete the development within the limited space of host intestines, and some synergistic or antagonistic effects occur among different Eimeria species. This study evaluated the impact of Eimeria mitis on the outcome of Eimeria necatrix or Eimeria tenella challenge infection. The severity of E. mitis/E. necatrix and E. mitis/E. tenella mixed infections were quantified by growth performance evaluation, survival rate analysis, lesion scoring, blood stool scoring, and oocyst output counting. The presence of E. mitis exacerbated the outcome of co-infection with E. tenella, causing high mortality, intestinal lesion score, and oocyst production. However, E. mitis/E. tenella co-infection had little impact on the body weight gain compared to individual E. tenella infection. In addition, the presence of E. mitis appeared not to enhance the pathogenicity of E. necatrix, although it tends to inhibit the growth of challenged birds and facilitate oocyst output and mortality in an E. mitis/E. necatrix co-infection model. Collectively, the results suggested a synergistic relationship between E. mitis and E. tenella/E. necatrix when sharing the same host. The presence of E. mitis contributed to disease pathology induced by E. tenella and might also advance the impact of E. necatrix in co-infections. These observations indicate the importance of accounting for differences in the relationships among different Eimeria species when using mixed infection models.