Relationship between Eimeria oocyst infectivity for chickens and in vitro excystation of E. acervulina, E. maxima, and E. tenella oocyst during long-term storage

Vaccination of chickens against avian coccidiosis in chickens often involves storing Eimeria oocysts for months after oocyst propagation and sporulation. The purpose of this study was to determine how long E. acervulina, E. maxima, and E. tenella oocysts remained viable when stored at refrigeration (4°C) or egg room (20°C) temperatures. Separate tubes containing E. acervulina, E. maxima, or E. tenella oocysts were stored at these temperatures and a sample removed every 3 mo for inoculating chickens for evidence of a patent infection. Also, an aliquot of each Eimeria species at each time–temperature combination was subjected to in vitro excystation to quantify the relative number of released sporozoites to intact (nonexcysted) sporocysts. Eimeria tenella appeared to be most susceptible to storage in that no oocyst production was observed at 9 mo at either temperature. Although E. maxima oocysts were viable at 9 mo, no oocyst production was observed at 12 mo storage at these 2 temperatures. Quite unexpected was that E. acervulina was much more stable than E. tenella and E. maxima remaining viable up to and including 27 mo at 4°C and up to and including 12 mo at 20°C. No consistent correlation was observed between in vivo oocyst production and in vitro excystation arising from these 2 respective temperatures (E. acervulina r = 0.58, r = 0.54; E. maxima r = 0.90, r = 0.54; E. tenella r = 0.38, r = 0.90). These data indicate that attention must be paid to time and temperature of Eimeria oocyst storage, and that sporozoite excystation may not be a good indicator of oocyst viability, particularly at later timepoints in incubation.