Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2014
Publication Date: 11/19/2014
Citation: Fetterer, R.H., Abrams, A., Jenkins, M.C. 2014. Protection of non-immunized broiler chicks housed with immunized cohorts against infection with Eimeria maxima and E. acervulina. Avian Diseases. 59:98-105.
Interpretive Summary: Poultry coccidiosis is caused by several different species of a protozoan intestinal parasite which causes considerable annual losses to the poultry industry. The primary control for the disease is through applied medications in the feed as birds are raised in confinement in broiler houses. The control by medications is becoming less effective because of increased resistance to the drugs and less desirable due to concerns about the possibility of drugs remaining in the meat and within the environment. As an alternative to drugs, immunization with a vaccine containing small numbers of live oocysts (the infective stage of the parasite) is currently used to induce immunity in birds and to protect them from disease. However,efficiency of vaccine delivery can vary greatly resulting in some birds having only partial protection. In order to optimize the use of live vaccines, an increased understanding of the development of immunity to coccidia in birds immunized with live oocyst vaccines is needed. The current study investigated if unimmunized birds housed with immunized cohorts were protected from detrimental effects of coccidosis by acquiring oocysts deposited in the litter by the immunized birds. The results indicate that when immunized birds were allowed 3 weeks after immunization to develop immunity, pens where only 75%of birds were vaccinated were protected from coccidiosis in a manner similar to that of control. However, if birds had 2 weeks in which to develop immunity, pens where 75% or 50% of birds were immunized were not well protected from coccidiosis. The results suggest that shedding of oocysts by immunized birds may play an important role in conveying infection to unimmunized cohorts.
Technical Abstract: The use of live oocyst vaccines is becoming increasingly important in the control of avian coccidosis in broiler chicks. Knowledge of the mechanisms of how chicks uptake oocysts and become immune is important for optimizing delivery of live vaccines. The current study tests the hypothesis that chicks that are not initially immunized may ingest oocysts by contact with litter containing oocysts shed by immunized cohorts. In experiment 1, day-old broiler chicks were housed in pens containing clean litter. In trial 1, 100 % of chicks in some pens were immunized with 2.5 x103 E. acervulina oocysts while in other pens 75% of chicks were immunized and remaining cohorts within the pens were not immunized. Other pens contained chicks that served as unimmunized, uninfected controls or unimmunized, challenged controls. On day 21 birds were given a homologous challenge of 6 x 105 oocysts. Birds were killed on day 27. A second identical trial was conducted, expect birds were immunized with 500 E maxima oocysts and were challenged with 3 x103 oocysts. In experiment 2, birds were placed in pens as above. In trial 1, 100% of chicks in some pens were immunized with 500 E. acervulina oocysts, while in other pens either 75% or 50% of the birds were immunized. On day 14, birds were challenged with 1 x 106 oocysts. Birds were killed on Day 20. Trial 2 was identical to trial 1 except birds were immunized with 100 E. maxima oocysts and challenged with 1 x 106 oocysts. For all experiments, weight gain, feed conversion ratio (FCR), plasma carotenoids and litter oocyst counts were measured. Experiment 1 results indicate pens of birds where only 75% of the birds were immunized with E. acervulina or E. maxima were protected from decreases in weight gain and cartenoids levels and increases in FCR and oocyst litter counts following challenge infection in a manner nearly identical to pens of birds where 100% of chicks were immunized. In experiment 2, pens where 50 or 75% of birds were immunized with either E. maxima or E. acervulina were not well protected from decreases in weight gain and plasma carotenoids and increases in litter oocyst counts following a challenge infection administered on day 14 relative to unimmunized controls. In addition, pens of birds where 100% of chicks were immunized were not well protected from challenged compared to unimmunized controls. These results in total suggest that when birds are challenged after 21 days, that unimmunized cohorts are protected from detrimental effects of challenge infection. However, when challenge infection is given at 14 days, unimmunized cohorts are not well protected. The results support a conclusion that protection to coccidiosis is conveyed to unimmunized cohorts by contact with oocysts shed into the litter by immunized chicks.