The aim of this study was to determine the immune response of broiler chickens to Eimeria tenella developmental stages Four hundred broilers divided into six groups (n=40) were used for the study. Each group was subdivided into two (n=20) as treated and non-treated and infected with different developmental stges of Eimeria tenella (local isolate). The molecular identification of the local Eimeria tenella isolate identity was done through polymerase chain reaction (PCR) amplification of the genomic deoxyribonucleic acid (DNA). Clinical signs, gross caecal lesions, humoral and cellular-mediated immune responses were determined in the infected broiler chickens with Eimeria tenella developmental stages. The faeces were processed using simple floatation technique and observed at x 10 and x 40 objectives of the Neiss microscope. Oocysts isolated from the caeca of birds naturally infected in Jos, Nigeria with the local strain were used to obtain the different developmental stages either in vitro or in vivo using bovine monocytes (schizonts), embryonated chicken eggs (gamatocytes) and two weeks old broilers (merozoites). To study the immune response elicited during the primary and secondary infection, each developmental stage was used to infect a group of two, three and half weeks old broilers, twenty of which were treated with the recommended dose of amprolium (250 mg/l (0.025%) for 5 days at the appearance of clinical signs. At the tertiary infection, all the experimental birds except the control group of forty birds were orally infecteded with 105 sporulated oocysts of known characterized virulent Eimeria tenella strain. The mean oocysts output or count was 37.07 x 106 in the infected birds non-treated than 25.65 x 106 in the treated groups, although there was a gradual reduction (groups II – 8.36 x 106 – 7.84 x 106 – 5.10 x 106; III – – 6.58 x 106 – 4.83 x 106; IV – 7.18 x 106 – 7.00 x 106 – 3.83 x 106; V – 6.59 x 106 – 5.87 x 106 – 4.20 x 106) in oocyst count from primary-secondary-

tertiary infections except group I (control). There was a significant difference in oocyst output between the groups (II and IV) ( p<0.05). Antibodies (IgG or IgY) titre values were higher in broilers sera infected with sporulated oocyst (0.265 ± 0.010, 0.282 ± 0.005;0.305 ± 0.002, 0.316 ± 0.010 and 0.252 ± 0.002, 0.281 ± 0.010) and merozoites (0.177 ± 0.001, 0.186 ± 0.003; 0.135± 0.010, 0.141 ± 0.002 and 0.069 ± 0.004, 0.139 ± 0.005 ) reaching a peak on day 10 of post primary and secondary infections and day 5 post tertiary infection in sera of broilers (treated and non- treated). At tertiary infection, antibodies increases at day 5, 7, 11 and 14 indicating that antibodies increases in broilers infected with the invasive or zoite stages, (sporozoite and merozoite) of the parasite.. There was a significant difference in the antibody output between the sera of the broiler groups (p<0.05). The study reveals the proliferation of cytokines in treated and non- treated broilers consisting of IFN- γ, IL-1, IL-2, IL-4, IL-6, TNF and TGF. The CD4 lymphocyte

count in the treated and non- treated broilers orally administered with various developmental stages of the parasite reached a peak at day 10 ((groups I – 198.0 x 103 µl,165.3 x 103 µl; 200.0 x 103 µl, 156 x 103 µl and 196.7 x 103 µl, 173.3 x 103 µl ; II – 199.0 x 103 µl, 186.0 x 103 µl ; 197.0 x 103 µl, 192.7 x 103 µl and 200.0 x 103 µl, 194 x 103 µl;

  1. – 198 x 103 µl, 153.3 x 103 µl ; 200.0 x 103 µ,l 160.0 x 103 µl and 188.7 x 103 µl, 166.7 x 103 µl ; IV – 193.3 x 103 µl, 183 x 103 µl; 198.7 x 103 µl, 183.3 x 103 µl and 190 x 103

µl , 188.0 x 103 µl ; V – 200.0 x 1 03 µl, 198.0 x 103 µl ; 187.3 x 103 µl , 174 x 103 µl and 188.7 x 103 µl, 175.3 x 103 µl respectively) at primary and secondary infections and day 24 at tertiary infection. There was significant difference in the CD4 cell count between groups of the infected broiler chickens (p<0.05). Caecal lesions were observed to gradually reduce from primary-secondary-tertiary infections. The lesions were significantly different between in groups (II and IV) (p<0.05). Oocyst output and caecal lesions were absent group VI (control). The current study observed a relationship between the different developmental stages of the parasite and immune responses (humoral and lymphocytes responses). The study also observed that broilers with high oocyst output had high antibody production, CD4 lymphocytes count and high levels of cytokines. Thus, the sporozoites and merozoites are the invasive stages that initiate infection of host cells and probably stimulation of the immune response and may be possible vaccine candidate against avian coccidiosis.