Study On Selection And Immunogenicity Of Precocious Strains Of Eimeria Flavescens In Rabbit

Rabbit coccidiosis is a parasitic protozoan that lives on the epithelial cells of the intestinal mucosa and the bile duct epithelial cells of the liver.To date,11 species have been found to cause related diseases,and all of them belong to the family Eimeria.The disease is very common and seriously harms the efficiency of rabbit breeding.As one of the most pathogenic species among the 11 species of rabbit coccidiosis,Eimeria flavescens can cause diarrhea,constipation,other clinical symptoms,and even death.Although rabbits can survive this disease,they require a long recovery time,so E.flavescens has a great impact on production performance.At present,the prevention and control measures of rabbit coccidiosis mainly use anti-coccidiosis drugs,but these often produce drug resistance problems.Therefore,generating immunity with a coccidiosis vaccine will be the most effective way to prevent rabbit coccidiosis.In this study,a precocious line of E.flavescens was selected from the Chongqing field strain by serial passaging.Then,the comprehensive biological characteristics and immunoprotective effects of the precocious line were studied,which provides a foundation for future research and the use of live attenuated coccidia vaccines for the control of E.flavescens.First,a precocious line of the E.flavescens Chongqing strain was passaged and selected.After 18 generations of selection for precocious development,the latent period of the E.flavescens Chongqing strain was successfully shortened,resulting in a decrease of 52h（from 220 h to 168 h）.Furthermore,this precocious strain was identified as a single,pure species by a specific PCR method.Therefore,a precocious line of E.flavescens Chongqing strain was successfully obtained.Second,the biological characteristics of the precocious lines of the E.flavescens Chongqing strain were determined,including latent period stability,oocyst morphology,size and shape,fertility,ovulation regularity and virulence.After 5 nonselective passages,the latent period of this precocious strain was determined to be 168 h,and it was found to primarily parasitize the epithelial cells of the cecum and the colon.The oocyst was inverted pear-shaped,the oocyst wall was yellow,and one end near the oocyst pore was determined to be thicker by microscope observation.There was no residue in the oocyst after sporulation,the size was（23.4～36.4）μm×（16.9～26.0）μm,and the shape index was1.31～1.78.After oral inoculation,most of the oocysts were discharged during the 8^th to 15^thdays,and the peak oocyst excretion occurred on the 10^th day.The rabbit was inoculated with 1.2×10³ sporulated oocysts of E.flavescens to reproduce 100 million oocysts.The pathogenicity of the precocious strain was significantly lower than that of its parent strain.The weight gain of rabbits infected with the precocious strains could quickly recover,but its parent strains showed a long period of negative growth.At autopsy,severe inflammatory reaction and intestinal damage were seen in the intestinal tracts of rabbits infected with the parent strains.However,mild inflammatory reaction with no other pathological damage was seen in the intestinal tracts of rabbits infected with the precocious strain,indicating that the pathogenicity of the precocious strain was lower than that of its parental strain.Third,the immune protective effect of the precocious line of E.flavescens was studied,including the optimal immunization dose,the immunization production time and the immunization duration.To explore the best immune times and production of immunization time,the same dose and different repeated immunization times were applied to immunize rabbits with the precocious line of the E.flavescens Chongqing strain.Subsequently,the different immune doses were compared to determine the optimal immune dose three times.After immunization with 5×10² spore oocysts/rabbit,the results showed that the first immunization could produce some immune protection,the second immunization was better than the first immunization,and the third immunization was superior to the second immunization;therefore,the third immunization produced the best protection.Based on the above research,we further studied the different immunization doses 3 times.The results indicated that higher doses produced better immune effects but slightly affected weight gain.In comprehensive consideration,immunization with 5×10²spore oocysts/rabbit produced the optimal immune protection.