Virulence determination of Iranian Escherichia coli isolates from poultry in day-old chicks using a lethality test

Colibacillosis due to Escherichia coli is one of the most important infectious diseases in poultry. While the influence of E. coli virulence factor in birds may differ due to interactions with other influential factors، the sole presence of such factors in E. coli does not determine its pathogenicity. Therefore، it is necessary to test the pathogenic capability of E. coli isolates on susceptible birds. The purpose of this study was to determine the virulence of three E. coli isolates from colibacillosis and compare their effects on day-old chicks using a lethality test. Seventy 1-day-old chicks were divided into 3 treatment groups of 20 chicks and 2 control groups of 5 chicks. Each treatment group was assigned to one E. coli isolate and divided into 4 sub-groups of 5 chicks. The overnight broth culture of each E. coli isolate was washed with PBS three times and diluted. Then، 0. 5 ml of undiluted culture (~109 CFU/mL)، and the dilutions of 0. 1، 0. 01، and 0. 001 of each culture were injected subcutaneously to each of the 5 birds in each subgroup behind the neck. One group of 5 birds was injected by PBS while negative control group did not receive anything. The chicks were monitored every 12 hours for 6 days. The dead chicks during the course of the experiment and all survived ones were necropsied and samples were taken from hearts and livers for bacteriological culture. Virulence of each E. coli isolate was evaluated based on a scoring system developed on death time، gross pathological observations، and bacteriological findings. All E. coli isolates of this study were capable of causing mortalities and producing the lesions typical of colibacillosis. There were no significant differences among the three E. coli isolates in their in vivo virulence abilities but the difference between each of the three E. coli isolates and each of the two control groups was significant (p|||£|0. 05). This study determined that our E. coli isolates were able to cause mortalities in day-old chicks. These findings increased our knowledge on the virulence characteristics of three Iranian E. coli isolates originated from avian colibacillosis.