Isolation of 4/91 Type of Infectious Bronchitis Virus as a New Variant in Japan and Efficacy of Vaccination Against 4/91 Type Field Isolate

Among field isolates of infectious bronchitis virus (IBV) from recent outbreaks, some isolates that were classified into the 4/91 genotype, by analysis of the S1 gene, have been confirmed to be a new variant in Japan. To elucidate the characteristics of these isolates, pathogenicity in chicks and the efficacy of vaccines against this new variant were examined. Severe respiratory symptoms were observed in 4-day-old specific pathogen free chicks inoculated with a 4/91 genotype isolate, either JP/Wakayama/2003 or JP/Iwate/2005; body weights 3 wk after inoculation were significantly lower than those of chicks inoculated with a 4/91 vaccine strain. These 4/91 isolates were neutralized with serum from birds immunized with 4/91 vaccine. In a challenge–protection test, five groups of chicks were immunized with C78, TM-86w, H120, Kita-1, or 4/91 vaccines and then challenged with JP/Iwate/2005 4 wk after vaccination. A protective effect in the 4/91 and TM-86w vaccine groups was indicated by evaluation of the ciliostasis score of the trachea, the respiratory symptom score, and virus isolation from trachea swab samples after challenge. The results of this study suggested that the 4/91 type of IBV, which is virulent to chicks when compared to vaccine strains, has emerged as a new variant in Japan, and vaccines containing the 4/91 strain or the TM-86w strain could be effective for this variant. Abbreviations: DPC = days postchallenge; DPI = days postinfection; EID50 = 50% embryo infectious dose; HVR = hypervariable region; IB = infectious bronchitis; IBV = infectious bronchitis virus; PBS = phosphate buffered saline; RT-PCR = reverse transcriptase–polymerase chain reaction; SPF = specific pathogen-free; VN = virus-neutralizing Aislamiento de un virus del tipo 4/91 como una nueva variante del virus de bronquitis infecciosa en Japón y eficacia de la vacunación contra una cepa de campo. Entre los aislados de campo de virus de bronquitis infecciosa ocurridos recientemente, algunos aislados fueron clasificados como del genotipo 4/91, mediante el análisis del gen S1. Este tipo ha sido confirmado como una nueva variante en Japón. Para conocer las características de estas cepas, se evaluó la patogenicidad de las cepas en pollos y la eficacia de las vacunas. Cuando se inocularon aves libres de patógenos específicos de 4 días de edad con aislados genotipo 4/91, tanto JP/Wakayama/2003 como JP/Iwate/2005; se observaron severos síntomas respiratorios, el peso corporal 3 semanas post