Infectious bronchitis in Brazilian chickens: current data and observations of field service personnel

摘要

The infectious bronchitis virus (IBV) was detected for the first time in Brazil by Hipólito in 1957 in chickens sold life in the municipal market of Belo Horizonte, MG, when commercial poultry production was just starting in that country. The Massachusetts (Mass) serotype was identified. However, the clinical disease was only observed in 1975, when poultry production was intensely growing. The extensive outbreak produced the classical condition in layers and breeders, affecting egg production and quality, whereas broilers presented respiratory and "nephritis-nephrosis" signs. The disease rapidly spread to all poultry-producing regions in the country, and in 1979, both the imports and the manufacturing of live vaccines against IB strains Mass, H120 and H52, were licensed. In 1980, inactivated vaccines were introduced. Molecular techniques, particularly PCR, started to bed in the identification of IBV. A retrospective analysis showed that, up to 1989, the main IBV strain circulating in Brazil was Mass. However, other studies shows the presence of a wide diversity of IBV strains in Brazil since the first strains were isolated, even before vaccination was introduced. Most researchers agree that the incidence of IBV different from Mass has increased, including of exclusively Brazilian genotypes, different from those described in other countries. Indeed, during the last few years, the number of genotypical variants has been much higher than that of the classical Mass serotype. Clinically, in addition of the classic presentations, atypical forms such as testicular atrophy and stones in the epidydimis associated to low fertility have been described. Serological techniques started to be used in vaccination monitoring and as a diagnostic tool. Serological response standards were developed, and have shown to be very useful to determine the expected profile in vaccination programs and when clinical disease is suspected. However, the immuno-enzymatic test ELISA is the most frequently used around the world due to its convenience. These situations led service people working in the field to suspect that vaccination programs using Mass strains were not providing the required protection because of the presence of variant strains. Some argue that this was expected, particularly in layers and breeders, because Mass-type vaccines have been used for a long time, whereas most agree that the emergence of variants is the primary cause of the increasing severity of the disease in the field. This is supported by the results using Ibv genotyping as diagnostic tool, independently of phenotype (pathotype x immune system x environment). Other argues that broiler carcass downgrading rates in processing plant are not consistent with the increase in IB clinical severity. Seroconversion in non-vaccinated flocks is acknowledged, but it occurs sporadically and not necessarily correlated with disease outbreaks. There is a general agreement that IBV has shown high variability in Brazil in terms of genotype, pathotype, and serotype. However, research should emphasize IBV phenotypical characteristics using birds as biological model.