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DESCRIPTION OF PROBLEMCoronaviruses infect a wide range of animals and cause respiratory, enteric, hepatic, and neurological diseases of varying severity [1]. They cause severe diseases in livestock animals and thereby lead to high economic losses [2]. Based on genotypic and serological character-ization, coronaviruses are divided into 4 distinct groups. Their tendency for recombination and high mutation rates may allow their adaptation to new hosts and ecological niches [3][4][5][6]. Infectious bronchitis virus (IBV), the coronavirus of chickens, is one of the foremost causes of economic losses for the poultry industry, affecting the performance of meat and egg-laying birds,
SUMMARY
Due to serotype variations among different avian infectious bronchitis viruses isolated inTunisia since 2000, protection of chicks, especially broiler flocks, with Mass H120 vaccine often fails. Therefore, association of CR88 (793B type) with H120 vaccines was used for better response. Challenge experiments were then conducted to evaluate tracheal and renal crossprotection in chickens immunized via nasal and eye drops. Conferred protection was measured by clinical signs and macroscopic lesions observed, based on scores attributed according to their severities. The results showed a low protection conferred by H120 alone, as vaccination did not reduce tracheal and kidney lesions (70% scored as 3) after TN20/00 virus challenge, which also led to 10% mortality. Conversely, the challenge results indicated that the combination of the 2 strains (H120/CR88) allow high protection. Based on the results of the challenge experiments, a vaccination protocol coupling CR88 to H120 was applied for industrial broiler flocks. Clinical observations and serological results confirmed that association of heterologous serotypes (H120 and CR88 vaccines) increased the levels of protection against infectious bronchitis viruses compared with the H120 vaccine given alone.