16.07.2011

Circulation of an antigenically variant lineage of highly pathogenic avian influenza (HPAI) H5N1 virus in chicken breeder flocks in Egypt is a continuing problem. A study published online by Vaccine suggests that avian influenza vaccines being used in Egypt's poultry (H5N2) may not adequately protect them from H5N1 avian influenza strain and may even contribute to spread of the disease. A nationwide vaccination campaign was launched after avian influenza first hit Egypt's poultry flocks in 2006, the report notes. In the study, a team of German and Egyptian researchers vaccinated breeder chickens at 6, 12, and 18 weeks with a commercial H5N2 vaccine and later exposed them to H5N1 viruses. The first dose generated a high level of antibodies against the H5N2 vaccine strain but a much lower level against the H5N1 strain circulating in Egypt. Antibodies against the H5N1 strain increased after the second dose, but the vaccine generated no antibody response to a newer      H5N1 variant circulating in the country (clade 2.2.1;  EGYvar/H5N1). Upon challenge at week 50 mild, if any, clinical signs were observed in the group infected with classic clade 2.2.1 (EGYcls/H5N1) although one of eight (12.5%) birds died. Mortality reached 6/8 (75%) in the variant lineage of clade 2.2.1 (EGYvar/H5N1) challenge group. The exposed chickens continued to lay eggs through most of the observation period, and virus was detected on the outer shell of 17 of 40 eggs. The majority of the infected eggs were derived from the EGYcls/H5N1 challenged animals; here the virus was detected also in the yolk and albumin. Repeated vaccination using a commercial H5N2-based vaccine broadened the antigen profile of induced antibodies but did not provide adequate protection against heterologous virus variant. In addition, the observation of AIV contaminated eggs from infected flocks highlights the risk of silent virus spread by vaccinated animals and point to eggs as a possible vector.