Avian metapneumovirus (aMPV) is a respiratory pathogen associated with the swollen head syndrome (SHS) in chickens. In Brazil, live aMPV vaccines are currently used, but subtypes A and, mainly subtype B (aMPV/A and aMPV/B) are still circulating. This study was conducted to characterize two Brazilian aMPV isolates (A and B subtypes) of chicken origin. A challenge trial to explore the replication ability of the Brazilian subtypes A and B in chickens was performed. Subsequently, virological protection provided from an aMPV/B vaccine against the same isolates was analyzed. Upon challenge experiment, it was shown by virus isolation and real time PCR that aMPV/B could be detected longer and in higher amounts than aMPV/A. For the protection study, 18 one-day-old chicks were vaccinated and challenged at 21 days of age. Using virus isolation and real time PCR, no aMPV/A was detected in the vaccinated chickens, whereas one vaccinated chicken challenged with the aMPV/B isolate was positive. The results showed that aMPV/B vaccine provided a complete heterologous virological protection, although homologous protection was not complete in one chicken. Although only one aMPV/B positive chicken was detected after homologous vaccination, replication in vaccinated animals might allow the emergence of escape mutants.321212571262Alvarez, R.L., Kapczynski, D.R., Njenga, M.K., Seal, B.S., Nucleotide and predicted amino acid sequence-based analysis of the avian metapneumovirus type C cell attachment glycoprotein gene: phylogenetic analysis and molecular epidemiology of U.S. pneumoviruses (2003) J. Clin. Microbiol, 41, pp. 1730-1735Arns, C.W., Hafez, H.M., Isolation and identi_ication of Avian Pneumovirus from broiler breeder flocks in Brazil (1995) Proceedings of the 44th Western Poultry Diseases Conference, pp. 124-125. , Jensen M.M. (Ed.), Sacramento, CaliforniaAung, Y.H., Liman, M., Neumann, U., Rautenschlein, S., Reproducibility of swollen sinuses in broilers by experimental infection with avian metapneumovirus subtypes A and B of turkey origin and their comparative pathogenesis (2008) Avian Pathol, 37, pp. 65-74Banet-Noach, C., Simanov, L., Perk, S., Characterization of Israeli avian metapneumovirus strains in turkeys and chickens (2005) Avian Pathol, 34, pp. 220-226Banet-Noach, C., Simanov, L., Laham-Karam, N., Perk, S., Bacharach, E., Longitudinal survey of avian metapneumoviruses in poultry in Israel: infiltration of field strains into vaccinated flocks (2009) Avian Dis, 53, pp. 184-189Catelli, E., Cecchinato, M., Savage, C.E., Jones, R.C., Naylor, C.J., Demonstration of loss of attenuation and extended field persistence of a live avian metapneumovirus vaccine (2006) Vaccine, 24, pp. 6476-6482Catelli, E., Lupini, C., Cecchinato, M., Ricchizzi, E., Brown, P., Naylor, C.J., Field avian metapneumovirus evolution avoiding vaccine induced immunity (2010) Vaccine, 28, pp. 916-921Chacó, J.L., Brandã, P.E., Buim, M., Villarreal, L., Ferreira, A.J., Detection by reverse transcriptase-polymerase chain reaction and molecular characterization of subtype B avian metapneumovirus isolated in Brazil (2007) Avian Pathol, 36, pp. 383-387Chacó, J.L., Mizuma, M., Vejarano, M.P., Toquí, D., Eterradossi, N., Patnayak, D.P., Goyal, S.M., Ferreira, A.J., Avian metapneumovirus subtypes circulating in Brazilian vaccinated and non-vaccinated chicken and turkey farms (2011) Avian Dis, 55 (1), pp. 82-89Cecchinato, M., Catelli, E., Lupini, C., Ricchizzi, E., Clubbe, J., Naylor, C.J., Evidence of aMPV attachment protein evolution coincident with mass live vaccine introduction in Italy. Proceedings VI International Symposium on Avian Corona- and Pneumoviruses and Complicating Pathogens, Rauischholzhausen (2009) Germany, pp. 278-284Cook, J.K.A., Huggins, M.B., Wood, M.A., Orbell, S.J., Mockett, A.P.A., Protection provided by a commercially available vaccine against different strains of turkey rhinotracheitis virus (1995) Vet. Rec., 136, pp. 392-393Cook, J.K., Avian pneumovirus infections of turkeys and chickens (2000) Vet. Journal, 160, pp. 118-125Cos Wig, L.T., Santos, M.B., Hafez, M.H., Ferreira, H.F., Arns, C.W., Propagation of avian metapneumovirus subtypes A and B using chicken embryo related and other cell systems (2010) J. Virol. Methods, 167, pp. 1-4Dar, A.M.T.K., Munir, S., Panigrahy, B., Goyal, S.M., Kapur, V., PCR-based detection of an emerging avian pneumovirus in US turkey flocks (2001) J. Vet. Diagn. Invest., 13, pp. 201-205D'Arce, R.C., Coswig, L.T., Almeida, R.S., Trevisol, I.M., Monteiro, M.C., Rossini, L.I., Di Fabio, J., Arns, C.W., Subtyping of new Brazilian avian metapneumovirus isolates from chickens and turkeys by reverse transcriptase-nested-polymerase chain reaction (2005) Avian Pathol, 34, pp. 133-136Eterradossi, N., Toquin, D., Guittet, M., Bennejean, G., Evaluation of different turkey rhinotracheitis viruses used as antigens for serological testing following live vaccination and challenge (1995) J. Vet. Medicine, 42, pp. 175-186Ferreira, H.L., Spilki, F.R., Santos, M.B., Almeida, R.S., Arns, C.W., Comparative evaluation of conventional RT-PCR and real-time RT-PCR (RRT-PCR) for detection of avian metapneumovirus subtype A (2009) Ciêcia Rural, 39, pp. 1445-1451Ganapathy, K., Jones, R.C., Vaccination of chicks with live attenuated subtype B avian metapneumovirus vaccines: protection against challenge and immune responses can be unrelated to vaccine dose (2007) Avian Dis, 51, pp. 733-737Gough, R.E., Jones, R.C., Avian metapneumovirus (2008) Diseases of Poultry, pp. 101-110. , Saif Y.M., Barnes H.J., Glisson J.R., Fadly A.M., McDougald L.R. & Swayne D.E. (Eds). 12th ed. Iowa State Press, AmesJirjis, F.F., Noll, S.L., Halvorson, D.A., Nagaraja, K.V., Shaw, D.P., Pathogenesis of avian pneumovirus infection in turkeys (2009) Vet. Pathol., 39, pp. 300-310Jones, R.C., Viral respiratory diseases (ILT, aMPV infections IB): Are they ever under control? (2010) Brit. Poult. Sci., 51 (1), pp. 1-11. , (Review)Naylor, C., Shaw, K., Britton, P., Cavanagh, D., Appearance of type B avian pneumovirus in great Britain (1997) Avian Pathol, 26 (2), pp. 327-338Juhasz, K., Easton, A.J., Extensive sequence variation in the attachment (G) protein gene of avian pneumovirus: evidence for two distinct subgroups (1994) J. General Virol., 75, pp. 2873-2880Jones, R.C., Naylor, C.J., Al-Afaleq, A., Worthington, K.J., Jones, R., Effect of cyclophosphamide immunosuppression on the immunity of turkeys to viral rhinotracheitis (1992) Res. Vet. Sci., 53, pp. 38-41Jones, R.C., Avian pneumovirus infection: Questions still unanswered (1996) Avian Pathol, 25, pp. 639-648Njenga, M.K., Lwamba, H.M., Seal, B.S., Metapneumoviruses in birds and humans (2003) Virus Res, 91, pp. 163-169Reed, J., Muench, H., A simple method for estimating _ifty percent end points (1938) Am. J. Hygiene, 27, p. 49Ricchizzi, E., Catelli, E., Cecchinato, M., Lupini, C., Brown, P., Naylor, C.J., Turkey rhinotracheitis outbreak in 7- week-old turkeys caused by a vaccine derived avian metapneumovirus (2009) Proceedings VI International Symposium on Avian Corona- and Pneumoviruses and Complicating Pathogens, Rauischholzhausen, Germany, pp. 260-264Santos, M.B., (2010) Experimental studies with avian metapneumovirus (aMPV) subtypes A and B isolates in broiler chickens, p. 32. , Dissertação de Doutorado em Genéica e Biologia Molecular, Universidade Estadual de Campinas, Campinas, SP, BrazilToquin, D., Bãon-Auboyer, M.H., Senne, D.A., Eterradossi, N., Lack of antigenic relationship between French and recent North American non--A/non-B turkey rhinotracheitis viruses (2000) Avian Dis, 44, pp. 977-982Toquin, D., Guionie, O., Jestin, V., Zwingelstein, F., Allee, C., Eterradossi, N., European and American subgroup C isolates of avian metapneumovirus belong to different genetic lineages (2006) Virus Genes, 32, pp. 97-103Van De Zande, S., Nauwynck, H., Cavanagh, D., Pensaert, M., Infection and reinfection with Avian Pneumovirus subtype A and B on Belgian turkey farms and relation to respiratory problems (1998) J. Vet. Medicine, 45, pp. 621-626Villarreal, L.Y., Brandã, P.E., Chacó, J.L., Assayag, M.S., Maiorka, P.C., Raffi, P., Saidenberg, A.B., Ferreira, A.J., Orchitis in roosters with reduced fertility associated with avian infectious bronchitis virus and avian metapneumovirus infections (2007) Avian Dis, 51 (4), pp. 900-904Villarreal, L.Y.B., Sandri, T.L., Assayag, L.J., Richtzenhain, L.J., Malo, A., Brandã, P.E., Field observations after natural infection of Brazilian layer chickens with a phylogenetically divergent lineage of subtype B aMPV (2009) Proc. VI International Symposium on Avian Corona- and Pneumoviruses and Complicating Pathogens, Rauischholzhausen, Germany, pp. 255-259