Acta Veterinaria Hungarica 53 (4), pp. 493–499 (2005) 0236-6290/$ 20.00 © 2005 Akadémiai Kiadó, Budapest HAEMATOLOGICAL AND HISTOLOGICAL FINDINGS IN BIRDS EXPERIMENTALLY INFECTED WITH HIGHLY PATHOGENIC H5N2 AVIAN INFLUENZA VIRUS Guadalupe RAMÍREZ 1 , T. FEHERVARI 2 , L. H. PAASCH 2 and Norma L. CALDERÓN 2* 1 Department of Pathology, Section of Clinical Diagnosis and 2 Department of Animal Production: Poultry, Faculty of Veterinary Medicine and Animal Science, National Autonomous University of Mexico, 04510 Mexico City, Mexico (Received September 20, 2004; accepted December 16, 2004) Experimental infection of specific-pathogen-free (SPF) Leghorn chickens with a highly pathogenic H5N2 avian influenza virus produced cellular hyperpla- sia in the bone marrow at 36 hours post infection (hpi) and haematological evi- dence of monocytosis, thrombocytopenia and heterophilia was also detected. An early, significant and progressive haematological change was thrombocytopenia starting at 24 hpi without an increase of prothrombin time. The findings suggest that highly pathogenic avian influenza virus interferes only with the primary haemostatic mechanisms by consumption of thrombocytes, while the secondary haemostatic mechanisms remain intact. Key words: Avian influenza, pathological findings, pathogenesis, chickens Avian influenza (AI) is caused by a type A orthomyxovirus of worldwide distribution. Highly pathogenic AI (HPAI) produces elevated infection and mortal- ity rates. The most frequently reported signs and lesions include lethargy, ruffled feathers, comb necrosis and conspicuous haemorrhages in the subcutaneous tissue, lungs, mucous membrane of the proventriculus and the skeletal muscle (Kobayashi et al., 1996; Easterday et al., 1997; Suarez et al., 1998; Capua et al., 1999; Capua and Marangon, 2000). Haemorrhages have been associated with primary viral repli- cation in vascular endothelial cells with subsequent thrombosis (Suarez et al., 1998). Several studies suggest that endothelial damage and thrombosis progress to a disseminated intravascular coagulation in which thrombocytes and coagula- tion factors are consumed, causing widespread haemorrhages in infected birds (Jain, 1993; Darien, 2000). In other viral infections such as classical swine fever (Calderón, 1998), thrombocytopenia caused by direct damage to the thrombocyte precursor cells * Corresponding author; Cerrada de Arenal # 44 Casa 5, Valle Escondido, 14600 Mexico City, Mexico; Phone/Fax: +52 (55) 56166923, +52 (55) 56225868; E-mail: nlca@servidor.unam.mx