Journal of Medical Virology 83:1418–1423 (2011) Viral Load of the Highly Pathogenic Avian Influenza H5N1 Virus in Infected Human Tissues Naraporn Sirinonthanawech, 1 Mongkol Uiprasertkul, 2 Ornpreya Suptawiwat, 1 and Prasert Auewarakul 1 * 1 Department of Microbiology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand 2 Department of Pathology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand The highly pathogenic avian influenza A (H5N1) virus is a virulent virus that causes an acute fe- brile respiratory disease with high mortality in humans. To gain a better insight of H5N1 viral distributions in infected human tissues, the lev- els of viral RNA were determined in the autop- sy tissues from two patients who were infected with H5N1 virus by using real-time reverse tran- scription-polymerase chain reaction. In one pa- tient who died on day 6 of the illness, the viral load in the lung was extremely high, whereas the levels of viral RNA in the other organs were more than 6 log lower. In the other patient who died on day 17 of the illness, the viral load was similar in the lung and other organs, and was comparable to the viral load in the extra-pulmo- nary tissues of the first patient. These results suggested that while the H5N1 virus can cause disseminated infection in humans, the lung is still the major site of viral replication, and viral replication in the lung in the later stages may decrease as a result of the depletion of the available target cells. In addition, the mRNA levels of the tumor necrosis factor-a (TNF-a) were found to be associated with the viral titers. J. Med. Virol. 83:1418–1423, 2011. ß 2011 Wiley-Liss, Inc. KEY WORDS: H5N1; viral load; autopsy tis- sues; tumor necrosis factor-a; interferon-induced protein-10 INTRODUCTION Most influenza viruses replicate only in the respira- tory and gastrointestinal tracts. This is because of the requirement of trypsin-like enzymes for the cleavage of the precursor hemagglutinin (HA) 0 molecule to produce functional HA1 and HA2 [Klenk et al., 1975; Lazarowitz and Choppin, 1975; Palese and Shaw, 2007]. Without cleavage by these enzymes which are present only in the respiratory and gastrointestinal tract, newly produced virions are not infectious [Zhirnov et al., 1984, 2002; Barbey-Morel et al., 1987; Kido et al., 1992]. The highly pathogenic H5N1 avian influ- enza viruses posses a highly cleavable HA cleavage site, which makes the molecule cleavable by furin, a ubiquitous enzyme [Stieneke-Grober et al., 1992; Horimoto et al., 1994; Klenk and Garten, 1994; Steinhauer, 1999; Guo et al., 2008]. Because of this, the H5N1 virus can cause disseminated infection in various avian and mammalian species [Lu et al., 1999; Keawcharoen et al., 2004; Kuiken et al., 2004; Govorkova et al., 2005; Thanawongnuwech et al., 2005; Amonsin et al., 2006; Songserm et al., 2006a,b]. However, in primates and humans, the infection has been shown to be limited more to the respiratory tract [Rimmelzwaan et al., 2001, 2003; Kuiken et al., 2003; Zhou et al., 2009]. Although viral RNA was found in various organs such as the intestine, heart, brain, spleen, kidneys, liver, lymph node, and placenta by reverse transcription-polymerase chain reaction (RT- PCR) and by in situ hybridization, viral antigen could be demonstrated only in the alveoli and a few other tissues [Uiprasertkul et al., 2005; Gu et al., 2007; Korteweg and Gu, 2008; Piwpankaew et al., 2010]. On the basis of these data, it was assumed that H5N1 virus replicated poorly in the extra-pulmonary tissues of humans [Uiprasertkul et al., 2005, 2007; Zhang et al., 2009]. In addition, strand-specific RT-PCR anal- yses of viral RNA from various tissues suggested that the presence of viral RNA in some tissues might be due to the contamination of virions and viral genomic RNA (vRNA) from the blood [Uiprasertkul et al., 2005] because viremia was demonstrated in patients infected with the H5N1 virus [de Jong et al., 2005; Grant sponsor: Faculty of Medicine Siriraj Hospital. *Correspondence to: Prasert Auewarakul, Department of Microbiology, Faculty of Medicine Siriraj Hospital, Mahidol University, 2 Prannok Road, Bangkok Noi, Bangkok 10700, Thailand. E-mail: sipaw@mahidol.ac.th Accepted 9 May 2011 DOI 10.1002/jmv.22146 Published online in Wiley Online Library (wileyonlinelibrary.com). ß 2011 WILEY-LISS, INC.