Experimental infection of mallard ducks with APMVs indicated that all APMV serotypes, including duck isolates, are avirulent in ducks. Differences were observed in the replication of various APMVs between chickens and ducks. For example, replication of APMV-1 strain BC was low in ducks compared to its efficient replication chickens, although APMV-1 has shown to infect waterfowls, such as geese and Muscovy and Pekin ducks [40] [41] [42] . Several APMV serotypes (APMV-3, -8, and -9) did not replicate in ducks, even though APMV-3 replicated well in chickens and the APMV-9 strain had been isolated from ducks. In chickens, APMV-4 poorly replicated in both 1-day-old and 2-week-old chickens in our study. In addition, the virus has been shown to induce low HI titers in chickens compared to other APMVs, which was taken as evidence of a low level of replication of this virus in chickens [26, 39] . However, we detected replication of APMV-4 in trachea and lungs of infected ducks, indicating its preference to ducks from which this strain had been isolated. moderate multifocal mucosal attenuation, and loss of tracheal alveolar mucous glands. Lung sections (B) exhibit moderate, multifocal, lymphocytic to lymphohistiocytic bronchitis with mild to moderate perivascular and peribronchial interstitial inflammation and focal perivascular cuffing with varying severity. The presence of antigens for most APMVs was detected in the epithelial lining of trachea (C) and in the epithelium surrounding the medium and small bronchi of the lungs (D). doi:10.1371/journal.pone.0034927.g007 Table 3 . Replication of APMVs in 3-week-old ducks. Three birds from each group were sacrificed on day 4, and tissues samples (brain, trachea, lung, and spleen) were collected and homogenized. To confirm the virus replication, each sample (100 ml) in the collected samples was inoculated into three eggs, and allantoic fluids were collected on 3 dpi. Virus replication was determined by hemagglutination assay. Note that APMV-5 was not analyzed because it does not replicate in the allantoic cavity of chicken eggs. doi:10.1371/journal.pone.0034927.t003