We next determined the replication and tissue tropism of APMVs in 1-day-old and 2-week-old chickens following intranasal inoculation. Chickens were sacrificed on 3 dpi for 1-day-old chicks and on 4 dpi for 2-week-old chickens, and the following tissues were harvested for quantitative virology: trachea, lungs, spleen, and brain (Fig. 6) . In 1-day-old chicks, APMV-1 strain BC replicated to high titers (.5.0 log 10 TCID 50 /g) in each of the sampled tissues in each of the birds, whereas LaSota replicated in trachea, lung and spleen, but not the brain, and its titers were less than those of BC (Fig. 6A) . APMV-3 replicated to moderate titers (.4.0 log 10 TCID 50 /g) in all of the collected samples, including the brain. However, replication of other APMVs was restricted to the trachea, and their titers were low, ranging from 1.5 to 3 log 10 TCID 50 /g (data for APMV-2 are shown in Fig. 6A ; the other serotypes are not shown). Chicks infected with APMV-1 strain BC began to show clinical signs on 1 dpi and distinctive neurological signs on 2 dpi, whereas chicks infected with other APMVs had no apparent clinical signs during the 3-day post-infection. In 2-week-old chickens, virus replication was more restricted and viral titers were lower compared to 1-day-old chickens (Fig. 6B) . APMV-1 strain BC was able to replicate in all of the collected tissues, but the virus titers in the brains of older birds (2.6 log 10 TCID 50 /g) were much lower than that of 1-day-old chickens (6.5 log 10 TCID 50 /g). APMV-1 strain LaSota replicated in the trachea, lungs, and spleen, but not the brain, as was observed in the 1-day-old chickens. Interestingly, APMV-3 replication was detected only in the trachea and the brain, indicating that it is neutrotropic in these older chickens despite this restricted replication. APMV-2 was also able to replicate in trachea of 2week-old chickens. However, replication of the other APMVs was not detected in any of the harvested tissue samples. Since APMV 2-9 can replicate much better in embryonated chicken eggs than in cell culture [20] , the collected tissue homogenates from this same experiment also were inoculated into 9-day-old embryonated chicken eggs to confirm the replication of viruses in various tissues ( Table 2 ; APMV-5 was not assayed because it does not replicate in the allantoic cavity of chicken eggs). This approach with increased sensitivity enabled us to detect replication of all of the tested APMVs in the trachea of most of the birds. In addition, APMV-2, 3, 6, 7, and 9 were detected in the lungs of at least one of the three chickens in each group, and APMV-3 was detected in the spleen of two chickens. Among the tested APMVs, APMV-4 showed the least replication in chickens both in vitro and in vivo ( Fig. 2 and Table 2 ), suggesting that it has a strong host range restriction in chickens. On 10 dpi, none of the APMVs were detected in any of the sampled tissues from any of the birds (not shown). Clinical signs of illnesses in any of the infected groups were not found up to 10 dpi.