Infection dynamics of highly pathogenic avian influenza and virulent avian paramyxovirus type 1 viruses in chickens, turkeys and ducks

A range of virus doses were used to infect 3-week-old chickens, turkeys and ducks intranasally/intraocularly, and infection was confirmed by the detection of virus shedding from the buccal or cloacal route by analysis of swabs collected using real-time reverse transcriptase-polymerase chain reaction assays. The median infectious dose (ID₅₀) and the median lethal dose (LD₅₀) values for two highly pathogenic avian influenza (HPAI) viruses of H5N1 and H7N1 subtypes and one virulent Newcastle disease virus (NDV) were determined for each virus and host combination. For both HPAI viruses, turkeys were >100-fold more susceptible to infection than chickens, while both these hosts were >10-fold more susceptible to H5N1 virus than the H7N1 virus. All infected chickens and turkeys died. Ducks were also much more readily infected with the H5N1 virus (ID₅₀≤10¹ median embryo infective dose [EID₅₀]) than the H7N1 virus (ID₅₀ =10^4.2 EID₅₀). However, the most notable difference between the two viruses was their virulence for ducks, with a LD₅₀ of 10³ EID₅₀ for the H5N1 virus, but no deaths in ducks being attributed to infection with H7N1 virus even at the highest dose (10⁶ EID₅₀). For both HPAI virus infections of ducks, the ID₅₀ was lower than the LD₅₀, indicating that infected birds were able to survive and thus excrete virus over a longer period than chickens and turkeys. The NDV strain used did not appear to establish infection in ducks even at the highest dose used (10⁶ EID₅₀). Some turkeys challenged with 10⁶ EID₅₀, but not other doses, of NDV excreted virus for a number of days (ID₅₀=10^4.6 EID₅₀), but none died. In marked contrast, chickens were shown to be extremely susceptible to infection and all infected chickens died (ID₅₀/LD₅₀ = 10^1.9 EID₅₀).