Age-related susceptibility to Japanese encephalitis virus in domestic ducklings and chicks

Abstract

Ardeid birds and pigs are known as major amplifying hosts for Japanese encephalitis virus, and ducklings and chickens have been considered to play at best a minor role in outbreaks because of their low or absent viremia. We hypothesized that viremia of sufficient magnitude would develop in young ducklings (Anas platyrhynchos) and chicks (Gallus gallus) for them to serve as reservoir hosts and thereby contribute to the transmission cycle. Infection was associated with reduced weight gain in both species, and ducklings infected at 10 days of age or less showed overt clinical signs of disease. The mean peak viremia in birds of both species decreased as the age at infection increased from 2 to 42 days, indicating the importance of age of infection on magnitude of viremia in birds from both species, and suggesting that young poultry may be amplifying hosts of importance in disease-endemic regions.

Figure 1.

Peak viremia as a function of age of ducklings and chicks when infected with Japanese encephalitis virus. The graph plots the peak viremia observed in each of the chicks and ducklings against the age of the bird at infection. The vertical axis is on a logarithmic scale so that the fitted model is presented as a straight line. The underlying equation is PFU^PEAK = α×β^AGE, which transforms to log₁₀(PFU^PEAK) = log₁₀(α)+log₁₀(β)×AGE, where AGE is the age of the bird at infection. PFU = plaque-forming units.

Figure 2.

Dynamics of viremia in ducklings and chicks in the days post-infection as a function of age when infected with Japanese encephalitis virus. The eight lines in each graph give the fitted model for the trend in viremia as a function of time post-infection by using the equation PFU = (α×AGE^β×f(t,μ,σ))^6.67, where f is the lognormal distribution with parameters α and σ. The individual points are the actual data observations of viremia count for all the infected animals on days 1, 2, 3, 4, 5 and 7 post-infection. PFU = plaque-forming units.