Methods to detect viable viral pathogens in poultry litter and implications for disease monitoring

Abstract

The broiler industry produces a large amount of spent litter. Repeated reuse of litter for rearing multiple batches of broiler chickens is common in North America and is becoming popular in Australia. However, this practice risks pathogen carry-over from batch to batch, particularly viruses. Pasteurizing litter by heaping or windrowing for several days between batches can help inactivate pathogens. Viable viruses in litter material can be detected using a chicken bioassay, an expensive and time-consuming method. A cheaper and easier method of detecting viable viruses in litter, particularly that undergoing pasteurization would be desirable. Detection of viral nucleic acid using PCR is an attractive alternative but the association between loss of viral infectivity and loss of detection of nucleic acids has not been established. Broiler chickens were reared on fresh pine shavings and infected with pathogenic Marek's disease virus, infectious laryngotracheitis virus, chicken anaemia virus, fowl adenovirus and infectious bursal disease virus to produce contaminated litter. Litter was then subjected mild to moderate heat in ovens (25, 35, 45, 55 and 65°C). In a subsequent experiment, chickens were exposed to heat treated litter at days 0, 5, 10 and 20 following heat treatment. After 35 days exposure, serum samples were collected and antibodies against the above viruses measured using ELISAs. Each viral genome was quantified from the heat treated itter co lected immediately before exposure using real-time PCRs. Most viruses were inactivated at higher temperatures (45-65°C) between 5 and 20 days but their nucleic acids tended to be detectable beyond loss of infectivity. Thus there is a lag between inactivation and disappearance of viral nucleic acid in litter.