Virus interference between H7N2 low pathogenic avian influenza virus and lentogenic Newcastle disease virus in experimental co-infections in chickens and turkeys

Abstract

Low pathogenicity avian influenza virus (LPAIV) and lentogenic Newcastle disease virus (lNDV) are commonly reported causes of respiratory disease in poultry worldwide with similar clinical and pathobiological presentation. Co-infections do occur but are not easily detected, and the impact of co-infections on pathobiology is unknown. In this study chickens and turkeys were infected with a lNDV vaccine strain (LaSota) and a H7N2 LPAIV (A/turkey/VA/SEP-67/2002) simultaneously or sequentially three days apart. No clinical signs were observed in chickens co-infected with the lNDV and LPAIV or in chickens infected with the viruses individually. However, the pattern of virus shed was different with co-infected chickens, which excreted lower titers of lNDV and LPAIV at 2 and 3 days post inoculation (dpi) and higher titers at subsequent time points. All turkeys inoculated with the LPAIV, whether or not they were exposed to lNDV, presented mild clinical signs. Co-infection effects were more pronounced in turkeys than in chickens with reduction in the number of birds shedding virus and in virus titers, especially when LPAIV was followed by lNDV. In conclusion, co-infection of chickens or turkeys with lNDV and LPAIV affected the replication dynamics of these viruses but did not affect clinical signs. The effect on virus replication was different depending on the species and on the time of infection. These results suggest that infection with a heterologous virus may result in temporary competition for cell receptors or competent cells for replication, most likely interferon-mediated, which decreases with time.

Figure 1

lNDV and LPAIV shedding in chickens and turkeys. Each data point and their equivalent EID₅₀/mL represents titers of lNDV and LPAIV detected in OP or CL swabs at different time points after inoculation, showing viral shedding of single infected birds (blue circles), simultaneously co-infected birds(red squares) and sequential infected birds (green triangles). Detection of lNDV in OP (a) and CL (b) swabs in chickens. Detection of LPAIV in OP (c) and CL (d) swabs in chickens. Detection of lNDV in OP (e) and CL (f) swabs in turkeys. Detection of LPAIV in OP (g) and CL (h) swabs in turkeys. In groups sequentially infected, the days post-inoculation (dpi) is based on the last virus given. Bars represent standard error of the mean.

Figure 2

Mean HI titers (log₂) in chickens and turkeys. Serum samples were taken at 11 and 14 days after infection with LPAIV, lNDV, or both. The number of birds with positive HI titers is shown (number of birds with titers ≥ the threshold of positivity/total animals per group). Different lowercase letters indicate significant differences in HI titers (P < 0.05) between groups. For statistical purposes, all HI-negative sera were given a value of 3 log2. Bars represent standard error of the mean.