19 October 2026
Reovirus continues to be detected, particularly in slower-growing broilers. During the first half of 2026, Royal GD diagnosed reovirus-associated tenosynovitis in 143 necropsy submissions. These comprised 10 submissions from conventional broilers, 129 from slower-growing broilers, and four from other poultry: one from broiler breeder pullets and three from layer pullets. Royal GD reported these findings in Veekijkernieuws Pluimvee.
The genotypes of the identified reoviruses were determined based on the σC gene. Under the revised classification system adopted by Royal GD, reoviruses are assigned to one of seven genogroups. The frequently detected genotype 4.7 is now classified within genogroup 6.
In 2025, genotype 4.7 accounted for 45% of the typed virus isolates. In the first half of 2026, this proportion fell to just 8%, while genotype 1 became dominant, accounting for 72% of all typed viruses. This increase was primarily attributable to a cluster of approximately 71 submissions with highly similar genotypes. These comprised 70 slower-growing broiler flocks and one conventional broiler flock, originating from 53 farms. The high genetic similarity within the cluster suggests spread from one or a few common sources.
The genotype 1 cluster appears to have emerged in the first quarter of 2026 and persisted into the third quarter. However, the cluster now appears to be stabilizing. At least three broiler breeder farms are associated with the cluster. Clusters are frequently observed in cases involving vertical transmission from parent birds to their offspring. In previous cases of vertical transmission, reovirus was detected in offspring for approximately six weeks. Subsequently, the outbreak subsided, and new offspring from the affected parent flock were protected against the strain. Depending on farm size, an outbreak may persist for a shorter or longer period. Vertical transmission lasting 12–16 weeks is unusual, as is the involvement of multiple farms. It therefore remains unclear whether other transmission routes are contributing to the spread, including repeated introductions through vertical transmission or contamination at the hatchery.
Reovirus antibody levels in broiler breeder hens are monitored at the end of rearing and/or at the start of production to assess the level of maternal immunity transferred to chicks. The measured antibody titers reflect a combination of antibodies induced by vaccination and those resulting from previous infections. Test results can vary considerably between different assays and, to a lesser extent, between laboratories. Reliable comparisons therefore require the same assay to be used by the same laboratory.
Until the end of 2025, the recommended benchmark was a titer group of at least 7 in 75% of parent birds. This guideline was based on historical data and the vaccines used at the time. In 2026, a new commercial vaccine offering broader protection was introduced. Following this change, lower antibody titers were observed using the reovirus IDEXX ELISA.
In 2026, only 3% of breeder flocks of conventional broiler breeds and 7% of flocks of slower-growing breeds met the previous benchmark. In earlier years, the proportion ranged from 24% to 61%. When the new vaccine was administered under controlled conditions, comparable titers were obtained using the reovirus IDEXX ELISA, while double vaccination resulted in slightly higher titers. These findings suggest that the new vaccines can explain the lower measured antibody titers.
Based on the available data, a titer group of at least 4 in 75% or more of samples appears to be an appropriate benchmark following one live and one inactivated vaccination. Following two inactivated vaccinations, a titer group of at least 5 should be achievable in 75% or more of samples. However, further research is needed to establish the relationship between specific antibody titers and protection against different reovirus strains and variants.
Royal GD, Veekijkernieuws Pluimvee (Poultry Veterinary Surveillance News), 2026. Related coverage: “Reovirus blijft opduiken bij trager groeiende vleeskuikens” - Sterke Erven, October 10, 2026.
