Interpretive Summaries

Epidemiology helps control ILT spread

Infectious laryngotracheitis (ILT) is a highly contagious respiratory disease in chickens. The ILT virus – a gallid herpesvirus – establishes latency within the host and can periodically reactivate in response to stress or immunosuppression. ILT has two forms: a mild form – which typically shows around 5% morbidity and 0.1-2.0% mortality – and a severe form – which can exhibit morbidity rates as high as 100% and mortality ranging from 5% to 80%.

While a global health concern in poultry production, Brazil’s Santa Catarina state had its first case in 2020 at a commercial layer farm in the county of São Ludgero. A team of researchers conducted a two-part epidemiological study to determine the serological, molecular, and pathological status of ILT in São Ludgero (Part 1, conducted in 2020) and all of Santa Catarina (Part 2, conducted in 2021). They described their findings in a recent edition of Poultry Science.

The researchers noted that the seropositivity (94.74%) found in São Ludgero county in Part 1 was “alarmingly high” for a newly diagnosed disease. All poultry farmers reported that they exclusively acquired chickens of known origin, and the vaccination program in the region predominantly included fowl pox, Mycoplasma sp., infectious bronchitis, pneumovirus, infectious coryza, and Salmonella sp.

In the subsequent year for Part 2, the seropositivity rate dropped to 65.3%, the researchers said.

According to the researchers, the serological results suggest the circulation and exposure to the ILT virus within the region’s poultry flocks was consistent with what would be expected during the seroconversion period, but the exact timing of primary exposure cannot be determined. Because of its status as an exotic disease in Santa Catarina, recombinant vaccines against ILT were not permitted until the first cases emerged, suggesting that the finding of anti-ILT antibodies in the São Ludgero region was associated with the circulation of virulent field strains or vaccine-derived strains from live attenuated vaccines that underwent virulence reversion.

The researchers noted that while their study examined ILT exposure across layer flocks, it did not include a separate analysis of commercial and rearing flocks. ILT during the rearing phase leads to lifelong seropositivity, which compromises the ability to determine when and where the infection occurred. They pointed out that the dynamics of rearing flocks may play a crucial role in the spread of ILT.

Once infected, the chicken remains a lifelong carrier, with periodic reactivations leading to viral replication in respiratory tissues, especially after stress or immunosuppression. When clinical signs of ILT do appear, they are often nonspecific and can resemble those of other infectious respiratory diseases such as avian influenza, Newcastle disease, infectious bronchitis, infectious coryza, and mycoplasmosis. The researchers pointed out that this highlights the importance of combining multiple laboratory diagnostic method.

Multivariate analysis confirmed that flock replacement with older chickens was a significant risk factor for ILT spread from the São Ludgero region to the rest of Santa Catarina state.

The researchers concluded that their study underscores the critical importance of implementing robust biosecurity measures in commercial layer farms, with particular attention to the type of replacement chickens introduced into flocks – the introduction of 90-day-old chickens significantly increased the likelihood of ILT seropositivity.

What does this mean for producers?

• Epidemiological surveys play a critical role in effectively controlling ILT and other respiratory diseases.

• Flock replacement with older chickens was a significant risk factor for ILT spread.

• Biosecurity measures important in preventing ILT outbreaks in commercial layer farms.

The full paper, “Two-year surveillance of infectious laryngotracheitis in layer farms from Southern Brazil: A seroepidemiological, molecular, and pathological approach,” can be found in Poultry Science and online here.

DOI: 10.1016/j.psj.2025.105382