Epidemiology plays critical role in controlling ILT

An epidemiological study reported the spread of infectious laryngotracheitis (ILT) among poultry, underscoring the critical role that epidemiology plays in detecting and controlling disease spread.

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 morbidity rates of around 5% and mortality rates of 0.1% to 2.0%; and a severe form, which can exhibit morbidity rates as high as 100% and mortality rates ranging from 5% to 80%.

Although ILT is a global health concern in poultry production, Brazil’s Santa Catarina state had its first case in 2020 at a commercial layer farm in São Ludgero county. A team of Brazilian 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.

Serology results

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 spp., infectious bronchitis, pneumovirus, infectious coryza and Salmonella spp.

The seropositivity rate dropped to 65.3% in the subsequent year for Part 2.

These results suggest the circulation of, 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 could not be determined, the researchers noted.

Because of ILT’s status as an exotic disease in Santa Catarina, recombinant vaccines against ILT were not permitted until the first cases emerged, suggesting that the detection 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 infection 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 ILT’s spread.

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 methods to confirm an ILT diagnosis.

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. Because the introduction of 90-day-old chickens significantly increased the likelihood of ILT seropositivity, there should be particular attention paid to the type of replacement chickens introduced into flocks.

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 are critical 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.

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