Layer producers urged to guard against ILT

Poultry disease specialist says preventing infectious laryngotracheitis outbreaks in layers remains challenging.

Despite a growing population of more than 9 billion broilers, 290 million table-egg layers and other farms raising turkeys, ducks, pheasants and quail, US poultry producers do a remarkable job keeping birds healthy and productive.

But make no mistake, managing infectious disease is very challenging, says Dr. Guillermo Zavala, Poultry Diagnostic and Research Center, University of Georgia.

The veterinarian is particularly concerned about the increasing prevalence of infectious laryngotracheitis (ILT), the costly respiratory disease that tends to strike long-lived birds such as layers and broiler breeders.

In a keynote address at the International Avian Respiratory Disease Conference, Zavala noted that one densely populated area may have large numbers of farms with unvaccinated broilers close to commercial layers that have been vaccinated with either chicken-embryo-origin (CEO) or tissueculture- origin (TCO) vaccines. The same area also may be home to rendering plants, hatcheries, feed mills, spent fowl and other types of processing plants.

“Thus, the opportunity for ILT outbreaks is considerable compared to the same areas decades ago,” Zavala said.

BIOSECURITY RISKS

Practices such as transporting spent table-egg layers that carry the ILT virus can increase transmission of ILT to other flocks. Zavala also noted that poultry litter can harbor a number of infectious agents, including ILT virus. Moving poultry litter from operations where CEO vaccines were routinely used or where an outbreak of ILT may have recently occurred can, therefore, aid in transmission of the disease.

“As long as the layer industry continues to vaccinate their pullets with CEO vaccines in areas where there are unvaccinated broilers, as long as the broiler industry continues to market millions of broilers at the peak of [ILT] virus shedding, as long as spent [hens] continue to be sold and transported in risky manners and as long as poultry litter continues to be moved around with disregard to biosecurity and disease control, there will be no vaccine capable of fully solving ILT problems in the field,” he said.

ROLE OF VACCINES

CEO vaccines remain popular, especially in layers, but some have disadvantages, such as horizontal spread and the risk for reversion to virulence. TCO vaccines are less likely to revert to virulence or spread horizontally but, like some CEO vaccines, can lead to the establishment of latency, causing ILT to recur weeks or months after vaccination, he said.

TCO vaccines can be applied only by individual methods such as eye drop. Although CEO vaccines can be applied by mass-application methods, they can be excessively reactive if not administered at the right time, and if vaccine coverage is poor, the opportunity for back-passaging to naïve chickens may be “quite significant,” Zavala said.

‘MOST INVASIVE’

“Clearly, the most protective ILT vaccine available today is the CEO type of vaccine, but it is also the most invasive, the most reactive and the most capable of spreading and reverting to virulence.

The fact that most viruses isolated from field ILT outbreaks are CEO-related is quite revealing,” he said.

The disadvantages of CEO and TCO vaccines helped initiate the development of recombinant products, such as the herpesvirus of turkey (HVT)- or fowlpox- vectored vaccines. The recombinants can be applied by mass-administration methods such as in ovo, and they reduce mortality if an ILT outbreak occurs.

Recombinant vaccines are “clearly being used more frequently,” and while they have eliminated several of the disadvantages associated with conventional ILT control, they cannot eliminate ILT, in part because so many other factors are involved, Zavala emphasized.

One other important tool for control of ILT is “a series of rapid, precise, specific and economic diagnostic tools,” he said. The gold standard for ILT diagnosis is virus isolation, but it’s no longer used for routine diagnostics because it’s timeconsuming and expensive and it could take weeks to get results.

Therefore, expedited histopathology and molecular detection have become the new standards for ILT diagnosis since they both allow for a more rapid and effective approach to control in the field, Zavala said.

Zavala: Most viruses isolated from field ILT outbreaks are CEO-related.

1: Severe catarrhal tracheitis caused by ILT virus

2: Congestive to hemorrhagic tracheitis caused by ILT virus

3: Severe (extreme) hemorrhagic tracheitis in chickens affected with clinical ILT and high mortality

Photos provided by Dr. Guillermo Zavala