Taking the guesswork out of ILT diagnosis

Layer producers who suspect infectious laryngotracheitis (ILT) in their flocks should confirm the diagnosis with laboratory testing because the disease can be easily confused with other respiratory conditions, cautions Dr. Rob Porter, veterinary pathologist at the Minnesota Veterinary Diagnostic Laboratory, University of Minnesota.

“The most common mistake with ILT diagnosis in poultry is a failure to use diagnostic methods to identify the disease,” he says.

Most poultry managers are adept at recognizing the tracheal plugs often associated with ILT that lead to suffocation, but those plugs can also be due to wet pox (poxvirus). In some instances, flocks can be infected with both poxvirus and ILT. Furthermore, the lesions and clinical signs of ILT can be confused with other respiratory viral infections such as Newcastle disease and infectious bronchitis, says Porter, also a clinical professor at the university.

“Flock managers have been known to vaccinate for a suspected ILT outbreak when, in reality, the problem wasn’t ILT, so be sure to get a diagnosis before initiating ILT vaccination. A diagnosis can be obtained in less than 24 hours after receiving tracheal samples,” he says.

Porter recommends a laboratory diagnosis even if layer flocks have only mild respiratory signs. “Producers might observe mild signs in a flock and not pursue a diagnosis, only to wish weeks later that they had known what infection went through,” he says.

The best time to identify the specific cause of a respiratory disease is early during infection, and the best methods of diagnosis are what Porter calls the “tried and true” — necropsy, bacteriology, virology and histopathology. Serology is inadequate for a definitive diagnosis of infection in a flock, he says.

COMMON SIGNS OF ILT

One of the most common signs of an ILT infection in a flock, Porter says, is increased mortality. Egg production numbers often do not change. Tracheal plugs consist of white to yellow inflammatory material that blocks the glottis — the opening of the trachea. In some instances, hens can have blood or a yellow, crumbly substance extending down the upper half of the trachea.

“If one listens carefully in an ILT-infected house, there might be intermittent, high-pitched coughing; you may see open-mouthed breathing and hens shaking their heads as well as conjunctivitis, characterized by reddening and tearing of the eyes,” he says.

Some flocks are more susceptible to ILT, Porter points out. Pullets, for instance, are generally the most susceptible to many respiratory infections, including ILT, but “I’ve diagnosed ILT in range-reared layers, layers in aviary systems and layers in cage systems,” he says.

Because ILT is a herpesvirus, it can be harbored for up to 16 months in the nervous tissue, particularly the trigeminal ganglion, of a clinically normal hen. If that hen is then added to a new flock, the stress of the transfer may result in multiplication of the ILT virus, which then spreads from hen to hen.

“For this reason, flocks that are on farms with poor biosecurity practices, such as adding new hens during lay or mixing ages in the same flock, might be more susceptible to ILT,” Porter says.

SAMPLE COLLECTION

When ILT is suspected, Porter urges producers to send a broad array of respiratory tissues to a lab where they can be tested for a variety of infectious agents. The ideal method is sending whole birds for necropsy and sample collection.

Producers who want to collect samples for respiratory-disease diagnosis should take samples from the lungs and from tracheal air sacs extending from the throat to the chest. The entire head should be sent, including the eyelids, nasal cavity and infraorbital sinuses. Samples should be placed in sealed, plastic bags and labeled accordingly.

For ILT, in particular, attention should be given to tracheas that contain blood or tracheal plugs, Porter adds. Tracheal samples can be placed in neutral, buffered formalin so they are ready to be processed for microscopic evaluation immediately upon arrival at the lab. Raw tissues can be surrounded by frozen gel packs and placed in a water-tight Styrofoam container for overnight shipping, Porter says, noting that it’s useful to alert the lab veterinarian or pathologist that samples are on the way.

The classical method for diagnosing ILT at the lab is by examining affected tracheas for typical ILT lesions, because the virus causes microscopic changes that in many instances are diagnostic. These changes include clustering of tracheal epithelial cells and eosinophilic intranuclear inclusions typical of ILT infection.

Some labs also offer real-time polymerase chain reaction (rtPCR), which detects the nucleic acid of the ILT virus in infected tissues and can be performed in less than 12 hours, he continues.

The gold standard for ILT diagnosis, Porter notes, is virus isolation by embryonated-egg inoculation, but this method requires up to 3 weeks and multiple egg passages to complete. The poultry diagnostician should be aware of the importance of reporting a definitive diagnosis of ILT as quickly as possible in case the producer intends to vaccinate the flock in the face of an outbreak.

Based on his perspective working in the diagnostic lab, he believes that the incidence of ILT outbreaks in layers has stayed the same or decreased slightly during the last 15 years.

“This may indicate that ILT outbreaks have truly decreased because of effective vaccination and biosecurity — or that lesions and clinical signs of ILT can often be quite subtle, likely because many field outbreaks are caused by live-ILT vaccine viruses heating up as they pass from hen to hen,” he says.

Outbreaks of ILT attributable to vaccine virus, however, often are associated with low-grade mortality and the classical tracheal plugs with little tracheal hemorrhage, Porter says.

“It’s the general opinion of many diagnosticians that these subtle ILT outbreaks with low-grade mortality, death due to tracheal plugs and no blood in tracheas occur in many poultry flocks as a result of the egg-adapted vaccine virus circulating through the flock and increasing in virulence over a period of time. However, this is no reason not to vaccinate a flock for ILT, since a field-challenge virus in unvaccinated flocks could cause severe clinical signs and death loss,” he says.

The most important step producers can take to control ILT is initiating an effective biosecurity program. The ILT virus can survive outside of the hen’s body on inanimate objects including equipment, clothing, boots, crates and building walls as well as on dander and feathers, enabling the virus to be carried from flock to flock. Vaccination of the pullet flock is an effective way to bolster the biosecurity program, Porter says.

Typical posture of a hen with respiratory distress due to ILT, characterized by extension of the neck and open mouth.

ILT in eggs, indicated by the arrow.

Tracheal plugs often associated with ILT, which can lead to suffocation.