Acute outbreaks can cause mortality so high that poultry producers face serious economic setbacks, warns Lloyd Keck, DVM, ACPV, a veterinarian for Zoetis Inc., in an article developed for Poultry Health Today.

Vaccination can help prevent the disease, but deciding whether to vaccinate and which vaccines to use can be a difficult balancing act, he writes.

ILT vaccines aren’t routinely used in flocks. They are generally reserved for flocks at risk for ILT. By at risk, I mean flocks on farms or in a geographic area that have had ILT outbreaks.

There are other risks for ILT. These include farms with less than ideal biosecurity, farms located near highways with poultry truck traffic and farms in areas with a high concentration of different flock types, including backyard hobby flocks and commercial layers. Because ILT tends to strike after 4 weeks of age, bigger birds are at increased risk too, and their producers stand to lose the most in an outbreak since more time and money have been invested.

Vaccine options

The most widely used ILT vaccines in broilers are the conventional, live chicken-embryo-origin (CEO) type. These vaccines are usually administered to broilers in water or by spray at about 2 weeks of age. They can cause respiratory signs and there’s a risk of spreading the virus from CEO-vaccinated to unvaccinated birds.

Because layer farms routinely receive live ILT vaccines, broiler farms located nearby have a higher risk of infection. However, studies show that CEO ILT vaccines provide better protection than the newer, recombinant vector ILT vaccines. [1,2]

The recombinant vector ILT vaccines can be applied in ovo or at the hatchery, which is a plus because it’s more convenient than administering vaccines on the farm. They cause no side effects and there is no risk that unvaccinated birds will be infected. However, recombinants cost more than CEO vaccines. They provide only partial protection and do not prevent field virus shedding, which has also been demonstrated in studies. [1,2]

In addition, not more than one herpesvirus of turkey (HVT)-based vaccine can be used at the same time — whether they are recombinant or not — because they will compete with each other.

As a result, if flocks receive an HVT Newcastle disease vaccine or an HVT infectious bursal disease vaccine, they shouldn’t be given an HVT vaccine for ILT.

The tissue-culture-origin ILT vaccine is labeled for eye-drop administration. This can be useful in long-lived birds like breeders but isn’t practical for broilers.

Individualised planning

It’s my belief that when vaccination for ILT is needed in a broiler operation, the exact plan needs to be customised. The history of ILT on the individual farm and in the general area needs to be considered as well as the age and weight of the birds being grown.

Birds being raised to less than 4 pounds or 5 weeks of age can be affected by ILT but seldom exhibit mortality from the disease. For that reason, it would seem unnecessary to vaccinate them against ILT, but it’s an arguable point.

There’s an incubation period with the virus of about 1 week, and during this time, these younger birds can appear clinically normal, but they’re shedding the virus, posing a risk to birds greater than 4 pounds or older than 5 weeks.

Younger birds can be vaccinated against ILT without much risk, which will reduce viral shedding. However, the performance cost of a CEO vaccine or the input cost of recombinant ILT vaccine are hard to justify in a small-bird operation.

In an ILT outbreak situation, especially in older birds where the disease can result in high mortality, it probably pays to use a CEO vaccine. Based on my experience in the field and on more than one occasion, I’ve seen producers with very high ILT mortality try to get things under control with a recombinant ILT vaccine.

Mortality has been reduced, but ILT bird deaths are still above the tolerance level. After the companies switch to a conventional CEO vaccine, mortality returns to acceptable levels within just a few weeks.

For poultry farms at very high risk for ILT or facing an outbreak situation, another option to consider is using both a recombinant ILT vaccine administered in ovo, followed by a CEO vaccine administered in the field. This plan obviously will raise vaccination costs but should help ensure enhanced ILT immunity. In fact, it’s possible that using both types of vaccine might save money in the long run if it prevents high ILT mortality.

Again based on my observations in the field, using the recombinant vaccine first in ovo appears to minimize the severity and duration of clinical signs associated with CEO vaccination in the field. It’s reasonable to infer that reduced severity and duration of clinical signs reflects reduced viral shedding.

Respiratory side effects from CEO vaccines are a concern, but they are generally mild and short-lived. Compare that to the clinical picture with an acute outbreak, when respiratory signs are severe and birds die, usually due to hemorrhagic tracheitis.

Vaccinal side effects can be further mitigated somewhat by management, such as good air quality and by keeping birds warm, because birds that feel chilled don’t want to eat and drink. Expectorants can also be used in water to help clear any vaccine reactions.

Biosecurity is imperative

Let’s not forget the importance of biosecurity and downtime, which is imperative for managing ILT. In an “LT zone,” all farms — vaccinated or not — are a biosecurity risk. As an industry, we’re not always as vigilant with preventive measures as we could be. I think that’s one reason that, in some areas, ILT is now occurring year-round instead of just during colder months.

Because the ILT virus can be transmitted by contaminated clothing and equipment, biosecurity must include restrictions on visitors and vehicles that come onto poultry farms. For example, anyone entering poultry houses should be required to have on clean clothes and use disinfectant footbaths going in and out. Equipment also should be disinfected before it is brought into the poultry house.

Another helpful measure we could take would be extending downtime between flocks. We tend to shorten the time between flocks in a house to operate more efficiently, but the ILT virus does not survive well without a host — the chicken. More downtime and improved cleaning and disinfection could help prevent ILT.

One more step to improved management of ILT would be cooperation and communication among producers. Vaccination of flocks can become a contentious subject if one company in an area wants to vaccinate with a CEO vaccine and another one doesn’t. The perceived level of risk isn’t the same for everyone.

Producers could make more headway by working together to assess risks for the disease, the advantages and disadvantages of various vaccination options and by planning an intervention strategy that includes biosecurity measures.

[1] Johnson D, et al. Protection against infectious laryngotracheitis by in ovo vaccination with commercially available viral vector recombinant vaccines. Avian Diseases. 2010;54(4):1251-1259.
[2] Vagnozzi A, et al. Protection induced by commercially available live-attenuated and recombinant viral vector vaccines against infectious laryngotracheitis virus in broilers chickens. Avian Pathol.. 2012;41:1, 21-31.