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Phil Stayer Cropped Alternative

Virulent ND outbreaks underscore need to rethink vaccination program

By Philip A. Stayer, DVM, MS, ACPV
Corporate Veterinarian
Sanderson Farms, Inc.


The USDA has confirmed more than 450 cases of virulent Newcastle disease (ND) in California, as of mid-December 2019.1 The outbreak started in backyard chicken flocks, but the disease has also reached at least three commercial layer operations.2

These cases of virulent ND are the first to occur in commercial poultry since 20033 and prompted California’s state veterinarian to order mandatory euthanasia of flocks in several neighborhoods to stop the spread of the disease.4

The situation underscores the importance of making sure our ND vaccination programs are up-to-date. It also brings to mind the difficulties that can be encountered — even when we’re not dealing with the highly fatal, virulent form of ND.

Triggered by IB

Fortunately, most commercial US flocks affected by ND still have the lentogenic or minimally virulent type. In fact, most respiratory diseases in commercial US broilers are triggered by infectious bronchitis (IB) virus and ND is considered rare. Some lentogenic ND virus, however, still persists.

I recall hearing observations made by another poultry production veterinarian who found that when he altered his vaccination program to address IB and minimized ND protection, the ND virus challenge increased.

Years ago, early in my career with broilers in south Mississippi, challenge with the Arkansas strain of the IB virus was a problem in unprotected flocks. My veterinary predecessors weren’t permitted to use aggressive commercial IB Arkansas vaccines because they were sometimes associated with detrimental effects on livability in flocks previously unchallenged by this pathogen. However, once we proved that milder IB Arkansas vaccines could be used successfully in our area of the country, airsacculitis condemnations decreased in processed flocks, and vaccination against IB Arkansas became standard fare.

Sudden change one winter

Our Mississippi broiler flocks continued to have excellent respiratory health for years with minimal vaccination changes, but our good fortune changed one winter. We experienced increased field mortality due to airsacculitis, as well as an increase in condemnations at one of the processing plants. The Poultry Research and Diagnostic Laboratory (PRDL) in Pearl, Mississippi, continued to isolate vaccinal IB Arkansas. However, in-house serology showed low IB virus titers and increased ND virus titers. At the time we were using a very mild, modified-live ND vaccine — a full dose administered in the hatchery and another full dose of the same vaccine at 2 to 3 weeks of age.

To address the increased ND virus challenge, we switched to a more aggressive modified-live ND vaccine for the field boost, which produced the desired result of keeping flocks mostly free of clinical airsacculitis. ND virus titers in vaccinated flocks rose and were considered a measure of increased protection.

This new vaccine program worked for several years and would have continued, but then the recombinant herpesvirus of turkey vaccines with ND virus (rHVT-NDV) inserts became commercially available. After a few successful trials with those products, we replaced both hatchery and field ND vaccinations with the single-dose rHVT-NDV vaccine given in ovo. As expected with rHVT-NDV vaccines, broiler ND virus titers plummeted to almost undetectable levels.

One might speculate that prolonged vaccination with an effective modified-live ND vaccine displaced the field challenge so that rHVT-NDV vaccines faced a less intense ND virus challenge. Regardless of the mechanism, flock health continued to be excellent in south Mississippi on this minimalist ND vaccine program, even up to the writing of this article.

Since the rHVT-NDV vaccination program used in Mississippi was both protective and cost-effective, we adopted the same vaccination plan for our Texas broilers. As in Mississippi, the broilers in Texas continued to have very few airsacculitis issues. When they did occur, they were mostly due to some type of IB virus, which was identified by both serology and virus isolation.

Another hiccup

Over the years and to minimize costs at our Texas location, we switched to another company’s rHVT-NDV vaccination, but the protocol remained the same — until one winter when broilers started dying in the field with complicated airsacculitis. Survivors had an increased prevalence of airsacculitis condemnations at processing.

A company veterinarian reviewed serology results and noticed both IB and ND virus titers were elevated, so the in ovo recombinant ND vaccine was removed and a modified-live ND vaccine at day-of-age was reintroduced. The more aggressive field vaccine we used before wasn’t available at the time, so we had to use another one. It had the IB virus variants needed and a reportedly mild ND virus strain. The vaccine was primarily used in the egg-layer industry, and we soon found out that broilers react more intensely to these vaccines than egg layers.

With the success of the rHVT-NDV vaccine in Mississippi, we had confidence in its ability to protect against field strains of ND virus. However, after a few more attempts in Texas using various modified-live ND vaccines at day-of-age with a field boost, we ended up trying the mild hatchery ND vaccine again at day-of-age preceded by rHVT-NDV given in ovo. This combination of in ovo rHTVT-NDV and day-of-age modified-live NDV vaccination has proved to be effective for control of ND at our Texas location for the past 3 years.

Delayed immunity

Researchers have documented that immunity takes about 4 weeks with the rHVT-NDV vaccines.5 ND virus antigen expression is directly linked to the slower replication of the host HVT Marek’s virus. In other words, ND virus immunity takes longer with the recombinant vaccines compared to immunity from modified-live ND vaccines.

It appeared the Texas field ND virus challenge occurred between the decay of maternal antibodies and the onset of rHVT-NDV immunity. Day-of-age modified-live NDV vaccination, even with a very mild vaccine, appeared sufficient to fill the gaps in ND immunity.

These two experiences with ND disease demonstrate several truisms. First, each occurrence of a disease may not respond the same way with the same tools. Second, experience with various vaccines helps direct expectations but must be accompanied by an open mind to accept new realities. Third, some trusted tools may not always be available, so having multiple options for the same vaccine may be essential to future survival of commercial broiler-chicken production.

Of course, the situation with ND could change dramatically if the virulent form of the virus spreads further among commercial flocks. Unfortunately, we can’t foresee the future, so we’ll have to monitor and make changes if our flocks are affected.



1 Virulent Newcastle Disease, USDA, Dec 13, 2019.
2 Virulent Newcastle disease spreads to California’s commercial flocks. Food Safety News. January 14, 2019.
3 USDA confirms virulent Newcastle Disease in a commercial chicken flock in California. Dec. 18, 2018.
4 Virulent Newcastle Disease. California Department of Food and Agriculture. January 8, 2019.
5 Palya V, et al. Advancement in Vaccination Against Newcastle Disease: Recombinant HVT NDV Provides High Clinical Protection and Reduces Challenge Virus Shedding with the Absence of Vaccine Reactions. Avian Dis. 2012;56:282-287.


Editor’s note: The opinions and advice presented in this article belong to the author and, as such, are presented here as points of view, not specific recommendations by Poultry Health Today.

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Outbreaks of virulent Newcastle disease are a reminder that a flexible and engaged approach to vaccination is required to manage the condition.

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Posted on November 1, 2019

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