By Guillermo Zavala, DVM, MAM, MSc, PhD, Dipl. ACPV
Adjunct Professor, University of Georgia
President, Avian Health International, LLC
Flowery Branch, Georgia (USA)
Two circumstances have initiated reconsideration about the value of Marek’s disease (MD) vaccination.
One is the explosion in no-antibiotics-ever poultry production and concern that in ovo vaccination for MD could lead to increased first-week mortality. There’s some truth to this concern — if needles are penetrating a contaminated eggshell. The solution, however, is not abandoning MD vaccination. It’s improving sanitation throughout the hatchery and especially egg sanitation.
The other circumstance that brings the value of MD vaccination into question is the imminent decision by USDA’s Food Safety and Inspection Service to allow carcasses with MD-induced leukosis lesions to be trimmed instead of discarded. Why bother with MD vaccination?
I cannot emphasize enough that abandoning MD vaccination could backfire — and badly. In fact, vaccination against MD has never been needed more than it is now.
Remember how it was
Consider that throughout the poultry-dense Delmarva region and Georgia in 1970, condemnations in broilers reached approximately 3%. Nationwide, they were in excess of 1.5%.1 Today, in an industry that produces approximately 9 billion boilers per year, leukosis condemnations remain at negligible levels nationwide.
Leukosis condemnations are as low as they are today simply because industry has been vaccinating 100% of broiler chickens (and layers and breeders) consistently for approximately 50 years.
When we think about MD, we picture a disease associated with increased mortality and tumors, which constitute the best-known negative effects of MD in unprotected chickens. We tend to forget that MD is a highly contagious disease caused by one of the most persistent primary immunosuppressive agents in the poultry environment of young and adult chickens.
Field viruses of moderate virulence can cause inflammation, mortality and tumors, but the more virulent MD viruses can induce significant immunosuppression. Although vaccination against MD does not prevent infection, it can prevent or reduce clinical disease, mortality and tumors, and it can prevent MD virus-induced immunosuppression that results in unrelated, opportunistic infections in young and adult chickens.
It wouldn’t take long after intentional or accidental interruption of MD vaccination to see the beginning of a progressively deteriorating situation. Mortality from MD may increase, and evidence of immunosuppression will become patent at some point. Broilers raised without antibiotics especially need protection from immunosuppression to help ward off invading bacterial pathogens.
A broad variety of diseases can invade immunosuppressed broilers, leading to serious health, welfare and economic problems. Coccidiosis and Escherichia coli are just a couple of examples.
Environmentally persistent
There’s another reason why the poultry industry cannot afford to relax MD vaccination, and that’s the environmental persistence of MD viruses. Intense cleaning, disinfection, extended downtime, insect and rodent control as well as single-age rearing strategies all reduce the concentration of MD virus in the environment.
Taken together, these strategies minimize field challenge, but they never completely eliminate MD viruses. Virtually every single commercially raised chicken will be exposed sooner or later to MD virus.
Furthermore, protection against various infectious diseases is now accomplished through the use of modern herpesvirus-of-turkey (HVT)-vectored, recombinant MD vaccines that protect against MD and also act as carriers for proteins that induce immunity against respiratory viruses like Newcastle disease and infectious laryngotracheitis.
If the industry reduces the use of HVT-vectored vaccines, it will be forced to go back to the use of live, attenuated respiratory vaccines that can potentially cause unwanted reactions and various forms of economic disadvantage.
Nearly 20 years ago, Richard L. Witter, PhD, a prolific, world-class MD researcher,2 said that vaccination for control of MD has been one of the great successes in veterinary medicine.3 He also said the poultry industry as we know it today could not possibly exist and thrive without the effective vaccines available for protecting chickens against MD. Let’s not forget those words. To abandon MD vaccination would be taking a significant step backward.
1 Witter RL, Protective Efficacy of MD Vaccines. In: Marek’s Disease, K. Hirai (Ed) Springer Verlag. 2001;57-90. https://link.springer.com/chapter/10.1007/978-3-642-56863-3_3. Accessed September 28, 2020.
2 Ibid.
3 Ibid.
