KUALA LUMPUR, Sept 19 — Despite more than seven decades of vaccines, outbreaks of Newcastle disease, a highly contagious poultry virus, continue to plague farms in Malaysia.
This raises concerns over the effectiveness of vaccination strategies for livestock when biosecurity and sanitation are weak.
Prof Dr Abdul Rahman Omar, professor of immunology and infectious diseases at Universiti Putra Malaysia (UPM), said vaccines may reduce illness, but do not completely prevent infection.
“Even if your livestock has been vaccinated, their immunity is still low, they still get an infection,” he told a technical session at the Livestock Malaysia 2025 conference last August 27.
“The same thing with Covid. You get vaccination but can you still get Covid? Yes. Why? Because the immunity is not strong enough. That’s what probably happened for Newcastle disease.”
Newcastle disease is caused by the Newcastle disease virus (NDV). The virus contains several important proteins, most notably the F, H, and G genes. The F gene forms the backbone for classifying the virus into different genotypes, while the H and G genes play a crucial role in vaccine development.
Unlike influenza, which has many serotypes, NDV has only a single serotype, which in theory should make vaccination straightforward. However, the virus is genetically diverse, with at least 21 genotypes identified so far. Genotype VII has emerged as the most prevalent worldwide, circulating in Asia, Africa, South America, and the Middle East.
The virus also varies in its pathotypes. Velogenic strains are highly virulent and cause severe disease, mesogenic strains are moderately virulent, while lentogenic strains are the mildest and form the basis for most vaccines in use.
Dr Abdul Rahman said studies in Malaysia show that genotype VII.2, which is prevalent locally, is genetically distinct from older vaccine strains such as LaSota and V1.
“When we did the analysis, we noticed that the genotype VII virus is the most diverse, with more than 20 per cent of this virus different compared to genotype II, typically like LaSota, V1 strain and so forth,” he said. This divergence, especially in the virus’s F gene, may reduce vaccine effectiveness and allow outbreaks to persist.
The virus is not confined to commercial farms. It also circulates in backyard chickens, village poultry, ducks, geese, and wild or migratory birds. Many of these birds carry the virus without showing symptoms, acting as hidden reservoirs of infection.
“The viruses can be detected in geese, in ducks – and these types of birds can be quite resistant against the virus, and they can become a carrier for the virus,” Dr Abdul Rahman explained. Migratory birds, he added, can spread the virus across regions and borders.
He stressed that recurring outbreaks are not solely due to the virus itself, but also to gaps in farm management and immunity.
Outbreaks often occur when the virus re-enters farms, bird immunity is weak even after vaccination, and sanitation or waste management is insufficient to clear remnants of infection.
“If you are unable to address all these things, you’re going to end up having Newcastle disease,” he warned.
Dr Abdul Rahman stressed that biosecurity is the most critical factor in prevention. Poor farm husbandry allows the virus to spread from previous outbreaks, neighbouring farms, or external sources.
Inadequate vaccination strategies make matters worse, particularly in day-old chicks that still carry maternal antibodies, which can interfere with vaccine effectiveness. “That’s why you have to make sure that you have the best strategy in terms of the vaccine you want to use,” he told participants.
Many farmers continue vaccinating without revisiting their programmes, even when outbreaks persist. “If you keep on with all this vaccination and outbreaks still happening, you may want to revisit your vaccination programme and see whether some parts probably may have gone wrong,” Dr Abdul Rahman said.
He added that concurrent infections and immunosuppression also play a role. Birds exposed to infectious bursal disease (IBD), including newer variants, may suffer early immune suppression. Other conditions such as infectious bronchitis or exposure to mycotoxins can similarly weaken the immune system and undermine vaccine performance.
Dr Abdul Rahman also warned that antigenic variation within genotype VII strains may not be reflected in current vaccine formulations.
“If these changes that we are detecting are not detected in the vaccine strains, the vaccine that we may have here may not be good enough to give robust protection,” he said.
This has driven the development of genotype-matched vaccines, which have shown promising results in some farms facing severe Newcastle disease challenges.
He also pointed to weaknesses in vaccine delivery methods. Drinking-water vaccination, though convenient, often results in uneven distribution and diminished potency.
“Try not to give the vaccine through water drinking,” he urged. Spray or eye-drop methods, while more labour-intensive, provide stronger and more consistent protection across flocks.
Poorly vaccinated flocks can continue secreting the virus, perpetuating its presence in farms and spreading it to other flocks if biosecurity is not enforced.
Genotype-Matched Vaccines Show Promise
Traditional live and killed vaccines remain the cornerstone of Newcastle disease control. Recombinant herpesvirus of turkeys (HVT) vaccines offer longer-lasting immunity but require specialised handling and take time to become effective.
Genotype-matched vaccines, designed specifically against genotype VII, offer a promising solution. “The vaccine has some superiority because it is able to increase the breadth and depth of the immunity,” Dr Abdul Rahman said.
South Korea, after introducing genotype-matched vaccines in 2010 alongside enhanced surveillance, saw a sharp drop in Newcastle disease cases.
Malaysian field trials likewise demonstrated stronger antibody responses, reduced virus shedding, and lower transmission rates.
Still, vaccines alone cannot solve the problem. Broiler farms need to ensure antibody titres – the measurable level of protection in the blood – reach protective levels through two to three doses administered correctly. Layers and breeders require even higher titres, with boosters every few months.
Yet even the best vaccines cannot compensate for poor farm management. “If your biosecurity, your sanitation programmes are not good enough, you will not be able to clear the virus from the farm,” Dr Abdul Rahman warned.
In layer flocks, outbreaks often reduce egg production without causing high mortality. Given the slow recovery, culling may sometimes be the most cost-effective option.
Dr Abdul Rahman said that while Newcastle disease remains endemic in Malaysia, improved biosecurity, vaccination strategies tailored to circulating strains, and consistent farm management can reduce its impact.
He said vaccines must be supported by strong sanitation and surveillance programmes if farms are to prevent recurring outbreaks.