Differentiating field strains of Marek’s disease virus and live vaccine virus in the commercial poultry house is essential to effectively control the disease, as Poultry World discovers

Vaccination has been successful in providing lifelong protection of chickens against Marek’s disease since the 1970s.

But it doesn’t stop them from becoming infected with virulent strains of the herpesvirus that causes the disease.

The ability to detect and measure the vaccine and the virulent virus, and to distinguish between them, is important for monitoring and thereby maintaining effective protection.

“There are several important reasons to be able to make this distinction,” says Stuart Andrews, poultry technical manager of Zoetis.

“We need to be able to confirm effective administration and replication of the vaccine virus, and to identify any causes of poor vaccination uptake. It is also helpful to be able to confirm the presence or absence of virulent virus, and to monitor its levels in the poultry house environment.”

Different strains

Marek’s disease is highly contagious and caused by virulent serotype-1 strains (MDV-1) of the herpesvirus.

Three closely related strains: MDV-2, herpesvirus of turkeys (HVT) and CVI988/Rispens – which do not cause disease in chickens, have for more than 40 years been used as live vaccines against Marek’s disease.

If the chickens have been vaccinated using MDV-2 or HVT, real-time quantitative PCR testing can be used to easily distinguish them from virulent MDV-1 strains, because they are genetically different.

However, if the chickens have been inoculated using CVI988/Rispens vaccine, it is very difficult to tell the difference because the vaccine is genetically so similar to virulent MDV-1.

Laboratory tests currently involve a lengthy two-stage procedure and are frequently inconclusive.

New tests

At the Pirbright Institute’s Compton Laboratory in Berkshire, researchers have been collaborating with Zoetis over the past 12 years to develop a new method to distinguish and measure CVI988 vaccine and virulent MDV, targeting a specific area of the virus genome.

“There are major advantages of the new quantitative PCR tests which we believe will help the poultry industry counter the constant challenge from Marek’s disease,” said Dr Andrews. “While there are two assays run on each sample – one detects CVI988 vaccine and the other virulent MDV field virus – each test is rapid and easy to perform.

“Each test is highly specific for the target virus, so both the CVI988 vaccine virus and virulent MDV field strains can be quantified, allowing comparison between samples.”

Dr Susan Baigent, who developed the test, said a major difficulty in tackling Marek’s disease is the ease with which the virus is spread.

“Although the vaccine protects the birds, it doesn’t stop the virulent virus replicating in them and being shed from the skin and feathers. Hence the virus can readily contaminate the litter and dust within the poultry house, and then spread to other houses through being carried on the air, on people and even vermin.

“Levels of vaccine and virulent MDV are greater in the feather tips than in other chicken organs. Feather tips are easily collected from young chickens, and we can readily isolate DNA. Feather samples are much more convenient than blood or spleen samples. We can also measure levels of vaccine and virulent MDV in samples of poultry house dust, which gives an overall picture of the level of the vaccine and virulent virus for the whole flock.”

The new technique, which is four times faster than a conventional test, is now being used at the Poultry Centre of Excellence which Zoetis opened at Torce in Brittany in 2013, providing PCR diagnostic facilities for customers in Western Europe.