Prevention

As there is no therapy for MD, the only way to fight the disease is by preventive measures. For this purpose we have several possibilities at our disposal:

1. Vaccination

At the moment there are several MD vaccines commercially available:
ท Monovalent vaccines:
HVT: Herpes Virus of Turkeys (MDV Group 3). Widely used in many countries. It has been given very good results in the past, but protection is now insufficient in many countries. HVT is marketed in cell-free (Lyophilized) and cell-associated (under liquid nitrogen) form.

CVI 988: This is the so called Rispen's strain. It is an attenuated Group I MD virus, which means that originally it was a pathogenic chicken Herpesvirus, weakened by attenuation. In the past, there was a fear that this vaccine could turn into a pathogenic form again and therefore it was not allowed in some countries.

Experience in the field during many years proved there was no risk whatsoever in using this vaccine. In some countries where Rispen's is not allowed, a cloned strain called CR6 is used on a small scale. Rispen's vaccine is a cell-associated strain, so it has to be stored under liquid nitrogen.

ท Polyvalent vaccines:
SBI: SBI is a group 2 MDV. As stand alone, it does not give sufficient protection but in combination with HVT, it gives better protection than HVT alone. This combination is used in some areas where HVT by itself, does not give sufficient protection.

HVT+CVI 988:
This combination is used in some countries where MD is a very serious clinical problem. There are field experiences that this combination gives a good protection for MD, however there is little experimental or laboratory work to support these experiences.

Besides the choice of the best vaccine for a certain area, the following aspects can also influence the result of MD vaccination considerably:

Dosage of vaccine
Generally, this only plays a role when the vaccine contains insufficient vaccine virus. The minimum required virus content of vaccines is 2000 PFU for HVT and 1000 FFU for the Rispens vaccine. Beyond a certain limit further increasing the PFU and FFU does not make a significant difference anymore.

Storage and reconstitution of the vaccine
The utmost accuracy is required: the manufacturer's instructions should be closely followed. A proper reconstitution is especially very important with the cell-associated vaccines. After reconstitution the vaccine has to be used quickly, preferably within a few hours time.


Administration of the vaccine
Also this should be done as accurately as possible. Quality, cleaning and disinfection of the material should be done perfectly. Automatic vaccinators have to be checked on a regular basis.

Maternal immunity
Maternal immunity in one day old chickens can delay immunity build-up after vaccination. This effect can especially be observed with cell-free HVT. To avoid this, we recommend to use HVT only for the last generation which is vaccinated (e.g. for great-grandparents: Rispens; for parent stock: Rispens+ HVT). In some countries birds are revaccinated after some time (at an age of 1 to 6 weeks) to avoid influence of maternal immunity. There is no scientific base for this system, but some organisation claims good response. To achieve a very early immunity for MD the vaccine can be given in ovo (in the egg). It is widely used for broiler now, but for layers and breeders, there is still insufficient data available to judge the effect.

1. Hygiene

Early strong MD infections:
As mentioned earlier, MD virus can be isolated in almost all poultry areas world-wide. It is present in all the dust inside and around poultry houses. Furthermore it requires some time (10-14 days) for vaccinated day-old chickens to develop a reasonable level of immunity. On top of this, young birds are very susceptible for infection with Marek's virus because the immune system is not fully matured at hatch.

These aforementioned facts show the high risk of young chickens getting the infection before immunity hence the day-old vaccination has been developed. To prevent this early, strong MD infection (against which no vaccination can really protect) , it is important to keep the young chickens under good hygienic conditions. The houses must be totally clean (to assist prevention of MD a proper cleaning procedure is even more important than good disinfection). This includes cleaning and washing of the houses, ventilation systems, and if possible the area around the house. No adult birds, which almost all spread MD virus, should be close to the rearing houses.

Other infections/ conditions influencing MD immunity. Some diseases, like Chicken Anaemia (CAV), IBD (Gumboro Disease) and REO viruses are known to have a negative influence on the immunity for MD. Also any immunosuppressive agents will delay the build-up of immunity. Examples here are stress factors and mycotoxins.

2. Genetics

For a long time, it is known already that there are differences in resistance to MD that are genetically determined. What these differences are exactly based on, are not known. The genetic resistance will probably become more and more important as protection by vaccination becomes more and more difficult. At the moment breeding for MD resistance is an important goal in the breeding programs. Different systems can be used for this:
ท Selection of resistant birds based upon blood-typing (B-alleles)
ท Selection of resistant birds based upon challenge: non-vaccinated birds are placed under strong infectious conditions
ท Selection based upon marker genes


Summary

MD is a very serious threat to the poultry industry world-wide. Unfortunately there are no easy answers to this problem. Any solution has to be based upon cooperation of all the following areas in the poultry industry:

laboratory:
ท proper diagnosis
ท virus isolation (to make the best choice of vaccine)

vaccine industry:
ท development of new vaccines (mono- or polyvalent)

hatchery:
ท proper storage of vaccine
ท proper vaccination procedure

farmers:
ท prevention of strong early MD exposure
ท prevention of other diseases influencing MD

breeding organisation:
ท selection of more resistant strains

Only with cooperation of all these levels, we will be able to keep Marek's Disease under control.