Western Meeting of Poultry

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Marek's Disease Condemnation and Vaccination

Dr. Teresa N. Cereno

Merial Canada

London, Ontario

 Meat inspectors in all provincially and federally inspected poultry Processing Plants regularly downgrade carcasses due to defects in quality. Most of the time carcasses are removed from the line due to air sacculitis and cellulitis (Infectious Process [IP] in the USA). Carcasses are also discarded when inspectors see lesions that may be associated with Marek’s Disease (MD).  

MD is a lympho-proliferative disease of chickens which affects the nerves, internal organs, iris of the eyes and skin. The causative agent is a herpesvirus that can be distinguished etiologically from other neoplastic diseases of birds. This very serious poultry disease has caused mortality rates as high as 60% in layers and up to 10% condemnation in broilers prior to the use of vaccines. Although losses still occur due to MD, they have been substantially reduced over time. However, the cost of vaccination and drops in egg production may still represent a significant cost to the industry worldwide. Downgrading of carcasses in the Plant is usually due to the skin lesions associated with feather follicles. The nodules may coalesce and appear scabby-like. In virulent MD, sometimes the blood vessels of the shanks are involved leading to the term “Alabama red leg”.

Fig. 1. Affected feather follicles of the legs   Fig. 2. “Alabama red leg” 

Almost every chicken hatchery in the world vaccinates against MD virus (MDV). Huge hatcheries employ in-ovo machines that are faster and are considered to be more efficient for vaccinating 18 day old embryonating eggs during transfer from incubators to hatchers. Smaller hatcheries still vaccinate at day of age by subcutaneous (SC) injection using a semi-automated machine (Accuvac, Bio-jector). Recently, the author was involved in an investigation following a report, from a hatchery, that some broiler carcasses were downgraded due to MD lesions. Other birds that were reared on the same farm but that originated from a different hatchery did not experience the same problem. On another farm that received birds from the same hatchery but on a different hatch date similar condemnation problems occurred. Since the hatchery had just brought in a new brand of HVT MD vaccine, management asked us for our assistance to help look into the condemnation problem. Here are some of the points that were observed during the investigation process:

1. A high number of staff members are performing the SC vaccinations, with a significant employee turn-over rate.
2. The machines being used are not properly maintained by the equipment supplier.
3. Vaccine mixing protocols are not followed properly.
4. Dye is not incorporated, making it difficult to determine inappropriate vaccinations.

 Proper vaccine mixing technique should be discussed, hands on, with the staff including the supervisors. Here are some of the points to keep in mind: 

1. The vaccine must be thawed properly and thoroughly using a water bath with distilled water and with a constant temperature (around 80 degrees Fahrenheit). Our experience has shown that a 90 second thaw cycle should be sufficient. A new water bath to meet this requirement was provided in this case.
2. If using any additive (dye, antibiotics), incorporate these into the diluent way ahead of the vaccine to allow time for these to stabilize.
3. Withdraw the vaccine from the ampoule and slowly inject it into the diluent. Withdraw some of the same diluent and use this to rinse ampoules and tips to ensure that all the vaccine PFU’s (plaque forming units) are harvested. Inject the final rinse back into the diluent bag. Mix slowly but thoroughly.
4. Ensure that the resuscitated vaccine is administered within 1 hour in order to maximize vaccine viability.
5. Periodic gentle agitation of the resuscitated vaccine solution will also help to ensure that the solution remains homogeneous and that all chicks will receive the correct dose. This will reduce the risks of sedimentation.

 It is important that vaccinators be regularly trained to ensure proper vaccination technique: 

1. Clean machines after every use and perform maintenance procedures on a regular basis. Check needles for dullness and kinks and change them regularly to prevent bird trauma.
2. The vaccination tubes in this case were long so it was suggested to change them for shorter ones as the length can contribute to damage and loss of vaccine PFU’s.
3. Ensure that all chicks are vaccinated. After incorporating a dye, it was determined that for some operators as many as 90% of the chicks were not being properly vaccinated. Misses can be easily determined by the dye showing up on feathers around the head, wings, chick boxes, walls and floor.    

Fig. 2. Vaccination misses

 Following several training sessions, the performance of the vaccinators improved as shown on the following graph. The hatchery also brought in a new machine which has been scheduled for regular maintenance and has a twin touch plate that ensures that the birds are always positioned properly for vaccination.

The MD vaccine is frozen and cell-associated and therefore fragile. Proper handling, mixing and application are very important to ensure that the chicks will have an increased chance of developing a high level of immunity to MD. Condemnation due to MD lesion may be a true concern and it is always essential to follow up with a broad ranged investigation. The author has presented some of the key elements that must be addressed in any such investigation. In most cases, MD problems are due to failures during the vaccination process. Adequate MD vaccination is still considered one of the best strategies for the prevention of clinical disease.

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Last modified: 5/28/2009