Marek’s disease virus “MDV” is a highly contagious herpes virus that can cause:
Other viruses in the retrovirus family are also capable of causing tumor diseases in heavy breeders such as leukosis and reticuloendotheliosis. There may also be sporadic mixed infections of Marek virus and avian retroviruses, making their diagnosis difficult; in addition, spontaneous tumors of non-infectious origin may occasionally be found.
It is important to recognise that, although vaccination can reduce the infection and transmission degrees of the virus , it is unable to completely prevent infection, replication, and release of pathogenic viruses into the environment. It is inevitable that healthy, productive birds free of clinical problems are carriers of pathogenic viruses, will carry pathogenic viruses and become a source of infection for other birds.
Historically vaccination has been a determining factor for the prevention of disease and the growth of the industry. The vast majority of vaccines available are live virus and cell associated.
Live virus vaccines
In the live virus vaccine, the virus replicates and remains within cultured cells for the production of the vaccines, which can be injected:
Commercial live virus vaccines
Commercial live virus vaccines include:
The HVT vaccine can be combined with vaccines of: Serotypes 2 -SB-1 and 301 / B1- and / or serotype 1 -CVI-988 (Rispens) –
With the combination of serotypes 1 and 2, the degree of protection is maximised. The combination of HVT with Rispens is the most used to protect breeders against highly virulent viruses.
Recombinant vaccines contain HVT, which functions as a vector for the insertion of genes that encode immunising proteins from other viruses. They induce immunity against the HVT vector and against the expressed proteins of the foreign virus. Recombinant vaccines should not be used alone in breeders, nor combined with other ordinary HVT or recombinant rHVT viruses, as the response to foreign virus proteins will be compromised.
Vaccine handling and preparation
Cell-associated vaccines are stored in liquid nitrogen (-196 ° C / -320 ° F) and are reconstituted in specific diluents supplied by the manufacturer. Cell-free vaccines are lyophilized and refrigerated, but are not used industrially. Reconstitution and administration of vaccines must be carried out strictly following the protocols recommended by the manufacturer. The strict hygiene of the vaccine preparation room and the vaccination equipment is an essential requirement to avoid contamination.
The importance of constant training
The processes of storage, thawing, reconstitution, hygiene and administration of vaccines require periodic training and routine audits by internal and external quality control specialists. Producers should consult with veterinary specialists familiar with available products, different vaccination strategies, local challenges, and vaccine selection, handling, preparation, and administration processes.
Administration of in-ovo vaccines has improved protection against disease and also suggested a positive effect on the non-specific immune response against other agents.
Reconstituted vaccines should be kept refrigerated and should be administered over a period of 30 to 60 minutes, in addition to being periodically gently shaken to prevent cell sedimentation and ensure a uniform dose.
The chicks will not be protected until the vaccine virus establishes viremia, which may take 4-5 days. It is essential to reduce the risk of early infection with Marek virus and other immunosuppressive agents through cleaning, disinfection, sanitary rest and biosecurity.
Factors influencing vaccination failures
Tumor lesions may occur as early as 3 weeks of age but are more common during development and around sexual maturity. Recently, problems have been observed around peak production, which is known as Late Marek. Virus isolation has little diagnostic value because even protected and healthy birds can become infected. Diagnosis should be based on clinical history, including mortality, lesions including thickened peripheral nerves, tumors in various internal organs, nodules on the skin, and irregularities in the shape and color of the eyes (gray eye), to be confirmed by histopathology.
It is important to collect a complete set of tissues to be examined by an avian pathology specialist.
Recommended samples for diagnosis
It is often necessary to use immunohistochemical or molecular methods that can detect specific proteins or high concentrations of Marek’s virus DNA in tissues and rule out other oncogenic viruses. The recommended samples for diagnosis are summarised below:
When mortality and possible tumors exist, cachectic and / or lethargic or paralysed birds, or those that have recently died, should be selected to perform the confirmatory methods described. A detailed medical history should be collected, including the vaccination history, weekly mortality, and percentage of birds with apparent tumor lesions. Once the diagnosis is confirmed, possible causes should be investigated.
It is possible to find mixed infections with Marek’s virus and other tumor viruses, which requires additional testing. Virus isolation is only occasionally used to characterise its pathogenicity in susceptible birds.
The control of Marek’s disease depends on the correct selection of vaccines; an excellent vaccine and vaccination management process; biosecurity, cleaning and disinfection; and the control of other immunosuppressive agents. Confirmatory diagnosis requires a correct approach in the laboratory and exclusion of other possible causes of tumours
Aviagen Brief – Marek’s Disease Control in Broiler Breeders by A. Gregorio Rosales DVM, MS, Ph.D, DACPV – January 2018
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