Marek s disease virus




















Photo by The New York Times. In recent years , experts have wondered if leaky vaccines were to blame for the emergence of these hot strains. Paralysis was more permanent; brains more quickly turned to mush. The hottest strains killed every unvaccinated bird within 10 days, and the team noticed that barely any virus was shed from the feathers of the chickens during that time.

The virus spreads via contaminated dust in chicken coops. In contrast, vaccination extended the lifespan of birds exposed to the hottest strains, with 80 percent living longer than two months. But the vaccinated chickens were transmitting the virus, shedding 10, times more virus than an unvaccinated bird.

This study argues yes. It did. The virus spread to sentinel birds nine days faster if it came from a vaccinated chicken versus an unvaccinated one.

In addition, sentinels died faster when exposed to vaccinated chickens versus unvaccinated chickens. The vaccination of one group of birds leads to the transmission of a virus so hot that it kills the other birds, said Read said.

The offspring are protected by the maternal antibodies of the mother and that allows the virus in the chicks to transmit before they kill the host. So they transmit and kill the other individuals. This trend persisted when the team tried the experiment in a setting meant to simulate a commercial chicken farm. When bird flu breaks out in these western chicken populations, farmers must cull their herds.

One important point to be borne in mind about MD diagnosis is that MDV is ubiquitous in many poultry farms and the detection of virus, viral antigens or nucleic acids alone in the absence of disease does not confirm the occurrence of MD.

Macroscopic lesions of tumours and enlargement of peripheral nerves together with histopathological confirmation is commonly used in the diagnosis.

More recently, PCR-based molecular diagnostic tests are more widely used than virus isolation. Availability of the nucleotide sequences of a number of pathogenic and vaccine strains of MDV have enabled the development of efficient PCR methods for precise detection and quantitation of pathogenic and vaccine strains. Because of the ubiquitous nature of the virus with the potential for long term survival both within and outside the host, vaccination is the only effective strategy for the control of MD, although additional measures of increased biosecurity and selecting for genetic resistance are considered valuable adjuncts to the control programme.

Several different types of MD vaccines are in use, both individually and in combinations. HVT is extensively used in broiler population, and is available both in cell-associated and cell-free forms 2.

Bivalent vaccines, comprising HVT and SB-1, demonstrated improved protection through synergistic effects of the two vaccines. HVT virus is also extensively used as a live recombinant viral vector for protection against other avian pathogens 16 , CVI strain is considered the most effective vaccine, and considered the gold standard in inducing protection, particularly against the more recent hypervirulent strains of MDV.

However, their efficacy can vary with virulence of the MDV strains, and this forms the basis of the pathotyping assay to classify strains according to their virulence ranks to break though different vaccination regimes 3. Continuing increase in virulence of MDV strains in spite of vaccination is a major concern to the poultry industry Recent study has demonstrated that the currently used MD vaccines such as HVT do contribute to increasing virulence by keeping the vaccinated hosts alive thereby encouraging transmission 1.

Contributed by: Prof. Venugopal Nair. PLoS Biol 13 , e, doi Poult Sci 77 , Even if the breeders are infected, the chicks will hatch clean if they do not come into contact with the dander. The symptoms of Marek's Disease depend on which tissues are attacked. In the classic form, Marek's Disease will cause inflammation and tumors in the nerves, spinal column, and brain.

In this form, birds will become paralyzed in the legs, or wings or may develop head tremors. Paralyzed chicken with Marek's Disease. Affected birds eventually die of starvation or are trampled or get severe sores on their body. They almost never recover from this. Marek's may also cause tumors in the internal organs, the eyes, and even the skin. Affected birds lose weight, become emaciated, and die. Marek's Disease affected eye picture on left. Liver tumors seen with Marek's Disease. Diagnosis of Marek's Disease is by typical symptoms, necropsy, and biopsy examination of the tissues.

Most diagnostic animal labs can test for this disease in necropsy specimens. So how can you prevent Marek's Disease in your flock?

Vaccination of day old baby chicks is the most dependable way to prevent the clinical disease. Birds must get the vaccine before they are exposed to the virus. Then the birds need about days for the vaccine to do its work. That means complete isolation of the chicks for at least this time.

The chicks should be kept away from other birds and have a separate caretaker for them. If a separate caretaker is not possible, the chicks should be cared for first before caring for the others.

Caretakers should shower and change clothes if needed to go back and forth between age groups. Again, this is for at least 4 days. The best way to vaccinate the chicks is to have the hatchery vaccinate them. This is the preferable method if purchasing from a hatchery. If vaccinating at home, the vaccine handling must be done exactly as the label directs.

This means following instructions on how quick to thaw, what temperature to thaw, and using the vaccine no longer than hours after reconstitution as indicated on the label. As hardy as this virus is in the feather dander, it is a live virus that dies rather quickly in vaccine form.

Unfortunately, once the vaccine is reconstituted, it must be thrown out after two hours. It cannot be stored for later use as the vaccine is no longer effective. The vaccine must be injected usually under the skin to work.

This can be tricky with tiny chicks, so having someone hold the chicks and another to inject is helpful. Other steps to get the best vaccine protection is to be sure the brooder is clean and disinfected before placing the chicks.



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