An oncogenic (tumor-inducing) herpesvirus
Occurrence and Economic Significance
Marek’s disease affects commercial chicken flocks from approximately 5 to 35 weeks of age in all areas of the world. Highly pathogenic (vvMD) strains of the virus are responsible for acute outbreaks of mortality which may attain 50% in exposed, non-immunized flocks up to 60 weeks of age. Generally, erosive losses of up to 20% occur in non-protected or inadequately vaccinated flocks. Marek’s disease virus is responsible for neural and visceral tumors. Marek’s disease virus
is immunosuppressive and infected broiler and pullet replacement flocks are susceptible to a wide range of viral and bacterial infections. Exposed broilers show increased mortality and condemnation rates at processing.
Exposure to MDV occurs by horizontal infection. The virus is resistant to environmental exposure and can remain viable for long periods in houses especially if units are not decontaminated between cycles. Infected birds shed “dander” (feather dust) contaminated with virus which can be distributed by wind, equipment, and personnel.
Involvement of the peripheral nerves results in paresis (weakness) of the legs or wings which progresses to paralysis. Death occurs in both caged and floor-housed birds as a result of dehydration and persecution.
Enlargement of the feather follicles is observed on the skin of de-feathered broilers and results in condemnation of carcasses in the USA, Canada, and Europe.
The characteristic MD lesion comprises enlargement of the peripheral nerves of the sciatic or brachial plexus. Occasionally visceral lesions are observed and the kidney, eye, proventriculus, ovary or other organs may be affected.
Diagnosis and Confirmation
The gross appearance of neural lesions is generally diagnostic. Histological examination of nerve and visceral lesions will show characteristic lymphocytic proliferation. The condition should be differentiated from botulism and from “transient paralysis”, an emerging condition of unknown etiology, but suspected to be an autoimmune response to vaccination in specific strains of commercial laying hens.
The causal virus may be isolated and identified by submitting tissues to a suitably equipped laboratory using specific tissue culture techniques.
Vaccination of broiler embryos using in ovo administration on the 18th day of incubation or by subcutaneous administration of vaccine to broiler, breeder or replacement egg-strain chicks at day old.
Three types of vaccine are available:
Type 1: attenuated chicken strain (e.g. Rispen’s)
Type 2: apathogenic chicken strain (e.g. SB1)
Type 3: apathogenic turkey, strain (e.g. HVT)
Due to the interfering effect of maternal antibody on HVT it is advisable to alternate vaccine types in successive generations. In countries where highly pathogenic MDV occurs, parents should be vaccinated with Rispen’s strain, allowing commercial progeny to be immunized using the less expensive HVT strain alone or in combination with the potentiating SB1 strain.
Cell-associated, frozen vaccines require special storage in a liquid nitrogen canister. Careful reconstitution using the diluents supplied by the vaccine manufacturer is necessary to maintain viability of the vaccine virus. Improper vaccination technique may lead to defective immunization with resulting “breaks”.
It is essential to place day old chicks in houses which have been thoroughly decontaminated to allow vaccinated flocks to develop immunity. Rearing farms and broiler growing units should be operated as single-age units with all-in-all-out cycles.
Fowl cholera is a contagious, bacterial disease that affects domestic and wild birds worldwide. It usually occurs as a septicemia of sudden onset with high morbidity and mortality, but chronic and asymptomatic infections also occur.
Newcastle disease is an infection of domestic poultry and other bird species with virulent Newcastle disease virus (NDV). It is a worldwide problem that presents primarily as an acute respiratory disease, but depression, nervous manifestations, or diarrhea may be the predominant clinical form. Severity depends on the virulence of the infecting virus and host susceptibility. Occurrence of the disease is reportable and may result in trade restrictions.
Fowl cholera is the reason for great economic loss of poultry in bangladessh