Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 26, 1999
Publication Date: N/A
Interpretive Summary: Marek's disease is an important infectious disease of chickens, caused by a herpesvirus. The virus most commonly causes death due to tumors in various internal organs, but also can affect the brain where it can cause nervous symptoms. One type of nervous disease has been called transient paralysis, because birds are suddenly paralyzed but rapidly recover. We report a variation of this form, which causes sudden paralysis but typically progresses to death within a few days. We suggest this form be called acute transient paralysis. This form is caused by more virulent strains of the virus, which have recently become prevalent. We infected several kinds of chickens with different viral strains and found that this acute form influenced by virus strain and dose, and by the genetic type of chicken. The acute form is prevented by prior vaccination against Marek's disease. These findings should help clarify the diagnosis of different forms of Marek's disease, and should also assist in developing better ways to classify the virulent strains and to design more efficient experiments to study these strains.
Technical Abstract: A novel syndrome was observed following inoculation of 3-week old chickens with highly virulent Marek's disease virus (MDV strains. This syndrome was characterized by the acute onset of neurological signs including flaccid paralysis of neck and limbs 9-10 days post inoculation, typically resulting in death 1-3 days after the onset of clinical signs. Most affected birds died and spontaneous recovery was rare. There were few if any gross tissue changes. Histologic brain lesions included acute vasculitis and vasogenic edema and perivascular cuffing. The syndrome was influenced by the virus strain and dose, and by chicken strain and B haplotype. It was prevented by vaccination with turkey herpesvirus. Chickens up to 18 weeks of age were susceptible. Based on clinical signs and histopathology, the syndrome was determined to be an acute form of transient paralysis (TP); its more acute nature and virtual lack of spontaneous recovery differentiated this syndrome from classical TP. Affected birds were viremic and brains were positive for viral DNA by polymerase chain reaction assays, but these tests were also positive in inoculated chickens without clinical signs and may have limited value for diagnosis. Although acute TP should only rarely in MD-vaccinated commercial flocks, this syndrome may be important in laboratory studies where it could not only interfere with pathogenesis trials but could also serve as a marker for highly pathogenic MDV strains. Acute TP may also be involved in the pathogenesis of the early mortality syndrome (EMS), a previously-described immunodepressive disease induced by inoculation of 1-day-old chicks with highly virulent MDV.