Increased Virulence of Marek's Disease Virus Field Isolates

Abstract

The continuation of an apparent evolutionary trend of Marek's disease virus (MDV) towards greater virulence may explain recent increased losses from Marek's disease (MD) in vaccinated flocks. To address this question, the virulence of 31 isolates of serotype 1 MDV obtained from layer or broiler flocks between 1987 and 1995 were characterized. Each isolate was cultured in duck embryo fibroblasts for four to six passages, and ascertained to be free from contamination with avian retroviruses, chicken anemia virus, and MDVs of other serotypes. The viruses, along with prototype viruses JM/102W and Md5, were tested for virulence by inoculation at 6 days of age into laboratory strain 15I5 x 7(1) chickens of three types: nonvaccinated, vaccinated with turkey herpesvirus (HVT) and bivalent (HVT + SB-1)-vaccinated. The results showed that three isolates did not differ from JM/102W and were classified in the virulent (vMDV) pathotype. Twenty-one isolates produced significantly higher levels of MD in HVT-vaccinated chickens than did the JM/102W control and were classified in the very virulent (vvMDV) pathotype. Seven isolates, five of which were isolated in 1994 or 1995, produced significantly higher levels of MD in bivalent-vaccinated chickens than did the Md5 (vvMDV) control. These isolates, provisionally designated as the vv+MDV pathotype, appeared to be at the high end of a virulence continuum. Several MD response parameters, including lymphoma mortality, early mortality with bursal/thymic atrophy, and frequency of visceral lymphomas or ocular lesions in nonvaccinated chickens were positively correlated with virulence. These findings support the continued evolution of MDV towards greater virulence.

Genomic Variation between Genetic Lines of White Leghorns Differed in Resistance to Marek’s Disease

“…Many factors affect genetic resistance to MD and vaccine protective efficacy. Some of these have been well investigated and reported [23][24][25][26][54][55][56]. We recently showed non-MHC genetic variation, in addition to MHC B haplotype that was primarily reported by Bacon and Witter in the 1990s [23][24][25][26]57], also plays an important role in modulating MD vaccine efficacy [31][32][33].…”

The industrialization of farming may be driving virulence evolution

“…The disease continued to cause paralysis, but was now accompanied by an unusually large presence of lymphomas in very young birds. This resulted in widespread outbreaks and very high mortality rates around the world (Benton & Cover, 1957; Witter, 1997). There was an urgent need for human intervention to combat economic losses, and the solution came in the form of vaccination (Churchill, Payne, & Chubb, 1969; Okazaki, Purchase, & Burmester, 1970).…”

Differential Cytokine Responses following Marek's Disease Virus Infection of Chickens Differing in Resistance to Marek's Disease

“…Clinical signs include immunosuppression, polyneuritis, and T-cell lymphoma formation in the visceral and ectoderm-derived tissues. Although MD has been controlled by vaccination for over 30 years, it continues to be a serious threat to the health and welfare of poultry, and there is growing evidence that intensive use of vaccines is driving the virus to increasing virulence (62). Infection with virulent strains of serotype 1 MDV causes an early cytolytic infection (3 to 7 days postinfection [dpi]), primarily in B lymphocytes with temporary, often profound, immunosuppression (14) and Tcell activation.…”