Marek’s disease

Emily Martin

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2022;26(1):13. 

Marek’s disease virus (MDV) is a cell-associated alphaherpesvirus belonging to the genus Mardivirus.  The classification of this virus can be confusing as it has evolved over time (Table 1), but the original terminology is still in use.  In general, MDV refers to Gallid herpesvirus 2 (serotype 1, prototype virus).  Tumour production is associated only with serotype 1 viruses that vary widely in virulence.  Serotype 1 viruses are further divided into pathotypes based on virulence (Table 1).  Virtually all chickens are susceptible to MDV infection and tumor development.  Quail, turkeys, partridges, pheasants, and some species of ducks and geese, are also susceptible to infection and disease.

Table 1. Marek’s Disease virus classification.

Marek’s disease virus is ubiquitous.  Feather follicle epithelial cells in feathers and dander are the main source of environmental contamination.  The virus can remain infective for 4-8 months at room temperature and years at 4C.  A variety of chemical disinfectants can inactivate the virus within a 10-minute application time.

Inhalation is the natural route of infection.  MDV is transmitted by direct contact (residual house dust and dander, fomites, personnel) or indirect contact (aerosolized from adjacent houses) between chickens.  Virus excretion begins approximately 2 weeks post infection and continues indefinitely with maximum shedding 3-5 weeks after initial infection.  Once the virus is introduced into a chicken flock, infection spreads quickly from bird to bird, regardless of vaccination status or genetic resistance.  Vertical transmission does not occur.

Marek’s disease begins as an inflammatory condition that will either regress, become latent within 7-8 days post infection but still detectable in lymphoid organs and peripheral blood lymphocytes, or progress to development of lymphomas.  Based on disease progression, clinical syndromes and lesions can be generally divided into non-neoplastic and neoplastic.  Non-neoplastic syndromes that may occur in unvaccinated flocks include: lymphodegenerative syndromes (early mortality syndrome, cytolytic infection, immunodepression); CNS syndromes (transient paralysis, persistent neurologic disease of limbs or neck); and atherosclerosis.

Neoplastic syndromes can have non-specific clinical signs related to peripheral nerve dysfunction including crop dilation or gasping (i.e., vagus nerve involvement), incoordination, stilted gait, or unilateral paralysis with a characteristic clinical presentation of one leg forward, one leg back (Fig. 1A).  Birds with lymphomas may have no clinical signs or may present with depression, weight loss, pallor, anorexia, diarrhea, torticollis, and/or unilateral or bilateral blindness, with or without ‘gray eye’ which is typified by a gray discolouration of the iris and an irregular pupil.  Lymphomas can occur in any internal organ (Fig. 1B), as well as brain, nerves, eye and skin.  Tumours can be nodular or cause diffuse enlargement of organs.  Nodules are white or gray, firm, and smooth on cut surface.  On histopathology, the tumours consist of mixed populations of neoplastic lymphoid cells and inflammatory cells that include lymphocytes (primarily T cells) and macrophages (Fig. 1D).

There is no effective or practical treatment for individual chickens or infected flocks.  Control is a challenge due to ubiquitous virus, latent infection, continuous viral shedding, and long-term survival of virus in the environment.  Vaccination, biosecurity, and genetic resistance are key components of control.  Vaccines in order of increasing efficacy are: HVT (FC126), mixed HVT + serotype 2 MDV (SB1, 301B), and attenuated serotype 1 MDV (CVI988).  Vaccines do NOT prevent infection but they do protect against early replication of virulent viruses in lymphoid organs, and reduce the level of latent infection.  Vaccines can be given in ovo at day 18 of incubation, or at hatch (SQ or IM).  At least 7 days are required to establish solid immunity after vaccination.

Increased cases in a flock can be caused by unvaccinated birds, improper vaccination, vaccine strain (weak strain of vaccine may not protect against very virulent strains), early exposure (before vaccinal immunity is established), stress (e.g., onset of lay), and immunosuppression due to other diseases and viral infections such as CAV, IBDV and reoviruses.

Diagnosis: Detecting virus alone is not sufficient to make a confirmatory diagnosis, unless this detection is associated with characteristic clinical signs and lesions including visceral tumours and peripheral nerve infiltrates.

Samples:

Histology: peripheral nerves (sciatic, brachial plexus, vagus), brain, tumours

PCR:    a) Send-out test to Quebec:

Samples and collection: (shipping and handling within Canada: $50 CAD, each PCR: $40 CAD)

1) 1 mL EDTA blood - collect blood into a purple-top tube

2) tissues, feather tip - sterile, leak proof container

3) postmortem – lymphoid tissue (spleen, liver) or tumours

b) Send-out test to Poultry Diagnostic Research Center – PDRC, Georgia, USA: provides more options for typing of virus.  Please contact AHL Specimen Reception for details and pricing.  AHL

Figure 1. Clinical presentation and histologic lesions of MDV. (Photos B, C, D by E. Martin, H&E stain.)  A. Clinical presentation of one leg forward/one leg back (AAAP Marek’s Disease Slide Set).  B. Thyroid lymphoma expanding space between follicles.  C. Normal sciatic nerve.  D. Severe peripheral neuritis, sciatic nerve.  Extensive cellular infiltrates, most consistent with MDV.

References

1. Nair V, Gimeno I and Dunn J. Marek’s Disease. In: Diseases of Poultry, 14^th ed. Swayne DE, ed. Wiley Blackwell, 2020; vol I:550-587.

2. Gimeno IM. AAAP slide study set: Marek’s disease. AAAP, 2021.