Poultry 1: Overview, Marek's Disease

Diagnosis of Marek's disease

-history, signalment (2-5mo) -classic clinical presentation (asymmetrical flaccid paralysis) -classic gross & histo pathology signs

Gross pathology of Marek's disease (4 things)

1) Enlargement +/- yellowing and loss of cross-striations of peripheral nerves (esp sciatic nerve) *this is called neural lymphomatosis 2) Visceral tumors and enlargement of organs (ovary, lung, heart, kidney, liver, skeletal muscle, intestines, iris, skin) 3) Enlargement of skin follicles (skin leukosis) 4) Discoloration of the iris (see pic)

10 general causes of disease in chickens

1) Nutritional (deficiency, toxicity, imbalance - eg rickets) 2) Metabolic (obesity, cage layer fatigue, fatty liver disease) 3) Genetic/hereditary (predispositions, susceptibilities) 4) Congenital (egg incubation - blindness) 5) Degenerative (muscle myopathy) 6) Toxicity (ionophore, arsenic, organophosphate) 7) Management (dehydration, starvation, cannibalism) 8) Neoplasia (Marek's disease, Leukosis) 9) Diseases of high production (ascites, sudden death syndrome) 10) Infectious agents (many!)

What is the hallmark histopathology lesion of Marek's disease?

1) Pleomorphic lymphocytes (mostly T cells, some B cells)

Prevention of Marek's disease: 2 main things to do

1) Vaccination: -HVT (serotype 3) -SB1 (serotype 2) -Rispens (serotype 1) **vx = combination of all serotypes; can do in ovo or SC 2) Biosecurity (AIAO, disinfection, etc)

What are the 3 serotypes of Marek's disease virus?

Serotype 1: ubiquitous in chickens, oncogenic Serotype 2: common in chickens, non-oncogenic Serotype 3: common in turkeys, non-oncogenic

Signalment of Marek's disease

Sexually immature chickens 2-7 mo (but can occur in any age >3wks)

Etiology of Marek's disease

herpesvirus induced neoplastic disease

Transmission of Marek's disease

Direct or indirect contact (airborne, fomites); infected chickens transmit indefinitely *not vertical transmission

Where does Marek's disease virus replicate?

Epithelial cells of the feather follicle

Clinical signs of Marek's disease

Incubation period 3-4 wks; then: -paralysis (ataxia, asymmetric flaccid paresis - classic sign is one leg forward & one back while lying on side) -gasping, crop dilation (d/t vagal involvement) -weight loss, anorexia, eventually starvation & dehydration -paleness *progression to neoplasms (classical form) or death within 24-72h following onset of paralytic signs (acute form)

Pathogenesis of marek's disease

Lymphotrophic herpesvirus --> lymphocytolytic infection --> immunosuppression --> changes in cytokine expression (increase IFN-γ, IL-6, IL-18)