Controller-approved source entry - manual-review caution required Poultry Infectious Disease Manual reviewFlock medicine

Poultry Marek Disease

Use age, unilateral paralysis, peripheral nerve enlargement, visceral tumors, feather-dander spread, and vaccination limits to answer cleanly.

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Overview

Marek disease is a highly contagious alphaherpesvirus disease causing T-cell lymphomas and peripheral nerve enlargement.

Signalment / Epidemiology

Chickens are the main host; virus spreads in feather follicle dander and persists in dust and litter.

Pathophysiology

Inhaled virus establishes infection and lymphoid neoplasia, with peripheral nerve enlargement and visceral tumors.

Clinical Signs

Asymmetric paralysis, one leg forward and one back, weight loss, ocular changes, enlarged nerves, and tumors are classic.

Diagnostics

Diagnose the tumor disease, not simple MDV exposure: use history, nerves, tumors, histopathology, and targeted advanced tests when needed.

Differential Diagnoses

Separate Marek disease from lymphoid leukosis, reticuloendotheliosis, Newcastle disease, botulism, nutritional neuropathy, trauma, and other paralytic flock diseases.

Treatment

There is no treatment for affected birds; prevention relies on hatchery vaccination, sanitation, and biosecurity.

Prognosis

Poor for affected birds; flock impact depends on vaccination, strain virulence, and environmental viral load.

NAVLE Pearls

Vaccination reduces tumors and clinical disease but does not reliably prevent infection, shedding, or all early-exposure failures.

Common NAVLE Traps

Do not choose drug treatment, diagnose disease from PCR alone, or confuse Marek nerve enlargement with lymphoid leukosis bursal tumors.

Core decision

Use age, nerve enlargement, bursa pattern, histology, and prevention status to choose Marek disease and flock-level control.

High-yield takeaways

Start with the safest next step, then narrow the case using signalment, timeline, exam findings, diagnostics, and response to treatment.
Use the traps, differentials, and practice questions to rehearse NAVLE-style reasoning instead of memorizing isolated facts.
This educational study page is not a clinical protocol; confirm patient-specific decisions with current references and clinician judgment.

30-second revision

OverviewContagious herpesvirus disease with T-cell lymphomas.

Clinical signsParalysis, one leg forward/one back, weight loss, tumors.

DiagnosticsNecropsy nerve enlargement, tumors, bursa pattern, and histopathology; do not diagnose from infection alone.

TreatmentNo effective treatment; prevention by vaccination and biosecurity.

TrapVaccine protects against disease better than infection; lymphoid leukosis is older and bursa-centered.

Exam core — read this first

NAVLE pearl → One leg forward and one leg backward with enlarged peripheral nerves is a classic Marek pattern.

Pathology pearl → Peripheral nerve enlargement and T-cell lymphomas separate Marek from many poultry neurologic diseases.

Diagnostic pearl → The testable diagnosis is tumor disease plus lesions; MDV infection alone is common and can mislead.

Vaccine pearl → Vaccination prevents tumors and disease expression but does not create sterile immunity or replace sanitation.

Differential pearl → Lymphoid leukosis usually affects birds older than 14-16 weeks, often involves the bursa, and does not classically enlarge peripheral nerves.

Exposure pearl → Early heavy exposure before vaccine protection is a common reason a vaccinated flock still has clinical disease.

Flock Control Note

Prevention depends on vaccine handling and biosecurity

Marek disease control is prevention-focused. Verify vaccination timing, storage, reconstitution, hatchery procedures, dust control, and early exposure pressure with current poultry guidance.

Pattern recognition

Core pattern

young chicken with unilateral leg paralysis or one leg forward and one backperipheral nerve enlargement at necropsy, especially sciatic, brachial, or vagus nervesvisceral lymphoid tumors in liver, spleen, gonads, heart, kidney, or proventriculusflock with dust/dander exposure and incomplete vaccination or early exposure pressurequestion asks to differentiate Marek from lymphoid leukosis, Newcastle disease, botulism, trauma, or nutritional neuropathy

Supporting clues

age and vaccination statusnerve enlargement versus bursal tumor patternwhether the question is asking for tumor diagnosis versus infection detectiongross necropsy and histopathologyflock exposure and sanitation historyneurologic versus neoplastic presentation

NAVLE trigger: The board trigger is nerve enlargement plus T-cell lymphoma logic, not treatment recall.

Decision core — what NAVLE actually asks

Paralysis with enlarged nerves

Choose Marek disease and confirm with pathology when needed.

Prevention question

Choose hatchery vaccination, sanitation, and exposure reduction rather than treatment of affected birds.

Tumor differential

Use age, nerve involvement, bursa pattern, and histopathology to separate Marek from lymphoid leukosis.

Vaccine failure concern

Assess vaccine handling, timing, virulent early exposure, maternal antibody, dust load, and sanitation pressure.

Key interpretation

Enlarged sciatic nerve

Marek anchor

Peripheral nerve enlargement is one of the most consistent gross lesions.

Visceral tumors

Neoplasia anchor

Lymphoid tumors can affect many organs.

Bursa pattern

Differential clue

Bursal tumors push lymphoid leukosis higher; Marek more often has atrophic or interfollicular bursal involvement.

Feather dander

Transmission clue

Dust control and sanitation matter because virus persists environmentally.

PCR positivity

Not enough alone

Field exposure is common, so diagnose disease from lesions and tumor context, not infection alone.

Skin or ocular lesions

Form clue

Skin leukosis and ocular changes can appear in addition to nerve signs.

Use current poultry references for diagnostic confirmation and hatchery vaccine handling.

Management and treatment

Affected birds

There is no effective treatment; manage welfare, culling decisions, and diagnostic confirmation under veterinarian guidance.

Do not choose antimicrobial or antiparasitic treatment as the primary answer.

Prevention

Use hatch or in ovo vaccination, sanitation, dust reduction, biosecurity, and genetic resistance when applicable.

Vaccination is highly protective but not sterile immunity.

When control fails

Review vaccine storage, thawing/reconstitution, administration timing, early virulent exposure, maternal antibody, flock genetics, and dust burden.

Do not blame the vaccine strain before checking the system.

Diagnostics

Use necropsy, histopathology, and advanced testing when differential uncertainty remains.

Diagnose tumors, not merely infection.

Prognosis

Poor for affected birds; flock impact depends on vaccination, strain virulence, early exposure pressure, and environmental control.

Subclinical production loss can occur.

NAVLE traps — where students lose marks

Trying to treat affected birds

Marek disease has no effective treatment; prevention is central.

Forgetting feather dander transmission

Environmental dust keeps exposure pressure high.

Calling vaccination sterile immunity

Vaccines prevent disease but do not fully stop infection or shedding.

Missing nerve enlargement

Peripheral nerve enlargement separates Marek from many differentials.

Confusing lymphoid leukosis

Age, bursa pattern, and nerve involvement help distinguish the tumor diseases.

Ignoring early exposure pressure

Early exposure before vaccine protection can break control.

Diagnosing from PCR alone

MDV exposure is common; NAVLE stems expect tumor and lesion reasoning.

Missing skin or ocular forms

Marek can present beyond the classic one-leg-forward posture.

Differentials — how to separate these on NAVLE

NAVLE discriminator: Marek means young chicken, paralysis, peripheral nerve enlargement, T-cell tumors, and prevention by vaccination plus exposure control.

Differential	Key clue	Decision bias	Trap
Marek disease	Peripheral nerve enlargement, unilateral leg paresis, visceral tumors	Vaccination and sanitation prevention	Treatment focus
Lymphoid leukosis	Older birds, bursal nodular tumors, lymphoid tumors without classic nerve enlargement	Necropsy and histology	Calling every tumor Marek
Reticuloendotheliosis	Tumors can mimic Marek; histopathology and virology may be needed	Confirm tumor type when lesions conflict	Assuming all T-cell-like tumors are Marek
Newcastle disease	Respiratory, neurologic, GI signs with high contagious/reportable concern	Biosecurity and official testing	Missing reportable context
Botulism or nutritional neuropathy	Flaccid weakness or diet-linked neurologic signs without lymphoid tumors or enlarged nerves	Exposure and nutrition workup	Forcing Marek without lesions
Avian encephalomyelitis	Tremors and ataxia in young chicks	Age and pathology discrimination	Ignoring peripheral nerves

Practice questions

Practice Marek disease diagnosis and prevention decisions

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Q1Classic lesion

A young chicken has one leg stretched forward and one backward. Necropsy shows enlarged sciatic nerves and visceral lymphoid tumors. What is the most likely diagnosis?

Q2Control

What is the best flock-level control principle for Marek disease?

Q3Differential

Which finding most strongly separates Marek disease from lymphoid leukosis?