Flock health

Marek's Disease in Chickens: Signs by Form, Spread, Vaccination and Diagnosis

Field guide · Updated November 11, 2026 · about 8 min read

Marek's disease is a highly contagious herpesvirus infection of chickens that causes tumors in the nerves, eyes, skin and internal organs; birds catch it by inhaling virus-laden feather dander, there is no cure or treatment, and vaccinating chicks at hatch or in the egg is the only control (University of Tennessee Extension D 185; Merck Veterinary Manual). This page summarizes that official guidance; HenClass does not give treatment or vaccine-dosing instructions.

Short version: buy chicks vaccinated at the hatchery, brood them in a disinfected brooder away from older birds while immunity builds, and have a lab confirm any suspected case. Marek's is not a risk to people, and eggs and meat from infected birds are safe to eat when cooked properly (Maryland Extension FS-1007; UT Extension).

Marek's disease signs by form: chart

Extension fact sheets sort the disease into four forms by where it does its damage; one bird can show more than one (Maryland Extension).

Merck also describes transient paralysis, in which birds are uncoordinated for several days and then recover; it is rare in immunized birds. Signs such as a gray iris are part of the eye check on the hen health check.

How it spreads

• Feather dander: the virus matures in the feather follicles and is shed in dander, which chickens inhale (Merck; UT). Darkling beetles can carry it too (Maryland).
• Before signs show: shedding starts about 2 to 4 weeks after infection, before the bird looks sick (Maryland). Infected birds stay infected and shed virus for life (UT; Penn State).
• On people: caretakers carry dander on shoes, clothing, hands, hair and skin, so the virus can reach birds even when new birds are quarantined (UT; Penn State).
• In the coop: the virus survives for months in litter and dust (Merck) and for years in dander; a contaminated coop may stay infectious for months to years after every bird is gone (Penn State; UT).
• Not through the egg: Merck, Penn State and UT all say it is not passed from hen to chick through the egg; chicks from infected breeders hatch clean if they are kept away from dander (Penn State).

Merck presumes every flock except strictly pathogen-free ones carries the virus; vaccination reduces shedding but does not prevent it.

Who is at risk, by age and breed

The disease can appear in chickens as young as 3 weeks (Merck) or 3 to 4 weeks (UT; Maryland), but the age ranges the sources give for most cases differ, as the chart below shows. Young chicks are highly susceptible, and females are affected more often than males (UT; Maryland).

• Breed: Leghorns and other light egg-type breeds are more vulnerable than meat-type breeds, and Silkies are highly susceptible (Penn State; UT). Maryland lists choosing a disease-resistant breed as part of prevention.
• Stress: UT says clinical disease often follows a trigger such as the onset of laying, pecking-order fights, extreme weather, a predator attack or heavy parasite loads.
• How many birds: illness in a flock ranges from 1% to 60% (UT), 1% to 50% (Penn State) or 10% to 50% (Maryland); among sick birds, deaths may reach 100% (UT; Maryland).
• Other species: quail and, rarely, turkeys (UT; Maryland). Penn State says the virus "will not make people sick."

Prevention: hatchery vaccination and brooder hygiene

1. Order vaccinated chicks. UT, Maryland and Penn State prefer a hatchery that vaccinates before shipping; Kentucky says all newly hatched birds should be vaccinated. Sources compared: buying chicks and pullets.
2. Clean the brooder first. Clean and disinfect the brooder before the chicks go in (Penn State). The rest of the setup is on the first 8 weeks page.
3. Isolate while immunity builds. Keep dander and debris from older birds away from chicks for at least one week (Penn State) and limit exposure for the roughly 2 weeks immunity takes (UT; Maryland).
4. Care for chicks first. Use a separate caretaker if possible; if not, tend the chicks before the older birds, and shower and change clothes between age groups for at least 4 days (Penn State).
5. Keep ages apart. UT and Maryland advise separating young and older birds; Utah State says to keep chickens under 6 months away from adults.
6. Do not restock a contaminated pen as-is. Do not house young or unvaccinated birds where affected birds were kept. Before restocking, remove dust, feathers and manure, clean thoroughly and sanitize non-porous surfaces with a 10% chlorine solution (Utah State).
7. Quarantine newcomers anyway. It helps, but caretakers can still carry the virus (UT). Quarantine steps are on adding new chickens and flock biosecurity.

What the vaccine does and does not do. Vaccination reduces clinical signs and tumors but does not prevent infection (Utah State); Merck puts efficacy usually above 90% and names HVT, SB-1 or 301B/1 and CVI988/Rispens as widely used strains. Vaccinating older birds is usually not effective because they have likely already been exposed (UT; Maryland). Penn State adds that vaccinating only some birds does not protect the rest. If you plan to vaccinate chicks yourself, UT and Maryland say to consult a veterinarian and follow the label.

No cure: when a bird has signs

Every source here says there is no treatment for Marek's disease (Merck; Kentucky; UT; Utah State), and Maryland says infected birds never recover. Utah State advises isolating affected birds and tending them last each day. Birds with neural signs cannot reach food and water, so UT advises removing them and euthanizing if needed to prevent starvation or trampling, and Maryland tells small-flock owners to plan for end-of-life care. Utah State suggests weak or paralyzed birds be considered for humane euthanasia and submitted to a diagnostic laboratory; ask a veterinarian or your county extension office.

Diagnosis goes through a lab

Marek's tumors have to be told apart from lymphoid leukosis, which Kentucky says is similar and typically starts at 16 weeks of age, and from the rarer reticuloendotheliosis (Merck). Diagnosis rests on flock history, signs and necropsy with tests on affected organs (Utah State; Penn State), and most animal diagnostic labs can test necropsy specimens (Penn State). Because the virus is everywhere, Merck says it is the tumors that are diagnosed, with tests such as PCR and immunohistochemistry for a definitive answer.

Maryland Extension, for example, tells owners to call the state agriculture department and, when deaths are unusually high with the same signs, to submit dead birds to a state Animal Health Diagnostic Laboratory. In California, UC Davis's California Animal Health and Food Safety (CAHFS) lab listed Marek's disease in 2019 among the most common diseases it diagnoses in backyard chickens. How to find your own state lab is on the hen health check page. Sudden deaths with breathing signs, swelling or purple color point to the emergencies on the respiratory diseases chart, which have their own reporting line.

The National Poultry Improvement Plan does not test for Marek's: the USDA APHIS reference guide lists avian influenza, pullorum disease, fowl typhoid, Salmonella Enteritidis and three mycoplasmas. A chick from an NPIP flock still needs its own Marek's vaccination.