Marek’s Disease Guide
Marek’s Disease Symptoms in Chickens
What to look for, what it is often confused with, and how testing confirms it.
This page is laboratory information, not veterinary treatment advice. If you have a sick bird, involve your vet. A PCR test tells you what is present so that conversation starts with a fact instead of a guess.
Marek’s disease is caused by a herpesvirus that spreads through dander and dust and is extremely common in backyard flocks. It usually shows up in birds between 6 and 30 weeks old, which is why it so often hits just as birds are coming into their own.
👀 What the signs look like
| Form | What you see |
| Classical, nerve form | Progressive paralysis, often one leg forward and one back, a drooping wing, or a twisted neck. The bird stays bright and keeps trying to eat |
| Visceral, tumor form | Wasting, pale comb, weakness, and sudden death in an otherwise healthy looking young bird |
| Ocular form | Iris turning gray, irregular pupil shape, and failing vision |
| Cutaneous form | Enlarged, roughened feather follicles, usually spotted at processing |
The pattern that makes people suspect it is a young bird going down in the legs while still eating, drinking, and looking alert. A bird that is simply unwell usually looks unwell all over.
❌ What gets mistaken for it
- Leg injury or a slipped tendon. Sudden, usually one leg, and it does not progress the way Marek’s does.
- Vitamin deficiency, particularly riboflavin, which causes curled toe paralysis and responds to correction.
- Botulism. Rapid onset flaccid paralysis, usually several birds at once and tied to a spoiled food source.
- Lymphoid leukosis. Produces similar tumors but generally in older birds, and it is a different virus entirely.
Every one of those has a different answer, which is the whole argument for testing rather than treating a guess.
🧬 Confirming it with a PCR test
A PCR test looks for the virus’s own genetic material in a sample from your bird. We test from a feather shaft or from blood.
| Result | What it means |
| Detected | Marek’s disease virus is present in that bird. Note that vaccinated birds can carry field virus without showing disease, so read the result alongside the bird’s actual condition and your vet’s assessment |
| Not detected | The virus was not found in that sample from that bird on that day. Worth looking at the alternatives above |
📦 Why people test even without a sick bird
The most common reason is new stock. A carrier can look perfectly healthy and shed virus into a flock for life, so screening incoming birds before they meet yours is far cheaper than what follows if you skip it. Breeders selling chicks and hatching eggs screen for the same reason.
Swabs, feathers, or blood spots go back to us in a plain envelope with no kit to buy first. Turnaround is 2 to 3 business days from the day your samples arrive at the lab, results load into your Pullidex account, and samples are held for one week after testing. Disease screening is currently available for chickens.