When something goes wrong in your flock, the symptoms don’t always point clearly to one disease. Chickens tend to show similar outward signs—weight loss, weakness, breathing problems, reduced egg production—but behind those signs could be one of several very different pathogens. That’s why accurate testing is so important.
At The Silkie Lab, we routinely test for Marek’s Disease (MD), Avian Leukosis Virus (ALV), Infectious Coryza, Mycoplasma gallisepticum (MG), and Mycoplasma synoviae (MS). These five are among the most common culprits behind losses in backyard flocks and breeding operations alike. But how do you tell them apart before you send in a sample? Let’s explore them one by one in depth.
Marek’s Disease: The Viral Cancer of Chickens
Marek’s is caused by a herpesvirus (Gallid alphaherpesvirus 2) and is sometimes described as the “chicken cancer virus.” It spreads through feather dust and dander, which can stay infectious in the environment for months. Once it’s in a flock, nearly every bird gets exposed. Vaccination prevents tumors and paralysis in most cases, but it doesn’t stop the virus from spreading.
When it shows up: Marek’s typically strikes young birds between three and sixteen weeks of age, though signs can sometimes appear later.
What you’ll see: The classic presentation is paralysis. One leg may shoot forward and the other stretch backward because the virus targets the sciatic nerves, enlarging them and preventing normal function. Birds may stumble, drag a wing, or simply waste away despite eating. Some cases show up in the eyes as “gray eye,” where the iris becomes cloudy and the pupil looks irregular. Because Marek’s virus causes tumors, you may also find lumps in the liver, spleen, or skin at necropsy.
Diagnosis: Marek’s can be confused with ALV because both cause tumors. The big difference is age (Marek’s = younger) and the involvement of nerves and eyes. Testing is essential. PCR and histology confirm the virus and differentiate it from other tumor-causing diseases.
Prevention & control: Vaccination at hatch (or in ovo at the hatchery) is standard practice. There’s no cure. Biosecurity—keeping dander from infected birds away from chicks—is key to protecting vulnerable young flocks.
Avian Leukosis Virus (ALV): The Hidden Retrovirus
ALV is a retrovirus, which means once it infects a bird, it integrates into the DNA and becomes a lifelong infection. Unlike Marek’s, ALV doesn’t survive well in the environment. Its main method of spread is through the egg, passed from hen to chick, though close contact can also transmit it horizontally.
When it shows up: ALV tends to affect older birds, usually after fourteen to sixteen weeks of age, though sometimes much later. That difference in timing is a major clue when trying to sort it out from Marek’s.
What you’ll see: Birds may appear thin, depressed, or pale, with chronic diarrhea and a droopy stance. Egg production may fall, and in breeding flocks, fertility can take a hit. Internally, ALV causes lymphoid tumors in the liver, spleen, kidneys, and gonads, and uniquely, it often involves the bursa of Fabricius (the immune organ near the cloaca). This “bursa involvement” is one of the main ways pathologists separate ALV from Marek’s.
Diagnosis: Necropsy and histology can raise suspicion, but confirming ALV requires molecular testing—PCR, virus isolation, or ELISA-based methods. Because so many flocks are exposed, testing must be interpreted carefully alongside flock history.
Prevention & control: There’s no treatment or vaccine. Control relies on breeding from clean flocks that are ALV-negative, strict culling of positives, and maintaining biosecurity. If your flock has birds over sixteen weeks with wasting and tumors but without nerve enlargement, ALV is a likely suspect, and testing should be ordered. While we don’t offer testing for ALV – we offer testing for an ALV Resistance Gene to help you breed your flock with resistance in mind!
Infectious Coryza: The Swollen Face Disease
Coryza is caused by the bacterium Avibacterium paragallinarum. Unlike Marek’s or ALV, it doesn’t cause tumors—it causes explosive respiratory disease. It spreads rapidly through contaminated waterers, feeders, or direct bird-to-bird contact. Carriers are common, which makes Coryza hard to stamp out once it’s in a flock.
When it shows up: Coryza doesn’t care much about age, though pullets and adult layers are the most frequent victims. Outbreaks can appear overnight in otherwise healthy flocks.
What you’ll see: The most distinctive symptom is facial swelling. Birds develop swollen sinuses around the eyes, sometimes so severe that the eyes can’t open. You’ll often see thick, smelly nasal discharge, sneezing, and bubbling at the corners of the eyes. Egg production drops sharply, and affected birds look listless and stop eating.
Diagnosis: On necropsy, sinuses are filled with gray or yellowish pus. Coryza can resemble MG or MS, but the rapid onset and dramatic swelling usually point toward Coryza. PCR or bacterial culture is used to confirm it.
Prevention & control: Antibiotics can lessen symptoms if given early, but they don’t clear carriers. Vaccines exist for certain serovars, but immunity is strain-specific. The best prevention is to keep carriers out of your flock and practice strict biosecurity. If sudden facial swelling is hitting your flock, a Coryza test is the best next step.
Mycoplasma gallisepticum (MG): The Chronic Respiratory Threat
MG is one of the most economically damaging poultry pathogens worldwide. It’s caused by a wall-less bacterium that colonizes the respiratory tract. Unlike Coryza, which burns fast and leaves carriers, MG smolders long-term in a flock and is very difficult to eradicate.
When it shows up: MG is most often noticed in birds older than four or five weeks, especially when stress (like moving, cold snaps, or mixing flocks) weakens the immune system.
What you’ll see: Birds develop persistent coughing, sneezing, and nasal discharge. Rales (raspy breathing) are common, and in turkeys, MG often causes severe sinus swelling. Chickens may develop conjunctivitis, swollen heads, or watery eyes. Productivity suffers: broilers may grow poorly, and breeders may have reduced hatchability.
Diagnosis: MG often overlaps with Coryza and MS, making testing essential. PCR provides direct detection, while serology (ELISA, HI, SPA) can reveal exposure. Culture is possible but slow.
Prevention & control: Antibiotics such as tylosin or tiamulin reduce symptoms but don’t cure infection. Once a flock is positive, it remains positive. The gold standard of prevention is to buy MG-free stock from NPIP-certified hatcheries, avoid exposure to wild birds, and consider vaccination where available. If your flock has ongoing respiratory issues that don’t resolve, MG testing is warranted.
Mycoplasma synoviae (MS): The Silent Problem with a Painful Twist
MS is a close relative of MG but behaves a little differently. Many flocks carry it without obvious signs, which makes it a silent spreader. However, when it does cause problems, it can be devastating.
When it shows up: MS can strike at any age but is often noticed in growers and layers under stress or with co-infections.
What you’ll see: MS is notorious for causing infectious synovitis. Birds develop lameness, sit on their hocks, or move stiffly. The hock and foot joints may be swollen and warm, filled with gray or creamy exudate. In layers, MS also causes “egg apex abnormalities”—odd-shaped, wrinkled, or fragile eggshells. Respiratory signs are usually mild, if present at all.
Diagnosis: Because symptoms can mimic other joint issues, testing is essential. PCR is the most reliable diagnostic method, though serology is useful for flock screening.
Prevention & control: As with MG, antibiotics may control clinical signs but won’t eliminate infection. Breeding from MS-free stock is the only true prevention. Vaccination is used in some countries but isn’t universally available. If your birds are going lame with swollen joints or producing strange-looking eggs, MS testing should be high on your list.
Making the Right Call: Which Test Do You Need?
The diseases we’ve covered can look similar, and in many cases more than one can be circulating in a flock at once. But some guiding differences stand out:
- Paralysis, tumors, or eye changes in young birds: Marek’s.
- Tumors in older birds, no nerve involvement: ALV.
- Sudden, dramatic facial swelling and stinky nasal discharge: Coryza.
- Chronic coughing, sneezing, and sinus infections: MG.
- Swollen joints, lameness, or wrinkled eggshells: MS.
Because overlap is so common, the most reliable path is to test. With the right diagnostic tools, you’ll know exactly what your flock is facing—and what steps you can take to manage it.
Final Thoughts
Marek’s, ALV, Coryza, MG, and MS represent very different categories of pathogens—viruses, retroviruses, bacteria, and mycoplasmas—but to the untrained eye, they can look deceptively similar in the coop. Knowing the key signs and the age groups they affect is the first step. The second step is testing, which removes the guesswork and allows you to protect your birds and breeding program with confidence.
At The Silkie Lab, we’re here to provide that clarity. Whether it’s a tumor in a young pullet, a layer with a swollen face, or a roo with a persistent cough, we offer testing designed to pinpoint the exact cause. Because the sooner you know what you’re up against, the sooner you can protect your flock.