Blog

One of the most commonly asked questions we get from growers is whether or not they should vaccinate against coccidiosis and Marek's disease.

Aside from avian influenza, these are two of the most ubiquitous diseases affecting poultry flocks. Both of them are incredibly devastating, but also highly preventable with the right care and management.


These diseases caused tremendous losses in the poultry industry throughout the early 20th century. Today, with the advent of vaccines and more understanding of how these diseases proliferate, growers can now take a more proactive approach in keeping their poultry healthy.

From personal experience, we believe the best protection is prevention. For some people, prevention means vaccination. For others, it means great hygiene and a solid biosecurity plan in place. It could also be a combination of the two. What works best for you might not work for someone else.

We will explain when vaccinations make sense, what you can do to prevent these diseases, and how to practice good biosecurity within your own flock.
 

What is Marek's Disease?

Marek's disease (MD) is a highly contagious viral disease caused by an alphaherpesvirus, known as the Marek’s disease virus (MDV). The virus attacks the bird’s immune and nervous systems, leading to T-cell lymphomas, a type of cancer that causes aggressive tumor growth and paralysis.

Marek's disease primarily affects chickens, although it can occasionally infect turkeys and other types of poultry. As an immunosuppressant, this disease not only devastating in itself, but makes infected birds more susceptible to various other ailments, including coccidiosis.

There are several different strains of the virus, so the severity of the disease and the outcome for each bird can vary widely. Some chickens may become infected and show no clinical signs, yet still carry and shed the virus. This is why Marek’s remains so widespread and difficult to detect early.

Once a chicken is infected, there is no treatment or cure for Marek’s disease; prevention relies on strict biosecurity practices or vaccination.

The Four Common Presentations of Marek's Disease in Chickens

The symptoms your birds exhibit will depend on the form of Marek's disease they are infected with. Some strains are highly virulent, while others are less severe.

Most commonly, chickens are infected with the neural form. Severe cases result in paralysis of the legs and wings, which leads to chickens dying of dehydration or starvation. Chickens in this state will not recover from the disease, and it is best to cull them before they continue spreading the disease.


Nerve (neural)- This is the most commonly observed form of Marek's. This strain affects the nerves and spine of the chicken, resulting in eventual paralysis. Symptoms include splayed legs, limited mobility, weight loss, and lameness.

Skin (cutaneous)- This form presents itself in the skin and feather follicles of birds. Symptoms include reddened follicles and scabby, irritated skin.

Eye (ocular)- This form is one of the rarer presentations, and is more commonly observed in adult birds. An eye infected with Marek's will be gray in color, with an irregular or misshapen iris. Eventually, chickens with this form of Marek's will go blind.

Organs (visceral)- This form attacks the internal organs, including the heart, liver, and lungs. The tumors eventually grow to the point where they suppress the function of these critical organs, leading to death.

How Does Marek's Disease Spread?

It is difficult to curb the spread of Marek's disease in chickens due to the ease in which it is transmitted among flock members.

The most common way other chickens become infected is by inhaling the dander of an infected chicken.

Marek's can also be spread via feces or bird to bird contact, but more commonly, it is transmitted via airborne particles.

Infected chickens constantly shed the virus from follicles in their skin and feathers. The virus can then survive on litter, dust, and surfaces in coops and poultry barns, making it difficult to completely eradicate. Even insects can carry and transmit the disease to other chickens that may ingest them.

Marek's can survive for several months (or even years, if the conditions are suitable), meaning a barn that was not sanitized or turned over properly can infect a brand new flock months later.

That's why good biosecurity and hygiene is one of the best preventive methods, aside from vaccination.

How to Prevent Marek's Disease in Poultry

With most poultry diseases, the best treatment is prevention by practicing great biosecurity or opting for vaccination.

Even with vaccination, most chickens will become infected or exposed to the virus (MDV), whether or not they show any symptoms. Vaccination can add a layer of protection, but getting the vaccine does not guarantee they will not contract Marek's and it will not cure the disease.

It's important to note that once a chicken is infected with MDV, they will carry and shed the disease for life, potentially infecting new birds later.

Vaccinate

Vaccinate your chicks at day-old. Vaccines can be administered in ovo, meaning in the egg, or given to day old chicks as a shot. The vaccine can take up to seven days to build efficacy and give your birds some level of immunity. Vaccines are only helpful if given prior to any exposure. 

Quarantine

Before making introductions, quarantine new birds separately from your current flock. It's best to wait a minimum of four weeks, but if possible, waiting longer is even better.

Biosecurity

The virus can spread if you have chicken dust or dander on your hands, clothes, or boots. If treating Marek's in your flock, biosecurity on your end can also help prevent the spread. Be vigilant about your own hygiene; wear disposable booties, gloves, and jumpsuits to prevent carrying it from one barn or coop to another.

Marek's Vaccine History and Efficacy

The disease was first noted by a veterinarian named Jozsef Marek in 1907, after he observed a strange illness afflicting his backyard flock. He performed autopsies on the deceased birds, noting that their nerves were inflamed.

He accurately described the pathological symptoms, although the exact cause of the disease wasn't discovered until 1967.

After much trial and error, the first iteration of vaccines were developed and became available in the late 1960s and early 1970s.

Before the advent of vaccines, commercial farms in the United States and United Kingdom were devastated by significant financial losses, with some figures quoting up to 150 million dollars annually.

Vaccines helped to curb these losses immensely, but the vaccine itself is not a cure, nor will it 100% eradicate Marek's from every flock.

Marek's vaccines are leaky, meaning that they inhibit symptoms and the severity of MDV, but will not stop that recipient from catching and spreading the disease to other birds.

While vaccination is not a guarantee for protection, it often leads to better outcomes for flocks.
 

What is Coccidiosis?

Coccidiosis is a parasitic disease that affects the function of the small intestine. In chickens, it's caused by parasitic protozoa, of the genus Eimeria (a type of coccidia). This is the most common cause of death for young chicks.

While all chickens have a small amount of coccidia naturally present in their intestinal tract, an overpopulation of this parasite can damage the intestines, causing poor nutrient absorption, bloody stools, and stunted growth.

Coccidiosis is not limited to chickens, as it can also affect other livestock, including cows, pigs, and even dogs. However, the coccidia parasite is host-specific, meaning that different species are infected by different species of Eimeria. For example, chickens cannot pass coccidiosis to cows, dogs, or vice versa.

Mortality rates are high if left untreated. One of the best preventatives is to ensure that you are raising your birds in a suitable environment.

How Does Coccidiosis Spread?

Coccidiosis and Environmental Conditions

Even after chicks leave the brooder, they will be exposed to coccidia in outdoor environments. Many growers raise chicks in the spring, which means they are transitioning outside just as late summer/early fall arrives.

Depending on your location, summer conditions can be especially challenging for young birds. Warm temperatures combined with rainfall or high humidity create ideal conditions for coccidia to thrive and multiply.

In general, temperatures between about 70°F and 90°F, along with adequate moisture, allow coccidia oocysts to sporulate rapidly and become infectious.

However, coccidiosis is not limited to any single season. No matter the time of year, it can be present outdoors or indoors whenever conditions are favorable—such as increased humidity, moisture buildup, or high bird density.

Outdoors, coccidiosis risk is highest when environmental conditions favor the parasite, which in many climates occurs in late summer and into autumn, especially following periods of rain. For this reason, starting chicks indoors and brooding them until they are more robust can help reduce early exposure and lower the risk of infection.


If you free-range your flock, you can reduce the risk of an outbreak of coccidiosis by rotating pasture areas. When birds forage in the same spot for too long, manure buildup increases the chance of coccidiosis spreading.

Additionally, regularly cleaning the coop is a must—removing soiled litter, sanitizing feeders and waterers, and maintaining good ventilation will keep parasite levels down.

As with Marek’s disease, growers can inadvertently spread coccidia if they don’t maintain proper hygiene. Because coccidiosis transmits easily from bird to bird, strong biosecurity practices are key to preventing it from spreading across multiple coops or barns.

Even with excellent biosecurity measures in place, chickens can still encounter coccidia from wild bird droppings. For this reason, it’s best to avoid placing wild-bird feeders near your poultry area, as they attract birds that may introduce the parasite to your flock’s environment.

Symptoms of Coccidiosis in Poultry

How to Treat Coccidiosis in Chickens

The coccidiosis vaccine is administered via a spray, which contains live Eimeria oocysts. This gradual exposure helps the birds build up their immunity and create a barrier against catching the disease.

While vaccines are highly effective, they are not a guarantee. Your chickens could still get the disease, so it's important to know the signs, get confirmation, and seek out treatment if you suspect you have a chick with coccidiosis.

Amprolium is the most common medication used to treat cocci. It can be administered in their water supply, or given orally if the chickens are too sick to eat and drink.


This medication helps to slow the multiplication of coccidia, but treatment needs to continue over a period of several days (typically up to a week) to become effective.

Some sources recommend treating the entire flock as a precaution, while others recommend isolating it to birds with visible symptoms, so as to avoid overmedicating or interfering with natural immunity. 

For best results and a treatment plan that is right for your flock, consult your veterinarian for proper dosage and ideal duration of treatment.

Should I Use Medicated Feed, or Non-Medicated Feed?

We generally recommend either vaccination or medicated feed to prevent coccidiosis. Both methods are effective at building immunity and improving outcomes for your flock.

An important note for growers: Do not give your birds medicated feed if you have opted for vaccination. If your birds have been vaccinated, the medicated feed will cancel out the vaccine.

If you choose not to vaccinate your poultry, then we highly recommend putting your chicks on medicated starter feed. This should give your chicks some immunity, but the best method will always be good prevention.


Biosecurity is of the utmost importance for any flock, whether you are a small backyard grower or a commercial operation. For more information about practicing good biosecurity, read our previous blog.

FAQ About Marek's and Coccidiosis

Can humans get Marek's disease from chickens? No, this disease cannot be transmitted from poultry to humans.

Can Marek's be spread via hatching eggs of an infected chicken? A common misconception is that Marek's can spread from affected birds via fertile eggs. Marek's spreads horizontally, rather than vertically.

Horizontal infection is transmission between bird to bird, while vertical infection is a parent transmitting the disease to offspring in utero or during birth.

So long as the chick doesn't come into contact with infected poultry or environments directly, they will not carry the disease at hatch.

At what age are chickens most susceptible to Marek's? Birds typically become infected between 6 to 30 weeks of age, with birds under 16 weeks being the most vulnerable. Even older birds can become infected—however, exposure and infection often occur early, so they are most vulnerable in the first several weeks of life.

If I vaccinate my birds, will they still be considered organic? Yes, vaccination does not mean that your chicks can no longer be certified as "organic." The USDA recognizes vaccines as a preventative, which still adheres to organic guidelines. Antibiotics and certain anticoccidial medications do not fall under this category, so be sure to reference organic poultry production guidelines before administering treatment.

I chose to vaccinate, but my birds still got sick. Why? Vaccination is not a foolproof method, and for it to be most effective, it should be supplemented with biosecurity and good hygiene. Vaccination typically makes symptoms less severe and improves outcomes for flocks, but it's not a guarantee that your chickens will never get sick.

Are certain breeds or types of poultry more susceptible to Marek's? Marek's primarily affects chickens. Certain breeds (backyard breeds in particular) are thought to be more susceptible to Marek's. While the validity of these claims is up for debate, some researchers believe that lighter breeds have less natural immunity or genetic hardiness against Marek's.

What are some of the other most common chicken diseases? Newcastle disease (NDV), salmonella, fowl pox, and avian influenza are also well-known diseases. Some can be prevented with vaccination, others can only be prevented with good biosecurity.

Can chickens with Marek's disease be consumed? Meat and eggs are safe for consumption, since the disease itself is not transmissible to humans. However, it depends on how badly the chicken has been affected by the virus.

The meat quality may be poor, especially if the bird is suffering from internal tumors or severe illness. If the chicken looks reasonably healthy and alert before processing, it is typically safe to assume the bird is ok for consumption. Use your best judgment by assessing the overall state of the bird.

Final Thoughts: Should I Vaccinate My Birds?

Many of our clients ask us whether it is worth it to vaccinate against these diseases. We would say that it depends on your unique situation.

If you are a commercial farmer, vaccination might be a crucial part of your biosecurity plan. The scale of your operation means that the risk of disease spreading is higher, and opting for vaccination can protect your flock from outbreaks that could lead to significant financial losses.

If you are a small-scale or backyard farmer, the decision to vaccinate may depend on factors such as the size of your flock, the presence of disease in your area, and your management practices. For smaller flocks, maintaining good hygiene and biosecurity measures might be enough to raise healthy chickens.


It's also important to consider the specific diseases that are prevalent in your region. Consulting with a veterinarian can provide valuable insights into which vaccines are most beneficial for your birds. They can help you assess the risks and benefits based on your flock's health history and environmental conditions.

Ultimately, the decision to vaccinate should be made with careful consideration of your flock's needs and the potential risks involved. By staying informed, you can make the best choices for the health and well-being of your birds.

Here at Myers Poultry, we offer vaccination options for day-old chicks and follow NPIP guidelines to ensure the health of our poultry. If you'd like to speak with a team member to discuss whether vaccination is right for you, don't hesitate to reach out to us directly.