Marek’s disease is a highly contagious viral condition primarily affecting chickens, caused by a type of herpesvirus known as Marek’s disease virus (MDV). This widespread infection leads to the development of T-cell lymphomas and nerve enlargement within affected birds. It is one of the most common avian infections, identified globally, and nearly every flock is presumed to be infected. The disease can impact chickens of any age, though clinical signs are most frequently observed in birds from 3-4 weeks to 30 weeks old.
Symptoms and Forms of Marek’s Disease
Marek’s disease can manifest in several distinct forms, each presenting with characteristic clinical signs. The classical or neurological form often results in asymmetric paralysis of one or more limbs. Birds might display a distinctive stance with one leg stretched forward and the other backward, sometimes referred to as “the splits,” or exhibit paralysis of a wing or the neck.
The acute or visceral form of the disease involves the rapid development of tumors on internal organs such as the liver, spleen, kidneys, lungs, heart, and proventriculus. These internal tumors can lead to general depression, significant weight loss, and sudden death in birds that previously appeared healthy. Outbreaks in unvaccinated flocks can result in high mortality rates, sometimes reaching up to 80%.
The ocular form of Marek’s disease affects the eyes, causing lymphocyte infiltration of the iris. This infiltration can result in a characteristic “grey eye” appearance or an irregular pupil shape, potentially leading to blindness.
In the cutaneous form, the disease causes the enlargement of feather follicles and the formation of firm, round lesions or bumps on the skin.
How Marek’s Disease Spreads
Marek’s disease is highly contagious and spreads readily among chickens primarily through the inhalation of contaminated dust. The virus matures into a fully infectious form within the epithelium of feather follicles, from which it is continuously shed into the environment in feather dander. This microscopic dander, composed of skin cells and feather debris, is a potent source of infection.
Once shed, the virus can survive for extended periods, potentially months, within poultry house litter or dust, especially in temperatures between 20 to 25 degrees Celsius. This environmental persistence means that coops, soil, and equipment can remain contaminated and infectious. Susceptible chickens become infected by breathing in this virus-laden dust.
Marek’s disease is not transmitted through the egg. Instead, the primary mode of transmission relies on the environmental spread of infectious feather dander, making airborne particles the main pathway for the virus to move from infected to uninfected birds.
Prevention Through Vaccination and Biosecurity
Vaccination is an effective method for preventing clinical Marek’s disease. The vaccine is typically administered to chicks at one day old, either via injection or in ovo (into the egg) before hatching, ensuring protection before potential exposure to the virus. While the vaccine does not prevent infection with the Marek’s disease virus itself, it significantly reduces or prevents the formation of tumors and paralysis, allowing vaccinated birds to live normal lives.
Vaccinated birds can still become infected with field strains and become lifelong carriers, shedding the virus. This means the vaccine protects individual birds from severe disease but does not stop viral spread within a flock. Therefore, vaccination is combined with strict biosecurity to minimize viral load and transmission.
Implementing biosecurity practices is also important for disease control. Maintaining a “closed flock” by avoiding the introduction of new birds is a fundamental step. When new birds must be introduced, a strict quarantine period of at least 30 days in a separate facility is recommended to monitor for any signs of illness before integration.
Dedicated clothing and footwear used only within the coop area can prevent the accidental transfer of viral particles from outside sources. Regular and thorough cleaning, followed by disinfection of coops and equipment between flocks, helps reduce environmental contamination. These measures, combined with vaccination, aim to create a barrier against the spread of Marek’s disease.
Managing an Infected Flock
Once a bird exhibits clinical signs of Marek’s disease, there is no effective cure or treatment available. The prognosis for symptomatic birds is poor, as the disease is typically fatal. Birds showing severe signs of paralysis or internal tumors will likely succumb to the illness, and humane culling may be considered to prevent prolonged suffering.
Even birds within an infected flock that are vaccinated or appear asymptomatic will become carriers of the Marek’s disease virus for life. These carriers continuously shed the virus through their feather dander, perpetuating its presence in the environment. This means that once Marek’s disease is confirmed in a flock, the entire premises is considered contaminated.
Culling symptomatic birds can help reduce the overall viral load in the immediate environment, potentially benefiting the health of remaining, less affected birds. However, the pervasive nature of the virus implies that any future chickens introduced to the same contaminated premises will be at high risk of exposure and infection, even if vaccinated.