There are several well-known viral diseases that affect pet and backyard chickens. Knowing the symptoms, potential treatment options, and methods of prevention is beneficial to anyone who owns a chicken or who is thinking of acquiring a pet chicken or starting a backyard flock.
The five most common viral diseases are: Marek's disease virus, lymphoid leukosis virus, avian influenza, infectious bronchitis and Newcastle disease virus.
Marek's Disease Virus
Marek's disease is caused by a type of herpes virus. It is spread by direct contact with an infected bird or by exposure to the ground or unclean flooring where an infected bird has been. Infected birds do not necessarily show symptoms of illness, so identification of sick birds is not always easy.
Symptoms of birds infected with Marek's disease virus are extremely varied.
- Young chickens often present with lameness and no evidence of an injury or swelling to the affected leg.
- Older chickens may show signs of a nervous symptom disorder (e.g., head tilt, unsteady gait, generalized weakness or lameness).
- Slow crop emptying is often noted, usually resulting in a swelling on the front of the chicken's neck just above the chest. The crop is a part of the digestive tract located just above the chest that is designed for mild digestion and food storage.
- Hens may show a variety of reproductive tract disorders.
- Some hens stop laying and may have a firm distension of the abdomen caused by swelling of the oviduct or have a fluid distension of the abdomen known as “water belly”.
When there is a firm distension of the abdomen, the oviduct is usually found to be impacted with debris, tumors, or fragments of egg material. Hens with fluid distension generally have cancer of the ovary and/or oviduct, causing fluid to build up in the abdomen. A build-up of abdominal fluid is a condition referred to as ascites. Blindness may occur if the virus invades the eyes.
Blood tests or swabs of the oral cavity on live birds may show exposure to the virus but may not tie the symptoms to Marek's disease virus. The only true way to diagnose a case of Marek's disease is for your veterinarian to send tissue samples (internal organs and a section of the sciatic nerve) from a deceased bird for analysis at a pathology laboratory.
There is no specific treatment for Marek’s disease. It can be prevented by vaccinating chicks within the first hour of hatching – before the chick touches the ground or before the chick is exposed to other chickens. Marek's disease virus may remain infectious in the ground for four to eight months.
Lymphoid Leukosis Virus
Lymphoid leukosis is a type of retrovirus. Chicks can be infected through the egg of an infected hen or by direct contact with an infected bird. The most common clinical sign is a weak, lethargic chicken with notable weight loss. Hens slow or stop egg laying.
"The most common clinical sign is a weak, lethargic chicken with notable weight loss."
Tumors may develop in the reproductive tract, causing firm abdominal distension. The spleen and liver may become enlarged due to tumors growing within these organs. Tumors may also affect the bone marrow, causing anemia that shows up as as pale comb and wattles.
Blood tests or swabs of the oral cavity on live birds may show exposure to the virus but may not tie the symptoms to lymphoid leukosis virus specifically. The only true way to diagnose is by analyzing internal organ samples from an exploratory surgery or a necropsy. Preferred tissues for necropsy specimens include the liver, spleen, oviduct, kidney, and a section of a leg bone.
Avian Influenza Virus
The avian influenza virus (AIV) is a member of the Orthomyxoviridae family of RNA viruses. There are two classifications of AIV: the low pathogenic avian influenza (LPAI) and the highly pathogenic avian influenza (HPAI). They are further grouped into two subtypes: hemagglutinin (HA) and neuraminidase (NA).
- LPAI generally causes weakness, decreased egg production in hens, diarrhea, and respiratory signs that include oculo-nasal discharge, sneezing, conjunctivitis, and difficulty breathing.
- HPAI is much more severe. Symptoms may show balance issues, necrosis (dead tissue) of the comb and/or wattle, head tilt, circling, convulsions, or death.
Sick birds that show any of the above signs should be examined by an avian veterinarian as soon as possible. AIV is a serious concern, since it is transmissible to other animals and humans (zoonotic disease). In Canada, avian influenza virus is reportable. The Canadian Food Inspection Agency (CFIA) must be notified when a positive case is verified by laboratory testing. In the United States, positive cases confirmed by a poultry diagnostic laboratory will be reported by the lab to the USDA.
There is no effective anti-viral treatment except supportive care. Blood tests are available at most veterinary schools and reference laboratories. If AIV is detected within a flock, it may be recommended to cull severely ill chickens. In Canada, the flock may be quarantined.
Infectious Bronchitis Virus
Infectious bronchitis is a type of coronavirus specific to chickens. It is an acute, highly infectious disease of chickens that generally causes respiratory signs that vary from sneezing to coughing, conjunctivitis, and tracheal rales (moist breathing sounds). Hens may show decreased egg production and abnormal shell formation. Some strains are known to cause severe kidney disease.
Vaccination in 2- to 14-day-old chicks is recommended in regions where the virus is circulating. For optimum protection, it is essential to use the proper strain of the vaccine.
Newcastle Disease Virus
Newcastle disease virus is an avian paramyxovirus (APMV). Specifically, Newcastle disease (ND) is caused by a highly virulent (infectious) strain of APMV known as virulent Newcastle disease virus (vNDV). The following subgroups are listed in increasing order of virulence (severity): asymptomatic enteric, lentogenic, mesogenic, neurotropic, velogenic and viscerotropic velogenic.
Symptoms may include lethargy, trouble breathing (dyspnea), dizziness, head tilt, facial swelling, and death. Newcastle disease is a reportable disease. Local, state, and federal authorities (USDA or CFIA) need to be notified when a positive case is verified by laboratory testing. Affected birds with confirmed ND must be culled. Consult the United States Department of Agriculture (USDA) or Canadian Food Inspection Agency (CFIA) websites for further information.
