RNA viruses in poultry (Part 2)
Wild birds, especially waterfowl, are the reservoir hosts for influenza viruses. Infection of domestic poultry occurs through direct contact with wild birds, and then spread throughout flocks. Different virus strains vary in their ability to cause illness. Occasionally, there are major avian influenza outbreaks, and the current outbreaks caused by H5; N1 (and H7) virus subtypes are affecting huge areas around the world. In addition to chickens, avian influenza viruses can infect turkeys. Viral particles that are ingested or inhaled by birds replicate in the respiratory and intestinal tracts. Infection with the most harmful strains cause the virus to spread throughout the blood and destroy immune tissues and organs, leading to inflammation of the pancreas, heart, muscles, and brain. The most devastating form of disease caused by avian influenza viruses is commonly called fowl plague, and can cause sudden death. If birds survive for more than 48 hours, the disease is characterized by respiratory distress, diarrhea, tearing, blue discoloration of the comb and wattles, as well as swelling of the head, neck, and face. The less damaging strains cause decreased appetite and egg production, with mild respiratory disease and inflammation of the sinuses.
Avian influenza viruses are potentially zoonotic. During the current H5;N1 outbreak, the transmission of avian influenza viruses from birds to humans has been reported, but the disease has not yet been documented to spread from a diseased human to a second person. Unless the virus mutates to a more readily transmissible form from human to human, the danger of a human pandemic is low.
There are a variety of different avian paramyxoviruses (PMV), with the most important being PMV-1 or Newcastle disease virus (NDV). NDV infects all birds, and produces disease varying from mild infection of the respiratory tract to severe disease and death in young chickens and turkeys. Other bird species are generally more resistant, but may serve as carriers which appear healthy while still being able to shed virus particles and infect other birds. Initial viral replication takes place in the lining cells of the respiratory and intestinal tracts, then the spleen and bone marrow. After spreading through the blood, the virus reaches its target organs: the lung, intestine, brain and spinal cord. It causes bleeding throughout the upper gastrointestinal tract, intestines, muscles, and heart. In the brain, NDV causes inflammation and kills neurons. The clinical signs depend on the specific strain of virus, as well as the age and immune health of the host. There is a loss of appetite, huddling, weakness, an enlarged crop, foamy mucus in the mouth, discharge from the beak, and yellow-green diarrhea. Nervous system signs include paralysis of wings and/or legs, torticollis or a crooked neck, circular movements, and spasms. In laying hens, NDV also drops egg production.
The clinical diagnosis of NDV must be confirmed by laboratory testing of tissue samples and blood. Vaccine administration within drinking water or aerosol is widely practiced against NDV.
NDV can produce conjunctivitis (pink eye) in humans, but it has been primarily seen in laboratory workers, and members of vaccination teams and poultry processing plants. It has not been reported in people who raise poultry or consume poultry products.
Turkey viral rhinotracheitis
In young turkeys, this disease is characterized by excessive mucus, sneezing, conjunctivitis, swelling around the head, and death. The disease also occurs in chickens and presents as a milder form of respiratory disease, termed “swollen head syndrome”. The virus replicates in and destroys lining cells of the trachea. Diagnosis is confirmed through the detection of antibodies in blood, and vaccines against this disease are available.
Avian lymphoid leukosis and sarcoma
Oncogenic (tumor-forming) retroviruses such as avian lymphoid leukosis can be transmitted between individual birds within a flock, and passed down from hens to their chicks. Chicks infected by other birds when they are at least 5 to 6 days old will produce antibodies to defend themselves against the virus, and will not develop leukemia, or cancer of the bone marrow. If they are infected earlier or within the egg, the virus will survive in their blood for long periods of time, increasing the likelihood that they will develop leukemia and other types of disease. These chickens also constantly shed virus and are a major source of infection. The virus causes immune cells to slowly infiltrate various organs, resulting in abnormally thickened bones, and malignant kidney tumors. Other strains of the virus cause rapidly-developing bone marrow cancers, anemia or an abnormally low red blood cell count, and bleeding tumors. In addition, egg production and quality in flocks infected with avian retroviruses may decrease.
Flock history, clinical signs, postmortem examination and microscopic evaluation of tissues are usually sufficient to diagnose avian leukosis, but infected birds can be detected based on the presence of viral antigen in cloacal swabs, semen, or egg albumen. There are no vaccines against avian leukosis, so its control is based on the genetic selection of chickens that are resistant to these viruses, and their eradication from selected flocks. Hygiene is important in minimizing the level of virus contamination, especially at hatching.
Source: all info is from my vet med school notes
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