Infectious Bronchitis

E. L. JUNGHERR AND J. F. SULLIVAN.

INFECTIOUS BRONCHITIS, a widespread respiratory disease of chickens in North America, attacks poultry of all ages.

A. F. Schalk and M. C. Hawn, at the North Dakota Agricultural Experiment Station in 1931, separated infectious bronchitis from the group of ailments known as the "common cold." Because they observed the disease primarily as a hatchery-transmitted condition, it became known as "chick bronchitis."

The disease since has become less common in brooder chicks but more common in growing and laying stock. That change in susceptible age is due probably to the widespread natural occurrence of the disease or artificial immunization for it. A transfer of temporarily protective substances--antibodies to the offspring follows occurrence and immunization.

The cause of infectious bronchitis is a tiny virus, which passes through bacteria-retaining filters and is visible only under the electron microscope.

The virus can be grown in embryonated chicken eggs, in which it kills or stunts the embryo. The inoculation of embryonated eggs is the only well-developed method of propagating the virus other than in live chickens. Propagation by tissue culture has been studied.

The virus is killed by ordinary disinfectants and dies in drinking water after 12 to 24 hours. Large amounts of water are effective for cleaning the premises. The virus is not affected by:sulfa drugs or by antibiotics. Different trains of the virus of infectious bronchitis vary in disease-making power. The disease is spread by direct or indirect contact with infected birds. The virus is present in the discharge from the respiratory tract and spreads from bird to bird or from contaminated feed, drink, and litter. It may live for a short time, probably not more than 2 or 3 days (except in cold, dry weather) on contaminated clothing, feedbags, crates, and utensils.

The air in an infected poultry house becomes contaminated heavily with virus, perhaps attached to dust particles or droplets, and thus can spread the disease far. The virus can enter all except specially constructed poultry houses.

The virus has been recovered from eggs laid by infected birds as long as 4 weeks after a naturally occurring outbreak or one initiated by a live-virus vaccine. Such eggs should never be set, as they may cause contamination of a hatchery.

Recovered birds carry the virus for a limited time, perhaps 5 weeks.

Recurrence of the disease in certain areas year after year is not fully understood. Only chickens are known to be susceptible to the disease.

The period between exposure and the onset of symptoms is 18 to 36 hours.

The shortness of that incubation period and the ability of the disease to spread through the air make infectious bronchitis the most contagious respiratory disease of poultry.

The symptoms of infectious bronchitis resemble those of other respiratory diseases and are not diagnostic. They consist of nasal discharge, rattling, sneezing, and coughing. Mild symptoms are best observed by pressing on the nostrils and holding the bird close to one's ear to detect gurgling in the windpipe.

The mortality in brooder chicks may be high because of suffocation. Growing birds rarely die from uncomplicated bronchitis but eat much less and have a setback in growth. Egg production drops. The eggs become bleached, thin-shelled, misshapen, rough on the outside, and watery on the inside. Hatchability drops. Some of those effects may persist weeks or months after recovery from the respiratory phase and even after return to full production. An early molt may ensue.

Research by D. I. Broadfoot, B. S. Pomeroy, and Wade M. Smith, at the University of Minnesota, disclosed that natural bronchitis infection in brooder chicks less than 3 weeks old may cause permanent injury to the oviduct so that such birds so-called false layers cannot produce fully formed eggs.

In brooder chicks dead of the disease, one can see yellowish casts in the windpipe and grayish-yellow turbidity in the air sacs. Older birds have increased mucus in the windpipe and in the large bronchi. The lesions are not diagnostic; furthermore, birds dead for some time often show accumulation of mucus without indications of a respiratory disease.

Accurate diagnosis can be made only in a well-equipped laboratory. The diagnosis during an acute outbreak is made by virus isolation on live birds submitted to the laboratory in person (to avoid shipment by common carrier) or on a whole windpipe shipped in a stoppered glass tube and packed with dry ice. Virus isolation is a long, costly technique, however, and does not always succeed.

Three to four weeks after recovery, a diagnosis can be made by demonstrating immunity. Two laboratory methods are available for that. Live recovered birds are exposed to infectious bronchitis virus and a "take" is not obtained. Blood serum from recovered birds is mixed with an embryo-killing bronchitis virus and the mixture is used to inoculate embryonated chicken eggs. These embryos are not killed or stunted. The latter is the serum neutralization test. Both tests indicate past exposure to infectious bronchitis virus and present immunity to the disease, even if previous symptoms have not been noted.

Treatment of infectious bronchitis is nonspecific and often is unsatisfactory. Extra moist heat, increased ventilation, and appetite stimulants are recommended.

It is unwise to resort to vaccination in places where infectious bronchitis does not exist, as proved by laboratory examination. Vaccination may introduce the disease, which, aside from its own damaging effect, may also act as a stress factor for other respiratory diseases, such as air-sac infection. Some poultry premises have a mild bronchitis infection, which immunizes the chicks early without causing severe symptoms. These strains protect the layers and even transfer parental immunity to the chicks. On such premises vaccination is not necessary, but the situation should be clarified by laboratory tests.

Timely vaccination is the only means of prevention in densely populated areas. Only live-virus vaccines are effective. Certain States offer the poultryman a fully virulent virus, which is inoculated into a few birds during the growing period to start an outbreak. This method generally gives good results, but it cannot be standardized. At times it induces severe reactions and endangers neighboring flocks. It is not applicable to broilers.

Commercial bronchitis vaccines utilize naturally mild or artificially weakened strains. The products are subjected to extensive tests under Government supervision for potency, purity, and safety. Progress has been made toward standardization. The resulting immunity is of shorter duration than that following contact with fully virulent virus.

Repeat vaccinations or booster shots should be applied according to the direction of the manufacturers. Even natural outbreaks occasionally fail to induce permanent immunity, especially for the second laying season. Vaccines may be applied individually into the nostril or eye, or by mass techniques as spray and dust or through the drinking water.

General rules should be followed, such as vaccination of only healthy birds, keeping the vaccine package under refrigeration, and using the vaccine immediately after dilution.

The need for bronchitis vaccination and the choice of vaccine must be decided by the poultryman. Diagnostic laboratories can advise as to the occurrence of the disease in the area and the status of immunity of the mature stock on the particular premises.

Any vaccination program should be planned for an entire year in advance consideration being given to the incidence of bronchitis in the area, to the immune status of birds on the farm, and to existing physical facilities.

Vaccination of susceptible birds causes a "take," or mild respiratory ailment similar to the natural disease. This "take" is noticeable within 2 to 3 days, but may be delayed in immune brooder chicks for 7 to 14 days.

The reaction to the vaccination lasts about 7 to 10 days. Everything possible should be done during that time to keep the birds comfortable and to maintain feed consumption. Replacement stock must be vaccinated at least 4 weeks before the expected beginning of egg production.

Unfavorable results from vaccination are usually due to unnoticed diseases in the vaccinated birds, such as coccidiosis, quail disease, or air-sac infection. Bronchitis vaccination acts as an extra burden on the birds and thus may precipitate other diseases. Vaccination itself does not improve the birds but constitutes an insurance policy for areas of increased risk.

E. L. JUNGHERR is professor and head of the Department of Animal Diseases at the Storrs Agricultural Experiment Station, University of Connecticut. He received the degree of doctor of veterinary medicine from the Vienna, Austria, Veterinary School in 1923.

J. F. SULLIVAN is a veterinarian employed by the Animal Disease and Parasite Research Branch of the Agricultural Research Service.