Infectious bronchitis (IB) in chickens can occur throughout the year, but is more common during the cold autumn and winter seasons. During the key autumn disease-prevention period in 10 month, the day–night temperature difference increases, and the conflict between maintaining warmth and ventilation in chicken houses becomes more pronounced, creating a high-risk window for infectious bronchitis.
The disease is characterized mainly by respiratory symptoms, deaths among chicks, reduced egg production in laying hens, and an increase in malformed eggs. Chickens of all ages can be infected. The mortality rate is high in infected chicks, while infection in growing chickens causes permanent damage to the oviduct. It is one of the main diseases currently affecting layer and broiler farming.
I. How can this disease be prevented and controlled correctly and scientifically?
Infectious bronchitis in chickens is caused by infectious bronchitis virus (IBV). It belongs to the genus Coronavirus in the family Coronaviridae and is a single-stranded RNA virus, transmitted mainly through the respiratory and digestive tracts. IBV has numerous serotypes (more than 30 serotypes/genotypes have been identified domestically), and readily undergoes mutation and recombination. Cross-protection between different serotypes is low; this is the fundamental reason infectious bronchitis is difficult to prevent and control. The virus is not highly resistant to external conditions and is sensitive to high temperatures and common disinfectants. It can be inactivated at 56°C for 15 minutes. Common disinfectants such as sodium hypochlorite and peracetic acid can effectively kill it. However, the virus can survive for a relatively long time in low-temperature environments, which is one reason it is more prevalent in autumn and winter.
1, Disease Characteristics
This disease has a short incubation period, generally 1–2 days, and spreads extremely rapidly. After 1 chickens in a flock become infected, the disease can spread to the entire flock in about 2 days, and the morbidity rate can exceed 90%. Chickens of all ages are susceptible, with chicks (especially those aged 2–6 weeks) affected most severely. The mortality rate for the respiratory form in chicks is generally 20%–30%; it rises further following secondary infection with Escherichia coli or Mycoplasma. The mortality rate for the renal form of infectious bronchitis can reach 30%–50%, and is relatively high.
After infection, laying hens may experience a 20%–50% drop in egg production and produce large numbers of soft-shelled, malformed, and faded eggs, with watery egg whites. If pullets (before 6 weeks of age) become infected, oviduct development may be impaired; after they begin laying, they may produce no eggs or very few eggs. This is known as a “false layer,” and the loss is irreversible. Recovered birds may continue to carry and shed the virus for a long time, becoming sources of infection.
2, Clinical Signs
Sick chickens appear depressed, huddle together and shiver, have reduced appetites, drooping wings, and ruffled feathers. The respiratory form is mainly characterized by open-mouth breathing with the neck extended, coughing, sneezing, and tracheal rales, which are more clearly heard when it is quiet at night. Some sick chickens have watery eyes and nasal discharge, and repeatedly shake their heads because of difficulty breathing. In the renal form, respiratory symptoms are brief and mild, followed by a sharp increase in water intake and the passing of loose, white, lime-water-like droppings. Feathers around the vent become soiled with feces, and in severe cases, dehydration is marked and the feet become shriveled. In laying flocks, mild rattling and coughing often appear first; 2—3 days later, feed intake declines and the laying rate drops “like a cliff,” while soft-shelled and sandpaper-shelled eggs increase markedly, and the egg whites become thin and watery.
3, Postmortem Changes
The main lesions are concentrated in the respiratory, urinary, and reproductive systems: the tracheal and bronchial mucosae are congested and edematous, and the lumens contain serous, mucoid, or caseous exudate. In severe cases, caseous plugs form at the bronchial bifurcations (bronchial obstruction); asphyxiation due to obstruction is the main cause of death in chicks. The air sacs are thickened and cloudy, and the lungs are congested and edematous. In the renal form, the kidneys are enlarged and pale, with large amounts of white urate deposits in the renal tubules and ureters, creating the typical “mottled kidney” appearance; the ureters are dilated. In the reproductive form, the follicles are congested, hemorrhagic, and deformed; the oviducts are atrophied, narrowed, or form watery cysts, and yolk falls into the abdominal cavity, causing yolk peritonitis.
二、Causes of Infectious Bronchitis in Chickens
Emphasis should be placed on both biosecurity and scientific immunization. The main reasons for the current outbreaks are:
1, IBV has many serotypes and mutates rapidly. The vaccine strains do not match the locally circulating strains, and cross-protection is insufficient, so disease can still occur after immunization. Poultry farms with the necessary resources should select a vaccine combination that matches local circulating strains based on pathogen testing and antibody monitoring.
2, the immunization program is unreasonable, immunization is administered too late, or procedures are not followed properly. Early infectious bronchitis infections are now relatively common. Mucosal immunity is not established promptly after chicks hatch, and immunizing after disease appears is too late.
3, sick chickens and recovered chickens carrying the virus are the main sources of infection. The virus spreads through people, vehicles, equipment, wild birds, and the air. The downtime between flocks is too short, and cleaning and disinfection are not thorough, allowing the virus to circulate and accumulate on the farm, with disease becoming more severe from flock to flock.
4, poor husbandry practices—including imbalanced insulation and ventilation in the chicken house, cold stress, excessive stocking density, excessive ammonia concentration, excessively high protein content in feed (which can trigger the renal form), and insufficient drinking water—are all factors that can cause and promote the occurrence of this disease.
Three. Preventive measures
1, ensure good biosecurity: maintain all-in/all-out, closed housing; keep brooding, growing, and adult chickens in separate areas and buildings where possible; allow a sufficient downtime period between flocks; thoroughly treat the chicken house following the “sweep—wash—disinfect—dry” process; refuse visitors; strictly disinfect people, vehicles, and equipment entering the premises; and prevent access by birds and rodents.
2, strengthen husbandry management: during the brooding period (1–3 weeks), maintain stable, uniform temperatures; in autumn and winter, ventilate without exposing birds directly to cold drafts; provide adequate ventilation to reduce ammonia concentration; control stocking density, keep litter dry, and provide sufficient clean drinking water; on farms with a high incidence of the renal form, appropriately reduce dietary protein levels.
3, implement immunoprophylaxis (core measure)
(1) Vaccine selection: Infectious bronchitis vaccines are primarily live and inactivated vaccines. Among live vaccines, strains such as H120 and MA5 are of low virulence and are suitable for primary vaccination of chicks; H52 is more virulent and is suitable for booster immunization of flocks over 21 days of age. In areas where nephropathogenic strains are prevalent, 28/86 and strain W93 may be selected; for QX-type prevalent strains, QX-L87 may be selected, and for the reproductive type, strain 4/91 may be selected. Inactivated oil-emulsion vaccines produce high antibody levels that persist for a long time, and are mainly used before the onset of lay and for booster immunization of breeding chickens. More strains are not necessarily better; a combination of conventional strains and locally predominant strains can be selected to broaden the protection spectrum.
(2) Immunization methods: Infectious bronchitis immunization primarily relies on local mucosal immunization. Methods include eye drop, intranasal drop, spray, and drinking water administration. Eye-drop and intranasal-drop immunization are more reliable and far more sensitive than drinking-water immunization, so eye-drop administration is the preferred choice. Before drinking-water immunization, withhold water for 2–4 hours (in hot weather, 2 hours; in cold weather, 4 hours). Use double the vaccine dose, and do not dilute it with tap water containing bleaching powder.
(3) Immunization timing: Immunize early for early protection. In general, give the primary vaccination at 1–3 or 7–10 days of age (with live attenuated vaccines such as H120), the second vaccination at 20–30 days of age, and a booster with H52 at around 60 days of age. Administer an inactivated vaccine such as the combined Newcastle disease and infectious bronchitis vaccine 2–4 weeks before the onset of lay to ensure protection throughout the laying period. After the onset of lay, breeding chickens should receive a live-vaccine booster every 2–3 months. When immunizing, ensure an appropriate interval from live vaccines such as Newcastle disease vaccine, or use a combined Newcastle disease and infectious bronchitis vaccine directly to avoid mutual antigenic interference.
Precautions during immunization (eye drops) (4):
Confirm that the flock is healthy before immunization. Postpone immunization for flocks with existing respiratory disease to avoid compounding environmental stress and immunization stress;
Be sure to use the designated diluent for the vaccine. Do not use tap water or other liquids as a diluent. Distilled water or deionized water may be used as a substitute;
When administering eye drops, ensure the vaccine has been fully absorbed before releasing the chickens. Hold the dropper tip perpendicular to the eye when dispensing to prevent the vaccine solution from flowing back, which could result in an insufficient dose and affect the chickens’ resistance;
Ensure the diluted vaccine is used within 0.5 hours. Using it for too long will reduce the vaccine’s potency and cause immunization failure;
After immunization, strengthen environmental control and reduce cold stress. If necessary, administer sensitive drugs to prevent secondary infections.
Four. Treatment measures
There is currently no specific treatment drug for this disease. After the disease occurs, adhere to the principle of “emergency immunization + symptomatic treatment + prevention of secondary infections”:
1, during the early stage of the disease, urgently vaccinate flocks that have not yet shown symptoms with a live infectious bronchitis vaccine matched to the circulating strain. At the same time, immediately isolate and cull sick chickens to prevent the outbreak from spreading.
2, raise the house temperature by 2—3℃, improve ventilation and air exchange, reduce the ammonia concentration, and minimize various stressors.
3, symptomatic treatment: For pronounced respiratory symptoms, drugs such as ammonium chloride and aminophylline can be used to reduce phlegm and relieve bronchospasm. Bronchial obstruction is the main cause of death in chicks; drugs such as aminophylline may be used to promote the expulsion of obstructions. For chickens with the renal form of the disease, add oral rehydration salts and sodium bicarbonate (baking soda) to their drinking water to promote urate excretion, while moderately lowering the dietary protein level.
4, prevent secondary infections: Use drugs to which Escherichia coli and Mycoplasma are sensitive (such as doxycycline, tylosin, florfenicol, etc.) for 3—5 days to reduce secondary losses.
5, add electrolytes, multivitamins, and vitamin C to the drinking water throughout the entire period to enhance the flock’s resistance. You can also mix traditional Chinese herbs such as Banlangen (Isatis root), Houttuynia cordata, honeysuckle, and licorice into the feed or brew them in water to help fight viruses and relieve respiratory inflammation.
Note: The treatment plan in this article is for reference only. Use specific medications under the guidance of a licensed veterinarian.
In short, preventing and controlling infectious bronchitis requires adhering to the principle that “rearing is more important than prevention, and prevention is more important than treatment”: biosecurity lays the foundation, husbandry management provides the safeguard, scientific immunization is the core, and antibody monitoring provides support. Only by coordinating all four can infectious bronchitis truly be kept out of the chicken house and autumn and winter farming returns be protected.