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 ensuring ventilation in chicken houses becomes more pronounced, creating a period of high risk for infectious bronchitis.
The disease is mainly characterized 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 major diseases currently affecting layer and broiler farming.
I. How to prevent and control it correctly
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. It is 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, which is the fundamental reason the disease is difficult to prevent and control. The virus is not highly resistant to environmental conditions and is sensitive to high temperatures and common disinfectants. It can be inactivated at 56℃ for 15 minutes. Common disinfectants such as sodium hypochlorite and peracetic acid can effectively kill it; however, it can survive for a relatively long time in low-temperature environments, which is one reason it is more common in autumn and winter.
1, Disease characteristics
The disease has a short incubation period, generally 1–2 days, and spreads extremely rapidly. After 1 chickens in a flock are infected, the disease can spread to the entire flock in about 2 days, and the morbidity rate can reach over 90%. Chickens of all ages are susceptible, with chicks (especially those aged 2–6 weeks) most severely affected. The mortality rate for the respiratory form in chicks is generally 20%–30%; it rises further following secondary infection with Escherichia coli and Mycoplasma. The mortality rate for the renal form of infectious bronchitis can reach 30%–50%, indicating a high mortality rate.
After infection, laying hens often experience a 20%–50% drop in egg production, and produce large numbers of soft-shelled, misshapen, and faded eggs, with watery egg whites. Infection in pullets (before 6 weeks of age) damages oviduct development; 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. Even after recovering, sick chickens may carry and shed the virus for a long time, becoming a source of infection.
2, Disease symptoms
Sick chickens show depression, huddling together and shivering, reduced appetite, drooping wings, and ruffled feathers. The respiratory form mainly presents as open-mouth breathing with the neck stretched out, 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 keep shaking their heads because of difficulty breathing. In the renal form, respiratory symptoms are brief and mild, followed by a sharp increase in water consumption and the passage of loose, white, limewater-like droppings. Feathers around the vent become soiled with feces, and in severe cases the chickens become severely dehydrated and their feet shrivel. In laying flocks, mild rattling and coughing often appear first; 2￣3 days later, feed intake declines and the laying rate drops “off a cliff.” Soft-shelled and sand-shelled eggs increase markedly, and the egg white becomes thin and watery.
3, Necropsy findings
The main lesions affect the respiratory, urinary, and reproductive systems. The tracheal and bronchial mucosae are congested and edematous, with serous, mucoid, or caseous exudate in the lumen. In severe cases, caseous plugs form at the bronchial bifurcations (bronchial obstruction); suffocation 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 deposited in the renal tubules and ureters, producing the characteristic “mottled kidney”; 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
Promote both biosafety and scientific immunization. The main causes of disease are as follows.
First, IBV has many serotypes and mutates rapidly. The immunizing strains may not match locally circulating strains, resulting in insufficient cross-protection, so disease can still occur after immunization. Where feasible, chicken farms should select a vaccine combination that matches locally circulating strains based on pathogen testing and antibody monitoring.
Second, the immunization program is unreasonable, the timing of immunization is too late, or the procedures are not carried out properly. Early IB infection is relatively common. Mucosal immunity is not established promptly after chicks hatch, and immunizing after disease develops is too late.
Third, sick chickens and recovered chickens that carry 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 incomplete, allowing the virus to circulate and accumulate on the farm, with the problem becoming more severe with each flock.
Fourth, improper husbandry and management practices—including an imbalance between heat retention and ventilation in the chicken house, cold stress, excessive stocking density, excessive ammonia concentrations, excessively high protein content in feed (which can trigger the renal form), and insufficient drinking water—can all trigger or promote the occurrence of this disease.
III. Preventive measures
1, maintain biosecurity
Follow all-in/all-out and closed-house practices. It is recommended that brooding, rearing, and adult chickens be housed in separate areas and separate buildings. Allow a sufficient downtime period between flocks and thoroughly treat the poultry house using the “cleaning—rinsing—disinfection—drying” process. Do not allow visitors. Strictly disinfect personnel, vehicles, and equipment before entry, and prevent birds and rodents from entering.
2, strengthen feeding management
During the brooding period (1－3 weeks), ensure stable, uniform temperatures. In autumn and winter, ventilate without allowing cold drafts to blow directly onto the birds; provide proper ventilation to reduce ammonia concentration. Control stocking density, keep the litter dry, and provide an adequate supply of clean drinking water. Farms with a high incidence of the renal form should moderately reduce dietary protein levels.
3, ensure immunization prevention
(1) Vaccine Selection
Live and inactivated vaccines are the main types used against infectious bronchitis. Among live vaccines, H120 and MA5 have low virulence and are suitable for primary vaccination of chicks; H52 has stronger virulence and is suitable for booster vaccination of flocks older than 21 days. In areas where the nephropathogenic form is prevalent, 28／86 and strain W93 may be selected. For prevalent QX-type strains, strain QX－L87 may be selected; for the reproductive form, strains 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 vaccination of breeding chickens. More strains are not necessarily better; combining traditional strains with locally predominant strains is sufficient to broaden the protection spectrum.
(2) Immunization methods
Avian infectious bronchitis vaccination primarily relies on local mucosal immunity. Methods include eye drops, nasal drops, spray, and drinking water. Eye-drop and nasal-drop vaccination are indeed effective and much more reliable than vaccination via drinking water, so eye drops are the first choice. Before vaccination via drinking water, 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 the vaccine with tap water containing bleaching powder.
(3) Vaccination timing
Immunize early for early protection. In general, give the first immunization at 1–3 or 7–10 days of age (with a live attenuated vaccine such as H120), the second immunization at 20–30 days of age, and a booster with H52 at around 60 days of age. Before the start of laying, administer an inactivated vaccine such as the Newcastle disease–infectious bronchitis vaccine 2–4 weeks beforehand to ensure protection throughout the laying period. After laying begins, breeder chickens should receive a booster with a live vaccine every 2–3 months. When immunizing, pay attention to the interval between this vaccine and live vaccines such as those for Newcastle disease, or use a Newcastle disease–infectious bronchitis bivalent vaccine directly to avoid mutual antigen interference.
(4) Precautions during immunization (eye drops)
Before immunization, confirm that the flock is healthy. Postpone immunization for flocks with existing respiratory disease to avoid compounding environmental stress with immunization stress. Be sure to use a dedicated vaccine diluent; do not use tap water or other liquids as the diluent. Distilled or deionized water may be used instead. When administering eye drops, allow the vaccine to be fully absorbed before releasing the chicken. Hold the dropper vertically over the eye to prevent the vaccine solution from flowing back, which could result in an insufficient dose and affect the chicken’s resistance. Use the diluted vaccine within 0.5 hours; prolonged use can reduce vaccine potency and cause immunization failure. After immunization, strengthen environmental controls and reduce cold stress. If necessary, administer a sensitive drug to prevent secondary infection.
IV. Treatment measures
At present, there is no specific drug treatment for this disease. After an outbreak, adhere to the principles of “emergency immunization, symptomatic treatment, and prevention of secondary infections.”
First, in the early stages of an outbreak, chicken flocks that have not yet shown symptoms can be urgently vaccinated 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.
Second, increase the house temperature to 2–3℃, improve ventilation and air exchange to reduce ammonia concentration, and minimize various forms of stress.
Third, provide symptomatic treatment. For pronounced respiratory symptoms, medications such as ammonium chloride and aminophylline may be used to loosen mucus and relieve bronchospasm. Bronchial blockage is the main cause of chick mortality; medications such as aminophylline may be used to promote the expulsion of obstructions. For birds with the renal form of the disease, add oral rehydration salts and sodium bicarbonate (baking soda) to the drinking water to promote urate excretion, while also appropriately reducing the dietary protein level.
Step four: Prevent secondary infections by using drugs effective against Escherichia coli and Mycoplasma (such as doxycycline, tylosin, and florfenicol). Administer continuously for 3–5 days to reduce secondary losses.
Step five: Add electrolyte-multivitamin supplements and vitamin C to the drinking water throughout the entire period to enhance the flock’s resistance. Chinese herbal medicines such as Banlangen, Houttuynia cordata, honeysuckle, and licorice may also be mixed into the feed or boiled in water to support antiviral action and relieve respiratory inflammation.