Infectious bronchitis (IB) in chickens is an acute respiratory infectious disease caused by infectious bronchitis virus, a member of the genus Coronavirus. The incidence is higher in winter and often causes deaths in chicks. It can also impair growth and development in chickens, reduce weight gain and feed efficiency, and, in particular, cause permanent damage to the reproductive organs of developing chickens. The disease is mainly characterized by coughing, sneezing, and tracheal rales. Chicks may have nasal discharge and pass white, watery droppings. Laying hens may show a decline in egg production, an increase in malformed eggs, delayed oviduct development (the oviducts are narrow and small), and egg-yolk peritonitis.
1, Epidemiological Features
Chickens of all ages are susceptible to infection, with the renal form of infectious bronchitis primarily affecting chicks. The main route of transmission is via respiratory droplets; the disease spreads rapidly and can occur throughout the year, with cases more common in autumn and winter. The younger the chickens, the more severe the disease. Colder ambient temperatures worsen the condition. Chickens fed diets with higher protein content are more likely to become ill than those fed diets with lower protein content.
2, Clinical Signs
Sick chickens show obvious respiratory symptoms, with rales or crowing sounds when breathing. Starting on day 4 of illness, the number of sick chickens with wet rales gradually decreases, while the number with dry rales gradually increases; by day 7, wet rales can basically no longer be heard. The earliest deaths occur on day 4 of illness, caused by bronchial obstruction (deaths usually do not occur during the first 3·d). Most sick chickens showing difficulty breathing also have obvious whistling sounds and dry rales. After chicks become ill, they are lethargic, with drooping wings, ruffled feathers, and tears and nasal discharge. After laying hens become ill, their laying rate decreases. During recovery, many of the eggs they lay are faded, shell quality declines, and the egg whites are thin and watery.
3, Necropsy findings
The deep tissues of the pectoral muscles in sick chickens show gelatinous edema, giving the carcass a “moist” appearance. The lower trachea and bronchi are flushed, with increased mucus; in severe cases, the trachea is blocked by caseous material. The lungs are edematous and congested, and the air sacs are thickened and cloudy. There is pericardial effusion. The intestinal mucosa is ulcerated and congested, and there is a large amount of viscous exudate in the abdominal cavity. The cecal tonsils are hemorrhagic; the proventricular papillae are grayish-yellow, with hemorrhagic spots along the edges. The kidneys are enlarged and dark red. The ovaries of most sick chickens are normally developed, but the oviducts are blocked; inflammatory exudate forms cysts in the oviducts, as large as a balloon. Mature follicles cannot enter the oviduct to form eggs and instead fall into the abdominal cavity, causing a laying disorder known as “false layer.”
4, Differential diagnosis
Differences between infectious bronchitis and Newcastle disease: Infectious bronchitis spreads quickly, whereas Newcastle disease spreads relatively slowly. Infectious bronchitis generally affects more young chickens, while Newcastle disease can infect chickens of all ages. Newcastle disease causes neurological signs, whereas infectious bronchitis does not. In laying hens infected with infectious bronchitis, the egg white is watery, but this symptom does not occur in flocks infected with Newcastle disease. Infectious bronchitis lesions are mainly found at the bronchial bifurcations, whereas Newcastle disease lesions are mainly found in the upper respiratory tract.
Differences between infectious bronchitis and avian influenza: In addition to the differences from Newcastle disease mentioned above, avian influenza mainly affects the oviducts of laying hens, causing white mucus to appear inside the oviducts; infectious bronchitis does not cause this symptom. Chickens with avian influenza have hemorrhagic spots in fatty tissue, which are generally absent in infectious bronchitis.
Differences between infectious bronchitis and Aspergillus infection: Young chickens infected with Aspergillus can show signs similar to those of infectious bronchitis. Infectious bronchitis mainly occurs in the lower third of the trachea, whereas fungal infection mainly occurs in the lungs and air sacs, with small yellow or yellowish-white fungal nodules.
5. Conventional vaccine classification and selection for infectious bronchitis virus (IBV)
Conventional IBV vaccines include live attenuated vaccines and inactivated vaccines. Live vaccines are generally used to immunize broilers and for the first immunization of breeder and layer chickens. Inactivated oil-emulsion vaccines are mainly used for booster immunization before breeder and layer chickens begin laying. IB is an endemic disease with many serotypes. At present, more than 30 serotypes have been identified in China. Strains circulating in different regions vary considerably, and the cross-protection rate between them is very low, making it difficult for farmers and veterinarians to determine which strain of vaccine is suitable for their locality.
5.1, respiratory-type strains
Strains H120 and H52: Mainly introduced from the Netherlands, they are primarily used to prevent respiratory and reproductive forms of infectious bronchitis. They are currently the most widely used because they provide good immune protection against the multiple IBV serotypes circulating in China and good cross-protection against strains that cause renal lesions. Both provide partial cross-protection against QX-like strains. The difference between H120 and H52 is that H120 was attenuated through 120 passages and is less virulent than H52, so it can be used for the first immunization of chicks under 7 days of age; H52, being more virulent, is used to immunize flocks over 21 days of age.
Strain M41: It is highly virulent. Its most notable feature is that it belongs to the same serotype as strains isolated in many countries, and it is widely used in the production of inactivated vaccines.
5.2, nephropathogenic strain
28/86 strain: This is a nephropathogenic strain from Italy. Its greatest advantage is that it is considered the most virulent strain in causing kidney lesions. Attenuation through passage in SPF chicken embryos can achieve a satisfactory level of virulence while retaining immunogenicity. This strain is used in chicken flocks of any age and provides good protection against kidney lesions.
W93 strain: This is a strain isolated domestically and provides good protection against nephropathogenic infectious bronchitis.
5.3, reproductive-type strain
4/91 strain and 91 strain: The main advantage is that it can prevent deep lesions caused by infectious bronchitis, such as a decrease in egg production and respiratory symptoms, and provides partial cross-protection against nephropathogenic infectious bronchitis.
LDT3-A strain: It can be used to prevent oviductal cysts containing watery fluid caused by reproductive-form infectious bronchitis, but provides poor cross-protection against respiratory-form infectious bronchitis strains.
QX-L87 strain: It can be used to prevent oviductal cysts containing watery fluid caused by QX-like strains, but provides poor cross-protection against respiratory-form and nephropathogenic infectious bronchitis strains.
6, Immunization timing and route
Mucosal immunity, humoral immunity, and cellular immunity are all essential for preventing and controlling infectious bronchitis, with local mucosal immunity playing an important role. This is especially true for chicks. A combined immunization approach using inactivated and live vaccines is generally adopted and is a classic practice in the industry for controlling this disease. The first immunization against infectious bronchitis is generally given at 1 to 3 days of age, and the second may be given at 7 to 10 days of age. Because early onset of infectious bronchitis is now relatively common, early immunization helps provide protection sooner. Breeder and laying hens receive another vaccination with an oil-emulsion inactivated vaccine before coming into lay. Live vaccines can be administered by intranasal drops, aerosol, or drinking water; inactivated vaccines can be administered by intramuscular injection.
7, Strengthen environmental control
Cold stress, poor ventilation, excessive stocking density, and an unfavorable environment are important factors contributing to infectious bronchitis outbreaks. A comprehensive approach to preventing and controlling infectious bronchitis should be adopted, starting with management measures such as providing a comfortable rearing environment and implementing an all-in/all-out system to reduce factors that may trigger the disease.
8, Current challenges facing vaccine-based control of infectious bronchitis
IBV is an RNA virus with complex serotypes/genotypes. It readily develops point mutations, and strains readily recombine, giving it a high degree of variability. Cross-protection between different serotypes/genotypes is incomplete, and the virus also has a geographic distribution pattern. The rapid mutation of coronaviruses makes vaccination-based prevention and control in chicken flocks difficult. For many years, strains with the QX strain as the predominant type have been the main targets of prevention and control in flocks. However, in practice, the combined use of QX and various other IB vaccine strains, as well as other vaccination strategies, has not produced ideal results, and disease appears to be becoming increasingly severe. Previously, infectious bronchitis in layer chickens occurred mainly on pullet farms. This was attributed to factors such as excessive stocking density, frequent brooding cycles, and short downtime between flocks. However, this issue has now also begun to occur at some farms that raise their own pullets. The main signs in the flock are an excessively high proportion of soft-shelled eggs at the start of lay and failure to reach a peak in egg production.
The possible causes of these problems vary, and there is currently no definitive conclusion. They include:
(1) The vaccine used may not match the infectious bronchitis strains currently circulating. Most infectious bronchitis strains detected in affected flocks are reported to belong to the QX strain, so some have recommended vaccination with a vaccine containing QX. However, in practice, prevention and control results have also been unsatisfactory after vaccination with a QX vaccine;
(2) The problem may be due to vaccination taking place too late and the flock becoming affected too early. Many hatcheries administer a spray vaccination when chicks hatch (1 days of age); farmers then vaccinate again at 7 days of age and give a booster at 20 days of age. However, the vaccination results are unsatisfactory, so this explanation is also unconvincing;
(3) The spray immunization method may be inappropriate. Some poultry farmers ask the hatchery not to spray-immunize newly hatched chicks, but instead to wait until the chicks are brought back and placed in the poultry house before vaccinating them themselves, at 1, 7, 20, and 40 days of age. However, the immunization results are still unsatisfactory;
(4) The empty-house period may be too short, causing the virus to persist on the farm for a long time and making it difficult to eliminate. As a result, some poultry farms began leaving the entire farm empty, carrying out strict disinfection multiple times, and bringing in chicks after an interval of 2 months, but found that the immunization results were still unsatisfactory;
(5) The IBV vaccine used may have insufficient potency. Therefore, some have tried selecting IBV vaccines produced by different manufacturers, rotating and combining different strain types, and have used vaccines from almost every manufacturer, including imported ones. However, the disease prevention results are still unsatisfactory, and infectious bronchitis continues to spread;
(6) Contamination of the hatchery equipment may have caused IBV in the flock. However, not all poultry farmers who bring in commercial chicks hatched in the same batch at the same hatchery experience infectious bronchitis. Therefore, this is only a speculation and is not supported by sufficient survey data;
(7)It is possible that chicks are already colonized by IBV in their oviducts before hatching; however, there is currently no effective way to resolve the IB problem in chicks. The only option is to strictly vaccinate parent-stock breeders according to the IB vaccination program, using vaccines that provide more targeted immunization, to prevent hatching eggs from being infected by the virus.
All of the above views are highly plausible, but obtaining clinical trial data to verify them is very difficult. Regardless of the reason commercial laying hens become infected with IBV, it is recommended that parent-stock breeding farms strictly follow the IB vaccination program and, after the onset of lay, administer both live and inactivated IB vaccines every two months.
At present, there are no reliable data on the extent of cross-antigenicity between circulating strains and vaccine strains; no one has done the foundational work, and much of the available data is theoretical and differs considerably from actual conditions. In short, avian coronaviruses will remain a difficult challenge for the livestock industry to overcome in the coming years, especially in the layer industry. The problem of infectious bronchitis must be thoroughly resolved; otherwise, even the best breeds and the highest-quality chicks will be a waste of resources.