As winter approaches and temperatures fall, temperature differences across the country increase, raising the risk of respiratory disease in flocks. Mycoplasma gallisepticum (MG), one of the main pathogens causing chronic respiratory disease in chickens, is known for its high contagiousness, wide range of transmission routes, and complex modes of transmission. This pathogen is highly pathogenic and can evade the host’s immune system, weakening the immunity of infected flocks and making them more susceptible to complications such as Newcastle disease and chronic respiratory disease. In addition, MG can affect chickens’ reproductive systems, causing poor embryonic development and death during incubation. This seriously harms the production performance of breeding chickens and causes significant economic losses to the poultry industry.
1, Etiology
MG is a microorganism belonging to the order Mycoplasmatales, family Mycoplasmataceae, and genus Mycoplasma. It is a highly pleomorphic prokaryote without a cell wall and can pass through bacterial filters. It grows slowly on artificial culture media, usually taking several days to several weeks to form visible colonies. Its optimal growth temperature is 37°C, and its optimal pH is 7.6-7.8. It primarily colonizes the epithelial cells of the respiratory mucosa, causing chronic inflammation and impaired ciliary function. MG has low resistance to various physical and chemical factors and is sensitive to drying, heat, ultraviolet light, and commonly used disinfectants. Therefore, standard disinfection methods such as ultraviolet irradiation, high-temperature steaming, and chemical disinfectants can effectively kill MG.
2, Epidemiology
Turkeys and chickens of all ages are susceptible to MG. Chickens aged 5–8 weeks have a high morbidity rate. When mixed infections occur, the mortality rate rises and can sometimes reach around 30%[2]. Adult chickens often have subclinical infections. They may develop the disease under conditions such as sudden temperature changes and environmental stress, but respiratory symptoms are generally mild; if treatment is delayed, the disease course is prolonged. The disease can spread horizontally through feed, drinking water, and equipment contaminated by respiratory and digestive tract excretions from sick birds. It can also spread vertically through infected hatching eggs. The disease can occur at any time of year and follows a chronic course. In adult chickens, cases are often sporadic and more severe in cold seasons. It commonly occurs as a mixed or secondary infection alongside other respiratory diseases, such as infectious bronchitis, Escherichia coli infection, Newcastle disease, infectious laryngotracheitis, infectious coryza, and infectious bursal disease.
3, Clinical symptoms
Chicks infected with MG
After infection, chicks develop mucoid or serous rhinitis. Sick birds show symptoms such as snicking, sneezing, coughing, and frequent head shaking, with difficulty breathing, nasal congestion, and respiratory rales. In prolonged cases, conjunctivitis, airsacculitis, and infraorbital sinusitis develop, forming large nodules in the infraorbital area. Some sick birds have varying degrees of eyelid swelling, and a few become blind. Hatching eggs from infected breeding chickens carry the pathogen, and their hatchability decreases. Chicks hatched from these eggs have poor vitality. This disease has a significant impact on egg production; laying hens show decreased egg production after infection. When chickens aged 5 to 8 weeks become ill, their growth and development are delayed[3].
4, pathological changes
Respiratory tract exudate during MG infection; caseous secretions in the air sacs
In chickens infected with MG, catarrhal inflammation occurs in the bronchi, trachea, sinuses, and respiratory tract, with increased exudate. The air sacs contain cloudy, viscous exudate; their walls are thickened and opaque. In severe airsacculitis, the sacs often contain grayish-white, cream-like or caseous secretions. In cases of concurrent Escherichia coli infection, fibrinous perihepatitis and pericarditis may be observed. The respiratory mucosa shows varying degrees of edema and congestion. Localized proliferation of lymphoid tissue can be seen beneath the respiratory mucosa, which becomes markedly thickened due to proliferation associated with mucous glands and mononuclear cell infiltration.
5, Diagnosis
MG is diagnosed primarily through clinical and laboratory diagnosis. Note that MG infection can be confused with Newcastle disease, influenza, infectious bronchitis, infectious laryngotracheitis, infectious coryza, Aspergillus infection, and poor air quality in poultry houses, among other problems; differential diagnosis should be considered in clinical practice. For laboratory diagnosis, samples such as tracheal, lung, and air sac tissue, and sinus secretions are inoculated into culture media for bacterial isolation. The isolates are then identified using MG-positive serum. MG infection in flocks can also be monitored using methods such as the agar gel precipitation test, hemagglutination inhibition test, and enzyme-linked immunosorbent assay. Large poultry farms can purchase a Mycoplasma gallisepticum PCR kit on the market and follow the instructions for rapid confirmation.
6, Prevention
Ø Vaccination at regular intervals: For chicks aged 8-15 days, an oil-emulsion inactivated vaccine may be used; subcutaneous injection in the neck of 0.2 mL/bird is recommended. Adult chickens may receive a subcutaneous injection in the neck of 0.5 mL/bird. Chickens will gradually develop immunity 15 days after vaccination.
Ø Strengthen daily husbandry management: Regularly inspect the poultry house ventilation system to ensure unobstructed airflow and fresh air, and reduce the levels of harmful gases. Regularly monitor humidity inside the poultry house and adjust it as needed to maintain a suitable level. Also regularly remove manure and debris from the poultry house to keep it clean and dry.
Ø Provide a nutritionally balanced feed: Formulate feed appropriately according to the chickens’ growth stage, breed, and physiological needs, ensuring it contains appropriate amounts of protein, vitamins, minerals, and other nutrients.
Ø Avoid stress: When adjusting the rearing environment (such as temperature and lighting), make changes gradually to avoid stressing the chickens through sudden changes. In particular, cooling and drafts can cause cold stress, an important factor that induces Mycoplasma infection. When transporting chickens, ensure the comfort and safety of the transport vehicle. Before and after transport, provide the chickens with sufficient drinking water and easily digestible feed to help them recover their strength and immunity. If necessary, anti-stress drugs or additives, such as vitamin C and electrolytes, may be used.
7, Treatment plan
After a flock is infected with MG, its immunity declines, making it susceptible to secondary infections with other respiratory diseases, including infectious bronchitis, Newcastle disease, avian infectious laryngotracheitis, and infectious bursal disease, which makes treatment more difficult. Doxycycline, tylosin, erythromycin, tiamulin, and tildipirosin are commonly used for treatment. However, given the growth characteristics of MG, it is advisable to select long-acting drugs to which it is sensitive to achieve more satisfactory results. Tildipirosin is a new option for controlling Mycoplasma. Because MG is often co-infected with other viral diseases in clinical practice, treatment should also enhance the flock’s immunity and disease resistance. For viral diseases such as Newcastle disease, a combination of traditional Chinese and Western medicine may be used, with antiviral Chinese medicines such as Jinchan Oral Liquid and Jingfang Baidu San as adjunctive treatments.