How to prevent and control avian tuberculosis? Understanding the prevention and control measures in one article
Avian tuberculosis is a chronic contagious disease caused by Mycobacterium avium. The disease is characterized by a chronic course, progressive emaciation, anemia, and reduced or absent egg production. On necropsy, tuberculous nodules can be seen in various tissues and organs, especially the liver, spleen and intestines. When an outbreak occurs on a poultry farm, it is mostly chronic, affecting growth and production performance, causing decreased egg production and death, resulting in serious economic losses. However, due to different rearing methods, mainly the short rearing period, broilers and force-fed ducks are slaughtered quickly, so it is rarely found; although breeding poultry are raised for a longer time, the contamination is not extensive and the incidence is relatively low.
Pathogen
The pathogen of this disease is Mycobacterium avium, a species of the genus Mycobacterium, characterized by being short and small with pleomorphism. The bacterium is slender, straight or slightly curved, sometimes rod-shaped, coccus-shaped or streptococcus-shaped. Both ends of the bacterium are bluntly rounded, 1.0-4.0µm long and 0.2-0.6µm wide. This bacterium has no spores, no capsule and no flagella, and is non-motile; it is not easily stained by ordinary aniline dyes, is Gram-positive, and has acid-fast staining characteristics. When stained with the Ziehl-Neelsen method, Mycobacterium avium appears red, while some other non-Mycobacterium bacteria are stained blue. This staining characteristic can be used for the diagnosis of this disease.
This bacterium is an obligate aerobe and has relatively strict nutritional requirements. It can only grow in special culture media containing serum, cow's milk, egg yolk, potato, glycerol and certain inorganic salts. The optimum growth temperature for culture is 39-40°C. The optimum pH of the medium is 6.5-6.8.
When isolating tubercle bacilli for the first time from materials from natural cases, a special culture medium must be selected. Adding 2% glycerol to the medium and culturing in an environment with 5%-10% carbon dioxide can promote its growth. Tubercle bacilli grow slowly. During primary isolation, after about 10-21 days, colonies can be seen growing that are round, raised, with a smooth, glossy surface, ranging from light yellow to yellow, and turning bright yellow as time goes on; some strains grow smooth, flat, translucent grayish-white colonies on certain media. In liquid media such as glycerol broth, a culture with granular sediment and a viscous pellicle grows.
When biochemical tests are performed on Mycobacterium tuberculosis, the results for nitrate reduction, urease, Tween 80 hydrolysis and niacin formation are all negative; glycerol promotes growth, it can reduce tellurite, and the catalase test is positive. These biochemical characteristics have differential diagnostic significance.
The surface of Mycobacterium tuberculosis contains large amounts of lipid and waxy components, giving it strong resistance to the external environment, especially the strongest resistance to drying. Bacteria in secretions can survive for 6-8 months in a dry environment and 18-31 days in sunlight; the bacteria survive for 5 months in feces and water, and can survive for 2 years or more in soil. Direct sunlight reduces them within 1 h, and after 4 h they all die; tubercle bacilli have weak resistance to moist heat, losing viability at 60°C for 30 min, and have strong resistance to low temperatures, surviving for 4-5 months below 0°C.
The bacteria also have relatively strong resistance to chemical agents and are sensitive to fat-dissolving ionic detergents; they are sensitive to 2% Lysol, 5% carbolic acid, 3% formaldehyde, 10% bleaching powder and 70%-75% alcohol. In 4% NaOH, 3% HCl and 6% H2S04 for 30 min, they have relative tolerance and their viability is unaffected, so in experiments this is often used to treat contaminating bacteria in diseased materials, and it is also often added to culture media to achieve the purpose of controlling contaminating bacteria.
Tubercle bacilli are insensitive to commonly used sulfonamides and antibiotics. Antibiotics such as streptomycin and cycloserine, and drugs such as isoniazid, para-aminosalicylic acid and rifampicin, have bacteriostatic or bactericidal effects.
Epidemiology
Mycobacterium avium mainly infects poultry and birds, and pigs are also susceptible; next are cattle and sheep, while humans, dogs, cats and deer are rarely affected. All birds can be infected with avian tuberculosis. Among experimental animals, rabbits are highly susceptible. Poultry of all breeds and ages can be infected, and because the course of the disease develops slowly, it is mostly found at the time of culling or slaughter of older birds.
Sick poultry are the main source of infection. The ulcerative intestinal lesions and tuberculous lesions of the liver and gallbladder in sick poultry shed bacteria, excreting large numbers of tubercle bacilli through feces, and respiratory secretions may also shed bacteria. The excreted bacteria contaminate feed, drinking water, poultry houses, soil, litter and the environment. After being ingested by healthy chickens, ducks, geese, etc., infection mainly occurs through the digestive tract. Infection can also occur through the respiratory tract by inhaling bacteria-laden dust. Raising sick poultry together with healthy poultry in the same flock spreads the disease. Humans, rearing and management equipment, vehicles, etc. can also promote transmission. If water sources are contaminated, ducks and geese grazing and playing in water, as well as wild birds perching there, may also be infected. A considerable portion of tuberculosis in waterfowl is infected from sick chickens.
Poor poultry house and environmental hygiene, inadequate disinfection, poor management, excessive density, damp and dark conditions, and poor ventilation can all promote the occurrence of this disease.
Epidemic characteristics
All avian species can be infected with Mycobacterium avium, and rabbits, pigs and mink are also highly susceptible. This disease is mostly sporadic, with a very low incidence. Chicks are more susceptible than adult chickens, but affected chickens are mostly adults.
Clinical characteristics
Sick chickens are depressed, have a normal appetite, but lose body weight. They become emaciated, with atrophy of the breast muscles, deformation of the sternum, a smaller body size, and fading and atrophy of the comb, wattles and ear lobes. Sick chickens often have diarrhea, and some are paralyzed.
Lesion characteristics
Sick chickens have an enlarged liver with yellow-white tuberculous nodules ranging from millet-seed to soybean size, some of which merge into large nodules. The spleen is enlarged several times, with numerous scattered yellow-white firm nodules. Tuberculous nodules can be seen in tissues and organs such as the small intestine, cecum, lungs and bones.
Prevention and control measures
Cull infected flocks, discard old chicken houses and old equipment, and carry out thorough disinfection. It is strictly forbidden to introduce sick chickens from other places, and isolation and quarantine must be carried out when introducing chickens. Treatment is generally not advocated for affected flocks; timely culling and disposal are sufficient. According to the characteristics of avian tuberculosis, the following preventive measures can be taken.
1. When tuberculosis is found in chickens, ducks, geese, etc. on poultry farms, they should be dealt with promptly. Dead poultry should be burned or buried.
2. Thoroughly clean and disinfect poultry houses and the environment. The removed feces should be piled up for fermentation and composting. For waterfowl raised on the ground, remove the feces and disinfect with caustic soda water; if the floor is dirt, the surface soil should be removed, disinfected and replaced with new soil; it is difficult to completely eliminate the pathogen from contaminated sites, as the pathogen retains its virulence in soil for several years.
3. If tuberculosis-affected poultry continually appear in a flock (for example, tuberculous lesions are seen on necropsy), the sick poultry and older poultry that are emaciated and lay few or no eggs should be culled. Because these birds (chickens) may be diseased birds (chickens), also lay few eggs and are old, from an economic point of view they have little rearing value, and culling the whole flock is more advantageous.
4. For laying hen flocks affected with tuberculosis, culling all the chickens in the flock after the first peak of egg production is the most economical and best measure. This is because old chickens lay few eggs, more of them are seriously ill, and more old chickens die, making them the most dangerous source of infection.
5. When necessary, use avian tuberculin to carry out an allergic reaction or rapid plate agglutination test on breeding poultry and rare poultry (birds). Birds showing a positive reaction should be culled to eliminate the source of infection. The rapid plate agglutination test is suitable for use by grassroots poultry farms. Culling the whole flock is even more ideal.
6. Eggs from sick poultry cannot be used for breeding.