Mycoplasma synoviae most commonly causes subclinical upper respiratory infections in chickens, turkeys, and other avian species. It can also cause exudative tendinitis and synovitis, known as infectious synovitis. Real-time PCR is becoming the most common diagnostic test. Antibiotics in feed may help prevent synovitis, but they are not effective in established cases. Control and serology-based surveillance programs have eliminated M synoviae from most breeder flocks of chickens and turkeys in the USA.
M synoviae was first recognized as an acute to chronic infection of chickens and turkeys that produced an exudative tendinitis and synovitis (infectious synovitis), with increased condemnation in the processing plant; it occurs most commonly as a subclinical infection of the upper respiratory tract, especially in multi-age layer flocks. M synoviae infection is also a complication of airsacculitis in association with Newcastle disease or infectious bronchitis. It is distributed worldwide and is seen primarily in chickens and turkeys, but ducks, geese, guinea fowl, parrots, pheasants, and quail may also be susceptible. M synoviae is even more fastidious than M gallisepticum. Serum (preferably swine) and nicotinamide adenine dinucleotide (NAD) are required for growth in M synoviae culture media.
M synoviae isolates vary widely in virulence, and suspected virulence factors include adhesins, sialidase, nitric oxide, cell invasion, and antigenic variation and immune evasion.
Epidemiology and Transmission of Mycoplasma synoviae Infection in Poultry
M synoviae is egg transmitted (transovarian), but the infection rate in breeder hens is low, and some hatches of progeny may be free of infection. Horizontal transmission is similar to that of M gallisepticum primarily via the respiratory tract, with direct and indirect routes.
The incidence of M synoviae infection in commercial poultry in the USA has decreased because of the National Poultry Improvement Plan control programs implemented for chicken and turkey breeders. However, from time to time, there are large outbreaks involving breeder and market birds, typically limited to one or a few states and related to one especially virulent isolate. M synoviae infections of multiple-age layer flocks are common and may contribute to decreased egg production.
Clinical Findings and Lesions of Mycoplasma synoviae Infection in Poultry
Although slight rales may be present in birds with M synoviae respiratory infection, usually no signs are noticed. Birds under stress or with concurrent infections are more likely to be clinically affected. The first signs of infectious synovitis include pale-bluish head parts and lameness in many birds with a tendency to sit. The more severely affected birds are depressed and found resting around feeders and waterers. Hocks and footpads are swollen, and sternal bursitis (breast blisters) may be seen. Morbidity is usually low to moderate with mortality of 1%–10%. In contrast to infection with M gallisepticum, the clinical signs of infection with M synoviae are more severe and more associated with joint lesions in turkeys. Effects on egg production are usually not apparent, but instances of transient egg production drops have occurred in layer flocks. M synoviae has been reported to cause eggshell abnormalities in egg layers, particularly in Europe. Respiratory lesions may be absent or consist of mild mucoid tracheitis or sinusitis with airsacculitis when birds are stressed from poor air quality or challenged with Newcastle disease or infectious bronchitis. Early in infectious synovitis, a creamy to viscous yellow-gray exudate is present in most synovial structures but most commonly seen in swollen hock and wing joints. In chronic cases, this exudate may become inspissated; livers are enlarged and sometimes green, spleens are enlarged, kidneys are enlarged and pale, and birds may be weak and thin with breast blisters from sternal recumbency.
Diagnosis of Mycoplasma synoviae Infection in Poultry
Skeletal abnormalities and trauma must be eliminated as the cause of lameness. Differential diagnoses include viral tenosynovitis, staphylococcal and other bacterial joint infections, and other respiratory pathogens such as Newcastle disease virus, infectious bronchitis virus and M gallisepticum.
A presumptive diagnosis based on clinical signs and gross lesions should be confirmed by laboratory tests. Serum plate agglutination or ELISA are used to detect M synoviae antibodies, but cross-reactions with M gallisepticum and other nonspecific reactions may occur. These reactors are confirmed as seropositive by hemagglutination-inhibition or by culture, isolation, and identification of the organism. PCR may be used to rapidly detect M synoviae DNA from pre- or postmortem specimens. In turkeys, the agglutination test for M synoviae may not be reliable, especially in birds with predominantly respiratory infection. Because of the fastidious nature of Mycoplasma and the difficulty of isolation, molecular diagnostic tests are becoming the most common method for detection and characterization of mycoplasma infections in poultry. Real-time PCR is a sensitive, specific, and fast detection test for M synoviae and can be performed directly on clinical swabs taken from infected sites (eg, choana, sinuses, trachea, airsacs, and joints). Sequence typing by targeting and amplifying specific sequences allows for differentiation between M synoviae isolates and outbreaks and can be particularly useful for epidemiologic investigations and to identify the source of infection.
Treatment, Control, and Prevention of Mycoplasma synoviae Infection in Poultry
The National Poultry Improvement Plan coordinates control and serology-based surveillance programs for M synoviae similar to those for M gallisepticum. These programs have resulted in eradication of the infection in most primary breeder flocks of chickens and turkeys in the USA. Chicks and poults should be obtained from M synoviae–free breeders and raised with biosecurity to prevent introduction. Antibiotics in the feed may be beneficial in prevention of synovitis but are expensive and not very effective in established cases. When M synoviae involvement in airsacculitis is an anticipated problem, preventive antibiotic therapy during the time of respiratory reaction to Newcastle disease and infectious bronchitis vaccines may be helpful. A live, temperature-sensitive vaccine (MS-H) is available in many countries but not commercially available in the USA. Availability of M synoviae-inactivated bacterins is becoming increasingly limited.
M synoviaeinfection can be transmitted vertically and horizontally. Once infected, individuals and flocks become chronic carriers (reservoirs).
Clinical signs caused by M synoviae are mild if uncomplicated, with low mortality and small drops in egg production. It is more severe in turkeys and known as "infectious synovitis."
Bacterial isolation, serology, and molecular diagnostic tests are commonly used in detection and characterization of M synoviae.
M synoviae-free breeding stocks are the method of choice for prevention.