Avian mycoplasmosis is one of the most economically important diseases in poultry. Losses result from decreased egg production, increased mortality, medications costs and carcass condemnation. However, the incidence of this disease could be reduced by the application of adequate prevention and control programs.
By Dr Hicham Sid and Dr Mustapha Oumouna, Algeria
Avian mycoplasmosis is caused by several pathogenic mycoplasmas of which Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS) are the most important. They belong to the class Mollicutes (mollis = soft, cutes = skin); they are very small prokaryotes (0.25 – 0.5 μm). One of their major characteristics is the lack of cell walls; they are limited only by a plasma membrane. Pathogenic mycoplasma species such as MS and MG tend to grow rather slowly (after 3-10 days at 37°C on agar media). Additionally, MS requires the presence of NAD (nicotinamide adenine dinucleotide) for its development. However, non-pathogenic mycoplasmas such as Mycoplasma gallinarum and Mycoplasma gallinaceum may develop colonies within one day.
The infection with MG is commonly known as ‘Chronic Respiratory Disease’ in chickens and ‘Infectious sinusitis’ in turkeys. While MS may cause air sac lesions when associated with viral infections; the infection of synovial membranes is also involved resulting in synovitis.
Pathogenicity and Immunology
Mycoplasma can escape from the host defensive immune mechanisms using the high frequency variation of the surface proteins. It also “hides” from host defences by invading cells which may explain the chronic infection despite the strong immune reactions. The portal of natural MG infection is the upper respiratory tract. The infection may spread to the air sacs and the oviduct. MG colonises the host cells after a successful attachment which is considered the most important virulence factor. The infection may induce release of mucous granules followed by destruction and exfoliation of ciliated and nonciliated epithelial cells.
Humoral immune response plays an important role in the protection against MG infection by production of respiratory tract antibodies, which inhibit bacterial attachment to the tracheal epithelial cells. On the other hand, cellular immune response could be involved by suppression or stimulation of B and T lymphocytes.
Clinical signs and lesions
MG infection aff ects almost all chickens in a fl ock. However the infection is variable in severity and duration and it depends on many factors. It is more severe and of longer duration in the cold months and aff ects younger birds more than mature birds. Clinical signs of naturally occurring mycoplasma infection in adult fl ocks are tracheal rales, nasal discharge, and coughing (Photo 1). In laying fl ocks, egg production declines but usually is maintained at a lower level. In broiler fl ocks, most outbreaks occur after four weeks of age. Signs are frequently more marked in young birds than those observed in mature fl ocks. Severe outbreaks with high morbidity and mortality observed in broilers are frequently due to concurrent infections. The infection with MS can cause lameness, swelling of joints and breast blisters.
Complications may occur in presence of other agents such as Newcastle disease (ND) and Infectious Bronchitis (IB) which may precipitate the outbreak of MG infection. E. coli is a frequent complicating organism. However, it does not readily infect the air sacs without previous infection by MG or in combination with either IBV or ND virus. Turkeys seem to be more susceptible to mycoplasma infection than chickens, and they may develop more severe clinical signs including sinusitis, respiratory distress, depression, decreased feed intake, and weight loss (Photo 2). Severe outbreaks with high morbidity and mortality could be observed with the involvement of complicating pathogens such as colibacillosis or diff erent environmental stressors. The infection of turkeys with MS could be associated with trampling and cannibalism. Gross lesions consist of catarrhal infl ammation of sinuses, trachea, bronchi, and air sacs. In turkeys, sinusitis is usually the most noticeable symptom; however, it could be also observed in chickens. Air sacs may contain caseous exudate (Photo 3, 4). Additionally, pneumonia could be observed. In severe cases of typical air sac disease in chickens or turkeys, three gross lesions are mostly observed: airsacculitis, fi brinous or fi brinopurulent perihepatitis (Photo 5), and adhesive pericarditis resulting in high mortality and extensive condemnations at slaughter.
Eggshell apex abnormalities
The EAA (eggshell apex abnormalities) could be associated with MS infection. The apex of the eggshell could be thin and fragile (Photo 6). The lesion could be explained by the colonisation of the oviduct by MS, which may interfere with eggshell formation. However, many other infectious and non-infectious conditions are responsible for the poor eggshell quality (for example, calcium/Phosphorus nutritional status, age of birds and other infectious diseases such as IB).
Prevention and control programs
Application of strict biosecurity procedures is essential to prevent horizontal transmission of mycoplasma from contaminated to healthy fl ocks. Due to the fact that mycoplasma could be vertically transmitted, it is important to maintain breeding stocks free from the infection. On the other hand, antibiotic treatment of fertile eggs either by ovo injection or by dipping eggs produced by genetic stocks in antimicrobial solutions could be eff ective. Application of control programs of breeding fl ocks is very important to reduce the incidence of this disease. Monitoring mycoplasma could be performed using several techniques such as: Plate Agglutination Test, ELISA and PCR. However, due to the intensifi cation of the poultry industry, it is sometimes diffi cult to guarantee the eff ectiveness of control strategies and maintain 100% mycoplasma free fl ocks.
Vaccination could, in some circumstances, be useful. Several types of vaccines have been used (inactivated and live- attenuated vaccines). The main eff ect of vaccination is the ability to reduce egg losses in commercial table-egg layers. However, the protection mechanism conferred by these vaccines is not fully understood.
Several antibiotics have been used for the treatment of mycoplasma infection such as macrolides, tetracyclines, tiamulin, spectinomycin and lincomycin. Antimicrobics may reduce egg production losses and the severity of clinical signs by decreasing the population of mycoplasma in the respiratory tract. Resistance and cross-resistance could be observed with commonly used antibiotics.