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M gallisepticum infections are commonly known as chronic respiratory disease in chickens and as infectious sinusitis in turkeys. These diseases affect poultry worldwide and may be more severe during cooler months, causing the most significant economic losses in large commercial operations. Infection may also be seen in pheasants, chukar partridges, and peafowl, and it has been reported in pigeons, quail, ducks, geese, and psittacine birds. Passerine-type birds are quite resistant, although M gallisepticum causes natural outbreaks of conjunctivitis in wild house finches (Carpodacus mexicanus) in the USA, particularly in the bird's eastern range.
M gallisepticum is the most pathogenic avian mycoplasma; however, strains may differ markedly in virulence. Primary isolation is made in mycoplasma medium containing 10–15% serum. Typical colonies on agar medium are identified by immunofluorescence using species-specific antibodies. PCR can also be used for detection of M gallisepticum DNA using swabs taken directly from infected sites (choana, sinuses, trachea, airsacs) or following growth in culture.
Transmission, Epidemiology, and Pathogenesis
In the USA, most commercial poultry breeder flocks are free of M gallisepticum due to control programs coordinated by the National Poultry Improvement Plan; however, in some parts of the world, egg transmission (transovarian) is a major source of infection. The incidence of egg transmission is highly variable and may reach 30–40% during the first 2 mo after infection of susceptible birds in production. The transmission rate then lessens and is inconsistent (0–5%) until the end of production. Birds infected before the onset of production transmit through the egg at a much lower rate, if at all. The infection may be dormant in the infected bird for days to months, but when the flock is stressed, lateral transmission occurs rapidly via aerosols and the respiratory route, and infection spreads through the flock. Flock to flock transmission occurs readily by direct or indirect contact from the movement of birds, people, or fomites from infected to susceptible flocks. Some potential reservoirs of M gallisepticum in the USA are backyard flocks, multiple-age layer flocks, and some free-ranging songbird species. Good management and biosecurity practices are necessary to ensure that M gallisepticum infections are not introduced from these and other sources. In many outbreaks, the source of infection is unknown. Live virus vaccinations, viral infections, cold weather, poor air quality or crowding may facilitate infection, disease, and transmission.
The epithelium of the upper respiratory passages is most susceptible to infection; however, in severe, acute disease the infection is also found in the lower respiratory tract. There is a marked interaction (polymicrobial disease) between respiratory viruses, Escherichia coli, and M gallisepticum in the pathogenesis and severity of chronic respiratory disease. Once infected, birds may remain carriers for life.
Clinical Findings
In chickens, infection may be inapparent or result in varying degrees of respiratory distress, with slight to marked rales, difficulty breathing, coughing, and/or sneezing. Morbidity is high and mortality low in uncomplicated cases. Nasal discharge and frothiness about the eyes may be present. The disease is generally more severe in turkeys than in chickens, and swelling of the infraorbital sinuses is common. Feed efficiency and weight gains are reduced. Broilers and market turkeys may suffer high condemnations at processing due to airsacculitis. In laying flocks, birds may fail to reach peak egg production, and the overall production rate is lower than normal.
Lesions
Uncomplicated M gallisepticum infections in chickens result in relatively mild catarrhal sinusitis, tracheitis, and airsacculitis. E coli infections are often concurrent and result in severe air sac thickening and turbidity, with exudative accumulations, fibrinopurulent pericarditis, and perihepatitis, particularly in broilers. Turkeys develop severe mucopurulent sinusitis and varying degrees of tracheitis and airsacculitis. The mucous membranes are thickened, hyperplastic, necrotic, and infiltrated with inflammatory cells. Lymphofollicular areas are found in the submucosa.
Diagnosis
Serology by agglutination and ELISA methods are commonly used for surveillance. Hemagglutination-inhibition is used as a confirmatory test because nonspecific false agglutination reactions may occur, especially after the inoculation of inactivated, oil-emulsion vaccines or M synoviae infection. M gallisepticum should be confirmed by isolation and identification or by PCR in samples from infraorbital sinuses, nasal turbinates, choanal cleft, trachea, air sacs, lungs, or conjunctiva. Isolates must be identified, because birds may also be infected with nonpathogenic Mycoplasma spp. Escherichia coli infection, Newcastle disease, influenza, and other respiratory diseases (eg, infectious bronchitis in chickens) should be considered in the differential diagnosis and can act as inciting or contributing pathogens.
Treatment and Control
Most strains of M gallisepticum are sensitive to a number of antibiotics, including macrolides, tetracyclines, fluoroquinolones, and others, but not penicillins or those that act on the cell wall. Tylosin or tetracyclines have been commonly used to reduce egg transmission or as prophylactic treatment to prevent respiratory disease in broilers or turkeys. Antibiotics may alleviate the clinical signs and lesions but do not eliminate infection. Regulations on the use of antibiotics in food animals are rapidly evolving and should be consulted prior to use.
Eradication of M gallisepticum from chicken and turkey breeding stock is well advanced in the USA and several other countries. The most effective control program is to identify breeders without serum agglutination or ELISA titers and to maintain seronegative stock with good biosecurity. In valuable breeding stock, treatment of eggs with antibiotics or heat has been used to eliminate egg transmission to progeny. Medication is not a good longterm control method but has been of value in treating individual infected flocks.
Laying chickens free of M gallisepticum are desirable, but infection in multiple-age commercial egg farms where depopulation is not feasible is a problem. Inactivated, oil-emulsion bacterins are available in most countries; these help prevent egg production losses but not infection. Three live vaccines (F strain, ts-11, and 6/85) have been licensed in the USA for use during the growing phase to provide protection during laying and may be used only with permission of the state veterinarian. F strain is of low pathogenicity for chickens but is fully virulent for turkeys. Vaccinated chickens remain carriers of F strain, and immunity lasts through the laying season. Live vaccine strains 6/85 and ts-11 offer the advantage of improved safety for nontarget birds and are widely used in commercial layers. A recombinant live poxvirus M gallisepticum vaccine has recently became available.
Last full review/revision March 2012 by David H. Ley, DVM, PhD, DACVM, DACPV
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