Mycoplasma gallisepticum Infection - The Merck Veterinary Manual

Mycoplasma gallisepticum Infection
(PPLO infection, Chronic respiratory disease, Infectious sinusitis)

Transmission, Epidemiology, and Pathogenesis

M gallisepticum infection is commonly designated as chronic respiratory disease in chickens and as infectious sinusitis in turkeys. Infection may also be seen in pheasants, chukar partridges, and peafowl. Infection in pigeons, quail, ducks, geese, and psittacine birds should be considered. Passerine-type birds are quite resistant, although M gallisepticum is the major cause of natural outbreaks of conjunctivitis in wild house finches (Carpodacus mexicanus) in the eastern USA. The disease is worldwide. Its effects are most severe in large commercial operations during winter.

M gallisepticum is the most pathogenic avian mycoplasma; however, strains may differ markedly in virulence. Primary isolation is made in enriched broth medium containing 10-15% serum, then plated on agar. Typical colonies are identified by immunofluorescence.

Transmission, Epidemiology, and Pathogenesis:

In the USA, most breeder flocks are free of M gallisepticum , and outbreaks are due to lateral transmission from infected chickens; however, in some parts of the world, egg transmission is a major source of infection. The incidence of egg transmission is highly variable, ranging up to 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 chick for days to months, but when the flock is stressed, aerosol transmission occurs rapidly and infection spreads through the flock. Live virus vaccination, natural virus infection, cold weather, or crowding may initiate the spread. In addition, the infection may be carried by personnel (especially from an infected to a clean flock), fomites, or introduction of infected birds. In many flocks, the source of infection cannot be determined.

The epithelium of the upper air 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 between respiratory viruses, Escherichia coli , and M gallisepticum in the pathogenesis of chronic respiratory disease. Once infected, birds 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. In turkeys, the disease is generally more severe than in chickens, and swelling of the paranasal sinus 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 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:

Agglutination reactions and ELISA are commonly used for diagnosis. M gallisepticum should be confirmed by isolation and identification, PCR, or hemagglutination-inhibition because nonspecific false agglutination reactions are common, especially after the inoculation of inactivated, oil-emulsion vaccines or M synoviae infection. Isolates must be identified, because birds may also be infected with nonpathogenic Mycoplasma spp . PCR is commonly used to rapidly detect the organism in upper respiratory tissues. Newcastle disease, infectious bronchitis, influenza, and other respiratory pathogens should be considered in the differential diagnosis.

Treatment and Control:

In the field, many cases of M gallisepticum infection are complicated by other pathogenic bacteria; thus, effective treatment must also attack the secondary invader. Most strains of M gallisepticum are sensitive to a number of antibiotics, such as chlortetracycline, erythromycin, oxytetracycline, spectinomycin, tiamulin, tylosin, or a fluoroquinolone such as enrofloxacin. Antibiotic is usually given in the feed or water for 5-7 days; however, in turkeys, antibiotic may be given initially by injection, followed by feed or water medication. Antibiotics may alleviate the clinical signs and lesions but do not eliminate infection.

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. In valuable breeding stock, treatment of eggs, usually with tylosin or heat, may be used to eliminate egg transmission to progeny. Medication is not a good long- term control method but is of value in treating individual infected flocks.

The use of birds free of M gallisepticum is desirable, but infection in multiple-age commercial egg farms where depopulation is not feasible is a problem. An inactivated, oil-emulsion bacterin is available in most countries; it prevents egg production losses but not infection. A live vaccine has been licensed in the USA for use in infected, multiple-age layer flocks but may be used only with permission of the state veterinarian. The vaccine consists of a mild strain of M gallisepticum (F-strain) and is usually given at ~10-14 wk of age. F-strain is of low pathogenicity for chickens but is fully virulent for turkeys. Vaccinated birds remain carriers, and immunity lasts through the laying season. Recently, 2 nonpathogenic live vaccine strains (6/85 and ts-11) have been introduced; these strains offer the advantage of improved safety and are in widespread use in commercial layers.