M meleagridis infection is a widespread, egg-transmitted disease of turkeys found worldwide. The primary lesion in the progeny is airsacculitis. M meleagridis is thought to be a specific pathogen for turkeys, and the organism is commonly found in the respiratory and reproductive tracts. It has been eradicated in most breeder and many commercial flocks.
Transmission and Pathogenesis
Infection occurs primarily through egg transmission, which can be as high as 30–50% or higher early in the production cycle. However, transmission of M meleagridis is also related to genital contact. Early infections usually become quiescent at sexual maturity. In the tom, the phallus and adjacent tissues are infected and contaminate the semen, thus infecting the vagina of the hen. Hens may retain infection in the bursa of Fabricius, which serves as a source of infection of the reproductive tract after rupture of the cloacal-vaginal occluding membrane at puberty. Infection ascends the reproductive system and may reach the surface of the ovary. The high rate of egg transmission of M meleagridis is from infection of the reproductive tract being incorporated into the egg after ovulation. Infection of the respiratory tract leads to horizontal transmission among birds in young flocks and may be a factor in the spread to flocks previously free of infection. Hatchery transmission is also possible.
The marked difference in the pathogenicity of various strains of M meleagridis results in variable clinical manifestations, with airsacculitis in poults being the most common. High prevalence of airsacculitis with low mortality in poults suggests a highly evolved host-parasite relationship. M meleagridis may be involved in crooked necks and leg deformities, but the pathogenesis of this syndrome is not clear. The vaginas of naturally infected hens are free of infection 1–3 mo after the source of infection is removed. However, hens can be reinfected with contaminated semen.
Embryo infection appears to reduce hatchability, poult quality, and growth rate. Superimposed stress, including other infectious diseases, may result in considerable mortality in poults during the first few weeks. Infection during early rapid growth of hock joints, periarticular tissues, cervical vertebrae, and adjacent bone may produce major bone deformities such as crooked necks and hocks. Rales may develop in poults 3–8 wk old and persist for several weeks without significant mortality or serious interference with growth.
Day-old poults have thoracic airsacculitis with variable degrees of thickening, turbidity, and caseous exudate. By 3–4 wk, the lesions may extend to the abdominal air sacs. These lesions recede with age. The air sac lesions of roaster and mature birds are probably related to other factors. The generalized skeletal lesions that may occur are characterized by chondrodystrophy or varus deformities and perosis.
Microscopic lesions in hens consist of lymphocytic foci in the fimbria, uterus, and vagina. In young poults, inflammatory lesions are seen in the air sacs and lungs.
A high prevalence of air sac lesions in day-old poults suggests M meleagridis infection. Serology by serum plate agglutination or ELISA can demonstrate antibody. Confirmation was generally by hemagglutination-inhibition, isolation and identification of the organism, or both. More recently, PCR methods are used to detect M meleagridis DNA from pre- or postmortem specimens. M gallisepticum, other Mycoplasma spp, and mixed infections (polymicrobial disease) must be considered in the differential diagnosis.
Treatment and Control
Turkey eggs or poults should be obtained from breeder flocks free of M meleagridis and monitored by serology and/or by examining pipped embryos or cull poults for airsacculitis. Semen used for insemination must be free of M meleagridis. Dipping eggs in tylosin or another suitable antibiotic reduces the incidence of transmission in infected flocks. However, this practice has been generally eliminated with the eradication of M meleagridis from primary breeder flocks, and is only used by multiplier breeders when there is an outbreak. Injection SC of a suitable antibiotic at 1 day of age or water medication for the first 5–10 days appears to reduce airsacculitis caused by M meleagridis and may improve weight gain.
Last full review/revision March 2012 by David H. Ley, DVM, PhD, DACVM, DACPV