Avian Mycoplasmosis

Mycoplasmosis is caused by several species of pathogenic Mycoplasma bacteria. The most relevant species in Pennsylvania include M. gallisepticum, M. synoviae, and M. meleagridis. Most disinfectants are effective against Mycoplasma, but once inside the host, the organism is very persistent. This genus of bacteria lacks a cell wall, unlike most typical bacteria. Therefore, when considering antibiotic treatment, it is important to avoid those antibiotics which target the cell wall. The antibiotics tylosin, tilmicosin, and tiamulin are useful to reduce the Mycoplasma load in the flock and alleviate clinical symptoms.

Clinical Signs

Typical respiratory signs, including conjunctivitis, facial edema, tracheal rales, coughing, and nasal discharge, are commonly seen in birds with mycoplasmosis. Sinusitis is especially common in turkeys. Nostrils often appear dirty due to feed particles adhering to the mucus on the nostril surface. M. synoviae tends to produce a more benign infection, but it can become systemic, resulting in synovitis affecting joints and tendon sheets. Inflamed and uneven hocks may be seen; severe cases will result in lameness. Mortality is usually low in uncomplicated cases, especially in mature animals. Co-infection of lesions with other agents (e.g. E. coli) can result in more severe clinical signs and increased mortality. In laying birds, egg production may be affected, especially in unvaccinated flocks. Infection of embryos results in reduced hatchability, particularly due to increased numbers of pipped embryos that are unable to complete the hatching process.

Occurrence

M. gallisepticum is distributed worldwide, affecting chickens, turkeys, ducks, pheasants, quail, and partridges. M. synoviae, which affects chickens and turkeys, is also widely distributed. M. meleagridis is only present in turkeys. The incubation period is slow and highly variable (1-3 weeks). Dissemination within the flock is also slow; thus in general, mycoplasmosis occurs in birds older than 4-5 weeks of age. The appearance of clinical signs seems to be related to stress factors. For example, it is common for layers to be infected at 6-10 weeks of age, but clinical signs may not appear until sexual maturity and egg production (after 18 weeks of age). The disease is most common in layer farms because animals remain carriers of the organism, even after treatment.

Transmission

The main portals of entry are the conjunctiva and the respiratory epithelium. Infected breeders transmit the infection to the eggs, contaminating the embryos. Horizontal transmission occurs after contaminated embryos hatch. Fomites are an important mechanism for horizontal transmission. It is important to note that wild songbirds can also be infected with M. gallisepticum and serve as reservoirs for the organism. Because previously infected birds will remain carriers of the organism even in the absence of clinical signs, this disease is impossible to eliminate in facilities that do not practice "all-in all-out."

Diagnosis

PCR and serology are the preferred techniques for diagnosis. Due to the slow transmission of the organism and the time necessary to develop an immune response, serology is not useful before 5 weeks of age. In adult birds, necropsy findings include: mucus and congestion in sinuses and trachea, airsacculitis (Figure 1), and in cases of secondary bacterial contamination, caseous exudate in the air sacs and abdominal cavity (Figures 2 and 3). In pipped embryos, the presence of caseous exudate in the thoracic air sacs (Figure 4) strongly correlates with the infection of the breeders with M. gallisepticum. Infections with M. synoviae will show creamy/grey exudate surrounding the synovial membrane of the tendon sheets and joints (Figure 5).

Differential Diagnoses

Viral respiratory diseases. Infectious bronchitis, low pathogenic avian influenza, Newcastle, infectious coryza, avian cholera. In turkeys, chlamydiosis, respiratory cryptosporidiosis, low pathogenic avian influenza, Newcastle, aspergillosis, and Ornithobacterium rhinotracheale (ORT).

Treatment

Mycoplasma infections can be treated with antibiotics to alleviate clinical symptoms. Avoid antibiotics that target the bacterial cell wall, as Mycoplasma do not have one. Tylosin, tilmicosin, and tiamulin are useful to reduce the mycoplasma load in the flock. However, antibiotic therapy cannot completely eliminate mycoplasma from the flock, which will remain Mycoplamsa-positive.

Control

Several vaccines are available for chickens (inactivated, ts-11, 6/85, and F strain). In general, vaccination does not prevent infection, but helps to reduce clinical signs and losses in chicken egg production. There are no good vaccines available to control Mycoplasma in turkeys. In fact, the F strain vaccine can be very pathogenic for turkeys. Ensure that birds are supplied from a Mycoplasma-free operation and test new birds before introducing to the existing flock. Protect the flock from wild birds, since they can carry M. gallisepticum. Incubation of eggs from Mycoplasma-negative flocks must be completely separated from eggs derived from positive flocks.

References

Diseases of Poultry, 13th ed. D. E. Swayne. Wiley-Blackwell.

Avian Diseases Manual. 7th ed. The American Association of Avian Pathologists.