(MG) infection is a significant disease in chickens, and is thought to be one of the most problematic of the various poultry mycoplasmal diseases. MG is a slow-spreading infection that often goes undetected in seemingly healthy flocks---until a stressful event occurs which has a detrimental effect on the bird's immune system, causing MG infection to present itself as a disease. MG spreads very quickly and easily throughout flock members and other bird species, including wild birds. A survey of commercial egg laying poultry in United States of America (USA) revealed that 37% of laying flocks (262.6 million layers) were infected with MG.
MG infection resembles many different respiratory diseases that affect poultry, including Newcastle disease, Infectious Bronchitis, and/or Escherichia coli
infections. It is also relatively common for chickens to be co-infected with multiple pathogens, such as E. coli
, M. meleagridis
, M. synoviae
, Ornithobacterium rhinotracheale
(ORT), Infectious Bursal Disease Virus (IBDV), and/or Newcastle Disease Virus (NDV). When chickens are co-infected with multiple pathogens, it often increases the severity of the clinical signs and makes it more difficult to diagnose.
Clinical signs of Mycoplasma gallisepticum Infection
Chickens are much less susceptible to MG infections than turkeys and suffer less severe clinical signs than that seen in turkeys. Some chickens infected with MG may show outward signs of infection, until a trigger stressful event. Younger chickens, between 4 to 8 months of age, and male chickens are more likely to develop classic more severe clinical signs of MG infection which includes nasal discharge, coughing, and tracheal rales. Occasionally, chickens can develop more severe clinical signs such as eyelid or facial swelling, conjunctivitis, increased lacrimation, and sinusitis. Conjunctivitis may be present in one or both eyes. The facial swelling is primarily concentrated around and just below the bird's eyes. The swelling is the result of pus accumulation in the bird's sinuses located in the area under the eyes. They can sometimes be so severe, that the chicken is unable to open their eyes and see. If left without treatment, the bird can permanently lose their eyesight as a result.
Transmission and Incubation Period
MG is transmitted vertically, from breeding birds to chicks through eggs, and horizontally---directly from bird to bird via respiratory secretions, as well as indirectly, from contaminated dust, feed, water sources, droplets, or feathers. Other animals such as rodents and wild birds serve as carriers of MG and are capable of transmitting the organism into the flock's environment. Humans are also able to transmit MG to their flocks, if they have been exposed to other birds or equipment contaminated with MG. MG is able to live in the environment for days to weeks, depending on the material it's living in and the temperature. It will remain active longer when present in chicken manure or eggs, therefore the cleaner the facility, the less likely MG will survive in the environment. The incubation period typically ranges from 6 to 21 days.
How MG Infection is Diagnosed
There are several different tests offered by veterinary diagnostic laboratories, used for detection of MG infection in chickens:
- Mycoplasma gallisepticum culture: Swabs are taken of the chicken's ocular or nasal discharge and mailed off to diagnostic laboratories to be cultured in order to identify the bacterium.
- Hemagglutination-inhibition (HI) assay: detects IgG.
- Serum plate agglutination (SPA) assay: This is a screening test used to detect IgM. It is prone to false positive reactions. Factors associated with false positive plate agglutination reactions include:
Plate agglutination is not useful for detecting antibodies in egg yolk and maternally derived antibodies that are primarily IgG.
- Administration of inactivated vaccines 2-5 weeks prior
- When birds are infection with avirulent or atypical strains that are poorly immunogenic
- Sera that have been frozen and thawed prior to testing
- Turbid contaminated serum
- Cross-reaction of MS-infected birds with the MG antigen
- Enzyme labeled immunosorbent assay (ELISA):
- Rapid serum agglutination (RSA) test: detects IgG.
- Polymerase chain reaction (PCR) assay: The PCR assay is a molecular based technique which has become increasingly popular. It works by targeting and detecting specific nucleic acid sequences, and is able to provide results in less than 24 hours.
Treatment and Control
species lack cell walls, it makes many types of antibiotics, such as penicillin, useless in treatment. Tylosin and tetracycline have been used successfully.
Documented cases of MG infection
|Causative agent||Affected||History||Clinical signs||Diagnostic Findings||Ref|
|M. gallisepticum||A flock of adult grey partridges kept at a game bird farm in Czech Republic.||The infection developed slowly with a 100% morbidity rate and 60% mortality, where the birds died within 3 to 4 weeks after initial appearance of clinical signs.||Nasal and eye discharge, open-mouth breathing, eyelid and infraorbital sinus swelling, poor feed intake, weight loss, drop in egg production, and lethargy.||M. gallisepticum was identified in a culture taken from tracheal swabs taken from the bird and PCR. E. coli was found in the lungs, infraorbital sinuses and air sacs. Serological tests for other diseases were negative. Necropsy revealed tracheitis, pneumonia and airsacculitis.||F Vitula et al., 2011|
|M. gallisepticum, ORT, and NDV.||Adult commercial egg laying hens from multiple flocks in Brazil||Associated with an average mortality rate of 15%||Significant swelling of the face and eyelids||Severe catarrhal sinusitis, tracheitis, bronchitis, and pneumonic changes of congestion and consolidation of the lungs.||Unknown|
|M. gallisepticum and E. coli||Adult commercial egg laying hens from multiple flocks in Brazil||Associated with a 14.8% mortality rate.||Respiratory disease ||Severe airsacculitis and egg peritonitis.||S Sivaseelan et al., 2013|
|M. gallisepticum||Backyard chicken flock||Unknown||Severely underweight, gasping, nasal discharge, and death||Severe airsacculitis, tracheitis, and pneumonia.||CAHFS Connection, March 2016|
|M. gallisepticum||Two flocks of adult chickens of the Babcock and Dekalb breed||All of the birds were previously vaccinated against Marek's disease, NDV, and IBV. The first signs appeared when the chickens were about 30 days of age, after giving the IBV vaccine. The disease gradually spread through the flock, despite treatment with antibiotics and additional vitamins. There was a 27.8% morbidity and 10% mortality.||Unilateral or bilateral swelling of the eyelids and facial skin, corneal opacity, excessive tearing of the eyes, conjunctivitis, closed or partially closed eyes, depression, respiratory rales, and death||High positive ELISA titers against IBDV, all the sera were positive for IBV, and six sera showed positive HI titers for NDV, apparently due to the vaccination. Laryngitis and tryophy of the thymus and bursa of Fabricius. ||Unknown|