avian mycoplasmosis (Mycoplasma gallisepticum)

Preferred Scientific Name

• avian mycoplasmosis (Mycoplasma gallisepticum)

International Common Names

• English: avian mycoplasma gallisepticum infection; avian mycoplasmosis; chronic respiratory disease of chickens; infectious sinusitis of turkeys; mycoplasmosis, avian

English acronym

• CRD

Four Mycoplasma species are recognised as pathogens of avian hosts, although more than 23 different Mycoplasma species have been recovered from birds. Mycoplasma gallisepticum is the pathogen addressed in this datasheet, but Mycoplasma synoviae may be seen in chickens and turkeys in association with synovitis and/or airsacculitis; Mycoplasma iowae may occur in several hosts but it is normally associated with mortality of turkey embryos but can give rise to joint and bone abnormalities and occasional airsacculitis; Mycoplasma meleagridis is usually found in turkeys causing airsacculitis, poor growth and skeletal abnormalities in progeny, and it has been associated with poor hatchability. M. gallisepticum causes chronic respiratory disease of domestic poultry, especially in the presence of management stresses and/or other respiratory pathogens. Disease is characterised by coryza, conjunctivitis, sneezing, and by sinusitis, particularly in turkeys and game birds. It can result in loss of production and downgrading of meat-type birds, and loss of egg production.

Mycoplasma gallisepticum and Mycoplasma synoviae are on the World Organisation for Animal Health (OIE) list of economically important diseases and infections are notifiable to them. The EU Directive 2009/198 includes Mycoplasma gallisepticum and Mycoplasma meleagridis and relates to animal health conditions governing intra-Community trade and imports from third countries of poultry and hatching eggs.

Mycoplasma gallisepticum (Mg) is the most economically significant mycoplasma pathogen of poultry and has a world-wide distribution (Levisohn and Kleven, 2000). Infection with Mg may manifest in different ways but chronic respiratory disease (CRD) and downgrading of carcasses in meat-type birds is probably the most severe forms. Mg is often one of the aetiological agents in a multi-factorial disease complex, which may include respiratory viruses, Escherichia coli, Haemophilusparagallinarum and other bacteria. Loss of egg production in laying birds may occur and is usually most marked at peak laying times. Conjunctivitis and sinusitis may occur with severe infections causing inflammation of the tissues around the eyes resulting in a swollen distorted face. Mg is transmitted both vertically from hen to progeny through the egg (in ovo), through the semen of infected roosters; and horizontally by the respiratory route. Mg has been reported in wild birds, particularly as a cause of conjunctivitis in house finches (Carpodacus mexicanus) in North America where the disease emerged in 1994 (Ley et al., 2006).

Antimicrobial treatment may reduce the impact of the disease, but can not be relied upon to eliminate the disease. Good biosecurity and obtaining birds from Mg free stock is a good way of preventing diseases. Some live attenuated vaccines are available, but some questions about their effectiveness and ability to cause disease still need to be addressed.

The Mycoplasma species that occur in avian host species are not zoonotic. 

The gross lesions of the respiratory tract may be mild and only consist of excess mucus or catarrhal exudate in the nares, trachea and lungs and oedema in the airsac walls. Caseous exudate may appear later in the airsacs or attached to their walls. Dilation of the infraorbital sinuses, particularly in turkeys, may be initially caused by mucus which may then be replaced by caseous material. In disease exacerbated by other pathogens, the lesions are more severe, pericarditis and perihepatitis may accompany the lesions of the airsacs and upper respiratory tract (Blaxland et al., 1982).

Encephalopathy can occur particularly in turkeys, but usually no gross lesions are visible. In salpingitis cases, caseous exudate occurs in the oviduct.

Mycoplasma gallisepticum infections vary from asymptomatic to severe depending on the infecting strain and other factors. More severe infections are seen when the birds are infected concurrently with other bacterial or viral pathogens, including Escherichia coli, Newcastle disease virus or infectious bronchitis virus. Infected birds initially develop respiratory signs that may include rales, coughing, sneezing, nasal discharges and dyspnea. Turkeys may present with more severe clinical signs which includes swelling of the infraorbital sinus. Conjunctivitis with a frothy ocular exudate occurs more frequently in turkeys, but does also occur in chickens. The route of exposure and the infectious dose of Mg, in addition to environmental and stress factors such as temperature and ammonia concentration, age and type of birds are factors that influence the course of the disease (In: Levisohn and Kleven, 2000). Production is lower in infected flocks, with decreased weight gain, feed efficiency and egg production. In chickens with uncomplicated infections the morbidity rate is high and the mortality rate low; however more severe disease occurs when the birds are co-infected with other pathogens. Mortality rates in turkeys are generally higher than in chickens. Mg has been implicated in salpingitis and other pathologies of the reproductive system, but it is not clear if this is the main or sole cause of reduction in egg production (Nunoya et al., 1997) which is particularly noticeable at times of peak lay.

Mycoplasma gallisepticum is in the Class Mollicutes which are phenotypically distinguished from other bacteria by their minute size and total lack of a cell wall. They have a small genome size (996?422 bp for R_low strain (Papazisi et al., 2003)) which accounts for their complex nutritional requirements and its obligate parasitic mode of life.

The primary habitats of mycoplasmas in general are the mucosal membranes of the respiratory tract, and/or the urogenital tract, eyes, mammary glands and joints (Levisohn and Kleven, 2000). Mg is one of the species of mycoplasma that can cause acute and chronic diseases at multiple sites, but is usually seen as a parasite of the airways of affected avian species. Uncontrolled proliferation of the organism in susceptible birds causes severe inflammation of the mucosa of the sinuses and/or trachea, and infection extends to the lungs and air sacs (Browning et al., 2010).

Transfer of the organism from hen to progeny through the eggs is an important means of spread in poultry. It is therefore important to source poults or chicks from Mg free breeding stock. Introduction of older birds into a flock can present a significant risk of introducing Mg, especially if purchased from mixed sources or through markets. Any stress, including the social stress of mixing birds together can precipitate an apparent healthy but infected bird to start shedding the organism. Direct spread from bird to bird via the respiratory route can occur readily, as well as on fomites, but spread within a flock is generally slow with an incubation period of 6 to 21 days. It is thought that an infection may persists for 18 months or even longer, but survival outside of the host under farm conditions was thought to be unlikely to exceed a few days, although Mg’s ability to form a biofilm may mean they can survive longer (Chen et al., 2013). Biosecurity is important to prevent spread from flock to flock, or farm to farm, as the organism can be carried and transmitted through contaminated footwear, clothing or equipment. The role of wild birds as a reservoir of disease and their role in transmitting Mg has been highlighted since the reports of Mg in house finches (Ley et al., 1996). If a flock becomes infected a complete depopulation followed by clean out and sanitization of the premises is required to ensure the disease is eliminated.

Despite its small genome Mg is genetically quite variable as it possesses between 30 and 70 variant vlhA genes, most of which are translationally competent (Baseggio et al., 1996). These genes have probably been acquired by lateral gene transfer between Mycoplasma species, but only one gene appears to be transcribed at a time, so only a single variant of this lipoprotein is expressed on the cell surface at any one time (Browning et al., 2010). This gene expression may change after initial infection, suggesting it has an initial role in adherence, but then the antigenic variation helps the organism evade the host’s immune response (Browning et al., 2010). Several approaches to molecular epidemiological typing have been used to differentiate isolates. Feberwee et al. (2005) used amplified fragment length polymorphism (AFLP) and random amplified polymorphic DNA (RAPD) analysis to give five clusters of Mg. Sprygin et al. (2010) demonstrated that Russian Mg isolates clustered more closely to each other than to isolates from USA, Australia, China and Iran using partial sequencing of a pvpA gene fragment. Ghorashi et al. (2010) used a PCR of the vlhA gene and high-resolution melting curve analysis to differentiate 10 Mg strains which included the ability to differentiate three vaccine strains.

Mycoplasma gallisepticum is believed to cost the worldwide poultry industry over US $780 million every year. Loss of egg production in the United States is believed to cost over US $120 million, without the cost of culling and restocking infected flocks to prevent further spread (Hennigana et al., 2011). Serious economic losses are also incurred through reduced growth production, carcass condemnations, and retarded growth in juveniles. Also, chickens have been documented to lose about 16 eggs over their laying cycle of 45 weeks (Peebles et al., 2012).

Earlier Mohammed et al. (1987) estimated 127 million eggs were lost during an Mg infection in 1984 in Southern California, with a financial loss of US $7 million

Mycoplasma gallisepticum is not a zoonotic pathogen as it only infects avian species.