avian mycoplasmosis (Mycoplasma gallisepticum)

Identity

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

Overview

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. 

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Pathology

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.

Diagnosis

Clinical signs, post mortem and histological lesions are not pathognomic for Mg. Isolation of the causative organism is the definitive confirmation of infection, but this requires mycoplasma culture media and incubation of cultures for three to four weeks. Specificity of the diagnostic tests does present some difficulties as birds are often infected with more than one mycoplasma species, sometimes as many as five Mycoplasma species have been detected in one sample; and many of these are considered to be non-pathogenic mycoplasmas, which may give a false negative or false positive result. Surveillance for clinical signs and lesions of mycoplasma infection must be ongoing. Testing should be carried out on a statistical representative number of birds in a flock and birds should be sampled at random from each part of the flock.

Serological detection of antibodies is recognised as a way of monitoring flocks and detection of disease, but this usually also requires confirmation by other diagnostic methods along with veterinary diagnosis of clinical signs in the flock. The haemagglutination-inhibition (HI) test is rarely used now, but is still a prescribed test listed in the OIE terrestrial manual. More commonly used is the rapid/serum plate agglutination test or rapid slide agglutination test. This is a flock test as the test does give some false reactions. The early guidelines were that 60 serum samples should be taken per house and a flock was only considered positive when more than 15% of the undiluted samples were positive or more than 3% with a titre of more than 1:8 (Intervet information sheet). However different manufacturers now make this test and the sensitivity and specificity may vary with between manufacturers and different batches of the antigen. The test is simple to perform, but manufacturer’s instructions should be followed and appropriate positive and negative control serum tested. Essentially the antigen and serum to be tested should be allowed to warm to 20-25°C. Equal volumes of the serum and the antigen are mixed for two minutes and any agglutination observed recorded as positive. Dilutions of sera should be tested to resolve any doubts about a positive test result. Only fresh sera should be tested.

More recently several manufacturers are producing ELISA tests and these are designed for laboratory use. These are generally a flock based test and users should be aware that some differences in test performance and batches do occur.

A species-specific confirmatory serological test using a Western blot/immunoblot method has been developed (Welchman et al., 2013).

Antigen detection using molecular tests have been developed, they are mainly polymerase chain reaction (PCR) based tests. A PCR method for detecting Mg is given in the OIE Manual (OIE, 2012), but many other methods have been published as referenced in Kleven (2008). A PCR method using the 16S rDNA gene followed by the use of denaturing gradient gel electrophoresis has been able to detect and identify the majority of Mycoplasma species including Mg. This test is sensitive and will identify all of the Mycoplasma species that affects avian species in one test and will also detect and identify mixed infections (McAuliffe et al., 2005).

Samples to be tested can be swabs from live birds or swabs/tissues from carcases. Tracheal, choanal, or air sacs swabs are tested. The same samples can be used for mycoplasma culture.

No single medium formulation has been accepted as optimum for growth of Mg. Mg ferments glucose and requires 10-15% horse or swine serum, and a yeast source to provide nutrients (Kleven, 2008). Broth culture is usually more sensitive than agar but both agar and broth are inoculated at the same time and incubated at 37°C ideally for growth on agar with an additional 5-10% CO₂ in a humid environment. Bradbury (1998a) describes methods to recover mycoplasma s from birds and gives details of suitable media formulations. Mg will ferment glucose producing an acid pH in the broth media and typical “fried egg” colonies can be observed on agar using a microscope under low magnification (approx 35X magnification). Mixed cultures are often obtained and identification need to be confirmed either by molecular methods or using a specific Mg antiserum in a growth inhibition test (Poveda and Nicholas, 1998) or using immuno-fluorescence antibody test (Bradbury 1998b).

List of Symptoms/Signs

Disease Course

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.

Epidemiology

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.

Impact: Economic

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

Zoonoses and Food Safety

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

Prevention and Control

Prevention of disease by ensuring birds are acquired from an Mg free source is the most effective approach to prevent introduction of disease into a flock. However biosecurity and sanitation are essential to prevent the spread of Mg into a susceptible flock. Biosecurity should keep wild birds away from the flock and visitor access to the flock should be restricted and strict sanitation procedures should be followed, with no equipment being shared with other bird owners, or thorough cleaning and disinfection applied. When a flock has been removed a thorough cleaning and disinfection should be carried out. If a flock has become infected then depopulation and an extended down time with thorough cleansing and disinfection should be instigated before repopulating from a clean source.

Mg is generally susceptible to a number of antibiotics, however antibiotic resistance is increasingly being reported and it is known that most antibiotics just suppress the clinical signs and do not eliminate the infection. Antibiotics that should be effective include the macrolides, tetracyclines, spectiniomycin, lincomycin and fluoroquinolones (Salami et al., 1992). However Gharaibeh and Al-Rashdan (2011) reported resistance to 8 antibiotics in three families of antibiotics and Gerchman et al. (2008) reported resistance to the fluoroquinolone enrofloxacin. Soaking of infected eggs in antibiotics may prevent the transfer of Mg infection in ovo, but it may reduce egg hatchability (Hall et al., 1963).

Inactivated bacterins have proved to be efficacious in some cases in reducing respiratory signs and lesions in chickens and reducing egg production losses and transmission, however other studies have shown minimal or no effect (Levisohn and Kleven, 2000). Three live Mg vaccines are available in different countries in the world. These are the F strain; 6/85 and ts-11, although not all countries officially permit use of live vaccines (Levisohn and Kleven, 2000). Some vaccines are for use in chickens only while others can also be used in turkeys. Some debate continues about the use of live vaccines, as some reports exist of introduction of disease into flocks by using the vaccines, whilst others report that the wild strain is eliminated by use of the vaccine. Feberwee et al. (2006) reported reduction in Mg infection but insufficient to prevent spread of disease. Ideally flocks should be kept free of Mg.

Mg is an OIE listed disease even though it occurs worldwide. 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.

References

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Distribution Maps