AGRICULTURE NEWS - Mycoplasmas are very small free-living organisms that differ from bacteria in that they lack a cell-wall.
This allows them to multiply very rapidly and also to alter receptors on the surface of the membrane which enables them to avoid the immune response of the host animals.
Infections are often chronic and difficult to eradicate from affected flocks.
Two species infect chickens and cause disease. Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS). Other species of Mycoplasma are known to infect chickens but have not been associated with serious disease.
Mycoplasma Gallisepticum (MG) (Chronic respiratory disease - CRD)
MG is commonly involved in "chronic respiratory disease" of chickens. The disease affects chickens worldwide, causing the most significant economic losses in large commercial operations, and are commonly seen in non-commercial flocks.
Mycoplasma synoviae (MS)
MS infection most commonly occurs as a sub-clinical upper respiratory infection and may also be isolated from CRD cases.
At other times MS becomes systemic and results in infectious synovitis (joint infections). It has also been associated with egg-shell abnormalities.
Transmission:
Mycoplasmas are transmitted vertically within some eggs (transovarian) from infected breeders to progeny, and horizontally via infectious aerosols and through contamination of feed, water, and the environment, and by human activity on fomites (shoes, equipment, etc).
Infection may be latent in some birds for days to months, but when birds are stressed horizontal transmission may occur rapidly via aerosols and the respiratory route, after which infection and clinical disease spread through the flock.
Flock-to-flock transmission occurs readily by direct or indirect contact from the movement of birds, people, or fomites from infected to susceptible flocks.
Some potential reservoirs of Mycoplasmas are non-commercial (backyard) flocks or multiple-age layer flocks. Good management and biosecurity practices are necessary to ensure that Mycoplasma infections are not introduced to commercial poultry from these and other sources.
In many outbreaks, the source of infection is unknown.
Cold weather, poor air quality or crowding, concurrent infections, and some live virus vaccinations may facilitate infection, disease, and transmission.
Once infected, birds may remain carriers for life.
There is a marked interaction (polymicrobial disease) between respiratory viruses such as Infectious Bronchitis, Escherichia coli, and MG in the pathogenesis and severity of the chronic respiratory disease.
Clinical Findings and Lesions:
In chickens, the infection may be inapparent or result in varying degrees of respiratory distress, with slight to marked difficulty breathing, coughing, and/or sneezing.
Nasal discharge and conjunctivitis with frothiness about the eyes may be present.
Uncomplicated MG (and MS) infections in chickens result in relatively mild sinusitis, tracheitis, and airsacculitis.
E. coli infections are often concurrent and result in severe air sac thickening and turbidity, with exudative accumulations, adhesive pericarditis, and fibrinous perihepatitis.
Joint infections caused by MS typically will affect a number of joints at the same time and should not be confused with bacterial joint infections secondary to footpad damage or bruising in injured birds.
Birds typically do not die from joint infections directly but become reluctant to move and have reduced feed and water intake.
This produces flocks that are uneven and in the case of laying birds creates poor laying flocks.
Diagnosis:
Mycoplasmas are notoriously difficult to culture in the laboratory so diagnosis is usually by PCR test. The best samples for diagnosis are the cranial sinuses, the trachea and the air sacs – in the case of MS, the joints can be sampled.
PCR is also used to ensure that vaccination has been done effectively. Most vaccines can be detected lifelong in the birds and can be differentiated from field strains using secondary PCR tests.
Serology (usually ELISA) is used to evaluate the immune response to Mycoplasma in birds – there is often a poor correlation between the serological response and the immunity of the birds, especially when live vaccines are used. Inactivated bacterin vaccines give strong antibody responses, but have proven generally less effective than live vaccines in controlling
Treatment, Control, and Prevention:
Most strains of MG are sensitive to a number of broad-spectrum antibiotics, including those in the macrolide group such as tylosin, tylvalosin, tiamulin and kitasamycin as well as the tetracyclines. Antibiotics are useful to control serious acute outbreaks of chronic respiratory disease associated with MG but do not easily eradicate the infection – therefore infected flocks may require repeated treatments.
MG prevention is based largely on obtaining chicks from MG–free breeder flocks. The most effective control program is to establish MG–free breeder flocks, managed and maintained under good biosecurity to prevent the introduction, and monitored regularly with serology to continually confirm infection-free status.
Commercial egg-laying chickens free of MG are desirable, but infection in commercial multiple-age egg farms where depopulation is not feasible is a problem.
Most commercial egg-laying hens are vaccinated in rearing using a live MG vaccine such as MG 6/85, MG ts11 or MG-F-strain vaccine.
They may also receive an inactivated bacterin vaccine. Recombinant MG vaccines are also available in the market and may be used by producers concerned with the side effects of the live vaccines on the health of the birds in rearing.
MS has proved even more difficult to eradicate from flocks than MG. Widespread use is made of the live MS-vaxsafe vaccine in commercial layers and broiler breeders.
Fortunately, MS infections are generally less severe than MG infections and some producers elect not to vaccinate against the disease, treating it with antibiotics if clinical signs develop.
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