By-Dr Ibne Ali, Livestock & Poultry consultant
CRD in Poultry Birds
Introduction to CRD: Mycoplasma infection in poultry birds causes great economic losses. Mycoplasma are prokaryotes and devoid of cell wall unlike other bacteria. Their nutrient requirements are also very complex and they are not easy to culture in labs. Due to this reason Mycoplasma are highly host specific and cross infections seldom occurs. Poultry is usually affected with two species Mycoplasma gallisepticum and Mycoplasma synoviae. These species commonly colonizes mucosal surfaces and non-invasive but M. gallisepticum is exception which penetrates inside cells. M. gallisepticum & M. synoviae causes heagglutination of chicken RBCs. Mycoplasma gallisepticum is most pathogenic and economically important species. Infection caused by M. gallisepticum is characterized by air sacculitis with or without complications. Infection manifests as reduces egg production, increased FCR, high medicine costs. It does not have any public health significance. Mycoplasma are usually sensitive to common chemical disinfectant and sanitizers.
However, molecular mechanism of mycoplasma pathogenicity is unknown but it works through weakening of immune system rather than direct cellular toxicity. Infection spreads through vertical as well as horizontal transmission. Direct and indirect spread of disease is well documented. Aerosol or droplet infection has ability to quickly enter and infect entry portals like upper respiratory tract, conjunctiva. However, it has to be noted that Mycoplasma do not survive outside host for long, so carrier birds plays an important role in spreading of infection. Fomites, contaminated dust, suspended aerosol particles & feathers contributes to widespread outbreak. Mycoplasma remain viable in chicken feces for 3 days and in feathers for 4 days at 20OC.
However extremely vast literature is available about CRD I don’t want to go into lab generated technical information, I just share some important points which helps in understanding the practical importance of the disease and how to tackle it?
- In most cases, the M. gallisepticum status of the progeny chicks is determined by the breeder flock from which they are derived.
- Infection with M. gallisepticum may have a wide diversity of clinical manifestations, of which chronic respiratory disease (CRD)
- Mycoplasma gallisepticum infection rarely cause respiratory disease in chickens but it is frequently present as one of several aetiological agents in a multi-factorial disease complex
- Vaccine against Newcastle disease or infectious bronchitis virus may give marked respiratory reactions in M. gallisepticum-infected birds, necessitating the use of milder vaccine strains under these conditions
- Depression is associated with Mycoplasma sp.
- Mycoplasma gallisepticum typically produces the most severe clinical signs in young birds, between the ages of 4-8 weeks.
- Ocular signs are frequently observed in Mycoplasma gallisepticum infections. These may include conjunctivitis, periorbital swelling, and eyelid edema, all of which are seen here.
- Early infection of pullets with M. gallisepticum can protect the birds against the pathogenic effects of later infection. This is the principle underlying the successful use of live M. gallisepticum vaccines in layer flocks
Clinical Signs:
• Most characteristic signs are tracheal rales
• Nasal discharge & coughing
• Reduced feed consumption
• Weight loss
• Egg production declines but maintained at lower level
• Serologically positive without clinical signs (because of infection occurs at younger age)
• Male birds are affected severely than females
• Disease is more severe in winters
• It has been ruled out tha E. coli do not effect air sacs unless it is previously infected with MG or IB/ND
• E. coli + Infectious Bronchitis + Mycoplasma gallisepticum = very severe outbreaks (airsacculitis) of CRD and CCRD
Vaccination:
There are majorly 2 types of vaccines used to prevent and control Mycoplama in flocks (used mainly in breeders and layers).
Killed (Non-living Vaccine):
• Non-living Mycoplasma gallisepticum vaccines in which Mycoplasma gallisepticum bacterins (killed organisms) with oil emulsion adjuvant protected young chicken from intra sinus challange from virulent Mycoplasma gallisepticum and commercial egg layers from Mycoplasma gallisepticum induced drops in egg production.
Live Vaccine:
• Live F strain of Mycoplasma gallisepticum is mild (originated from connecticut F strain) used in pullets.
• In broilers some protection from air sacculitis following aerosol challenge with virulent R strain is seen
Treatment:
• Mycoplasma gallisepticum has shown sensitivity in vitro and in vivo to several antimicrobials including macrolides, tetracyclines, and fluoroquinolones
• Antimicrobials have been used to treat MG respiratory diseases and to reduce egg production losses and transmission. Antimicrobials may reduce the severity of clinical signs and lesions and significantly reduce populations of MG in the respiratory tract.
• Use of oxytetracycline or chlortetracycline at 200g/ton feed for at least several days. Tylosin has been injected subcutaneously at 8-10mg/kg body weight or administered at 2–3g/4lit drinking water for 3–5 days.
