Staphylococcosis in Chickens

Introduction

Staphylococcal infection of poultry. This infection is commonly associated with infection of skin, bones, joints, and navel. There is a potential risk for human food poisoning. 

Etiology

Staphylococcus aureus is the most common species that causes disease in poultry.  However, other species may also be found.  Among the most frequently isolated species are S. lentus, S. simulans, S. cohnii, S. gallinarum, and S. capit

Features of the agent

Staphylococcus is a genus of coccoid, facultative anaerobic bacteria. In stained smears, it tends to aggregate, giving the appearance of clusters of grapes. Staphylococcus species are Gram-positive, although cultures older than 24h may stain Gram-negative. They are readily isolated in blood agar after a standard culture period of 24h. The colonies are generally b-hemolytic, 1-3 mm in diameter, and often white or creamy in color (but can also appear orange).  Some poultry isolates are capable of human food poisoning.

Susceptibility of the agent

Staphylococcus species are fairly resistant to chemical and physical agents.  In particular, they are very resistant to high concentrations (7.5%) of common table salt (NaCl), which can be used to isolate the agent from heavily contaminated material.  Disinfectants containing chlorine are effective against the organism (in the absence of abundant organic matter).

Occurrence 

Infections with Staphylococcus occur worldwide and affect all species of birds. Outbreaks are generally more important in chickens and turkeys.

Pathogenesis

Most species of Staphylococcus are considered benign organisms on skin and mucosal membranes. Coagulase positive isolates of these bacteria are considered pathogenic for poultry.  The pathogenesis and port of entry of these bacteria have not been completely understood.  It is almost certain that bacteria can gain entry inside tissues through cuts, lacerations, scratches, injections contaminating the skin (localized abscesses, cellulitis, and also gangrenous dermatitis). In addition, it is thought that Staphylococcus can translocate into the blood through mucosal surfaces affected by severe inflammation (enteritis). From the blood, these bacteria can reach articular surfaces (causing arthritis and synovitis), and bones (causing osteomyelitis).  In the case of contamination of the navel in the hatchery, newly hatched chicks will develop omphalitis.  The incubation period is in general short (2-3 days). However, when hard tissues are affected the incubation period can be longer. Some cases of Staphylococcus have been linked to previous infections with immunosuppressive agents (infectious bursal disease, infectious anemia, Marek's disease).

Clinical manifestations

They depend on the affected system.

• Locomotor system:  Swollen joints and tendons are common; lameness or gait alterations may develop.  Osteomyelitis of the femur and/or tibiotarsus can produce lameness.  In severe cases, the birds can be prostrated. If the developing abscess in the thoracolumbar vertebrae compresses the spinal cord, paraplegia may occur. Reduction of body weight and dehydration can be expected due to increased difficulty to access feed and water.
• Skin:  localized abscess or diffuse cellulitis may develop with the accumulation of purulent exudate.  In immunosuppressed birds, the dermal infection may progress to a gangrenous dermatitis.  When the skin infection occurs through cuts in the skin of the feet, a plantar abscess with a very large inflammatory component may develop (bumblefoot). 
• Omphalitis:  In case of navel contamination, the navel appears wet and may fail to heal.  Abdominal distention may be apparent.  Affected chicks fail to thrive, and many will die during the first week of life.

  
Picture 1. Swelling of the joints. Picture credit. Dr. I. Rojas

 
Picture 2. Bumblefoot. Picture credit Dr. E. Pendleton

Necropsy findings

Necrosis and congestion may be present in the internal organs.  In osteomyelitis, the head of the femur may be necrotic or partially degenerated due to septic necrosis.  Fracture of the femoral head is a common finding.  Purulent material and necrotic tissue are evident at the site of the fracture.  Caseous material may also be found in the proximal end of the tibiotarsus and sometimes in the thoracolumbar vertebra.

In young animals with omphalitis, there is congestion of the subcutaneous blood vessels.  The yolk inside the abdominal cavity is not absorbed and blood vessels irrigating the yolk appear engorged.  Lumps of purulent material may be present in the yolk.

 
Picture 3. Bacterial chondronecrosis with osteomyelitis (BCO). Picture credit. Dr. I. Rojas

 
Picture 4. Bacterial chondronecrosis with osteomyelitis (BCO). Note the caseous exudate in the proximal tibia. Picture credit. Dr. G. Lorenzoni

Diagnosis

Lesions are suggestive. Culture of the organism from affected tissue is necessary for diagnosis.

Relevant differential diagnoses

E. coli, Mycoplasma synoviae, Reoviruses, Salmonella, Pasteurella. 

Prevention and treatment

To reduce the incidence of Staphylococcosis, it helps to use an appropriate vaccination program to protect chickens from immunosuppressive diseases.  There is a vaccine available for turkeys that is produced with an avirulent strain of staphylococcus (S. epidermidis 115) that has shown some efficacy on reducing the incidence of the disease in the field.

Minimize the risk of skin lesions by eliminating sharp objects from the area used by birds.  If toe trimming is a regular practice in your operation, make sure to properly disinfect blades and skin.  Provide sufficient feeder space to prevent birds from climbing on each other's backs to gain access to feed.  Scratches are a common port of entry for Staphylococcus.  Control intestinal diseases or respiratory diseases that could compromise the epithelial barriers.  Antibiotic treatment may be effective if the infection is in soft tissue but is largely ineffective for osteomyelitis and omphalitis. 

References

Diseases of Poultry, 13^th ed. D. E. Swayne.  Wiley-Blackwell. 

Avian Diseases Manual.  7^th ed. The American Association of Avian Pathologists.