Staphylococcosis is a bacterial disease that can affect a wide range of avian species, including poultry, and is seen worldwide. Staphylococcus aureus is most commonly isolated from staphylococcosis cases, but species such as S hyicus have also been reported as the causative agent of osteomyelitis in turkey poults. The disease conditions associated with staphylococcosis vary with the site and route of inoculation and can involve the bones, joints, tendon sheaths, skin, sternal bursa, navel, and yolk sac. Economic losses may result from decreased weight gain, decreased egg production, lameness, mortality, and condemnation at slaughter.
S aureus is a gram-positive, catalase-positive, coccoid bacteria that appears in grape-like clusters on stained smears. S aureus is the most common isolate recovered from clinical cases. Most pathogenic strains have been coagulase-positive; however, coagulase-negative Staphylococcus, including S hyicus, S epidermidis, and S gallinarum have been reported from clinical cases. Phage typing has been used to distinguish among the different strains. Staphylococcus toxins can enhance the pathogenicity of a strain.
Transmission, Epidemiology, and Pathogenesis
Because Staphylococcus is part of the normal skin and mucosal flora, many infections are the result of a wound, mucosal damage, or both. Infection can also occur in the hatchery as a result of contamination of an open navel. Birds that are immunocompromised are also more prone to staphylococcal infections. Once in the host, S aureus invades the metaphyseal area of the nearest joint, which leads to osteomyelitis and localization within that joint. Alternatively, the bacteria can invade the bloodstream and lead to a systemic infection in multiple organs.
Omphalitis, or infection of the yolk sac, caused by S aureus has been reported in young chicks and poults (see Omphalitis). Its prevalence depends on its presence in the breeder flock and the hatchery environment. Navel infections can occur in young hatchlings if the navel becomes infected with Staphylococcus. Gangrenous dermatitis can occur after skin trauma and subsequent contamination with Staphylococcus. Immunocompromised birds have a higher prevalence of gangrenous dermatitis. Because Staphylococcus is found on the skin, skin injuries can also lead to localized abscesses, such as on the foot (bumblefoot). Finally, Staphylococcus can be dispersed systemically and can cause arthritis, synovitis, osteomyelitis, and endocarditis. Most Staphylococcus infections in poultry cause synovitis, with lameness being the most common clinical presentation. The bones and associated joints most frequently affected are those of the leg, especially the stifle and tibiotarsus. In addition, septicemia can result in green livers or livers with multifocal necrosis and/or granulomas. In acute infections, a sudden increase in mortality may be noted.
Chicks with omphalitis have navels that are moist and dark, and affected birds are often lethargic. Infected yolk sacs are retained longer than uninfected yolk sacs, which are normally resorbed by the developing chick within the first week of life. Infected yolks are abnormal in color (dark green to brown), have a doughy consistency, and are odorous. Gangrenous dermatitis is often reported in immunocompromised chickens and is often due to a combination of S aureus and Clostridium septicum and/or Escherichia coli. Affected areas are usually hemorrhagic and crepitant. Lesions associated with septicemia are most common. Within the musculoskeletal system, affected bones often have focal yellowish areas of necrosis, while lesions in the joints contain purulent exudate. At the processing plant, green liver has been reported in turkeys that have had osteomyelitis and synovitis. Liver spots and granulomas resulting from septicemia have been a cause of liver condemnation. In acute infections, necrosis and vascular congestion are seen in the liver, spleen, kidneys, and other internal organs. Vegetations on the heart valves have been reported.
Although some lesions may be suggestive of a Staphylococcus infection, diagnosis is confirmed by identifying the organisms from stained smears of the lesion and by culture on blood agar plates. Phenotyping and genetic techniques have been used to classify strains of poultry S aureus. Differential diagnoses include E coli and Pasteurella multocida as well as other septicemic diseases of poultry.
Treatment and Prevention
Staphylococcosis can be successfully treated with antibiotics, but an antimicrobial susceptibility test should be performed because antibiotic resistance is common. Antibiotics used to treat Staphylococcus infections include penicillin, erythromycin, lincomycin, and spectinomycin. Proper management to prevent injury and immunocompromised poultry helps prevent staphylococcosis. Because wounds are the primary route by which Staphylococcus can enter the body, it is important to reduce all potential sources of injury to the bird. Wood splinters in litter, protruding wires from cages, and fighting/cannibalism have been associated with skin wounds and staphylococcosis. Because beak and toe trimming procedures in young chickens and turkeys could result in a staphylococcal septicemia, ensuring that equipment is sanitary will help to prevent outbreaks. Good litter management is important in controlling foot-pad injuries to prevent bumblefoot. Hatchery sanitation and good egg management practices are also important to reduce navel infections and omphalitis.
S aureus can cause food poisoning in people. Enterotoxin-producing strains are found on clinically healthy poultry, and proper precautions should be taken during the handling and cooking of poultry products. Methicillin-resistant S aureus (MRSA) has been isolated from poultry meat in a number of countries, but the prevalence and significance for human health are incompletely understood at present.
Last full review/revision November 2013 by Claire B. Andreasen, DVM, PhD, DACVP