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Overview of Streptococcosis in Poultry

By Teresa Y. Morishita, DVM, MPVM, MS, PhD, DACPV, Professor of Poultry Medicine & Food Safety, College of Veterinary Medicine, Western University of Health Sciences

Streptococcosis has been reported in numerous bird species throughout the world. There are two forms of the disease, an acute septicemic form and a chronic form. Flock mortality can be as high as 50%. Because streptococci are part of the normal flora of the intestinal mucosa of most avian species, infections are often thought to occur secondarily to other diseases.

Etiology and Epidemiology:

Streptococci are nonmotile, gram-positive, catalase-negative coccoid bacteria that occur singly, in pairs, or in short chains when observed on stained smears. Streptococcus spp commonly associated with disease in avian species include S zooepidemicus (S gallinarum), S bovis, S dysgalactiae, S gallinaceus, and S mutans. Streptococci have been associated with acute septicemia, joint infections, cellulitis, osteomyelitis, and endocarditis. Transmission is via oral or aerosol routes as well as through skin injuries.

Clinical Findings:

Streptococcal infections can be localized or septicemic. Endocarditis and lameness occur during the subacute or chronic stage of the infection. In S zooepidemicus infections, clinical signs are typical of an acute septicemic infection, and lethargic birds are often prostrate. In affected layers, egg production may drop by 15%. In pigeons, S bovis infection produces acute mortality with lameness, inappetence, diarrhea, and the inability to fly. Acute fibrinopurulent conjunctivitis has been noted in infections caused by other Streptococcus spp.


Lesions in the acute septicemic form include splenomegaly, hepatomegaly (with or without reddish tan to white multifocal necrotic foci), and enlarged kidneys. There may also be serosanguineous fluid in the subcutaneous space and in the pericardium. Bloodstained feathers around the mouth and head due to blood from the oral cavity have been reported occasionally. Cellulitis involving the skin and subcutaneous tissues has been associated with both Escherichia coli and S dysgalactiae.

Chronic streptococcal infections result in arthritis and/or tenosynovitis, osteomyelitis, salpingitis, pericarditis, myocarditis, and valvular endocarditis. Lesions on the heart valves appear as small yellowish white or tan raised areas on the valvular surface. Focal granulomas, as a result of septic emboli, can be found in many tissues. Gram-positive bacterial colonies are readily observed in thrombosed vessels and within necrotic foci microscopically.


History, clinical signs, and lesions, along with demonstration of Streptococcus-like bacteria in blood films or impression smears of affected tissues, allows for a presumptive diagnosis of streptococcosis. Isolation of Streptococcus spp from lesions confirms the diagnosis. Streptococci can be cultured easily on blood agar.

Differential diagnoses include other bacterial septicemic diseases, including staphylococcosis, enterococcosis, colibacillosis, pasteurellosis, and erysipelas. Infectious laryngotracheitis, pasteurellosis, avian influenza, and exotic Newcastle disease should be considered as differential diagnoses when blood is noted coming from the mouth.

Treatment and Control:

Antibiotics, including penicillin, erythromycin, novobiocin, oxytetracycline, chlortetracycline, and tetracycline, have been used to treat acute and subacute infections. Clinically affected birds respond well early in the course of the infection, but this response decreases as the disease progresses within a flock. Antimicrobial susceptibility testing should be performed to select a suitable antibiotic.

Because streptococcosis often occurs secondary to other diseases, it is important to prevent immunosuppressive diseases and conditions. In addition, because skin wounds can provide an entry for Streptococcus, it is important to reduce this risk factor. Proper cleaning and disinfection can reduce environmental sources of infection.