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Streptococcosis in Poultry


Teresa Y. Morishita

, DVM, PhD, DACPV, College of Veterinary Medicine, Western University of Health Sciences

Reviewed/Revised Nov 2023
Topic Resources

Streptococci are opportunistic pathogens, often causing secondary infections. Clinical manifestations of avian streptococcosis in poultry include an acute septicemic form and a chronic form. Diagnosis is confirmed with isolation of the organism via culture. Treatment has been effective for early acute infections, with effectiveness decreasing as the disease becomes chronic.

"Streptococcosis" refers to disease caused by bacteria of the genus Streptococcus. Streptococcal bacteria are opportunistic pathogens, with many species being a normal part of the microbiota of humans and other animals, including poultry and other bird species. Streptococci are the cause of various acute and chronic diseases in commercial and noncommercial poultry.

Etiology of Streptococcosis in Poultry

Streptococci are nonmotile, gram-positive, catalase-negative coccoid bacteria that occur singly, in pairs, or in short chains when observed on stained smears.

The genetic diversity of Streptococcus spp is expansive, and multiple species either have been reclassified to the closely related genus Enterococcus or have had their nomenclature in Streptococcus updated. Several classification systems exist for streptococci, such as the Lancefield grouping and classification by hemolytic activity on blood agar.

Streptococcal bacteria are normal commensals in a wide variety of species, inhabiting the skin, nasopharynx, oral cavity, genital tract, and GI tract. Streptococci can act as facultative pathogens, and streptococcosis is often secondary to other diseases.

Streptococcus spp associated with disease in avian species include the following:

  • S equi subsp zooepidemicus (S gallinarum)

  • S gallolyticus subsp gallolyticus (formerly S bovis)

  • S dysgalactiae

  • S gallinaceus

  • S mutans

  • S pluranimalium

  • S suis

Epidemiology of Streptococcosis in Poultry

Streptococcosis has been reported in numerous bird species, including turkeys, ducks and geese, and chickens. It has a worldwide distribution.

Flock mortality rates of streptococcosis can be as high as 50%.

Streptococcal infection is transmitted via oral or aerosol routes, as well as through skin injuries.

Clinical Findings of Streptococcosis in Poultry

Depending on the species and strain of Streptococcus, infection may result in a wide range of disease conditions, including the following:

Streptococcal infections can be localized or septicemic. Endocarditis and lameness occur during the subacute or chronic stage of the infection.

  • In S equisubsp 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 gallolyticus infection produces acute death with lameness, inappetence, diarrhea, and the inability to fly.

  • Acute fibrinopurulent conjunctivitis has been noted in infections due to other Streptococcus spp.


Lesions in the acute septicemic form of streptococcosis include the following:

  • splenomegaly

  • hepatomegaly (with or without reddish-tan to white multifocal necrotic foci)

  • enlarged kidneys

In addition, the subcutaneous space and the pericardium may contain serosanguineous fluid. 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 can result in the following:

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.

Diagnosis of Streptococcosis in Poultry

  • Bacterial isolation via culture

History, clinical signs, and lesions, along with demonstration of Streptococcus-like bacteria in blood films or impression smears of affected tissues, enables a presumptive diagnosis of streptococcosis.

Isolation of Streptococcus spp from lesions confirms the diagnosis. Streptococci can be cultured on blood agar.

Differential diagnoses of streptococcosis include other bacterial septicemic diseases, including the following:

When blood is noted coming from the mouth, the following differential diagnoses should be considered for streptococcosis:

Treatment and Control of Streptococcosis in Poultry

  • Antimicrobial treatment

  • Prevention of immunosuppressive diseases or conditions

Antimicrobials, including erythromycin and chlortetracycline, have been used to treat acute and subacute infections of streptococcosis. For poultry, current FDA-approved food animal drugs should be used.

Clinically affected birds respond well early in the course of the streptococcal infection; however, this response decreases as the disease progresses within a flock. Antimicrobial susceptibility testing should be performed to select a suitable antimicrobial.

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, this risk factor should be minimized.

Proper cleaning and disinfection, including water sanitation, can decrease environmental sources of streptococcal infection.

Zoonotic Risk of Streptococcosis in Poultry

To date, there has been no conclusive evidence of Streptococcus spp transmission between poultry and humans. However, an isolate from a human case of Streptococcus gallolyticus subsp gallolyticus endocarditis was reported to share characteristics with isolates found in a layer flock under the patient's care (1 Reference Streptococci are opportunistic pathogens, often causing secondary infections. Clinical manifestations of avian streptococcosis in poultry include an acute septicemic form and a chronic form... read more Reference ).

The specific transmission route for Streptococcus needs to be further elucidated.

Key Points

  • Streptococcus species are considered part of the normal microbiota in poultry.

  • Streptococcal infections usually occur secondary to other primary infections.

  • Infections can be acute, or they can be subacute or chronic forms due to septicemia, and they can be treated effectively.

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