Veterinary Significance of Staphylococcus Infections: Identification, Pathogenesis, and Clinical Implications

Introduction

Staphylococci are Gram-positive, spherical bacteria, approximately 1 µm in diameter, that typically appear in irregular clusters resembling grape bunches. The name originates from the Greek words staphyle (meaning “cluster of grapes”) and kokkos (meaning “berry”). There are at least 30 species of Staphylococcus that naturally inhabit the skin and mucous membranes, with some acting as opportunistic pathogens capable of causing pus-forming infections.

Figure 1: Identification of Staphylococcus aureus isolates using an electron microscope reveals round (cocci) cells arranged in irregular clusters resembling grape bunches (Salah et al., 2022)

Most Staphylococcus species are facultative anaerobes and catalase-positive. They are non-motile, oxidase-negative, and do not produce spores. However, two species, S. aureus subsp. anaerobius and S. saccharolyticus are anaerobic and lack catalase enzyme activity.

Species	Hosts	Clinical Conditions
Staphylococcus aureusa	Cattle	Mastitis, udder impetigo
	Sheep	Mastitis, tick pyaemia (lambs), benign folliculitis (lambs), Dermatitis
	Goats	Mastitis, Dermatitis
	Pigs	botryomycosis of mammary glands, impetigo on mammary glands
	Horses	Arthritis and septicemia in turkeys, bumblefoot, and omphalitis in chicks
	Poultry	Arthritis and septicemia in turkeys, bumblefoot, omphalitis in chicks
	Dogs, Cats	Pyoderma, endometritis, cystitis, otitis externa, and other suppurative conditions
Staphylococcus intermedius	Dogs, Cats	Suppurative conditions similar to those caused by S. aureus
Staphylococcus hyicusb	Cattle	Mastitis (rare)
	Pigs	Exudative epidermitis (greasy pig disease), arthritis
Staphylococcus aureus subsp. anaerobius	Cattle, Sheep	Mastitis (rare) Lymphadenitis
Staphylococcus delphini	Dolphins	Suppurative skin lesions
Staphylococcus schleiferi subsp. Coagulansc	Dogs	Otitis externa

Notes:

a S. aureus can cause neonatal septicemia and wound infections across multiple species.

b Approximately 25-50% of S. hyicus strains exhibit coagulase activity.

c S. schleiferi subsp. coagulans was first described by (Igimi et al., 1989).

Among the pathogenic species, S. aureus subsp. aureus (or S. aureus), S. intermedius, and the coagulase-variable S. hyicus are significant pathogens in domestic animals. Coagulase production is linked to their ability to cause disease. While coagulase-negative staphylococci generally have low virulence, some can occasionally lead to infections in animals and humans.

Natural Habitat

Staphylococcus species are widely distributed and naturally present on the skin of both humans and animals. They are also commonly found on the mucous membranes of the upper respiratory tract, lower urogenital tract, and temporarily in the digestive system. These bacteria exhibit remarkable environmental stability.

Certain strains of Staphylococcus show a preference for specific animal species. The transmission of S. aureus strains between different animal species, as well as between animals and humans, is relatively uncommon.

Differentiation of Staphylococcus Species

In clinical samples, Staphylococcus species must be distinguished from Streptococcus and Micrococcus species. One key difference is that staphylococci are generally catalase-positive, whereas streptococci are catalase-negative. Staphylococcus species are primarily identified based on their colony morphology, hemolytic characteristics, biochemical reactions, and ribosomal RNA gene restriction patterns. Differentiating S. aureus from S. intermedius is particularly crucial in certain clinical cases, especially in dogs and cats.

In veterinary diagnostic laboratories, the precise identification of coagulase-negative staphylococci is typically performed only when these organisms are found in nearly pure cultures or isolated from normally sterile sites, such as joints or cerebrospinal fluid.

Colonial Characteristics

Staphylococcus colonies are typically white, opaque, and can grow up to 4 mm in diameter. However, S. aureus strains from both cattle and humans often appear golden yellow. Some coagulase-negative staphylococcal species also produce pigmented colonies.

Hemolysis on Sheep or Ox Blood Agar

Four distinct hemolysins—alpha, beta, gamma, and delta—have been identified in staphylococci. These hemolysins vary in their antigenic properties, biochemical nature, and effects on red blood cells from different animal species. The ability to produce hemolysins differs among strains, with animal isolates of S. aureus and S. intermedius commonly producing both alpha- and beta-hemolysins.

When grown on ruminant blood agar, alpha-hemolysin creates a narrow zone of complete hemolysis around the colony, while beta-hemolysin generates a wider zone of partial or incomplete hemolysis, a phenomenon known as double hemolysis. These hemolysins act as toxins in living organisms. Coagulase-negative staphylococci show variability in hemolysin production, which usually develops at a slower rate. Isolates of S. hyicus are typically non-hemolytic.

Figure 2: Double zone hemolysis on 7 % blood agar (Padmaja and Halami, 2013)

Slide and Tube Coagulase Tests

These tests assess the ability of Staphylococcus species to produce coagulase, an enzyme that converts fibrinogen into fibrin. The test is conducted using rabbit plasma mixed with a bacterial suspension, either on a slide or in a tube.

Slide Test:

Detects bound coagulase (clumping factor) present on the bacterial surface. A positive result is indicated by bacterial clumping within 1–2 minutes.

Tube Test:

Identifies free coagulase (staphylocoagulase), which is secreted into the plasma. This is considered the definitive test for coagulase production, with a positive result indicated by clot formation after incubation at 37°C for 24 hours.

Biochemical Tests for Differentiating S. aureus and S. intermedius

Species	Colony Color	Hemolysis on Sheep Blood Agar	Coagulase Production (Tube Test)	Coagulase Production (Slide Test)	Acetoin Production	Maltose Utilization
Staphylococcus aureus	Golden yellow (bovine & human strains)	+	+	+	+	+
Staphylococcus intermedius	White	+	+	v	–	±
Staphylococcus hyicus	White	–	v	–	–	–
Staphylococcus aureus subsp. Anaerobiusc	White	+	+	–	–	NA
Staphylococcus delphini	White	+	+	–	–	NA
Staphylococcus schleiferi subsp. coagulans	White	+	+	–	+	NA

Notes:

• (a) Maltose utilization tested using 1% maltose in a purple agar base.
• (b) Golden yellow colony color is observed in bovine and human strains only.
• (c) S. aureus subsp. anaerobius is anaerobic.
• (+) Over 90% of strains are positive.
• (-) Over 90% of strains are negative.
• (v) Variable reactions.
• (±) Poor utilization.
• (NA) Data not available.

• A rapid acetoin detection test has been developed (Davis et al., 1973).
• Purple agar, which contains bromocresol purple as a pH indicator and 1% maltose, is used to distinguish S. aureus from S. intermedius. S. aureus ferments maltose, producing acid that changes the medium from purple to yellow, whereas S. intermedius, a weak maltose fermenter, does not alter the color.
• Commercially available biochemical test kits can be used to confirm Staphylococcus species.

Molecular Techniques

Advanced molecular methods, such as polymerase chain reaction (PCR), are typically performed in research or reference laboratories for precise identification of Staphylococcus species.

Pathogenesis and Pathogenicity

As pyogenic bacteria, Staphylococcus species frequently cause suppurative (pus-forming) infections. Minor injuries or a weakened immune system can increase susceptibility to infection. The virulence factors of S. aureus and their pathogenic effects are summarized:

Virulence Factor	Pathogenic Effects
Coagulase	Converts fibrinogen to fibrin, which helps staphylococci evade phagocytosis by forming a protective fibrin shield.
Lipase, Esterases, Elastase, Staphylokinase, Deoxyribonuclease, Hyaluronidase, Phospholipase	Enzymes that enhance virulence by breaking down host tissues, facilitating bacterial spread.
Protein A	Binds to the Fc region of IgG, preventing opsonization and interfering with immune response.
Leukocidin	Destroys phagocytic cells (white blood cells) in certain animal species, aiding immune evasion.
Alpha-toxin (Alpha-hemolysin)	The primary toxin in gangrenous mastitis. Causes smooth muscle spasms, tissue necrosis, and can be lethal.
Beta-toxin (Beta-hemolysin)	A sphingomyelinase that disrupts cell membranes, contributing to tissue damage.
Exfoliative Toxins	Cause desquamation (skin peeling) in staphylococcal scalded skin syndrome in humans.
Enterotoxins	Heat-stable toxins that cause staphylococcal food poisoning in humans.
Toxic Shock Syndrome Toxins (TSST)	Cause excessive lymphokine production, leading to tissue damage. TSST-1 is produced by bovine and human strains of S. aureus, while sheep and goat strains produce a variant. The significance of TSST in animals is unclear.

While the significance of some virulence factors remains unclear, most are encoded within the bacterial genome, though some may be regulated by plasmids or bacteriophages.

Certain structural components, such as capsular polysaccharides, teichoic acids, and protein A, play a role in evading the immune response by inhibiting opsonization and subsequent phagocytosis. Additionally, staphylococcal cell wall proteins that bind to fibronectin and fibrinogen may promote bacterial adhesion to tissues damaged by the toxins secreted by these bacteria.

Coagulase production is a key indicator of Staphylococcus pathogenicity. Other important markers of virulence include DNase activity and the production of protein A.

Diagnostic Procedures

Among domestic animals, Staphylococcus species are specifically linked to exudative epidermitis in piglets and tick pyaemia in lambs. In cases of suppurative infections, staphylococcal involvement should be considered, and appropriate samples, such as exudates and mastitic milk, should be collected for laboratory analysis.

Gram-stained smears of pus or other relevant specimens may reveal the characteristic grape-like clusters of Staphylococcus. Culturing is performed on blood agar, selective blood agar, and MacConkey agar, followed by aerobic incubation at 37°C for 24–48 hours. Selective blood agar, supplemented with nalidixic acid and colistin, helps suppress the growth of Proteus species and other Gram-negative contaminants.

Identification Criteria for Isolates:

• Colony morphology
• Presence or absence of hemolysis
• No growth on the MacConkey agar
• Catalase production
• Coagulase production
• Biochemical profile

Phage typing is useful for epidemiological investigations, particularly in cases such as staphylococcal food poisoning outbreaks in humans.

Clinical Infections

Since Staphylococcus species are naturally present on skin and mucous membranes and can also be found in the environment, infections may originate from endogenous sources (normal flora) or exogenous sources (external contamination). Many infections are opportunistic and occur in association with trauma, immunosuppression, concurrent parasitic or fungal infections, allergic reactions, or metabolic and endocrine disorders.

Coagulase-positive Staphylococcus species account for most infections. However, some coagulase-negative strains with low virulence can also cause disease in animals. Currently, no effective vaccines exist to prevent staphylococcal infections, so antibiotic susceptibility testing is recommended before initiating treatment.

Major Staphylococcal diseases in domestic animals are:

• Mastitis
• Tick pyaemia
• Exudative epidermitis
• Botryomycosis
• Pyoderma

Bovine Staphylococcal Mastitis

Staphylococcal mastitis, primarily caused by Staphylococcus aureus, is a prevalent form of bovine mastitis worldwide. The disease can manifest in subclinical, acute, or chronic forms, with the majority of infections being subclinical. However, peracute and gangrenous forms are associated with severe systemic reactions and can be life-threatening.

In cases of gangrenous mastitis, the affected quarter of the udder turns cold and develops a bluish-black discoloration before eventually sloughing off. Tissue necrosis is attributed to the alpha-toxin, which induces contraction and necrosis of smooth muscle in blood vessel walls, leading to impaired blood circulation in the affected quarter. Additionally, this toxin triggers the release of lysosomal enzymes from leukocytes, further exacerbating tissue damage.

Tick Pyaemia

Tick pyaemia is an S. aureus infection affecting lambs, primarily occurring in hill-grazing regions of Britain and Ireland, where the tick Ixodes ricinus is prevalent. Lambs may carry S. aureus on their skin and nasal mucosa, with infection typically entering through minor skin wounds, including tick bites. Ixodes ricinus also serves as a vector for Ehrlichia phagocytophila, the causative agent of tick-borne fever. This can lead to immunosuppression in lambs and increase their susceptibility to Staphylococcus infections.

Tick pyaemia presents in two primary forms:

1. Septicemic form (septicaemia), leading to rapid death.
2. Localized form is characterized by abscess formation in various organs.

Clinical signs include arthritis, posterior paresis, and poor growth. This condition can have significant economic consequences, with infection rates reaching up to 30% in lambs aged 2 to 10 weeks during spring and early summer.

Diagnosis

In lambs grazing on rough pasture in Britain or Ireland, clinical symptoms can provide a strong indication of tick pyaemia. Confirmatory diagnosis involves microscopic examination of pus for bacterial presence, followed by isolation and identification of S. aureus from infected lesions.

Treatment and Control

Treatment is generally ineffective in severely affected lambs, so prevention and control strategies are crucial.

Control Measures:

• Prophylactic antibiotic treatment: Long-acting tetracycline can be administered to lambs at one week of age to help prevent infection. Tetracyclines also offer protection against E. phagocytophila.
• Tick control: Measures such as dipping should be implemented to reduce tick infestations and minimize the risk of transmission.

Exudative Epidermitis (Greasy Pig Disease)

Exudative epidermitis, commonly known as greasy pig disease, is a highly contagious skin infection in piglets caused by Staphylococcus hyicus. It affects suckling and recently weaned pigs up to three months old and is characterized by excessive sebaceous secretion, exfoliation, and exudation on the skin. Affected piglets exhibit anorexia, depression, and fever, developing a widespread, non-itchy dermatitis with a greasy exudate.

In severe cases, piglets under three weeks old may die within 24 to 48 hours. Morbidity ranges from 20% to 100%, while mortality can reach 90% in severely affected litters. S. hyicus can be found on the vaginal mucosa and skin of healthy sows and likely enters the piglets’ skin through minor abrasions, such as bite wounds.

Predisposing Factors:

• Agalactia in the sow
• Concurrent infections
• Stress due to weaning

Experimental studies have shown that injecting S. hyicus toxin into young pigs’ skin can trigger exfoliation.

Diagnosis

A high mortality rate in piglets with exudative, non-itchy skin lesions indicates the disease. Isolation and identification of S. hyicus from skin lesions confirm the diagnosis.

Treatment and Control

• Early systemic antibiotic therapy and topical antiseptic or antibiotic treatments may be effective.
• Strict isolation of affected piglets is necessary to prevent the spread.
• Thorough cleaning and disinfection of contaminated facilities should be implemented.
• Sows should be washed with an antiseptic soap before farrowing to reduce bacterial transmission.
• Preventive colonization of piglets’ skin with an avirulent S. hyicus strain has been shown to protect against infection (Allaker et al., 1988).

Botryomycosis

Botryomycosis is a chronic, suppurative granulomatous disease, often caused by Staphylococcus aureus. It typically develops within a few weeks after castration in horses due to infection at the spermatic cord stump (scirrhous cord). The condition may also affect the mammary tissues of sows.

Characteristics of Botryomycosis:

• Lesions consist of fibrous tissue masses containing pus-filled foci and sinus tracts.

Staphylococcal Infections in Dogs and Cats

Staphylococcus intermedius is frequently isolated from various infections in dogs and cats, including:

• Pyoderma (skin infections)
• Otitis externa (ear infections)
• Mastitis
• Endometritis
• Cystitis
• Osteomyelitis
• Wound infections

Occasionally, similar infections may also be caused by Staphylococcus aureus.

Conclusion

Staphylococcus species, particularly coagulase-positive strains such as Staphylococcus aureus, Staphylococcus intermedius, and Staphylococcus hyicus, play a significant role in veterinary medicine due to their ability to cause a wide range of infections in domestic animals. These bacteria are associated with conditions such as mastitis in cattle, tick pyaemia in lambs, exudative epidermitis in pigs, and pyoderma in dogs and cats. Their pathogenicity is influenced by various virulence factors, including coagulase production, hemolysins, and exotoxins. Accurate identification and differentiation of Staphylococcus species are crucial for effective diagnosis and treatment, with laboratory methods such as biochemical tests, hemolysis patterns, and molecular techniques aiding in their classification. Since no effective vaccines currently exist for staphylococcal infections in animals, antibiotic susceptibility testing is essential to guide appropriate treatment. Preventive measures, including hygiene protocols, tick control, and early antibiotic intervention, are key strategies in managing staphylococcal diseases in veterinary practice.

References

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