Introduction

Staphylococcus is a gram-positive coccus that divides in all planes forming grape-like clusters. It belongs to the Terrabacteria Group which are gram-positive bacteria adapted to life on land. They have a thick peptidoglycan outer membrane which gives them environmental resistance.

Staphylococcus is a halophile and one of 10 genera in the Family Staphylococcaceae.  Only important genera containing animal pathogens are shown in the taxonomy ranking tree.

Identification of the Staphylococci

Identify members of the genus Staphylococcus are nonmotile, nonsporulating, gram-positive cocci occurring in clumps that are facultatively anaerobic and catalase-positive.  Like other members of its family (Staphylococcaceae), which are essentially marine bacteria, Staphylococcus will grow in a saline environment = halotolerant. The pathogenic staphylococci usually produce the enzyme coagulase.

Unlike the staphylococci, streptococci form chains and are catalase-negative. They are not usually halotolerant.

Below is an interactive table comparing the major identification characteristics of some of the pathogenic staphylococci.

Below is an interactive picture showing S. aureus in a purulent exudate

Identify potential sources of pathogenic staphylococci.

Staphylococci are commensals on the skin and mucosa of all animals and people.

They can survive in dry environments for days to weeks i.e. up to 3 months on clothing and cleaning cloths. Pathogenic staphylococci are only a small part of the normal skin and oral-upper respiratory microflora. However, due to their ability to cause disease, they are the most common cause of opportunistic infections in traumatic lesions of the skin and mucosae.

Diseases caused by the Staphylococci

Being common skin and mucosal bacteria, the more virulent coagulase-positive staphylococci are often locally invasive causing both acute and chronic opportunistic infections. In cows and ewes S. aureus is a common cause of contagious mastitis. It will be discussed in The Chapter on mastitis.

Below is a table listing the more important diseases of animals caused by Staphylococcus species.

Some more details on coagulase positive. pathogenic Staphylococcus are below.

Staphylococcus aureus (meaning the “golden grape-cluster berry”)

S. aureus is the most common of all the pathogenic staphylococci in animals causing pyogenic infections of the skin and related structures. Infections are usually initiated by traumatic wounds. Once established locally S. aureus can spread from these sites causing systemic infections, the most notable being septic arthritis and osteomyelitis. This occurs more common in dogs and horses.

S. aureus is also contagious with animal-to-animal and animal-to-human (zoonosis) or vice versa (reverse zoonosis) being common. Of concern is when multiresistant drug strains i.e. methicillin-resistant staphylococci (MRSA) is transmitted between animal species and to and from people. There are several published examples i.e. MRSA in the nasopharynx of pigs can infect cattle causing mastitis, so too can human-associated staphylococci. This agent has also been implicated as a cause of nosocomial (hospital-acquired) infections in horses and dogs. Transmission is by contact with either infected animals or with objects that have been in contact with infected animals or contaminated surfaces i.e. hands and hair clippers.

Being a wound invader, it also can cause chronic infections of the skin and subcutaneous tissues, such as bumblefoot in poultry and a pyogranulomatous lesion known as botyromycosis in horses. Bumblefoot and botyromycosis can have more than one bacterial species present.

Staphylococcus pseudintermedius

S. pseudintermedius is a skin and mucosa commensal found in canids and is the most common cause of skin-associated infections in dogs, cats and occasionally other animals and people. Like S. aureus, it  can exhibit multidrug resistance. Refer to the paragraph on canine staphylococci for further information. (Its species name is interesting. It was originally named S. intermedius as it was related to S. aureus, but not quite S. aureus. Later on it was discovered that the rabbit-associated S. intermedius bacterium was genotypically different from the canid S. intermedius strain. So the canid strain, not being the type strain, become S. pseudintermedius. In the laboratory it is hard to distinguish the two strains, so for practical purposes, all canine isolates are named S. pseudintermedius.)

Staphylococcus hyicus (Pig Staphylococcus)

S. hyicus is the cause of exudative epidermitis or greasy pig disease in pigs and can affect poultry and rarely other animal species. Refer to the paragraph on porcine staphylococci for further information.

Antibiotic therapy of the staphylococci

In small animals, amoxycillin with clavulanic acid (potentiated beta-lactam drug) is the antibiotic of choice. Nowadays most staphylococci derived from companion animals is resistant to amoxicillin alone. For skin infections cephalexin, and for surgical prophylaxis cephazolin, both first generation cephalosporins, are recommended. If MRSA or MRSP is diagnosed (presence of the mecA gene) then amoxycillin potentiated with clavulanic acid should be used. Penicillins are highly synergistic with the aminoglycosides and therefore in serious infections a combination therapy of amoxicillin-gentamicin is often administered in dogs and cats. For livestock procaine penicillin (long-acting penicillin G), and in horses, a penicillin-gentamicin combination is used as first choice (for empirical use) antibiotics.

Then blaZ gene encodes for beta-lactamases and the mecA gene for an altered penicillin-binding protein2a (PBP2a). Staphylococci that have the mecA gene are often multidrug resistant = superbug. Refer to antibiotic resistance notes.

important staphylococcal toxins

To establish themselves pathogenic staphylococci produce a number of virulence factors, dependent on their species and strain. Well-described virulence factors are shown in the picture and table below.

greasy pig disease = exudative epidermitis in pigs=Marmite disease

Exudative dermatitis is a contagious skin disease of predominantly piglets caused primarily by Staphylococcus hyicus (occasionally S. chromogens and S. aureus).

While exudative epidermitis causes a low mortality rate, it manifests itself as explosive outbreaks lasting for about 3 months, where affected piglets grow more slowly and are susceptible to secondary bacterial infections.

This bacterium reaches high numbers in the sow’s vagina prior to partus and then colonises the skin and oral cavity of piglets during birth. However, disease only manifests when there trauma to the skin i.e. biting, rough floors or straw bedding. High infestation of irritant skin mites i.e. Sarcoptes mites and infection with porcine circovirus 2 and porcine parvovirus also predispose to the disease. Thus the disease is more common in 1 to 5-week-old pigs where it can spread to most piglets.

For more detailed information on this disease refer to: Foster, A. (2012). Staphylococcal skin disease in livestock. Veterinary Dermatology, 23: 342–363. Available online via JCU library – search Google Scholar

Pathogenesis and Clinical Signs

Staphylococcus hyicus, a skin commensal proliferates on the skin in a humid, warm environment. It will invade damaged skin, proliferate (more than 10⁵ CFU/cm²) and produce exotoxins. The most important being exfoliative toxins (encoded by exhA, exhB, exhC and exhD), which destroys desmoglein 1 found in the intracellular cement of cells of the stratum spinosum and cleaves these cells. Furthermore, a severe inflammatory response in the skin is elicited. Consequently, there is exudation of a protein-rich fluid which if severe enough can lead to weight loss and dehydration. Pigs that die also have secondary bacterial infections especially of the kidneys and lungs. (This disease resembles scalded skin syndrome in humans which is caused by S. aureus).

The disease is most severe in suckling pigs with a mortality rate of up to 50%. Outbreaks of exudative epidermitis often occur 2-3 days after weaning with the skin starting to redden, become brown and later develop a greasy texture. Up to 15% of this age group may die.  Outbreaks usually last 2-3 months. In older pigs and immune pigs the disease is mild with only localised crusting lesions. Infection often starts on the face, feet or knees where the skin is easily injured.

Treatment of exudative epidermitis

• Determine the antibiotic susceptibility and inject affected piglets daily for 5 days, or on alternate days with a long-acting antibiotic to which the organism is susceptible to.
• Topical antibiotics/antiseptics. Mix novobiocin intramammary preparation with mineral oil and spray onto the skin. Chlorhexidine soap wash can also be effective.
• Piglets become very dehydrated and should be offered electrolytes by mouth.
• If present, treat mange (Sarcops).
• If indicated, metaphylactic treatment i.e. at weaning with either long-acting penicillin or tetracycline.

Management control and prevention

• Examine the pigs to see where abrasions are taking place. Consider upgrading those surfaces, but in the meanwhile they should be disinfected.
• Make sure teeth clipping, tail docking and iron injections are done with clean/sterile and sharp implements.
• Make sure farrowing pen/crate is clean and disinfected between farrowings.
• Wash the sow thoroughly before she enters the farrowing pen. A cow teat dip can be used to dip her teats 3 days before and 3 days after farrowing.
• If concrete surfaces are poor, brush these over after cleaning with hydrated lime that contains a phenol disinfectant.
• Treat the piglets for mange, if present.
• If mange is present in the herd treat the sow prior to entering the farrowing house.
• Adopt an all-in all-out policy in the weaning accommodation. Have the pens bacteriologically checked after they have been washed out and disinfected.
• When mixing litters at weaning put them in the dark, spray with deodorant or wash with an antiseptic soap. This minimises fighting. Have toys.
• Take particular care in environments that have a humidity of greater than 70%.
• Autogenous vaccine (prepared from bacteria cultured from affected pigs on the farm) and administered to the gilt/sow 2 and 4 weeks prior to farrowing to provide colostral immunity. If the disease is a problem in litters of gilts, then allow the piglets at birth to suckle from older sows for 4 to 5 hours.

Staphylococcal skin disease in dogs

Staphylococcus pseudintermedius  (formerly S. intermedius) is an unpigmented, coagulase positive Staphylococcus that is found on the skin and mucous membranes of dogs (all canids) and cats. It is the primary cause of skin infections such as pyoderma as well as acute ear infections. It will also infect other organ systems like the bladder causing cystitis and bone causing osteomyelitis. Note that dogs can also be infected with Staphylococcus aureus. S. aureus is the cause of 20% of staphylococcal infections in dogs and is usually more invasive than S. pseudintermedius where it is the most common cause of osteomyelitis in dogs. Staphylococcus schleriferi and S. coagulans can also cause skin lesions and ear infections in dogs and cats.

Infection occurs when there is overgrowth of mainly S. pseudintermedius. Wet and moist areas are colonised by a higher number of bacteria and are therefore more likely to become infected. Any skin disease i.e. demodicosis, diseases causing itchiness that changes the normally dry, desert-like environment to a more humid environment can predispose the host to over-colonisation of the skin with bacteria. Pressure points (i.e. elbows on concrete), possibly as a result of follicular damage, are also more prone to infection. The most common underlying triggers of superficial pyoderma include fleas, flea allergy dermatitis, atopy, food allergy, hypothyroidism, hyperadrenocorticism, and poor grooming.

Clinical signs depend on the lesions’ depth and can be as mild as scaling to papule and pustule formation with ulceration and exudation. Areas of alopecia (hair loss) are found.  In deep pyodermas skin tracts draining pus or sero-haemorrhagic fluid (haemorrhagic bullae) may develop.

Deep pyodermas are the hardest of all the skin infections to treat. Below is some background information on deep pyoderma. No need to learn it at this stage:

Deep pyoderma includes folliculitis, furunculosis, abscesses or cellulitis where there is an accumulation of pus within the dermis and subcutis.  Clinical signs include localised hair loss, pustules, epidermal collarettes, ulcerations, haemorrhagic bullae, nodules (see white dog below) and draining sinuses. Predisposing factors include immunosuppression, follicular hyperkeratosis, demodicosis, or a foreign body. It can be secondary to superficial pyoderma (caused by an allergy, seborrhoea, parasitic infections, hormonal factors, local irritants, matted fur). German Shepherd Dogs and Bull Terriers have a genetic predisposition to infection.

Gram’s stained direct smear of intact pustules, areas underlying crusts will reveal gram-positive cocci, usually found in clumps. Also, look for the yeast Malassezia as it can infect 50% of staphylococcal infections.

First-time bacterial pyoderma can be treated with empiric antibiotic therapy such as cephalexin or amoxycillin plus clavulanic acid. Avoid using penicillin and tetracyclines as they usually don’t work.

Staphylococcus pseudintermedius can be transmitted to dog owners. Infections, especially with antibiotic-resistant strains and in immune-compromised people can be problematic.

Other diseases caused by Staphylococcus aureus

Necrotising fasciitis/exudative epidermitis in possums

Brushtail possums (Trichosurus vulpecula) in the Tropics can develop a severe and often fatal skin infection caused mainly by Staphylococcus aureus. Infections are more likely secondary to wounds, especially those associated with territorial fighting. Most possums that develop it are young male possums. The prevalence of this disease increases when possums are stressed i.e. when there are high population densities and when young possums leave the nest and have to find a territory for themselves and forage for food. Oral treatment with cefaclor (a first generation cephalosporin) for 10 days is effective in mild cases. Those with lesions greater than 30% of their skin don’t usually respond well to treatment.

Very little is published on this disease. A link to the article by Jim Pollock (2006) provides you with some more information.

Below is a Gram stain of a smear made from the skin exudate of a brushtail possum suffering from necrotising fasciitis.

bumblefoot (pododermatitis) in birds

Pododermatitis is an inflammatory disease of the sole of the feet of individual birds. it occurs when there is injury to the feet and bacteria on the skin or in the environment can enter these wounds and stimulate a pyogenic inflammatory response. Staphylococcus aureus is the most common cause either alone or in a mixed infection. It is more common in heavy birds that are kept in captivity or intensively reared. Predisposed to bumblefoot are heavy birds, those with leg deformities, dietary deficiencies (vitamin A, biotin), overgrown toenails and bullied birds.

Early stages of disease can be managed with environmental changes such as providing a better walking surface. Heavily infected wounds will require surgical debridement, foot soaks, protective bandaging, protective shoes, and antibiotics. Cloxacillin is a good choice for empirical treatment. Otherwise, antibiotics should be chosen based on antibiotic susceptibility tests.

botryomycosis

Botryomycosis is a chronic suppurative inflammation of the skin and soft tissues giving the appearance of swellings, nodules or growths. It is rare and is characterised by focal areas of pyogranulomas with central Splendore-Hoepli bodies that contain bacteria. Any pyogenic bacteria can cause it, however, the skin bacterium S. aureus is the cause of most cases.

Below are pictures showing botryomycosis of the spermatic cord remnants in a gelding.

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