Bacterial Skin Infections in Dogs

Infections of the skin typically are all thought of as bacterial infections, but, virus infections, fungal infection and parasitic infections are all common.

Bacterial skin infections in dogs are classified in two groups, either superficial or deep. Superficial infections affect only the top layer of the epidermis and possibly the hair follicle. Deep infections have tracts that extend deep into the dermis.

Bacterial infections are mostly caused by staphylococcus infections. Staphylococcus organisms are located on the body normally. The bacteria are able to cause an infection due to reduced immune levels during stress. Damage to the skin from allergy induced itching and scratching will cause hair follicles to become infected. Areas of the skin that are moist, such as a “hot spots”, under the front legs, under the tail, between the toes, or in the ears are predisposed to staphylococcus infections.

Deep bacterial infections are uncommon but severe when they occur. They can also be present with Demodex mange infection. Severe trauma to the skin such as in a burn or abscess can also result in a deep bacterial infection or furunculosis. Antibiotics and drainage are necessary to treat these problems. Antibiotics are necessary, yet staphylococcus has the ability to develop resistance to the antibiotic. Powerful antibiotics have been developed such as Clavamox®, Tribrissen®, Cephalexin®, and Baytril®. Local therapy for staphylococcus infections of the skin can involve soaking the area in diluted Betadine® or Nolvasan® solution, or, topical application of benzoyl peroxide (Pyoben® or Oxydex gel®) to the affected areas. (Note: These stain carpet and clothes and may be irritating to the skin of cats). Oxydex® or Pyoben® shampoo can also be used to help reduce surface infections.

Recurrent Pyoderma

Recurrent Pyoderma is a common syndrome seen in dogs. The term Idiopathic Recurrent Pyoderma is given to a bacterial skin disease that responds to antibiotics but relapses three to four weeks later. The disease may not have any underlying reason but the list of possible reasons is long. Hypothyroidism, immunodeficiency, flea allergy, sarcoptic mange, food allergy and seborrhea are all possible causes. The hallmark of recurrent, superficial pyoderma is repeated episodes of papular to pustular eruption, with or without formation of epidermal collarettes, which completely resolve with antibiotic therapy. Lesions can be generalized or confined to one body region. The loose skin of the ventral inguinal areas and under the arms is usually involved. Itching is usually severe. Relapses occur two to four weeks after therapy has stopped. This pyoderma is usually a life-long disease. It is important that the owners realize that the disease can be managed but rarely cured. Many owners will switch veterinarians frequently in search of a cure that simply does not exist. The owner should be made aware that the pet will likely need antibiotics for its entire lifetime. This can pose a severe economic burden for the client as well as a time consuming, arduous task of giving medications to the pet. Euthanasia may be considered by the owner if the veterinarian is not careful to have an optimistic attitude.

Initial Management of Bacterial Skin Disease

  1. Use the correct antibiotic (see below)
  2. Use the antibiotic for long enough
  3. Avoid corticosteroids
  4. Topical adjunctive therapy

Poor antibiotic choices (staphylococcus resistance)

  • Ampicillin
  • Amoxicillin®
  • Hetacillin®
  • Penicillin
  • Tetracycline

Good initial antibiotic choices – Poor long-term choices

  • Chloramphenicol®
  • Antirobe®
  • Erythromycin
  • Lincomycin®
  • Tribrissen®
  • Primor®

Good initial antibiotic choices – Good long-term choices

  • Clavamox® – 20mg/kg T.I.D.
  • Cephalexin – 20mg/kg B.I.D.
  • Oxacillin – 20mg/kg T.I.D.
  • Baytril®

Poor antibiotic choice (toxic)

  • Gentamicin®

A common “rule of thumb” for duration of initial antibiotics is to treat for two weeks past clinical resolution. This may be from four to six weeks of therapy.

Long Term Therapy for Bacterial Skin Disease

    1. Topical— Shampoos are easy and inexpensive methods to reduce the bacterial numbers on the surface of the skin. Oxydex® has been shown to be the most effective in reducing the numbers of bacteria for the longest time. Once to twice weekly applications are necessary in most cases. This may be drying to the skin and Humilac ® or hot oil treatments may be helpful. If the pet is too irritated with this therapy, then Nolvasan® shampoo is recommended on the same schedule. Localized areas can be treated with Bactoderm® cream, Oxydex® or Pyoben gel®.
    2. Extended antibiotics (PULSE THERAPY) — The pyoderma is first brought under control as mentioned above under initial therapy. Then, once the patient is in remission, the antibiotic is given in full daily doses on a one-week-on/one-week-off basis. This is done for two months. If the patient is still in remission, the regime is extended to one-week-on/two-weeks-off. Then after another two months, the number of “drug-free” weeks is increased until the minimum dose frequency is achieved.

Fungal Infections in Dogs

Fungal infections in Dogs are common. There are many types of fungus that infect the skin, but the most common are the ones that cause “ringworm”. Ringworm is not a worm. Ringworm is a fungus that grows only on the epidermis and only on the dead part of the keratinized, horny layer. The hairs in this area that are dead will also be infected. Infection of this layer of skin is related to the immune status of the pet, so young or stressed animals are most likely to have this infection.

The fungal growth of ringworm is in a circular manner with hair loss and a crusty area of skin at the edge of the lesion. A Woods light can be used to make a rough diagnosis. Since only 40% of ringworms react to a Woods light, the most accurate method of diagnosis is to cultivate the fungus on D.T.M. medium.
D.T.M. (Dermatophyte Test Medium) is a test medium (a substance that ringworm will grow in) that has added to it antibiotics to kill bacteria and other substances. The medium is kept refrigerated before use. This culture medium will change color (to red) when an infectious fungus is growing. The sample should be plucked from the edge of the lesion and placed into the medium. Use clean, dry forceps – no alcohol – do not prep the area. The lid should be left ajar and the medium kept at room temperature for the culture to grow.

Treatment of ringworm involves shaving the pet, as the hairs are the mode of transmission, and dips, along with oral medications. Treatment is difficult because the ringworm grows only on dead tissue and it is difficult to treat dead tissue. Four to six weeks of treatment is sometimes necessary.

Ringworm is contagious, primarily to children and aged adults, but anyone can contract it from, or give it to, a pet. Other pets are susceptible and contact should be prevented. Cats can be carriers without showing signs and all cats in a household should be suspect. All cats should have the hair combed with a toothbrush, then the brush contents applied to the D.T.M. for diagnosis. All positive animals are treated.

Ringworm may be infectious to hospital personnel. Equipment used to shave an infected animal should be disinfected with Betadine® or chlorine bleach. The entire area and all hair should be disinfected immediately. REMEMBER! THE HAIR IS THE TRANSMITTING AGENT!

Malassezia Dermatitis in Dogs

A relatively new diagnosis has been made as a cause of chronic dermatitis, mostly in West Highland White Terriers and Cocker Spaniels. This is a yeast infection (Malassezia Pachydermatis) that is growing on the skin of dogs. This yeast is commonly found in the ears but recently has been discovered as a cause of skin disease. The signs are typically on the ventral abdomen, feet, face (around mouth), anterior limbs or posterior thighs. Musty odor and moist dermatitis are the presenting complaints. Oral ketoconazole (Nizoral®) (10mg/kg twice daily for 30 days) is the treatment. KetoChlor® Shampoo contains ketoconazole and should be used weekly for dogs with Malassezia.