Furunculosis in dogs – painful skin lumps and treatment

Overview

A hair follicle ruptures deep in the skin

A hair follicle is the structure in the skin from which a hair grows. An inflamed lump called a furuncle can form when a hair follicle ruptures within the skin. Hair and keratin, the structural material in hair, entering the tissue can sustain a foreign body reaction. Bacterial furunculosis is classified as deep pyoderma, a bacterial skin infection.

Deep infection is more serious than superficial skin infection because it extends into the dermis and can spread into the bloodstream. Lesions may be localised or spread widely across the body and limbs. Furunculosis lesions can also occur in the armpits, on the chest and around the hips. In the rare eosinophilic form, white blood cells called eosinophils are prominent in the inflammation. The initial inflammation may be sterile, meaning that no bacterial infection is identified.

Causes

Skin damage and secondary bacterial infection

Progression of superficial folliculitis can cause an inflamed hair follicle to rupture. A bite, cut or penetrating foreign body can also carry bacteria deep into the skin. Repeated pressure and friction can damage hair follicles around the elbows and hocks, for example. Licking can perpetuate damage and encourage microbial overgrowth.

An underlying cause may be an allergy, demodicosis, a disease caused by Demodex mites, or a hormonal disorder. In furunculosis between the toes, paw conformation and weight bearing can be central factors. Hairy skin bearing weight can thicken into a false pad, with hair follicles that become blocked and rupture.

The form that follows bathing or grooming

Post-grooming furunculosis is deep bacterial folliculitis associated with bathing or grooming. Clipped or vigorously brushed skin can be exposed to opportunistic bacteria that take advantage of skin damage. The cause is often Pseudomonas aeruginosa. Serratia marcescens has also been found in dogs and washing products.

Contaminated shampoo or another washing product may be the source of infection. Similar infections have also occurred after swimming in natural water and using an underwater treadmill. The post-grooming form is rare considering how frequently dogs are bathed.

The eosinophilic form

The exact mechanism behind eosinophilic furunculosis is unknown. The distribution of lesions and tissue findings suggest a possible hypersensitivity reaction to bites from insects or other arthropods. The treatment approach therefore differs from that for an ordinary bacterial infection.

Risk factors

Repeated skin damage, itching and licking predispose dogs to deep skin lesions. Allergic skin disease is a common underlying cause. Demodex infestation, an underactive thyroid and Cushing's syndrome are also investigated according to the pattern of signs.

The short, stiff hairs and wide spaces between the toes of breeds such as the Shar Pei, Labrador Retriever and English Bulldog can predispose them to paw furunculosis. Excess weight and an abnormal gait increase the load on hairy paw skin. This paw-related risk does not mean the same predisposition to every form of furunculosis.

The eosinophilic form is described particularly in large dogs aged 2–5 years. No clear sex predisposition has been described. Many affected dogs have spent considerable time outdoors, for example in parks and gardens.

Typical symptoms

Typical signs of deep bacterial infection include pain, crusting, odour and discharge of blood and pus. The location of lesions can help identify the form of the disease. You may notice:

• Red or bluish-red skin lumps.
• Swelling and tenderness to touch.
• Hairless, broken or ulcerated areas of skin.
• Openings in the skin draining pus mixed with blood.
• Crusts formed from dried discharge.
• Paw licking and pain when walking if the lesion affects a paw.

A deep furuncle may look like a reddish or bluish bump more than 3 millimetres across. Signs of the post-grooming form usually begin within 24–48 hours. Lesions often concentrate over the upper neck, back and trunk. A dog may show signs of general illness before obvious skin lesions develop.

The eosinophilic form often starts rapidly with swelling, lumps and ulcers on the bridge of the muzzle. Lesions can also appear around the eyes, on the ear flaps or elsewhere on the body. The degree of itching and pain varies between dogs.

Complications and consequences

Scarring and recurrence

Severe deep skin infection can heal with scarring. Long-standing lesions can leave the skin darker and thicker. A foreign body reaction to hair keratin can prolong paw inflammation. Recurrence is often sustained by an undetected skin disease or another underlying factor. An unsuitable medicine or missed follow-up examinations can also lead to treatment failure.

Serious systemic infection and outlook

Deep bacterial infection can spread into the bloodstream. In an individual case, post-grooming furunculosis was associated with sepsis and a widespread clotting disorder. Most post-grooming cases nevertheless respond well to treatment. Permanently altered paw conformation also affects the outlook for chronic paw furunculosis.

In a reported eosinophilic case, the active skin lesions had settled within about 3 weeks, but scarring and hair loss remained. The same dog remained free of signs during more than 5 months of follow-up. One dog's recovery time does not define how long every form of furunculosis lasts.

Diagnosis at the vet

History and examination of all the skin

Your vet examines the type of lesions and their distribution across the body. Long or matted hair may be clipped to reveal the extent of the skin lesions. For paw lesions, the underside between the pads and the weight-bearing skin are also examined. Excess weight, an abnormal gait and possible joint disease are assessed where necessary.

If the post-grooming form is suspected, your vet asks when bathing, swimming, grooming and brushing took place. How washing products were diluted and stored, and whether other bathed animals have signs, may also help the investigation.

Cytology helps identify the nature of inflammation

A cell sample is taken from the skin lesions before antimicrobial treatment starts. It is examined under a microscope for inflammatory cells, bacteria and possible yeasts. A deep lesion may require a needle sample or tissue biopsy. This obtains a sample from the inflamed tissue rather than the skin surface.

Staphylococcus pseudintermedius accounts for around 60% of the bacteria causing deep bacterial skin infections. In superficial pyoderma, its share exceeds 90%. Other bacteria and mixed infections are therefore important when choosing medicines for deep infection. A deep bacterial infection is sampled for culture and susceptibility testing to select a suitable antibiotic.

A cell sample consistent with the eosinophilic form contains numerous eosinophils without identified microbes. A biopsy may show eosinophilic inflammation concentrated around hair follicles and follicular rupture. A persistent ulcerated eosinophilic lesion can also develop a secondary infection.

Looking for the underlying cause

Skin scrapings can investigate Demodex mites, while hair or fungal samples can investigate ringworm. Allergies, hormonal diseases and tumours are also considered in the investigation plan. Diseases resembling the eosinophilic form on the muzzle include bacterial infection, demodicosis and juvenile cellulitis in puppies. Treatment decisions depend on the signs, cytology, culture and, where necessary, a biopsy.

Treatment

Treating bacterial infection

Confirmed deep bacterial furunculosis usually needs a systemic antibiotic, which acts throughout the body. The medicine is selected using bacterial culture and susceptibility testing. This helps avoid a medicine that is ineffective against the particular bacterium. Your vet prescribes treatment tailored to the nature of the infection and your dog's other health needs.

Cytology from the same skin lesion helps interpret the significance of the culture result. A superficial swab may miss the organism responsible for a deep infection. A very ill dog may need treatment before the culture result is available. The choice of medicine is reviewed once the result arrives.

Topical care and pain relief

Topical antiseptic care supports treatment of deep bacterial infection. It starts once the affected area can be handled without pain. Pain relief is always considered when skin lesions are painful. Shampoos and wipes can remove crusts, discharge and microbes from the skin surface. The formulation is chosen according to the extent of lesions, the coat and how readily treatment can be applied.

Topical care often uses a chlorhexidine product suitable for animals, for example. Antiseptic products are generally preferred to topical antibiotics. Yeast overgrowth may also need separate attention. Surgery may be considered for severe chronic paw lesions. Surgery does not remove the need to manage the underlying skin disease.

The underlying cause and eosinophilic form

Treating an allergy, Demodex mites or a hormonal disease reduces recurrence. The aim is usually to bring deep bacterial infection under control before starting treatment that suppresses the immune response. Inappropriate corticosteroid use can underlie recurrent bacterial infections.

In confirmed eosinophilic furunculosis, however, the main drug treatment is a systemic glucocorticoid. An antibiotic is added if cytology or culture identifies a secondary bacterial infection. This highlights the importance of sampling before treating a skin lump on appearance alone.

Follow-up examinations guide further treatment

The first follow-up examination for a deep infection is usually arranged at around 3 weeks. If infection persists but is improving, it is reassessed at roughly 2-week intervals. Alongside examining the skin lesions, your vet checks whether cytology still shows bacterial infection. If improvement stops and bacteria remain visible, another culture may reveal drug resistance.

A remaining lump may represent sterile inflammation or a lesion caused by a chronic underlying disease. An antibiotic is therefore not automatically continued simply because the skin does not yet look completely normal. Topical antiseptic maintenance care may continue if the risk of recurrence remains high.

Prevention

Shampoo requiring dilution is prepared on the day of use. Leftover diluted products are not saved for the next bath. Bathing equipment, brushes and combs are kept clean. Vigorous hand stripping or rough brushing can damage the skin before bathing. After such grooming, bathing may be postponed for about 2 weeks to avoid skin damage.

Long-term management of allergies and other diseases predisposing to skin infection helps prevent recurrence. Softer surfaces for resting and moving around can reduce friction at pressure points. Weight management in an overweight dog reduces the load on fragile skin. Where paw conformation contributes to the problem, an abnormal gait and any joint problems are also addressed.

Licking of affected skin is addressed because it perpetuates damage and microbial overgrowth. Recurrent infections may require topical maintenance care planned by your vet.

Important for dog owners

Record when the lumps appeared and which areas of skin are affected. Tell your vet about bathing, swimming, grooming and every product used. Also mention whether other bathed dogs in the household have signs. You may find it helpful to photograph the same areas between follow-up visits.

Do not squeeze, puncture or dig into skin lumps at home. Pain may prevent topical care, in which case handling is replanned with pain relief. Monitor appetite, alertness and fever as well if the skin infection causes general illness. Rapid deterioration requires an emergency assessment.

Agree a follow-up schedule and check which signs mean you should stop topical care and contact the practice. Treatment success is assessed using the skin and cell samples. A remaining scar or hair loss does not necessarily mean bacterial infection persists.

Frequently asked questions

Does furunculosis always affect the paws?

No. Furunculosis can also affect skin elsewhere, such as the chin, muzzle or back. The form associated with paw conformation is one localised manifestation. The location, appearance of lesions and samples help distinguish bacterial infection from other deep skin lesions.

Why did my dog develop skin lumps after a bath?

In post-grooming furunculosis, tiny skin injuries can allow bacteria into hair follicles. Signs often develop within 24–48 hours of bathing or vigorous grooming. Bacterial overgrowth in old or previously diluted shampoo may be responsible.

Does every furuncle need an antibiotic?

Deep bacterial furunculosis is usually treated with a systemic antibiotic and topical care. In the rare sterile eosinophilic form, treatment instead centres on controlling the inflammatory reaction. The need for an antibiotic in that form depends on whether cytology or culture shows secondary infection.

Why might a sample be taken from deep in the tissue?

A swab of the skin surface may miss the cause of a deep infection. A needle sample or biopsy may therefore be taken directly from the inflamed tissue within the lump. Cytology and culture from the same lesion are compared so that medication targets the identified infection.

Why does furunculosis return after treatment?

An untreated allergy, demodicosis or another underlying disease may sustain recurrence. Hair keratin remaining within the skin can also perpetuate inflammation in the paws. Follow-up therefore investigates whether there is a new bacterial infection, a persistent deep infection or sterile inflammation.