Clinical and histopathologic features of dorsally located furunculosis in dogs following water immersion or exposure to grooming products: 22 cases (2005–2013)

Christine L. Cain Department of Clinical Studies-Philadelphia, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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Elizabeth A. Mauldin Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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Abstract

Objective—To describe clinical and histopathologic features of furunculosis in dogs following water immersion or exposure to grooming products.

Design—Retrospective case series.

Animals—22 dogs with skin lesions consistent with furunculosis and a history of water immersion or grooming prior to onset.

Procedures—Information collected from the medical records of affected dogs included signalment, clinical signs, bathing or grooming procedure, diagnostic tests, treatment, and outcome.

Results—German Shepherd Dogs (4/22 [18%]) and Labrador Retrievers (4/22 [18%]) were most commonly affected. Skin lesions, particularly hemorrhagic pustules and crusts, were dorsally located in all dogs and occurred a median of 2 days (range, 1 to 7 days) following water immersion or exposure to grooming products. Twenty (91%) dogs were bathed at home or at a commercial grooming facility prior to lesion onset; 1 dog developed skin lesions following hydrotherapy on an underwater treadmill, and 1 dog developed peri-incisional skin lesions after surgery. Lethargy, signs of neck or back pain, and fever were common clinical signs. Pseudomonas aeruginosa was the most common bacterial isolate from dogs with bacteriologic culture performed on skin samples (10/14). The main histologic feature was acute follicular rupture in the superficial dermis with suppurative inflammation and dermal hemorrhage. Systemic antimicrobial treatment, particularly oral administration of fluoroquinolones, resulted in excellent clinical response in 16 of 22 (73%) dogs.

Conclusions and Clinical Relevance—Acute-onset furunculosis with characteristic clinical and histopathologic features in dogs following water immersion or exposure to grooming products was described. Knowledge of the historical and clinical features of this syndrome is essential for accurate diagnosis and appropriate treatment of affected dogs.

Abstract

Objective—To describe clinical and histopathologic features of furunculosis in dogs following water immersion or exposure to grooming products.

Design—Retrospective case series.

Animals—22 dogs with skin lesions consistent with furunculosis and a history of water immersion or grooming prior to onset.

Procedures—Information collected from the medical records of affected dogs included signalment, clinical signs, bathing or grooming procedure, diagnostic tests, treatment, and outcome.

Results—German Shepherd Dogs (4/22 [18%]) and Labrador Retrievers (4/22 [18%]) were most commonly affected. Skin lesions, particularly hemorrhagic pustules and crusts, were dorsally located in all dogs and occurred a median of 2 days (range, 1 to 7 days) following water immersion or exposure to grooming products. Twenty (91%) dogs were bathed at home or at a commercial grooming facility prior to lesion onset; 1 dog developed skin lesions following hydrotherapy on an underwater treadmill, and 1 dog developed peri-incisional skin lesions after surgery. Lethargy, signs of neck or back pain, and fever were common clinical signs. Pseudomonas aeruginosa was the most common bacterial isolate from dogs with bacteriologic culture performed on skin samples (10/14). The main histologic feature was acute follicular rupture in the superficial dermis with suppurative inflammation and dermal hemorrhage. Systemic antimicrobial treatment, particularly oral administration of fluoroquinolones, resulted in excellent clinical response in 16 of 22 (73%) dogs.

Conclusions and Clinical Relevance—Acute-onset furunculosis with characteristic clinical and histopathologic features in dogs following water immersion or exposure to grooming products was described. Knowledge of the historical and clinical features of this syndrome is essential for accurate diagnosis and appropriate treatment of affected dogs.

Community-acquired skin infections, particularly folliculitis (hot tub folliculitis) or painful nodules on the hands and feet (hot hand-foot syndrome), are well documented in humans as the result of exposure to contaminated swimming pools, hot tubs, whirlpool baths, and saunas.1,2 Infections are generally associated with ubiquitous gram-negative bacteria such as Pseudomonas aeruginosa, and common-source exposure may result in outbreaks of several individuals.1–4

A similar syndrome of acute-onset furunculosis following water immersion or grooming has been described in dogs.5–7 Furunculosis has been proposed as a sequela to follicular trauma from vigorous manipulation of the skin and coat (ie, brushing or coat-stripping), followed by bathing with contaminated shampoos or conditioners, particularly bulk formulations that are diluted for use in commercial grooming facilities. Pseudomonas aeruginosa and other gram-negative bacteria have been isolated most commonly from skin lesions of affected dogs.5 Gross and Crowa reported 2 cases of acute, dorsally located furunculosis in dogs following use of grooming products. In one dog, P aeruginosa was isolated from skin lesions and diluted conditioner; in the other dog, Serratia marcescens was isolated from skin lesions and undiluted shampoo, supporting bacterial contamination of grooming products as the infection source for these dogs.a

Although this clinical syndrome is a recognized entity in dogs, reports of acute-onset, dorsally located furunculosis following water exposure are primarily anecdotal at this time. To our knowledge, no published case series has specifically described furunculosis in dogs with a recent history of grooming or water immersion. The purpose of the study reported here was to determine the clinical, histopathologic, and microbiological features as well as treatment of dogs with compatible clinical lesions of furunculosis and a recent history of water immersion or exposure to grooming products.

Materials and Methods

This study was designed as a retrospective analysis of canine patients with a histopathologic or clinical diagnosis of acute furunculosis following water immersion or exposure to grooming products. Computer searches of the University of Pennsylvania Laboratory of Pathology and Toxicology biopsy database and the Matthew J. Ryan Veterinary Hospital of the University of Pennsylvania patient hospital discharge system were performed for medical records of dogs with a morphological or clinical diagnosis of furunculosis from January 1, 2005, to January 31, 2013. Inclusion criteria consisted of acute onset of skin lesions consistent with furunculosis (papules, pustules, plaques, nodules; with or without erosions, ulcers, and crusts) and a history of water immersion or grooming within 7 days prior to onset of skin lesions.

Medical records review—Medical records were reviewed for the following information: patient signalment, month of onset of clinical signs, location and description of skin lesions, time of lesion onset in days following water immersion or grooming, location of bathing or grooming (home vs commercial grooming facility), shampoo type if used (over the counter vs prescription; medicated vs nonmedicated), history of brushing prior to lesion onset, associated clinical signs, clinical diagnostic tests performed and any abnormal results, results of bacteriologic culture of the skin samples (if performed), histopathologic features (if performed), treatment, and treatment response. For specimens of affected dogs submitted to the University of Pennsylvania Laboratory of Pathology and Toxicology by outside veterinarians, submitting veterinarians were contacted by email, fax, or telephone for additional follow-up information.

Results

Twenty-two dogs met the inclusion criteria. Nine (41%) dogs were castrated males, 3 (14%) were sexually intact males, 9 (41%) were spayed females, and 1 (4%) was a sexually intact female. The median age was 4 years (range, 0.75 to 13 years). Ten breeds were represented, with 4 (18%) German Shepherd Dogs, 4 (18%) Labrador Retrievers, 3 (14%) Golden Retrievers, and 3 (14%) Welsh Corgis. There were 3 mixed-breed dogs. The remainder of the patient population consisted of 1 each of the following breeds: Beagle, Bloodhound, Weimaraner, Jack Russell Terrier, and Rottweiler. Three of 4 German Shepherd Dogs and 1 of 4 Labrador Retrievers were working police dogs.

Onset of clinical signs was perennial, with all months represented except for February and June. The most common month for diagnosis was September (6/22 [27%]), followed by August (3/22 [14%]) and December (3/22 [14%]).

Skin lesion distribution and description—Skin lesions were dorsally distributed on the neck and trunk in all dogs; 1 dog was also affected on the dorsal aspect of the tail. In addition to dorsally located skin lesions, 2 dogs had lesions involving the inguinal region and hind limbs, 1 had lesions on the head and lateral aspect of the abdomen, 1 had lesions on the flanks and lateral aspect of the thorax, and 1 had lesions on the cranial aspect of the thorax.

The most commonly reported skin lesions were hemorrhagic crusts (15/22 [68%]), followed by papules and pustules (10/22 [45%]), plaques (8/22 [36%]), pinpoint ulcers or draining tracts (6/22 [27%]), and nodules (3/22 [14%]) or hemorrhagic furuncles (3/22 [14%]). For 3 dogs, oozing of hemorrhagic to purulent exudate from hair follicle ostia was specifically noted in the medical records. Intense macular erythema surrounded skin lesions along the dorsum of 2 dogs (Figure 1).

Figure 1—
Figure 1—

Photographs of a 6-year-old castrated male Golden Retriever (A and C) and a 1-year-old castrated male Cardigan Welsh Corgi (B and D) that had skin lesions consistent with furunculosis and a history of water immersion or grooming prior to onset. Intense macular erythema surrounded skin lesions found along the dorsum of these 2 dogs. A and C—Hemorrhagic papules, pustules, and crusts surrounded by a zone of intense erythema are evident. These lesions were photographed 3 days after bathing with a prescription cleansing nonmedicated shampoo. Bathing was followed by brushing with an antishedding brush. B and D—Erythematous plaques, pinpoint ulcers draining hemorrhagic and purulent exudate, and crusts are evident. These lesions were photographed 2 days after bathing with an unknown shampoo. Hair has been clipped from the coat to reveal skin lesions.

Citation: Journal of the American Veterinary Medical Association 246, 5; 10.2460/javma.246.5.522

Grooming history—Water immersion or exposure to grooming products occurred a median of 2 days prior to onset of clinical signs (range, 1 to 7 days). Of the 22 dogs, 10 (45%) were bathed at home (Figure 1), 9 (41%) at commercial grooming facilities, and 1 (4.5%) while boarding at a veterinary hospital; 1 dog was immersed in an underwater treadmill 1 day prior to onset of clinical signs, and 1 dog had hair clipped from the dorsal aspect of its coat and its skin scrubbed in preparation for hemilaminectomy 2 days prior to onset of clinical signs. In this dog, lesions were only found on the dorsal portion of the back where hair was clipped surrounding the surgical incision.

The shampoo type was unknown for 12 of 22 (55%) dogs. For the remaining 10 dogs, 5 were bathed with over-the-counter nonmedicated shampoos, and 1 each was bathed with an over-the-counter flea and tick shampoo, a prescription nonmedicated cleansing shampoo, and a prescription antiseborrheic shampoo. One dog was scrubbed with chlorhexidine-based surgical scrub prior to hemilaminectomy. The remaining dog developed lesions after physical therapy on an underwater treadmill and was towel-dried only (no products used). For the dog bathed with a prescription nonmedicated cleansing shampoo, the shampoo was known to be within the manufacturer's recommended use-by date. It is unknown whether the other shampoos were expired or within recommended use-by dates. It is also unknown whether shampoo concentrates were diluted prior to use.

Two dogs were brushed with a commercial brushb meant to remove undercoat to prevent shedding. One dog was brushed immediately prior to bathing; skin lesions were noticed 1 day after brushing and bathing in this dog. The other dog was brushed immediately following bathing; skin lesions were observed 3 days after bathing and brushing in this dog.

Associated clinical signs—Twenty of 22 (91%) dogs had clinical signs of systemic illness in addition to skin lesions. Clinical signs of systemic illness were acute in onset and temporally related to skin lesions in all 20 dogs. Lethargy was reported for 15 of 20 (75%) dogs. Signs of back, neck, or tail pain were reported for 15 of 20 (75%) dogs: 12 dogs had signs of back pain, 2 had signs of neck pain, and 1 had signs of tail pain (the same dog with skin lesions involving the tail). Fever (body temperature range, 39.67° to 41.17°C [103.4° to 106.1°F]) was documented for 14 of 20 (70%) dogs during the physical examination at the veterinary hospital. Ten of 14 dogs had body temperatures > 40.0°C (104°F), and 6 of these 10 dogs had body temperatures > 40.56°C (105°F). Other reported clinical signs of systemic illness included inappetence (10/20 dogs), stiff gait with reluctance to walk upstairs or jump (2/20 dogs), dehydration (2/20 dogs), diarrhea (3/20 dogs), and head tremors (1/20 dogs).

Clinical diagnostic tests and abnormalities—Clinical diagnostic tests, other than histologic evaluation or bacteriologic culture of skin samples with antimicrobial susceptibility testing, were performed for systematic diagnostic evaluation in 14 of 22 (64%) dogs. A CBC with or without serum biochemical analysis was performed for 13 of these 14 dogs, urinalysis was performed for 8 dogs, radiography (vertebral, shoulder, tail, thoracic, or abdominal) was performed for 8 dogs, tick-borne disease testing was performed for 6 dogs, bacteriologic culture of urine was performed in 4 dogs, and abdominal ultrasonography was performed for 3 dogs. One of 14 dogs each underwent Brucella canis serologic testing, resting cortisol concentration determination, semiquantitative canine pancreas-specific lipase testing,c fecal flotation, bacteriologic culture of blood, coagulation panel (including prothrombin time, partial thromboplastin time, d-dimers concentration, and platelet count), CSF evaluation, Cryptococcus latex agglutination test, Neospora immunofluorescence test, and Toxoplasma gondii ELISA.

Laboratory abnormalities were reported in 12 dogs undergoing diagnostic testing. The most commonly reported laboratory abnormality was leukocytosis (10/12 dogs). Leukocytosis was further characterized as neutrophilia in all 10 dogs, with neutrophilia detected in 5 dogs and neutrophilia with a left shift in 4 dogs. Monocytosis was also detected in 4 of the 10 dogs. Other CBC abnormalities in the 12 dogs included lymphopenia in 6; mild thrombocytopenia in 5; mild normocytic, normochromic anemia in 2; and eosinopenia in 1. Serum biochemical analysis revealed abnormal values in the 12 dogs including high alkaline phosphatase enzyme activity in 5, mild hypokalemia in 3, and high alanine aminotransferase enzyme activity, hypophosphatemia, hypocalcemia, and hyponatremia in 1 dog each. The coagulation panel performed in 1 dog revealed mildly elevated d-dimers concentration (0.36 μg/mL; reference limit, < 0.2 μg/mL). Cytologic analysis of CSF from the dog with head tremors revealed mild mixed pleocytosis. Two dogs had splenomegaly and 1 dog had prostatomegaly diagnosed via abdominal radiography or ultrasonography.

Bacteriologic culture of skin samples—Punch biopsy specimens from lesional skin or culture swab samples of the contents of pustules or furuncles were submitted for aerobic bacteriologic culture and antimicrobial susceptibility testing for 14 of 22 (64%) dogs. Pseudomonas aeruginosa was the most common bacterial isolate (10/14 dogs). Pseudomonas aeruginosa was isolated alone on bacteriologic culture of skin in 5 of 10 dogs, there was coisolation of other bacterial species on bacteriologic culture of skin in 5 of 10 dogs (with Staphylococcus pseudintermedius from 1 dog, Staphylococcus epidermidis from 1 dog, Enterobacter cloacae and Staphylococcus epidermidis from 1 dog, and S marcescens from 1 dog). Staphylococcus hominis was isolated alone from 1 patient that was concurrently receiving enrofloxacin. Aerobic bacteriologic culture of samples from the remaining 3 dogs revealed pure growth of Klebsiella oxytoca from the skin of 1 dog, pure growth of Burkholderia cepacia from the skin of 1 dog, and no bacterial growth from the remaining culture, which was performed while the dog was receiving enrofloxacin. All P aeruginosa isolates were susceptible to fluoroquinolones. In the 2 dogs receiving enrofloxacin at the time of bacteriologic culture of skin, the antimicrobial had been prescribed for associated clinical signs of systemic illness prior to the discovery of skin lesions.

Bacteriologic culture was performed for the shampoo used for bathing prior to onset of skin lesions in 2 of 22 dogs. Shampoo concentrate alone was submitted for bacteriologic culture for one of these dogs, and both shampoo concentrate and diluted shampoo for the other dog. No bacteria were isolated from shampoo cultures. For 1 dog, bacteriologic culture was also performed from one of the antishedding brushes used following bathing, with isolation of very few colonies of Bacillus spp.

Cytologic findings—Cytologic evaluation of impression smears from skin lesions was performed for 9 of 22 dogs. Inflammatory cells (neutrophils and macrophages, with or without eosinophils) were observed on impression smears from all dogs. Red blood cells were observed on impression smears from 2 of 9 dogs. Intracellular or extracellular bacteria were observed on impression smears from 6 of 9 dogs; in 3 of these dogs, bacterial cocci were present, and in 3 of these dogs, bacterial rods were present.

Histopathologic findings—Histologic evaluation was performed on punch biopsy specimens of lesional skin in 16 of 22 (73%) dogs. Histopathologic features were similar in all dogs. Typical histopathologic findings included acute follicular rupture with suppurative to pyogranulomatous furunculosis, most pronounced within the superficial dermis (Figure 2). Superficial dermal hemorrhage was also a frequent finding, as was superficial dermal edema and vascular congestion. Epidermal and follicular outer root sheath necrosis with associated erosions, ulcers, and serosuppurative to hemorrhagic crusts was found in several dogs. Bacterial rods were documented within the inflammatory infiltrate on H&E-stained sections in 1 dog that developed skin lesions after hemilaminectomy. Pseudomonas aeruginosa and S marcescens were cultured from skin lesions of this dog.

Figure 2—
Figure 2—

Representative photomicrographs of sections of skin lesions from a dog with furunculosis and a history of water immersion or grooming prior to onset. Typical histopathologic findings included acute follicular rupture with suppurative to pyogranulomatous furunculosis, most pronounced within the superficial dermis. A—Suppurative inflammation and hemorrhage associated with acute follicular wall necrosis and perforation (arrows). H&E stain; bar = 400 μm. B—Higher magnification image showing hemorrhagic and suppurative inflammation in the superficial dermis adjacent to the follicular rupture. H&E stain; bar = 200 μm.

Citation: Journal of the American Veterinary Medical Association 246, 5; 10.2460/javma.246.5.522

Periodic acid-Schiff and Gram staining was performed on sections from all skin biopsy specimens. Results of periodic acid-Schiff staining were negative for fungal organisms in all dogs. Gram staining showed multifocal clusters of gram-negative rods and gram-positive cocci within inflammatory infiltrate and follicular ostia in one dog (the same dog in which bacterial rods were present on H&E-stained sections), large numbers of mixed gram-negative rods and gram-positive cocci within a serocellular crust and small extracellular gram-positive cocci within the superficial dermal inflammatory infiltrate under an ulcer in another dog, and moderate numbers of small gram-positive cocci in areas of hemorrhage and follicular perforation in a third dog. Results of Gram staining were negative for bacteria in all other dogs.

Treatment—Systemic antimicrobial treatment was administered in 20 of 22 (91%) dogs. Antimicrobial selection was made on the basis of bacteriologic culture and antimicrobial susceptibility testing in 11 of 20 dogs; fluoroquinolones (marbofloxacin or enrofloxacin) were prescribed for treatment in all dogs. Antimicrobials were selected in 9 of 20 dogs without bacterial culture results; the most common of these empirically prescribed antimicrobials were fluoroquinolones (marbofloxacin or enrofloxacin; 5/9 dogs) and cephalosporins (cephalexin or cefpodoxime; 4/9 dogs). Marbofloxacin dosage ranged from 3.7 to 6.3 mg/kg (1.7 to 2.9 mg/lb), PO, every 24 hours; enrofloxacin dosage ranged from 10 to 12.2 mg/kg (4.5 to 5.5 mg/lb), PO, every 24 hours. Treatment was unknown for 1 dog, and 1 dog was not treated with antimicrobials. Median duration of antimicrobial treatment was 30 days (range, 5 to 42 days). Treatment duration was unknown for 2 of 20 dogs treated with antimicrobials.

Excellent clinical response to systemic antimicrobial treatment, with full resolution of skin lesions, was documented for 16 of 20 (80%) dogs. Treatment response was unknown for the remaining 4 dogs treated with antimicrobials. Clinical signs such as lethargy, signs of neck or back pain, inappetence, and fever also resolved in all 16 dogs, often within 24 to 48 hours after initiating antimicrobial treatment. One dog had head tremors on initial examination; these were completely resolved at reevaluation 2 weeks after the start of antimicrobial treatment. Hematologic evaluation was repeated after 1 week of antimicrobial treatment in 1 dog that had had mild normocytic, normochromic anemia, mild thrombocytopenia, leukocytosis with neutrophilia and monocytosis, lymphopenia, hypophosphatemia, hypocalcemia, hyponatremia, and hypokalemia; all abnormalities were resolved.

In addition to systemic administration of antimicrobials, 5 of 20 dogs were treated with NSAIDs (carprofen, deracoxib, or aspirin). Of 22 dogs, 5 were treated with tramadol, 2 were treated with diphenhydramine, 1 was treated with famotidine, and 1 was treated with sucralfate. Three dogs were treated with topical antimicrobial treatment in addition to systemic antimicrobials, 2 were bathed twice weekly with medicated shampoos containing ethyl lactate and 4% chlorhexidine, and 1 was treated with 1% silver sulfadiazine cream applied twice daily to lesional skin. Three of 22 dogs received tapering courses of oral administration of corticosteroids (prednisone) at anti-inflammatory dosages (starting dosage, 0.5 mg/kg [0.23 mg/lb], PO, q 12 to 24 hours). One dog was treated solely with prednisone at a starting dosage of 0.5 mg/kg, PO, every 24 hours, tapered over 28 days, with full resolution of skin lesions.

Discussion

To our knowledge, the case series presented here included the largest number of dogs with clinical signs and histopathologic findings of a distinct syndrome of acute, dorsally located furunculosis to date. All dogs in the present case series had a history of water immersion or exposure to grooming products prior to onset of skin lesions, contrasting with a prior report by Hillier et al8 of 7 cases of deep pseudomonal pyoderma in dogs with similar clinical and histopathologic features, but in which a history of bathing or grooming prior to onset could be confirmed for only 2 of 7 dogs. Since the inclusion criteria for the present study required a recent history of water immersion or grooming preceding skin lesion onset, it is possible that a subset of dogs with similar clinical features of this syndrome, but without a history of water or grooming product exposure, were excluded from the patient population. The predilection for skin lesions over the dorsal aspect of the neck and back may be related to typical grooming and bathing behavior by owners or groomers with most vigorous application of shampoo, brushing, or coat-stripping occurring over the dorsum as well as the increased hair follicle density and hair shaft size in this region. Subsequent follicular trauma may then predispose to folliculitis and follicular rupture. By contrast, most humans develop folliculitis following exposure to contaminated water (hot tub folliculitis) in the axillae, groin, perineal region, and other areas where wet bathing suits may lead to prolonged follicular occlusion.4

A limitation of the present study was the failure to identify a source of bacterial infection for these patients. By isolating the same bacterial species from both grooming products and the skin of affected dogs, Gross and Crowa supported bacterial contamination of grooming products as the likely source of infection for these patients. In the present study, a similar association could not be confirmed. Because of its retrospective design, bacteriologic culture of the shampoo (concentrate or diluted shampoo) was performed for only 2 dogs in the present study, with no bacterial growth in both cases. This could indicate another source of bacterial exposure, such as other grooming products, sinks, tubs, faucets, or showerheads, for these patients. It is also possible antimicrobial agents or detergents from the shampoos may have interfered with bacterial growth on routine aerobic bacteriologic culture. Interestingly, 1 dog in the present case series was not reported to be bathed or groomed, but had physical therapy on an underwater treadmill 24 hours prior to onset of skin lesions. Bacterial contamination of water in small enclosures, like hot tubs, whirlpools, or hydrotherapy tanks, is a common source of infection for humans developing postexposure folliculitis.1 In this particular case, towel-drying may have contributed to microabrasions of the skin surface, hair follicle trauma, and subsequent furunculosis.

Previous reports5,6 of furunculosis arising after grooming have suggested an association with do-it-yourself dog washing facilities or grooming parlors that use bulk shampoo concentrates diluted with tap water. Dilution and redilution of shampoos may lead to bacterial contamination. Frequent handling of the same shampoo bottle over time may increase the opportunity for contamination, whether from tap water or another source. In the present study, nearly 50% of affected dogs were bathed at home. Because of the retrospective nature of the present study, it was not possible to ascertain how many dogs were bathed at commercial self-serve facilities or whether shampoos or conditioners used for bathing at home or at grooming parlors were diluted with tap water prior to use. Similarly, only 1 shampoo used for at-home bathing could be confirmed to be within the manufacturer's recommended expiration date. Expired shampoos or conditioners may also be more likely to be contaminated by bacteria. An association of furunculosis development with vigorous brushing or coat-stripping (alone or prior to bathing) has also been suggested.5,a Brushing history was unknown for most dogs in the present case series, although 2 dogs were brushed prior to or following bathing with a commercial anti-shedding brush, and 1 had its hair clipped in preparation for surgery. Presumably, aggressive brushing or hair clipping may also cause follicular trauma and increased susceptibility to opportunistic infection following exposure to contaminated grooming products or water sources.

As has been suggested previously,5 most dogs included in the present case series were large-breed dogs with thick wiry or short coats. Some smaller dogs with short, bristly coats (Beagles and Jack Russell Terrier) were also represented in the patient population. Short- or wire-haired dogs may be predisposed to follicular perforation by thick or coarse hair shafts following trauma from vigorous bathing or brushing.5,6 German Shepherd Dogs and Labrador Retrievers comprised most of the patient population, although major conclusions about breed predisposition are precluded by the fact that several of these patients were working police dogs. Working dogs are frequently evaluated at the authors’ institution through a contractual arrangement.

Most dogs in the present case series had other constitutional signs, particularly signs of back or neck pain, fever, and lethargy. In several dogs, evaluation of signs of back or neck pain and fever preceded discovery of skin lesions. It should be mentioned that alopecia was not a typical clinical feature in these dogs. Skin lesions are often masked by the thick dorsal coat until the hair is clipped; clipping of the coat or collection of skin biopsy specimens may necessitate sedation or anesthesia to minimize discomfort. Without knowledge of skin lesions, initial differential diagnoses in patients with fever and signs of back or neck pain included diskospondylitis and meningitis. Results of the present study suggested that dorsal furunculosis should be considered as a differential diagnosis for any dog with fever of unknown origin or signs of back or neck pain. Owners should be asked specific historical questions concerning recent swimming, bathing, or grooming, and the skin overlying the neck and back should be examined closely for maculopapular, pustular, or crusted lesions prior to performing diagnostic tests such as radiography. Although all of the dogs included in the present study had a history of water immersion or grooming prior to onset of clinical signs, deep pyoderma associated with P aeruginosa has been previously described as a cause of dorsally located furunculosis and signs of back pain, even in dogs without a reported history of bathing or grooming within the 48 hours preceding the onset of skin lesions.8 Signs of systemic illness, such as headache, gastrointestinal upset, sore throat, and lymphadenomegaly, may also be seen in humans with pseudomonal folliculitis secondary to contaminated water-source exposure.2

In addition to constitutional signs, several affected dogs also had clinical laboratory abnormalities. The most common hematologic abnormalities were leukocytosis, characterized by neutrophilia with or without a left shift and monocytosis; lymphopenia; and mild thrombocytopenia. Taken together, these hematologic abnormalities, along with fever in several affected dogs, are potentially consistent with early systemic inflammatory response syndrome. Lipopolysaccharide, a component of the cell membrane of gram-negative bacteria, may play a major role in inciting the proinflammatory cascade in patients with gram-negative bacterial infections.9,10 In dogs in the present study that underwent bacteriologic culture of skin samples, gram-negative bacteria, particularly P aeruginosa, were isolated most commonly. In healthy dogs receiving infusions of lipopolysaccharide to model endotoxemia, release of proinflammatory and fever-inducing cytokines, such as tumor necrosis factor-α, interleukin-1, and interleukin-6, occurs within the first 3 to 6 hours.9,10 Progressive thrombocytopenia is a common change; with time, coagulation abnormalities may occur.9,10 One of the earliest markers of coagulation abnormalities in systemic inflammatory response syndrome is elevation of d-dimers concentrations.10 d-dimers are cross-linked products of fibrin breakdown; increased production of d-dimers may be found in a variety of diseases that result in fibrin formation and degradation (including inflammatory disease and hemorrhage).11 Concentration of d-dimers was assessed in only 1 dog in the present case series (a 6-year-old spayed female Labrador Retriever bathed at a commercial grooming facility 1 day prior to initial evaluation at the veterinary hospital) and found to be mildly high. This dog also had other evidence of a serious systemic inflammatory response, including fever (41.17°C [106.1°F]), neutrophilia with a left shift, and mild thrombocytopenia. Pseudomonas aeruginosa was isolated from this dog's skin lesions, lending support to lipopolysaccharide-associated systemic inflammatory response syndrome.

Pseudomonas aeruginosa was the most common bacterial isolate from skin lesions of affected dogs in the present study, either alone or in combination with Staphylococcus spp or other gram-negative bacteria. Pseudomonas aeruginosa is ubiquitous in the environment and is found most commonly in soil and water. Opportunistic infection of the skin can occur following colonization with exposure to contaminated water sources and abrasions of the skin or through hair follicle ostia.1 Contamination of water sources, such as swimming pools and hot tubs, may be more likely to occur with increased water temperature, high pH, low chlorine concentration, or poor disinfection.1,3 However, P aeruginosa has been commonly isolated from swimming pools and hot tubs even when chlorination is considered adequate.3 Piping or fixtures that are small or difficult to disinfect, such as sinks or showerheads, may also predispose to P aeruginosa contamination by facilitating biofilm formation.3

In addition to P aeruginosa, S marcescens was isolated from the skin lesions of 1 dog, which developed skin lesions following hair clipping and scrubbing for a hemilaminectomy. Serratia marcescens is a gram-negative facultative anaerobe that can also be found commonly in water and soil and has been isolated from skin lesions and shampoo of a dog with acute, dorsally located furunculosis following bathing.a In humans, opportunistic nosocomial infections have been reported, particularly in immunocompromised patients.12,13 In dogs and cats, S marcescens has been implicated as a contaminant of indwelling IV catheters.14,15 In a study15 of widespread bacterial contamination of IV catheters and other sites throughout a veterinary hospital, contamination was traced to a common source of aqueous benzalkonium chloride solution stored with cotton sponges. In the infection described in that study,15 the chlorhexidine-based surgical scrub used following clipping of hair at the surgical site may have been the source of P aeruginosa and Serratia marcescens contamination, considering that both organisms may survive at room temperature in areas of high humidity.15 Pseudomonas aeruginosa has been documented as a contaminant of antimicrobial hand soaps and soap dispensers in human hospitals.16–18 Isolates obtained from soap dispensers containing chlorhexidine-based products may be multidrug resistant and resistant to chlorhexidine; biocide resistance is intrinsic in Pseudomonas spp and other gram-negative organisms.18

Like Pseudomonas spp, Burkholderia spp are also commonly found in water and soil. Although infection is poorly documented in veterinary patients, B cepacia is an important cause of opportunistic infections in humans, particularly severe respiratory infections in cystic fibrosis patients.19 To the authors’ knowledge, this is the first report of B cepacia as a cause of furunculosis in a dog. Contamination of commercial ear cleansing solutions used for home treatment of dogs with B cepacia has been documented, supporting the organism's ability to survive in liquids.20 In another dog described in the present study, only K oxytoca was isolated from skin lesions. To the authors’ knowledge, this organism has not been previously described as a cause of furunculosis following water immersion but has been rarely associated with necrotizing fasciitis in humans.21,22

Histopathologic features of furunculosis were similar in skin biopsy specimens from affected dogs, including superficially oriented acute outer root sheath rupture with suppurative inflammation, superficial dermal hemorrhage, and epidermal and follicular outer root sheath necrosis. Although other differential diagnoses for folliculitis and furunculosis, including dermatophytosis, should be considered, the acute and superficial nature of the furunculosis, dermal hemorrhage, and characteristic lesion distribution (dorsal aspect of trunk and midline) should increase the suspicion for furunculosis following water immersion or grooming product exposure among clinicians and pathologists.6 Results of Gram staining of skin biopsy specimens were negative in most cases, illustrating the limitations in identifying bacteria, particularly gram-negative bacteria, within heavily inflammatory skin lesions by histologic evaluation. Bacteriologic culture and antimicrobial susceptibility testing of lesional skin is recommended in all cases of furunculosis following water immersion or grooming, even in cases with negative findings on Gram staining of histologic sections.

Most dogs in the present study were treated systemically with antimicrobials, with excellent clinical response. In several dogs, improvement in attitude, appetite, and fever was observed within 24 to 48 hours after initiating antimicrobial treatment. As has been reported for dogs with pseudomonal pyoderma,8,23 fluoroquinolones administered orally for approximately 4 weeks had excellent clinical results in the present study. Bacteriologic culture and antimicrobial susceptibility testing is recommended to base antimicrobial treatment in cases of suspected pseudomonal pyoderma. Although all P aeruginosa isolates from dogs in the present study were susceptible to fluoroquinolones, Pseudomonas spp isolates frequently have resistance to multiple antimicrobials as the result of altered cell membrane permeability, drug efflux pumps, and production of β-lactamases and cephalosporinases.3 A recent study3 demonstrated isolation of multidrug-resistant P aeruginosa from privately owned hot tubs and swimming pools. In addition to the unpredictable antimicrobial susceptibility of gram-negative organisms, bacteriologic culture and antimicrobial susceptibility testing is also indicated, given that bacterial furunculosis is a form of deep pyoderma. Although most dogs in the present study were clinically normal within 4 weeks of antimicrobial treatment, antimicrobial treatment should be extended 1 to 2 weeks beyond clinical resolution of deep pyoderma and may be required for 6 to 8 weeks or more.24

Interestingly, several dogs, particularly those that did not have bacteriologic culture and antimicrobial susceptibility testing performed on samples from skin lesions, were reported to be treated successfully with cephalosporins (cephalexin or cefpodoxime) that typically have poor antipseudomonal activity.25 None of the P aeruginosa isolates from dogs in the present case series were susceptible to cephalexin or cefpodoxime. In humans, pseudomonal folliculitis is also frequently self-limiting and may resolve without treatment.2,4 The skin lesions of these dogs may have self-resolved regardless of the antimicrobial administered. In 1 dog, cephalexin and a tapering course of prednisone at an anti-inflammatory dose were started pending bacteriologic culture and antimicrobial susceptibility testing results. Bacteriologic culture and antimicrobial susceptibility testing from skin lesions isolated P aeruginosa resistant to cephalexin and methicillin-susceptible S pseudintermedius. In this dog, skin lesions were so improved with initial treatment that cephalexin was continued for 4 weeks while prednisone was tapered. In this dog, the dramatic clinical response may have occurred with antimicrobial treatment targeting S pseudintermedius or may be related to prednisone administration. Another patient was treated with only a tapering course of prednisone at an anti-inflammatory dose, despite clinical and histopathologic features typical of bacterial furunculosis following physical therapy on an underwater treadmill. In these dogs, systemic treatment with corticosteroids may have reduced inflammation associated with hair follicle rupture and the foreign body response to keratin, facilitating spontaneous resolution of bacterial infection and skin lesions. Because deep pyoderma typically requires extended systemic treatment with antimicrobials24 and bacteriologic culture and antimicrobial susceptibility testing was not performed in the latter dog, sterile furunculosis secondary to manipulation of the skin and coat cannot be ruled out for this dog or, possibly, other dogs included in the present case series. Cytologic assessment of impression smears and bacteriologic culture and antimicrobial susceptibility testing from skin lesions should be performed in all dogs with acute-onset furunculosis both to establish a definitive diagnosis of deep bacterial infection as well as to provide targeted antimicrobial treatment.

a.

Gross TL, Crow DW. Postgrooming furunculosis due to Serratia marcescens and Pseudomonas aeruginosa in two dogs (abstr), in Proceedings. 21st Annu North Am Dermatol Forum 2006;176.

b.

Furminator, United Pet Group Inc, Cincinnati, Ohio.

c.

SNAP cPL test, Idexx, Westbrook, Me.

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