Pediatric dermatology (Proceedings)

Normal pediatric integument

The skin and hair of newborn dogs and cats undergoes significant change between birth and six months of age. The thicknesses of the epidermis and dermis increase two to threefold as the skin matures. During the same period, the integument as a percentage of body weight decreases from 24% to 12% in the dog. Meanwhile, there is a rapid replacement of reticulum fibers composed of Type III collagen by mature Type I collagen fibers in the dermis. These collagen fibers, as well as elastic fibers, increase in size and number during the first months of life.

The coat of kittens and puppies consists principally of fine hairs. At 12 to 16 weeks of age, hairs begin to thicken and decrease in curvature in a breed-specific manner, giving rise to the adult-type coat. Pigment of the skin and hair continues to develop into the adult phenotype until approximately 3-6 months of age.

Signs of congenital and hereditary skin disorders are most often observed during the first 2-3 months. In many cases, the genetic defect resulting in the disorder has not been fully characterized. Examples of hereditary skin disorders include black hair follicular dysplasia, ichthyosis, congenital hypotrichosis, and cutaneous asthenia.

Young animals may be predisposed to infectious skin diseases due to either an immature immune system or a hereditary primary immunodeficiency. As the epidermal thickness increases and the protective function of the skin improves, puppies and kittens become less susceptible to certain viral, bacterial and fungal infections.

Mucocutaneous viral papillomas are common in puppies. Smooth white lesions quickly progress to grey verrucous nodules that are often pedunculated. They are seen most often in the oral cavity and on the lips, but may also occur on haired skin and conjunctiva. It is believed that various papilloma virus types have site predilections. The virus appears to be spread most often by direct contact, but can survive for 2 months in the environment. There is a 1-2 month incubation period. Diagnosis can most often be made by clinical recognition in a puppy. Histopathology, if performed, reveals a hyperplastic and hyperkeratotic epidermis.

Papillomas typically regress in 2-3 months without therapy. In cases that fail to resolve or when treatment is otherwise warranted, cryotherapy is the treatment of choice. At least two freeze-thaw cycles are suggested. As long as the majority of lesions are frozen, all generally resolve, presumably by stimulating a host immune response.

Impetigo and neonatal pyoderma

Staphylococcus impetigo is a non-pruritic superficial infection recognized commonly in puppies and rarely in kittens. Impetigo is characterized by non-follicular pustules that occur most commonly in the non-haired skin of the ventral abdomen. In contrast, in the adult dog, impetigo often results in larger pustules that span follicular units and is commonly associated with immunosuppression.

Neonatal pyoderma may be seen in puppies 2-4 weeks of age. Multiple puppies in a litter are often affected with pustules, erythematosus macules, and alopecia. The ventrum is most often affected, but widespread involvement may occur.

Cytology of the pustules will demonstrate neutrophils and intracellular cocci. Bathing every 3-7 days with a gentle shampoo containing an antibacterial ingredient such as chlorhexidine or triclosan is often sufficient to resolve impetigo. In severe cases that fail to respond to topical therapy, a 14 day course of a systemic antibiotic (amoxicillin-clavulinic acid or cephalexin) is indicated.

Dermatophytosis

Young animals carry an increased risk of developing dermatophytosis, reflecting their immature immune status and potential for exposure to carriers. Microsporum canis is the most frequent cause of ringworm in kittens and puppies. Entire litters can develop lesions, typically multifocal alopecic, mildly erythematous patches that progressively develop papules, scale and hyperpigmentation. Frequently affected areas of kittens and puppies include the head, muzzle, pinnae, and distal limbs.

Approximately fifty percent of M. canis infections will cause hairs to fluoresce with ultraviolet (Wood's) light examination. Microscopic examination of plucked hairs for ectothrix fungal spores also provides a rapid method of confirming the diagnosis. Incubating the hair for 15-30 minutes in KOH digests the keratin and may aid in visualization of the spherical spores. Fungal cultures should be performed to identify the species of ringworm.

In general, treatment of pediatric patients should be limited to topical products until they are approximately 16 weeks of age. Topical application of 2% lime-sulfur dip every 5-7 days is suggested. A protective collar is applied until the patient is dry, to prevent excessive ingestion through grooming. Care should be taken to ensure that body temperature is maintained after applying a full-body dip to a pediatric patient. Small, localized lesions can also be treated with topical clotrimazole or terbinafine creams, applied daily. When possible, affected animals should be isolated and the patient's environment cleaned with 0.5% bleach. Caretakers should be educated regarding the zoonotic potential of dermatophytosis.

Treatment is continued until there is both a clinical resolution and a microbiological cure, as determined by follow-up fungal culture. Once the patient reaches 16 weeks, systemic therapy can be considered, following recommendations for adult dogs and cats. Oral itraconazole (5-10 mg/kg/day), fluconazole (5 mg/kg/day), terbinafine (30-40 mg/kg/day), or microsized griseofulvin (50-100 mg/kg/day) are commonly prescribed.

Demodicosis

Demodex canis, the common mite species causing demodicosis in dogs, is acquired by pups from their dams within the first days of life. As normal skin inhabitants, the mites reside in hair follicles without causing cutaneous changes in most dogs. Commonly, young dogs develop localized demodicosis, defined by convention as 5 or fewer lesions. These are non-pruritic, alopecic lesions that often develop on the face or limbs, although other areas may be involved. Generalized demodicosis is diagnosed when more than 5 lesions are present. This is not a hard and fast rule, as several extensive lesions would also warrant a diagnosis of generalized demodicosis. Purebred dogs are at increased risk of generalized demodicosis. Breeds reportedly predisposed include the American Staffordshire Terrier, Shar Pei, English Bulldog, French Bulldog, and a number of terrier breeds.

Diagnosis is readily made with multiple deep skin scrapings. The skin is pinched to force mites to the surface and the area is scraped with a mineral oil-coated scalpel blade or stainless steel spatula until capillary bleeding is observed. A 4x objective provides sufficient magnification to recognize mites easily and speeds scanning of the entire slide. Mites are more easily visualized with the condenser lowered into a position that increases contrast. Although occasional mites can be found on normal dogs, in the presence of characteristic lesions, skin scrapings with multiple mites observed should be considered positive.

Treatment of juvenile-onset localized demodicosis is usually unnecessary as most cases respond without therapy. Topical treatment with daily benzoyl peroxide gel or rotenone ointment (Goodwinol®) is of unproven efficacy, but may provide some benefit while carrying little risk. Patients that don't resolve in 2-3 months require more aggressive therapy.

The safety of some of the products used to treat generalized demodicosis has not been established in dogs less than four months of age. In cases of generalized demodicosis that are diagnosed in dogs younger than four months, weekly topical therapy with benzoyl peroxide-containing shampoo is recommended until the patient is old enough to tolerate generalized therapy safely. Thereafter, treatment with amitraz dips (every 7-14 days), oral ivermectin (0.3-0.6 mg/kg/day), or milbemycin (1-2 mg/kg/day) follows the recommendations for adult dogs and is continued until clinical resolution and failure to demonstrate mites twice by skin scrapings performed four weeks apart. Recently, a spot-on product containing amitraz (Promeris®; Fort Dodge Animal Health) has been approved for the treatment of demodicosis in puppies older than 8 weeks of age.

Sarcoptic mange

Due to the highly contagious nature of Sarcoptes scabiei var. canis infestation, entire litters of pups may be presented with typical lesions. These include a ventrally-oriented papular rash, crusted and alopecic pinnal margins, and excoriations of the lateral elbows. Scratching usually is elicited by rubbing the ear margin. Pups are often exposed by an infested bitch. The mite has a 21-day life cycle. Once infested, pruritus usually begins within several days and steadily worsens over a period of weeks. Superficial skin scrapings covering broad areas of affected skin may reveal the mites, characterized by unjointed pretarsi with terminal suckers. Looking for the mites' rapid movement on the slide helps identify them among the copious keratinaceous material, crusts, and hair often collected on the scrapings. However, negative skin scrapings are common, especially in early infestation, necessitating therapeutic trials when clinical suspicion warrants. A response to treatment confirms the tentative diagnosis.

Selamectin provides a convenient and effective treatment of sarcoptic mange for pups over 6 weeks of age. Two doses four weeks apart are normally effective, although more frequent applications are often recommended in adults with severe infestations. Lime sulfur dips applied every 5-7 days are a safe and effective alternative therapy for young puppies. If caretakers are affected, their lesions most often resolve with treatment of the patient.

Cheyletiellosis

Cheyletiella spp. mites may infest puppies, kittens, and rabbits. The three species of mites are not highly host-specific. The mites complete their life cycle on the host, with eggs attached to hairs. These large mites are recognized by their hooked mouth parts. Pruritus varies from absent to moderate. Dog and cats may not display any lesions or may be heavily scaled and crusted, particularly over the dorsum. Mites may transiently infest humans resulting in pruritic papules, often on the abdomen and forearms, similar to sarcoptic mange.

Superficial skin scrapings, trichograms (microscopic hair examination), and acetate tape preparations are used to demonstrate mites or eggs. All in-contact animals must be treated. Selamectin or fipronil are applied every two weeks 3-4 times. Alternatives, though less effective, include dips, powders, or sprays containing lime-sulfur, pyrethrin, carbaryl, or organophosphates. Lime-sulfur dips are the safest option when the age of the kitten or puppy precludes the use of other insecticides. In heavy infestations, the home environment should be treated with an insecticide to minimize zoonosis.

Pediculosis

Lice are host-specific, obligate parasites. Infestations of young dogs and cats are commonly associated with poor nutrition, overcrowding, and direct transmission from infested animals. Biting lice (Trichodectes canis and Felicola subrostratus) are recognized by their relatively broad heads, as compared to those of sucking lice (Linognathus setosus).

Infested pediatric patients display variable pruritus, progressive alopecia, and scale. Matting, erythematous papules, and anemia may develop in severe infestations. Lice and their eggs (nits) may be grossly visualized.

Treatment should be directed at all in-contact dogs or cats. Selamectin administered every 2 weeks for four treatments is safe, effective, and convenient. Alternatives include spot-on formulations, dips, powders, or sprays containing imidocloprid, fipronil, lime-sulfur, pyrethrin, carbaryl, or organophosphates, following manufacturer recommendations. The environment, bedding, and grooming tools of infested animals should be cleaned or replaced.

Fleas

Puppies and kittens are very susceptible to infestations with the cat flea, Ctenocephalides felis. Due to the flea's high reproductive potential, heavy infestations can develop rapidly. Litters of puppies and kittens can quickly become infested when the bitch or queen is not maintained on preventative flea control. Young animals may develop pruritus, erythematous papules, scale, alopecia, and excoriations. The severity of pruritus is often inversely related to age, as hypersensitivity to fleas develops over time. Areas that are commonly affected are the neck and ventral abdomen regions of cats and the ventral abdomen and dorsal lumbar regions of dogs.

Heavy infestations can lead to severe, life-threatening anemia, particularly in pediatric patients with relatively low blood volumes. Further, young kittens do not yet groom appreciably, a mechanism that helps reduce the flea burden of mature cats. Intensive care, including blood transfusions, may be required in severely anemic animals.

Flea products with excellent adulticidal activity are now available for use on puppies and kittens. Spot-on products are generally approved for kittens 8 weeks of age or older and for puppies 6-8 weeks or older. Nitenpyram, available as an oral tablet, can be used in animals as young as 4 weeks and has a very rapid kill, appropriate for heavy infestations. For animals younger than the minimum approved age of these products, manual removal of fleas with a fine-toothed comb is recommended.

Juvenile cellulitis

Also referred to as "puppy strangles," juvenile cellulitis is an idiopathic dermatitis and lymphadenitis, most commonly affecting 3 to 16 week old puppies. Dachshund, golden retriever, and Gordon setter breeds are predisposed. Attempts to demonstrate organisms or transmit the disease have been unsuccessful. Erythematous papules and pustules develop on the lips, muzzle, eyelids and ear canals. Submandibular lymphadenitis is profound. Pinnal edema, alopecia, and erythema may be the first lesions noticed.

Most pups respond well to 2.2 mg/kg prednisone daily. Within several days to a week, erythema, swelling and lymphadenopathy improve substantially. However, to prevent relapse, the dose should be tapered slowly according to the response, over the course of 4-6 weeks. Refractory cases may respond to dexamethasone therapy in place of prednisone. Owners or breeders should be warned that some scarring can be expected due to the deep nature of the lesions.

References

Medleau L, Hnilica KA. Small Animal Dermatology: A Color Atlas and Therapeutic Guide, 2nd ed. 2006, Saunders Elsevier.

Nagle T. Topics in Pediatric Dermatology, Vet Clin Small Anim 26 (2006): 557-572.