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The pruritic dog

THE PRURITIC DOG
BVSc (Hons) MMedVet (Med) PhD Dipl. ECVIM (Internal Medicine) PO Box 67092, Bryanston, 2021, South Africa
INTRODUCTION

Pruritus can be defined as the sensation that elicits the desire to scratch and/or chew and is assumed
to occur in dogs that show erythema, excoriations, alopecia, lichenification, or hyperpigmentation.
Pruritus is the most common clinical manifestation of disease that causes owners to bring their pets in
for treatment; however, the diagnosis of pruritic dermatitis is not always easily, partly because of the
variation in clinical presentation and partly because of the range of different aetiologies. The diagnostic
approach must thus be through and methodical and should include certain fundamental diagnostic steps.
PATHOPHYSIOLOGY
The skin functions as an "external nervous system," providing continuous sensory input to the central
nervous system (CNS) through a network of free nerve endings responsible for transmitting the
sensations of touch, temperature, pain, and pruritus. In humans, "sensory spots" or "itch points"
coincide with areas of increased density of free nerve endings, which may be true in the dog as well.
Pruritus commonly stimulates self-trauma; however, the mechanisms by which self-trauma relieves
itching are unclear. Possible mechanisms would be that self-trauma may disturb the amplified,
reverberating spinal pathways that perpetuate the sensation of itch and that self-trauma could
substitute pain for pruritus.
Endogenous mediators that can trigger pruritus include histamine, peptides, proteases,
prostaglandins, leukotrienes, monohydroxy fatty acids, and opioid peptides. Proteolytic enzymes and
leukotrienes are thought to be more important mediators of pruritus than histamine. Bacterial and
fungal endopeptidases also can initiate pruritus. The CNS can also amplify or reduce the sensation of
pruritus: stress or anxiety may amplify pruritus by releasing opioid peptides; boredom or other
cutaneous sensations such as pain, heat, cold, or touch can alter the perception of pruritus; and
factors such as increased skin temperature, diminished skin hydration, and low humidity can heighten
the sensitivity of the skin to pruritic stimuli. The pruritic threshold is often reduced at night in both
humans and animals when other sensory inputs are diminished.
The concepts of threshold phenomenon and summation of effect are paramount in understanding and
managing pruritus. The threshold phenomenon is where a certain pruritic load may be tolerated
without initiating clinical signs, but a small increase in that load can provoke clinical signs. Summation
of effect
occurs when additive pruritic stimuli from coexistent skin diseases raise an animal above
threshold. As an example, pruritus from mild flea allergy is additive to pruritus from other skin
diseases during flea season, thus exacerbating and perpetuating itch-scratch cycles.
DIAGNOSIS OF PRURITUS
Signalment, history, clinical examination, diagnostic testing, and, occasionally, response to therapy
are the cornerstones of diagnosis. Because many pruritic skin diseases are visually similar, clinical
history coupled with signalment may offer more direct clues to diagnosis than the actual clinical
examination.
Signalment
Age
Age provides critical information for prioritizing differential diagnoses. Scabies and demodicosis are
pruritic skin diseases seen more commonly in young dogs whereas atopy, food allergy, and pyoderma
occur more commonly in adult animals.

Breed
Certain skin diseases are breed specific: golden retrievers, Dalmatians, retrievers, beagles, and many
small terrier breeds are at increased risk for the development of atopy; the West Highland white terrier
for secondary Malassezia dermatitis; and the Chinese Shar Pei seems predisposed to atopy, food
allergy, pyoderma, and demodicosis.
Sex
Although sex predilections are not common in pruritic skin diseases, pruritus may be seen with Sertoli
cell tumours, male-feminizing syndromes, and female hyper-oestrogenism.
Historical findings
General history
A general history pertaining to diet, environment, use of home skin care, recent exposures, and the
presence or absence of pruritus in other animals or people in the environment should be obtained,
which may aid in prioritizing differential diagnoses.

Diet

Food allergy or intolerance can cause pruritus; however, adverse reactions to food frequently coexist
with other allergic skin diseases such as atopy and flea allergy dermatitis. Lipid-deficient diets may
exacerbate seborrhoea.
Environment and exposure
Flea allergy, scabies, and cheyletiellosis are all seen more frequently in dogs that are permitted to
roam free. Acquisition of a new pet, sheltering a stray animal, recent kennelling, and grooming can all
increase the likelihood of contagious disease.
Other household pets
Pruritus or lack of pruritus in other animals may offer clues to contagion. However, even though dogs
and cats share the cat flea as a common ectoparasite, flea allergy is much more common in dogs. A
seemingly unaffected cat is often the source of fleas in indoor dogs with flea allergy dermatitis.
Although uncommon, asymptomatic carriers of canine scabies do exist because clinical disease
requires hypersensitivity.

Human contacts
A pruritic papular rash in an owner with a pruritic pet may suggest zoonotic infestation with scabies or
cheyletiellosis. Annular, erythematous lesions may suggest dermatophytosis.
Specific history
Specific history relates to the current pruritic skin disease. The initial site of skin lesion development,
onset and progression, intensity of pruritus, seasonality or other pattern, and response or lack of
response to previous therapy may aid in establishing a diagnosis.
Site, onset, and progression
Knowledge of the initial sites of skin lesions may be useful, especially if the disease has generalized
before veterinary care is sought. Scabies often begins on the margins of the pinnae before
generalizing. Rapid-onset pruritus should increase suspicion for ectoparasitic diseases and, less
commonly, adverse drug reactions. Pruritus of insidious onset is more suggestive of slowly
progressive, chronic skin diseases such as atopy, food allergy, pyoderma, seborrhoea, and
Malassezia dermatitis.
Intensity
Most animals do not exhibit pruritus in the examination room with scabies and flea allergy dermatitis
being notable exceptions. Frequency and intensity of pruritus may be inferred from asking the owner
how many times the animal will scratch (or chew or lick) if it is ignored while the owner observes the
animal at home.
Seasonality
Atopy and flea allergy dermatitis are often seasonal; Malassezia dermatitis may occur more frequently
during months of higher humidity; cyclical pruritus without seasonality can sometimes signify contact
dermatitis associated with change of environment; and psychogenic pruritus may begin as a
predictable, attention-getting device. Pruritus seen with food allergy should be continuous unless the
diet is changed.
Response to previous therapy
Response or lack of response to previous medications, particularly corticosteroids, antibiotics, or
parasiticides, may offer additional clues. Although allergic diseases all respond to corticosteroids to
some degree, food allergy may be less responsive to corticosteroids than atopy or flea allergy
dermatitis. Prior diminished pruritus in response to antibiotics in dogs is often overlooked and
indicates the likelihood of pyoderma. Pruritus as the result of pyoderma may also diminish in
response to corticosteroids.
Clinical examination
A complete clinical examination is extremely important when evaluating any animal with skin disease
as skin disease may be an indicator of systemic disorders. Examination of the skin, muco-cutaneous
junctions, oral cavity, ears, genitals, and lymph nodes should be emphasized. Objective signs of
pruritus include excoriations and broken or barbered hairs with a dry lusterless hair coat.
Pruritus may occur with or without primary skin lesions. If present, primary skin lesions such as
papules or pustules may be helpful in establishing a diagnosis. Coexistent alopecia may offer
additional clues. Unfortunately, self-trauma often leads to the obliteration of initial, more diagnostic
primary skin lesions substituting excoriations, lichenification, and alopecia. The concept of "a rash that
itches" indicates primary skin lesions that are itchy, and "an itch that rashes" indicates that pruritic
patients without primary lesions traumatize themselves. Ectoparasitic skin diseases, pyoderma, and
cornification abnormalities are among the more common pruritic skin diseases where primary skin
lesions are identified. Conversely, primary lesions are much less common in atopy and food allergy.
The distribution of lesions, presence or absence of bilateral symmetry and major foci of pruritus can
be valuable aids to diagnosis. Primary or secondary lesions, if present in a particular site, may be
highly suggestive of specific diseases.
Diagnostic plan
Diagnostic plans should be formulated based on prioritization of differential diagnoses using
signalment, history, and physical findings with specific diagnostic procedures selected based on the
most likely differential diagnoses. Skin scrapings, fungal culture, exfoliate cytology, trial therapy for
ectoparasites, and skin biopsy, are the most cost-effective diagnostic procedures for the pruritic
animal.
Skin scrapings
Multiple skin scrapings should be performed on all pruritic dogs. Affected areas should be gently
clipped, and then either a scalpel blade or glass slide dipped in mineral oil, should be scraped
perpendicular to the skin surface in the direction of hair growth. The acquired debris is then placed on
a slide, a cover slip applied, and the specimen examined microscopically using low light. Demodex
mites usually are readily demonstrable (except in chronic pododemodicosis and in the Chinese Shar
Pei). Scabies mites are documented in less than half of affected dogs, underscoring the need for trial
therapy in suspected cases. Dry scrapings may be stained as smears to look for Malassezia.
Exfoliate cytology
Affected skin, intact pustules, or exudates should be smeared, stained with a rapid stain such as Diff
Quik®, and examined microscopically for the presence of bacteria, Malassezia, and inflammatory
cells. Clear tape preparations may demonstrate Cheyletiella and stained tape preparations may
demonstrate bacteria or Malassezia.

Faecal examination
Faecal examination may document endoparasite infestations and may reveal the presence of mites.
Skin biopsy
Skin biopsy is especially valuable if primary skin lesions are free of self-traumatic excoriations. If only self-traumatic lesions are present, definitive diagnosis is less likely, but results may aid in prioritizing or ruling out various differential diagnoses.
Response to trial therapy
Trial therapy with parasiticidal agents is used routinely in suspected cases of scabies or flea allergy
dermatitis. Despite the availability of effective modern flea control products, flea allergy dermatitis still
remains the most common cause of pruritus. As lesions seen with superficial pyoderma may be
pleomorphic, trial use of antibiotics may be indicated in an undiagnosed pruritic crusted papular
dermatoses. Although response to corticosteroids is suggestive of underlying allergic disease,
superficial pyoderma may partially respond to corticosteroid therapy.

Elimination diets
Animals suspected of having food allergy or food intolerance as a cause of pruritus should be fed an
exclusive hypo-allergenic diet for 8-12 weeks.
In vitro testing
In vitro testing (allergen-specific IgE serology [ELISA] or radioallergosorbent test [RAST]) for atopy
offers convenience and accessibility. Reproducibility of test results has increased dramatically over
the past decade. However, problems still remain with antigen selection, grouped testing, and
standardization of results.
Environmental restriction
If allergic contact dermatitis is suspected, an animal may be housed in a markedly different
environment (water-rinsed kennel) for 10 days.
MANAGEMENT OF PRURITIS

The general considerations in managing the pruritic dog are avoidance of allergens (whenever possible),
using topical and/or systemic medications, considering desensitisation therapy, and treating concurrent
problems.
Environmental control is usual y aimed at limiting exposure to offending al ergens. For pol en al ergies,
keeping the dog indoors most of the day is partially effective. Bathing in cool water wil soothe the skin
directly but wil also wash away pollens or moulds that might stil be present on the skin or hair coat.
Topical anti-pruritic agents include colloidal oatmeal, hydrocortisone, antihistamines (diphenhydramine),
and anaesthetics (pramoxine).
Corticosteroids are often effective at managing the pruritus but must be used cautiously for long-term
therapy. A starting dose of prednisolone is usually 0.5 to 1 mg/kg/day for 3-5 days. Long-term therapy
should only be used on an alternate-day basis.
Cyclosporine at 5 mg/kg/day is often sufficient in controlling clinical signs without many of the long-term
side effects of corticosteroids. Treatment is given daily for 6 weeks; after that, it is often possible to treat
on an alternate-day basis, and sometimes even twice per week.
Antihistamines are reported to be satisfactory anti-pruritics in approximately 25-35% of atopic dogs with
clemastine, hydroxyzine, chlorpheniramine, and diphenhydramine giving the highest percentage of
control.

Combinations of omega-3 and omega-6 fatty acids (eicosapentaenoic acid and gamma linolenic acid),
have been shown to have beneficial effect in approximately 20% of atopic dogs. Treatment should be
based on the eicosapentaenoic acid (EPA) content of these products and dosed at 5-40 mg/kg of EPA
daily. It takes approximately 12 weeks of supplementation to change leukotriene levels in the skin and
blood.
Desensitisation therapy is a biologic method used to treat atopic dogs. Although the exact mechanism by
which immunotherapy works is unknown, it is likely that it “down regulates” the allergic response, and
may raise the pruritic threshold. Desensitisation therapy should be considered when the al ergic signs last
for 4 months or more each year; when there are side effects from medical treatment, regardless of the
duration of allergic signs; and if the pruritus is not adequately control ed with medical treatment. Success
rate of desensitisation therapy is approximately 60-75% and requires, on average, 4-12 months for the
beneficial effects to be seen.
MANAGEMENT OF PYODERMA
In general pyoderma requires systemic treatment with antibiotics, although topical anti-microbial
shampoos may also be effective. In all cases precipitating underlying disease should be sought and
recurring previously well-managed atopic dogs may require antibiotic therapy to eliminate secondary
bacterial aetiologies and not an increase in steroid dosage (bacterial “summation of effect”).
General principles
The antibiotic should have known spectrum of activity against Staphylococcus intermedius and be
beta-lactamase resistant. Only about 40% of dermal blood flow reaches the dermal/epidermal junction
and therefore in vitro sensitivity testing may be poorly correlated with clinical response. For cultures
proper sampling technique is essential with superficial skin swabs generally unacceptable. A tissue
biopsy is usually required.
Proper therapeutic dosing for the full duration of treatment is essential and the minimum treatment
time for a superficial pyoderma is 14-21 days and for a deep pyoderma 8-12 weeks.
Antibiotic selection
• Erythromycin (10 - 15 mg/kg tid). • Lincomycin (22 mg/kg bid, at least 2 hrs away from food). • Clindamycin (10 mg/kg bid). • Potentiated sulphonamides (20 –30 mg/kg bid). • Cloxacillin (20 mg/kg bid). • Amoxicillin with clavulanic acid (20 mg/kg bid). • Cephalosporins (20 mg/kg bid-tid). Poor choices include penicillin, ampicillin, amoxicillin, streptomycin, tetracycline, and non potentiated sulphas.

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