Canine External Parasites Study Guide

Overview and Clinical Importance

External parasites represent one of the most common reasons for veterinary visits in small animal practice. These ectoparasites cause significant morbidity through direct tissue damage, hypersensitivity reactions, and vector-borne disease transmission. The NAVLE frequently tests knowledge of parasite identification, life cycles, clinical presentations, and treatment protocols.

This study guide covers the three major categories of canine external parasites: fleas (Siphonaptera), ticks (Acari: Ixodidae), and mites (Acari: various families). Understanding the pathophysiology, diagnosis, and treatment of these parasites is essential for board examinations and clinical practice.

Section 1: Fleas (Order Siphonaptera)

Biology and Life Cycle

Ctenocephalides felis (the cat flea) is the most common flea species affecting both dogs and cats, accounting for greater than 90% of flea infestations. Despite its name, it is the predominant flea on canines in North America.

Flea Life Cycle Stages

Flea Allergy Dermatitis (FAD)

Flea allergy dermatitis is the most common skin disease in dogs and represents a Type I and Type IV hypersensitivity reaction to antigens in flea saliva. A single flea bite can trigger a severe reaction in sensitized animals.

Clinical Distribution Pattern

The classic distribution of FAD in dogs follows the "flea triangle" or "pants pattern":

• Lumbosacral region (dorsal tail base) - most commonly affected
• Caudomedial thighs
• Ventral abdomen and inguinal region
• Caudal aspect of rear limbs

Clinical Signs by Stage

Diagnosis of Flea Infestation

• Visual identification: Adult fleas (laterally compressed, 1-2mm) or flea feces ("flea dirt")
• Flea comb: Fine-toothed comb through hair coat, especially over dorsal lumbosacral area
• Wet paper test: Place debris on wet white paper; flea dirt (digested blood) produces red-brown streaks
• Response to treatment: Resolution of clinical signs with strict flea control supports diagnosis

Section 2: Ticks (Order Acari, Family Ixodidae)

Ticks are obligate blood-feeding ectoparasites of significant veterinary and public health importance due to their role as vectors for numerous pathogens. Understanding tick species, their geographic distribution, and associated diseases is frequently tested on the NAVLE.

Major Tick Species and Associated Diseases

Pathogen Transmission Timing

Understanding transmission timing is critical for client education about the importance of rapid tick removal and fast-acting preventatives:

Tick-Borne Disease Clinical Presentations

Ehrlichiosis (Ehrlichia canis)

• Acute phase (1-3 weeks): Fever, lethargy, lymphadenopathy, thrombocytopenia, petechiae
• Subclinical phase: Asymptomatic carrier state; can persist for months to years
• Chronic phase: Pancytopenia, bone marrow hypoplasia, epistaxis, uveitis, glomerulonephritis
• Diagnosis: SNAP 4Dx (antibody), PCR, morulae in monocytes on blood smear

Rocky Mountain Spotted Fever (Rickettsia rickettsii)

• Clinical signs: Acute fever (104-105°F), thrombocytopenia, petechiae, edema (especially limbs, scrotum, prepuce)
• Pathophysiology: Vasculitis from endothelial cell infection
• Treatment: Doxycycline (5 mg/kg PO BID for 14-21 days) - treat based on clinical suspicion, do not wait for serology

Lyme Disease (Borrelia burgdorferi)

• Clinical signs: Shifting leg lameness (polyarthritis), fever, lethargy, lymphadenopathy, glomerulonephritis (Lyme nephritis)
• Breed predisposition: Labrador Retrievers, Golden Retrievers at higher risk for Lyme nephritis
• Diagnosis: SNAP 4Dx, C6 antibody quantification (Lyme Quant C6)

Section 3: Mites (Order Acari)

Mite infestations cause significant dermatologic disease in dogs. The two most important mange conditions are sarcoptic mange (highly contagious, zoonotic) and demodectic mange (non-contagious, associated with immune dysfunction).

Sarcoptic Mange (Sarcoptes scabiei var. canis)

Sarcoptes scabiei is a burrowing mite that causes extremely intense pruritus - often out of proportion to visible lesions. It is highly contagious between dogs and is zoonotic (can cause temporary papular dermatitis in humans).

Clinical Presentation

• Distribution: Ear margins (pinna), elbows, hocks, ventral chest, ventral abdomen
• Lesions: Papules, crusts, excoriations, alopecia, thick yellowish crusts on ear margins
• Pruritus level: SEVERE - often 10/10 on pruritus scale, poorly responsive to corticosteroids
• Secondary changes: Peripheral lymphadenopathy, secondary bacterial pyoderma

Diagnostic Tests

Demodectic Mange (Demodex canis)

Demodex canis is a cigar-shaped follicular mite that is part of the normal skin fauna in small numbers. Clinical disease (demodicosis) occurs when mites proliferate excessively, typically due to immunosuppression or immunodeficiency. Demodicosis is NOT contagious between adult dogs and is NOT zoonotic.

Classification of Demodicosis

Breed Predispositions

Memory Aid - "SOAP BED": Shar-Pei, Old English Sheepdog, American Pit Bull Terrier, Boston Terrier, English Bulldog, Doberman Pinscher

Additional predisposed breeds include: West Highland White Terrier, Scottish Terrier, Pug, Boxer, German Shepherd Dog

Clinical Presentation

• Distribution: Face (periocular, perioral), forelimbs common initially; can become generalized
• Lesions: Alopecia, erythema, scaling, comedones, follicular casts, hyperpigmentation
• Pruritus: Usually MINIMAL unless secondary bacterial infection present
• Secondary infection: Deep pyoderma, furunculosis, draining tracts common with generalized form
• Pododemodicosis: Interdigital swelling, pain, discharge - often most resistant to treatment

Diagnosis

Deep skin scraping is the diagnostic test of choice. Scrape until capillary bleeding occurs to reach follicular mites. Unlike Sarcoptes, Demodex mites are usually easy to find on scraping.

• Squeeze skin before scraping to extrude mites from follicles
• Examine for adults, immature forms, and eggs
• Trichography (hair plucks) useful for periocular areas and Chinese Shar-Pei
• Skin biopsy if scraping negative but clinical suspicion high (especially pododemodicosis)

Other Important Mite Infestations

Otodectes cynotis (Ear Mites)

Ear mites are the most common cause of otitis externa in puppies and account for approximately 50% of ear infections in young dogs. They are highly contagious between animals.

• Clinical signs: Intense ear pruritus, head shaking, dark brown to black "coffee ground" ceruminous discharge
• Diagnosis: Otoscopic visualization of mites, ear swab cytology (mites and eggs)
• Treatment: Isoxazolines (highly effective), selamectin, ivermectin; treat all in-contact animals

Cheyletiella yasguri ("Walking Dandruff")

• Appearance: Large surface-dwelling mite; visible as moving "dandruff flakes" on dorsum
• Clinical signs: Excessive dorsal scaling, variable pruritus, "walking dandruff" appearance
• Zoonotic: YES - causes papular dermatitis in humans (areas of animal contact)
• Diagnosis: Superficial skin scraping, acetate tape preparation, fecal flotation (mites ingested during grooming)

Mite Comparison Summary

Section 4: Treatment Protocols

Isoxazoline Antiparasitics

The isoxazoline class represents the current gold standard for ectoparasite control in dogs. These compounds act as GABA and glutamate-gated chloride channel inhibitors in arthropod neurons, causing paralysis and death.

Key Points about Isoxazolines:

• Highly effective against fleas, ticks, Sarcoptes, Demodex, and Otodectes
• FDA warning for potential neurologic adverse events in dogs with seizure history
• Kill ticks before transmission of most pathogens (within 24-48 hours)
• For treatment of demodicosis: continue until 2 consecutive negative skin scrapings 1 month apart

Other Treatment Options

Macrocyclic Lactones

• Ivermectin: 0.3-0.6 mg/kg PO daily for Sarcoptes and Demodex (off-label)
• CONTRAINDICATED: MDR1 (ABCB1) mutant breeds - Collies, Shelties, Australian Shepherds, Old English Sheepdogs
• Selamectin (Revolution): Safe for MDR1 mutants; effective for Sarcoptes, Otodectes, some flea control
• Moxidectin + Imidacloprid (Advantage Multi): Topical; effective for Demodex, Sarcoptes