• Definition
• Cause
• Signalment and history
• Clinical signs
• Diagnosis
• Treatment and management

Definition

Many parasites may cause pruritus.

• Dermatitis occurs as a reaction to a parasite bite, or as a hypersensitivity reaction. Ectoparasites include flies, mosquitoes, ticks, mites, and lice
• The larval stages of some parasites such as Ancylostoma (hookworm) and Cuterebra fly species cause skin inflammation as they migrate through or burrow into the skin
• Some systemic parasitic diseases may also cause skin lesions (for example Leishmaniasis, a parasite spread by sand flies)

There are geographic variations in the species, distribution and frequency of these parasites.

Cause

Mites

• Ear mites (Otodectes cynotis) complete a 3 week life cycle in or around the ear of the host, causing otitis externa
• The fur mite Cheyletiella yasguri, also known as 'walking dandruff' completes a 2 week life cycle in the fur of dogs, cats and rabbits. Handlers of infested animals may also be bitten and develop reactions. This is a zoonosis
• 'Harvest mites' or 'chiggers mites' (Trombiculids) include several mite species which live most of their 50-70 day life cycle in vegetation. Only the 6-legged larval stage is parasitic, climbing on to passing animals (or people) and attaching to the skin surface. Larvae eventually drop off the host in order to continue their life cycle

Lice (Pediculosis)

• Lice complete their life cycle on the host in 3-6 weeks
• There are both chewing and sucking species of lice: chewing lice (for example Trichodectes canis) attach to the base of the hair and feed on hair and skin scales; sucking lice (for example Linognathus setosus) attach to the skin and feed on blood

Ticks

• There are many species of ticks, each with a different geographic distribution. The brown dog tick (Rhipicephalus sanguineus) is found in most parts of the world
• Ticks are either 2-host or 3-host obligate parasites. Eggs are laid in the ground by females. Each subsequent stage of the life cycle climbs onto a host to feed before dropping to the ground to moult (or, for adults, to reproduce)
• Ticks feed on blood and lymph, and tick bites may cause skin irritation
• Several tick species have a neurotoxin in their saliva that may cause paralysis in the host
• Ticks can also act as vectors for many pathogenic diseases. Examples include Rocky Mountain spotted fever (Rickettsia) and Lyme disease (Borrelia)
• Other common tick species include Ixodes (for example sheep or deer ticks), Amblyomma (for example Lone Star tick) and Dermacentor (for example American dog tick)

Flies

• Adult biting flies such as the Stable fly (Stomoxys calcitrans) cause painful bite wounds on the skin of animals (and humans) as they feed on blood. Hypersensitivity to the bite may increase lesion severity
• Black flies or gnats (Simuliidae spp.) are smaller flies that tend to cause more severe lesions
• Bot flies (Cuterebra or rarely Hypoderma and Dermatobia spp.) are 'opportunistic' parasites (dogs are not their normal host). Eggs are laid in the environment and adhere to passing animals. Hatched immature stages enter the host through the nose, mouth, or open wounds and migrate to the subcutaneous tissue where they develop, breathing through a tiny pore in the skin. After about 30 days, immature flies or larvae emerge from the skin and drop to the ground to complete their life cycle

Nematodes

• Hookworms (Ancylostoma spp.) are intestinal parasites of the dog
• Eggs are passed in the faeces of infected dogs. Infective larvae develop in the soil and can infect dogs (and sometimes people) by burrowing through the skin, leaving pruritic inflammatory tracts. Ancylostoma species can complete their life cycle in 15-28 days
• Dogs can also be infected while feeding from their mother as puppies, directly ingesting the larvae, or by ingesting infected paratenic hosts (for example a rodent)
• Humans are not normal hosts for canine hookworm. In people, the parasite may cause disease through migration in the skin, migration through the internal organs, or complete migration to the intestine, resulting in eosinophilic enteritis

Protozoa

• Leishmaniasis is a serious disease that affects multiple body systems and is caused by the Leishmania protozoan
• The protozoa complete their life cycle between 2 hosts: sandflies and vertebrate hosts (such as cats, dogs and people)
• In dogs, the parasite infects the lymphatic cells. Skin damage is usually the result of immune-complex deposition and appears as granulomatous inflammation

Signalment and history

Mites

• Ear mites (Otodectes) are common in young dogs or cats
• Fur mites (Cheyletiella, or 'walking dandruff') are highly contagious and can be zoonotic
• Harvest mites (Trombiculids) are most prevalent in autumn (harvest time) although seasonality depends on climate. Trombiculid mites only occur in certain geographic regions

Lice (Pediculosis)

• Are highly host-specific
• Most common in young, old or debilitated animals, or those kept in unsanitary environments
• More common in cooler months
• Chewing lice may be found worldwide; sucking lice are generally found in cooler climates

Flies

• Bot fly (Cuterebra) skin lesions are most commonly seen from summer to autumn. The flies occur only in certain geographical regions
• Biting flies (Stomoxys calcitrans) are common around livestock rearing facilities, stables and pastures. Adult flies generally emerge in spring and numbers peak in warm weather
• Black flies or gnats tend to be found in spring to summer near shaded, running water

Ticks

• Prevalence and distribution of ticks vary with species, climate, host numbers and surrounding vegetation
• Numbers often peak through warmer months. In some areas this may occur as 2 peaks, for example spring and autumn, consistent with the life-cycle progression
• Ticks climb up vegetation to enable attachment to passing hosts, increasing the risk of exposure for dogs spending time in rural areas. Some ticks (such as the brown dog tick) have adapted to housing and can also be found in indoor environments such as kennels

Nematodes

• Hookworms can infect dogs of any age. Puppies may become infected through their mother's milk. Hunting or outdoor animals are at greater risk of infection
• Skin migration of the immature hookworm, cutaneous larval migrans, results from contact with soil or ground which has been contaminated with infected faeces. Hookworm larvae are susceptible to drying, and completion of the life cycle is temperature-dependent so infection risk may be higher in warm, humid climates

Protozoa

• Leishmaniasis commonly occurs in dogs less than 5 years old, particularly in rural dogs or those that sleep outdoors at night. Leishmania are only present in certain geographical areas such as the Mediterranean
• Some dogs may be asymptomatic
• Incubation periods vary from weeks to years

Clinical signs

Mites

• Ear mites (Otodectes) are usually confined to the ears and are a common cause of otitis externa. Pruritus can result in self-trauma around the ears and head
• Cheyletiella are also called 'walking dandruff' due to the visible movement of mites among the debris in the fur. They cause mild pruritus with excessive scale production
• Harvest mites (Trombiculids) are commonly found around the eyes, ears, legs, toes, perineum and belly, i.e. areas in contact with grass and vegetation. Multiple larvae will fix in one area and cause pruritus, papules, crusts, scaling, and alopecia in affected areas. Repeated exposure may result in hypersensitivity reactions

Lice (Pediculosis)

• Chewing lice attach to the base of the hair particularly around the head, neck and tail
• Sucking lice are found primarily on the neck and shoulders, especially under the collar
• Lice may also be found near body openings or skin abrasions
• The effects of infestation are usually mild. Movement of the lice through the coat causes intense pruritus leading to self-trauma (chewing, scratching, alopecia) and restlessness. Heavy infestations may cause skin inflammation with crusts and alopecia. Coat may appear rough and matted due to presence of eggs and lice in the hair. Urticaria-like lesions have been reported with sucking lice, and secondary infections can occur
• Severe infestations may result in anaemia especially in young animals

Ticks

• Ticks are commonly found on the face and ears, around the anus, in the axillae and inguinal regions, and interdigitally but they may be found anywhere on the body
• Irritation and pruritus may occur at the site of tick attachment. Secondary bacterial infections can occur. Incomplete removal of ticks can lead to the formation of a micro-abscess around any mouthparts remaining in the skin
• Heavy infestations, particularly in young animals, can cause anaemia
• Tick paralysis is an acute, ascending, flaccid paralysis. The most common offending tick species are the Dermacentor andersoni, D. variabilis, Amblyomma americanum, A. maculatum, Ixodes scapularis, I. pacificus and I. holocyclus, Rocky Mountain wood tick, American Dog tick, Lone Star tick, Gulf Coast tick, Deer tick (Black-legged tick), Western black-legged tick, and the Paralysis tick

Flies

• Bot fly (Cuterebra) bites are seen as fistulous subcutaneous swellings about 1 cm diameter, mostly around the head, neck and trunk. Hair over the lesion may be matted. Secondary bacterial infection increases the degree of pain and causes a purulent discharge
• Biting flies generally attack the face or ears of dogs. Bites result in erythema and haemorrhagic crusts. Hypersensitivity to the bites may increase severity of lesions
• Black flies generally attack areas with little or no hair. Lesions may be more severe than other biting flies with intense pruritus, ulcers, bleeding and severe self-trauma in addition to crusting papules

Nematodes

• Hookworms migrating through the paws can cause pododermatitis, thickened pads, or nail growth abnormalities. In other areas of skin, inflamed tracks may be seen in the skin where the larva has entered. Intestinal hookworm infection can cause weight loss, haemorrhagic diarrhoea, weight loss and anaemia

Protozoa

• Leishmaniasis can affect multiple body systems, including the skin
• Skin lesions most commonly include exfoliative dermatitis, particularly around the head, ears and extremities. Hyperkeratosis of the nasal planum and footpads, hair coat thinning or alopecia in affected areas, and ulcerative lesions, particularly at pressure points, may also be seen. Other lesions include nail growth abnormalities, sterile pustular dermatitis, nasal erosion, depigmentation and nodular dermatitis
• Secondary bacterial infections or generalised demodicosis may occur
• Other affected body systems may include eyes, musculoskeletal, renal, haematological (epistaxis, anaemia, thrombocytopaenia) lymphatic and gastrointestinal systems
• Increased aggression has also been reported

Diagnosis

Mites

• Diagnosis is achieved through history and identification of mites
• Ear mites (Otodectes) may be seen on microscopic examination of wax from the ear. Moving mites may also be visible in the ear when viewed with an otoscope
• Fur mite (Cheyletiella) movement may be observed in the coat ('walking dandruff'). Mites may also be seen on microscopic examination of coat brushings. History from owners or handlers may reveal evidence of both the pet and pet owner being affected
• Differential diagnoses for chiggers or harvest mites include seasonal skin infections (for example atopic dermatitis) or ear mites. Diagnosis is based on observation of the immature mites, which are easily differentiated from other mites through their orange-red colouration. Note: if examined under microscope, mites should be placed in mineral oil to prevent escape

Lice (Pediculosis)

• Diagnosis is based on clinical signs and on demonstration of the parasites or their eggs ('nits')
• Species and type of lice may be differentiated based on location (skin or hair attachment) and microscopic examination

Ticks

• Diagnosis is achieved through identification of the parasite

Flies

• Differential diagnoses for bot flies (Cuterebra) include abscesses or foreign bodies. Diagnosis is made by finding and identifying larvae (do not squeeze lesions). L3 are the most commonly identified and are 5-10mm in length, dark, thick and heavily spined
• Diagnosis of fly bite dermatitis (biting or black fly) is based on history, lesions and distribution

Nematodes

• History of poor housing and parasite control +/- gastrointestinal signs may be indicative. Hookworm eggs can be identified on faecal flotation

Protozoa

• Leishmania may be suspected based on clinical presentation (for example multisystemic involvement) and geographic location. Diagnosis is confirmed with a specific antibody assay and visualisation of the parasite on appropriately stained tissue samples (biopsy or aspirate particularly of bone marrow or lymph nodes)
• Note: immune-mediated diseases (such as lupus erythematosus) are important differential diagnoses. Tests for these diseases may be positive in dogs with Leishmaniasis

Treatment and management

Treatment for most parasites includes an appropriate parasiticide for a duration that considers the life cycle of the parasite, and environmental management where appropriate.

Mites

• Ear mites: generally treated with an appropriate topical parasiticide. Treatment may be repeated after 3-4 weeks to remove any newly hatched mites. All in-contact animals should be treated
• Fur mites (Cheyletiella) are treated with an appropriate topical parasiticide. All in-contact animals should be treated
• Harvest mites (Trombiculids) are treated with topical parasiticides. Reinfestation is common when animals return to their normal environment. Exposure to source of infection should be limited if possible

Lice (Pediculosis)

• Treat with an appropriate topical insecticide
• Place treated animals in a clean, disinfected living space
• Potential sources of re-infestation should be cleaned or discarded (for example brushes, combs, bedding). Several hours of hot, dry conditions will kill eggs (for example clothes drier)
• Treatment may be repeated after a week to ensure that any new lice developing from eggs are also killed
• Look for underlying disease or source of debilitation if appropriate

Ticks

• Application of appropriate acaricide
• Considerations in selection of treatment include safety for breeding animals (if appropriate), ease of application for owner, and effect on efficacy of swimming or bathing for dogs with regular water exposure
• Avoidance of high infestation areas during tick seasons

Flies

• Bot fly treatment involves physical removal of the maggot and cleaning of the site: (i) remove maggot by careful probing of the fistula with mosquito forceps. Do not squeeze lesion (rupture of the maggot may result in a chronic foreign body reaction and secondary infection); (ii) flush with saline, debride if required, and leave open to heal by granulation. Failure to heal may indicate that not all of the maggot was removed or reinfestation
• Fly bite dermatitis: (i) apply topical antibiotic or anti-inflammatory agents as required; (ii) apply insect repellents every day; (iii) avoidance of flies may aid recovery, for example housing indoors; (iv) environmental management by reducing preferred fly breeding areas (often a combination of moist decaying organic material and rotting vegetation)

Nematodes

• Regular administration of an appropriate wormer to at-risk animals (and/or regular faecal monitoring)
• Regular collection and disposal of dog faeces from environment
• Concrete areas of flooring may be treated with chemicals, which can reduce or remove infective stages (for example chlorine based bleach) where appropriate

Protozoa

• Leishmaniasis is extremely difficult to treat and relapses may occur. Appropriate anti-Leishmania treatments may control infection. Maintenance treatment may reduce the risk of relapse
• Prevention is preferred and includes control of the sand fly vector, avoidance of high-risk areas if possible, and use of insect repellent treatments