Mites

Astigmata (mites) introduction

Mange on a dog - Wikimedia Commons

Mites are one of the most successful and diverse vertebrate groups. The species of veterinary importance are parasitic, although a few non-parasitic mites are also important, such as oribatid mites (intermediate hosts for Anoplocephala and Moniezia) and forage mites.

Mites are very small, on average under 0.3mm in length and for this reason often go unnoticed. They spend the majority of their life cycle on the host and cause mange.

The body shows no segmentation but can be divided into two sections, the idiosoma and the gnathosoma. Adult and nymphs have four pairs of legs, whereas larvae have only three pairs of legs.

The taxonomy of mites is complex as there are over 8 families. In this article the families are split according to their location on the host into sub-surface (burrowing) and surface (non-burrowing) mites.

Burrowing Mites

A single nest in a scraping may yield many mites

Non-Burrowing Mites

Live on the skin surface

Feed on either skin scales and tissue or suck blood

Psoroptes

Causes psoroptic skin infestation

Recognition

Oval shaped

Long legs

Funnel shaped suckers on segmented pedicels

1-2mm in length

Life cycle

Confined to skin surface

Feed on serous exudate by siphoning

Adult female can lay up to 100 eggs during her life time (1 month)

10 day life cycle

2 nymphal stages

Psoroptes cuniculi

Parasite of rabbits

Common among conventional rabbits

Transmitted via contact

Adapted to living in an aural environment

Pathogenesis

The ears are painful and intensely pruritic

Affected rabbits shake their heads and scratch their ears

The inner surfaces of the pinnae are covered with brown, scaly, fetid material, and the skin beneath is raw

Mites are grossly visible

Histologically, there is chronic erosive and proliferative eosinophilic dermatitis
- The mites are non-burrowing and thus are found only in the exudate, not in the tissue

Diagnosis

Microscopic examination for mites (low magnification)

Appearance

Control

Infestations are difficult to eliminate from a colony
- Ivermectin is usually effective

Psoroptes ovis

Adult females are large mites at 750μm in length

Males identified by copulatory suckers and paired posterior lobes

Males attach to deutonymphs (second moult after larval stage) in a process called copula
- Males remain in copula until females moult for the last time
- Copulation occurs

Life cycle last 14 days

Transmitted by direct contact between sheep

Indirect transmission can also occur

Pathogenesis

Economically important ectoparasite of sheep

Causes sheep scab
- Wool loss, restlessness, biting, scratching of infested area and decreased productivity through decreased weight gain
- Usually seen in late autumn and early winter (although may also occur in late summer)
- Population numbers decline after shearing due to a change in the micro-climate, then build up again as the fleece grows
- Notifiable in UK

Mites found under scabs and in skin folds

Lesions most common on flanks, neck, back and shoulders

Causes pruritic condition of cattle

Active in keratin layer

Mouthparts abrade the skin

Antigenic material in mite faeces can lead to hypersensitivity reactions

Diagnosis

Skin scraping

KOH added

Warm slide over a bunsen flame

Examine under a microscope

Treatment

Sheep
- Plunge dipping; no less than 1 minute and must dip head at lease once
- Can treat with avermectins or milbemycins by injection

Cattle, horses and rabbits
- No licensed product for horses in the UK
- Cattle and rabbits can be treated with avermectins, milbemycins or topical acaricides

Chorioptes bovis

These cause parasitic skin infestation

Surface parasite of horses and cattle

Less pathogenic than Psoroptes
- Mouthparts cannot pierce the skin

Life cycle takes 3 weeks

Recognition

Oval body

Long legs

Cup shaped suckers on unsegmented pedicels

Females about 300μm in length

Pathogenesis

Chorioptic mange
- Often seen in rough-legged horses with heavy feathering
- Induce crusty skin and lesions below the hocks and knees

Mild condition in cattle
- Rubbing and scratching
- Hide damage
- Usually affects the base of the tail, perineum and udder

Usually found on legs of sheep
- Mild condition

Otodectes cynotis

Causes otodectic skin infestation

Commonest mange of dogs and cats in the world

Inhabits the inner ear

Also found in the fox and the ferret

Closed keratinous bars (apodemes) on ventral surface

Life cycle takes 3 weeks

Feeds on ear debris

Pathogenesis

The majority of cats harbour the mites, however only a few show symptoms
- Transmission occurs whilst kittens are suckling

Common cause of otitis externa in dogs

Brown waxy exudate produced

Can lead to secondary infection

Clinical signs are apparent
- Head shaking
- Ear scratching
- Aural haematomata

Treatment

Acaracidal ear drops
- Massage base of ear to disperse drops after treatment

Most treatments need to be repeated in 10-14 days to kill newly hatched mites

Selamectin can be used as a spot-on treatment
- Prolonged duration of action

Treat all in-contact animals
- These may be asymptomatic carriers

Cheyletiella spp.

Surface mite of cats and dogs

Also found on humans and rabbits
- C.yasguri (dogs)
- C.blakei (cats and humans)
- C.parasitivorax (rabbits)

Causes parasitic skin infestation

Recognition

Waisted body

Claw like palps on head

Combs at ends of legs

Pathogenesis

Highly contagious

Mild pathogenesis

Causes very scaly dermatitis

Can be transferred to humans

Diagnosis

Clinical signs
- Excess scurf

Brush scurf onto dark paper
- 'Walking dandruff' as mites will move when present in large numbers

Skin scrapings

Hair pluckings from scaly areas
- Eggs may be present

Dermanyssus gallinae

Red mite of poultry

Spends most of time off the host
- Adults and nymphs visit poultry at night to feed

Life cycle takes 1 week

Adults can survive several months without feeding so reservoirs can build up

Appearance

Spider like mite with long legs

White or grey
- Becomes red when engorged with blood after feeding

Few hairs on body

Hooks on legs

Pathogenesis

Blood sucking mite

Lesions usually found on the breast and legs

Irritation, restlessness, decrease in egg production

Anaemia can result if mites are present in large numbers

Newly hatched chicks can rapidly die if infested

Treatment

Acaricide

Environmental treatment

Remove wild bird nests

Ornithonyssus

Also called the Northern mite or Northern feather mite

Closely related to Dermanyssus

Hairy

Spends entire life cycle on the host

Occurs in caged birds and poultry

Causes feathers to become matted and severe scabbing can develop
- Scabs particularly seen around the vent

Decreases egg production

Grey or black discolouration of feathers when large numbers of mites are present

Trombicula autumnalis

Causes parasitic skin infestation

Also called the harvest mite

Not host-specific
- Will parasitise any animal, including humans

Only the larval stage is parasitic
- Nymphal and adult stages are free-living in the soil

Mite numbers are highest in late summer in temperate climates
- Mite numbers are constant all year in tropical regions

Recognition

Six legs

Bright orange in colour

Hairy

No spiracles
- Breath through cuticle

Pathogenesis

Larvae insert mouthparts into skin and inject cytolytic enzymes

Feed on partly digested host tissue

Causes irritation

Can cause a hypersensitivity reaction

Mites found on head, ears and flanks of pets

Mites found on face and limbs of grazing animals (depending upon host height)

Control

Very difficult; try to restrict access of animals to 'hot-spot' areas

Treatment

Fipronil spray applied to affected areas

Leporacarus

Known as the rabbit fur mite

Found on rabbits (domestic and wild) and on hares

Common

Clings to individual hairs

Feeds on sebaceous secretions and skin debris

Non-pathogenic

May cause dermatitis in humans handling infected animals

Forage Mites

Pests of stored food products, hay and straw

May cause skin reactions, respiratory and intestinal disturbances in animals and humans

Control relies upon
- Identifying and destroying infected feed and bedding
- Thoroughly cleaning feed storage bins
- Keeping feed storage areas clean and dry