The skin

Chapter 5


The skin





Chapter contents



INTRODUCTION


PARASITIC SKIN DISEASES



PROTOZOAL SKIN DISEASES



BACTERIAL SKIN DISEASES



VIRAL SKIN DISEASES



FUNGAL SKIN DISEASES



CONGENITAL/HEREDITARY SKIN CONDITIONS



PIGMENTARY DISORDERS



ENVIRONMENTAL DERMATOSES



WOUNDS



ALLERGIC SKIN DISEASES



OTHER IMMUNE-MEDIATED CONDITIONS



NEOPLASTIC SKIN CONDITIONS



CUTANEOUS ENDOCRINOPATHIES



MISCELLANEOUS SKIN CONDITIONS




INTRODUCTION


Diseases of the skin are commonly encountered in equine practice and frequently present diagnostic and therapeutic problems. Many conditions do not have a pathognomonic clinical presentation, with diseases of different etiology having a similar appearance. A logical and systematic approach to the investigation of dermatologic disorders is essential in order to reach a definitive diagnosis and initiate appropriate therapy. Failure to do so results in the use of remedies on an empirical basis, which all too often wastes time, money and the owner’s patience, as well as having welfare implications for the patient.


A thorough history is required and it may be helpful to use a customized dermatology questionnaire to assist in this respect. After completing a general examination and detailed dermatologic examination, appropriate samples should be taken for laboratory examination. In many cases this will include collection of surface debris by brushing or scraping, for identification of ectoparasites or for fungal culture; surface adhesive tape strips are useful for collection of surface ectoparasites and identification of microorganisms by cytologic evaluation. Other samples might include bacterial swabs and skin biopsy.


Skin biopsy is essential to differentiate between the many causes of papular and nodular lesions. Multiple biopsies should be taken and the use of disposable biopsy punches facilitates this, with the patient sedated and use of local anesthetic. Larger nodules, masses and ulcerated lesions may require the use of excisional biopsy. Correct choice of biopsy site is important in gaining the most helpful diagnostic information, and no surgical preparation should be performed, apart from clipping, in order to preserve surface pathology. Full thickness skin should be removed, excess blood blotted away and the tissue placed on stiff paper or card to prevent distortion during fixation. A complete history and clinical details must be supplied to enable the pathologist to give an informed interpretation of the pathologic features observed in the tissues examined.


Biopsy should not be viewed as a last resort in the investigation of skin disease and should be performed within 3–4 wk of lesions appearing if a definitive diagnosis has not been made already. Early diagnosis allows early introduction of the most appropriate therapy and accurate prognostication for the owner of the horse.



PARASITIC SKIN DISEASES



MANGE



Sarcoptic mange



Etiology

Sarcoptic mange is a now rare disease of horses, notifiable in the UK and in many other countries, caused by the mite Sarcoptes scabei. The disease has been eradicated from the USA and currently is not specifically listed in the US Department of Agriculture list of Reportable Diseases and Conditions, although any confirmed or strongly suspected case should be brought to the attention of State or Federal veterinarians. Current opinion is that the various mites found on different hosts, although phenotypically different and adapted for a particular host, are genotypically identical. Cross-infestation among animals and humans is not uncommon.


The female sarcoptic mite lays 40–50 eggs in epidermal burrows tunneled into the skin. These hatch and, after passing through larval and nymphal stages, reach maturity within 2–3 wk.


Infestation is usually transmitted by direct contact between animals, but may also be transmitted on fomites such as riders’ clothing, stable blankets, harness, etc. In temperate regions and under conditions of poor hygiene, mites and eggs may survive away from the host in grease and debris within buildings, vehicles or on tack and grooming equipment for 1–4 wk.



Clinical signs and diagnosis

Initial infestation is asymptomatic, but within 2–4 wk there is evidence of papule formation and scaling accompanied by a very severe pruritus in sites of infestation, leading to rubbing, biting and the production of self-inflicted skin damage. Lesions spread rapidly, finally involving all areas of the body. The skin becomes increasingly thickened and corrugated with marked hair loss. Rest and feeding patterns are severely disrupted, leading to serious generalized debility, weight loss, weakness and even death.


Infestation is suspected in animals showing the typical history and clinical picture, and particularly when humans in contact present with a papular rash over the exposed areas of the body. Diagnosis may be confirmed by microscopic examination of scale and debris obtained by scraping affected areas, although mites may be difficult to find. Serologic tests for other sarcoptic mange antibody titers in other species may be adaptable for horses.



Treatment and prevention

A very strict code of hygiene must be enforced. All infested animals and those suspected of infestation must be isolated; their blankets, harness, grooming material and living accommodation should also be isolated. Animal attendants must change clothing and wash thoroughly before tending other animals.


Ivermectin has been shown to be effective in the treatment of sarcoptic mange. Infested animals may also be treated using lime sulfur, bromociclen, lindane or organophosphorus preparations, where available, used as a dip, spray or wash at 7–10-day intervals on at least three occasions. These preparations may also be used when necessary for treatment of infested buildings, vehicles and equipment after prior cleaning. In advanced cases or when scaling and skin thickening is excessive, it may be necessary to treat over a much longer period of time.


It is essential that infested animals be kept on a high plane of nutrition. Secondary infections such as by Dermatophilus congolensis may require concurrent treatment.


In areas where sarcoptic mange is still endemic, all newly acquired horses must be carefully examined and preventive measures taken where deemed necessary.



Psoroptic mange



Etiology

Psoroptic mange mites are non-burrowing, living on the skin surface and feeding by puncturing the epidermis, which causes serous exudation, inflammation and scab formation accompanied by irritation. The female mite lays eggs in the surface debris that hatch within 2–3 days and develop to maturity within 11–14 days. Mite activity, and the development of lesions, is greatest during autumn and winter and is probably associated with periods of higher humidity. Transmission is by direct or indirect contact.


Several species of psoroptic mite are reported to infest the horse: Psoroptes equi, P. natalensis, P. ovis and P. cuniculi. Psoroptes mites show little host specificity and appear to be genetically homogenous and it is probably best not to assign specific clinical manifestations to any individual species.





Chorioptic mange




Clinical signs and diagnosis

Chorioptes mites have a predilection to infest the skin of the legs from below the knee and the hock. Infestation may also occur on the tail and ventral trunk, and occasionally generalized lesions occur. Draft horses and cobs with feathered limbs are predominantly affected. Pruritus is very variable, from intense to absent.


Skin lesions may show a marked seasonal variation. Characteristic regionalized scaling lesions accompanied by marked irritation, stamping and rubbing are noted during the autumn/fall, winter and early spring, but apparently resolve spontaneously during the late spring and early summer, with a corresponding reduction in mite numbers, only to recur again the following autumn. The skin becomes grossly thickened and may become secondarily infected with bacterial or yeast organisms.


Chorioptic mange may be suspected on clinical examination and can be confirmed by microscopic examination of scale and debris obtained from active lesions.




Demodectic (follicular) mange




Clinical signs and diagnosis

Demodex mites are a not infrequent finding during routine microscopic examination of skin material. On rare occasions, local massive increases in mite numbers may occur and lead to the production of either:



Diagnosis may be confirmed either by finding large numbers of mites on microscopic examination of scale or substantiating the presence of mites in the caseous material obtained from opened follicular cysts or in skin biopsies.




FORAGE AND OTHER MITE INFESTATIONS




Etiology

Free-living members of the suborder Astigmata, which occur in livestock bedding and stored foods, are often known as forage mites. Certain species, including Tyrophagus spp., Glycyphagus spp. and Pyemotes spp., may produce cutaneous disease by inoculation or deposition of irritant or pharmacologically active substances into the skin or by induction of hypersensitivity reactions.


Many species of larval Trombiculidae (harvest mites, red bugs or chiggers) are capable of producing skin lesions and irritation in the horse. These include Trombicula (formerly Eutrombicula) alfreddugesi, T. (formerly Neotrombicula) autumnalis, T. (formerly Eutrombicula) splendens, and T. (formerly Eutrombicula) sarcina. The larvae are active for 4–6 wk in the year in late summer and early autumn/fall. Once they have obtained a meal of lymph and disintegrated skin cells they progress to free-living nymphal and adult stages, and the problem is therefore limited.



Clinical signs and diagnosis

The cutaneous responses associated with forage mite infestation in the horse are extremely variable and depend on the particular mite, its mode of infestation and the reactivity of the host. Lesions may present as areas of multiple, pinpoint crusted papules; patchy, irritant scaling; papular urticaria or urticarial plaques. Lesions associated with infestations contracted at pasture most frequently involve the heels, legs, brisket, ventral and lateral chest and abdomen, face and lips.


Harvest mites produce variable numbers of asymmetrical patches of small, focal weeping lesions over areas of predilection. In early lesions harvest mite larvae may be recognized by their orange/red appearance. Confirmation of diagnosis in older lesions is difficult as larvae feed and vacate their host within a few days.


Horses at pasture can also pick up other pathogenic forage mites. The lesions they produce may be similar to those of harvest mites or may present as localized irritant scaling, papular urticaria or urticarial plaques.


In stabled and yarded horses the distribution of cutaneous lesions associated with forage mite infestation will vary according to the source of that infestation. Infestations associated with food troughs and hay nets frequently only involve the face, neck and brisket, while those associated with bedding frequently only involve the lower parts of the body.


Mites such as Trombicula and Pyemotes species, which produce a mainly irritant response rather than an allergic response, induce skin changes on all in-contact animals and humans. Other forage mites, such as some Tyrophagus species, only produce changes in individual animals. Lesions due to forage mite infestation must be differentiated from papular urticarial reactions caused by biting insects in temperate summers or throughout the year in subtropical and tropical climates. Similar cutaneous responses may be encountered in association with parasitic mites of birds, fleas and bed bugs in housed animals.


Survival of forage mites on the horse is variable. Many vacate their host after feeding but some species, such as Pyemotes, often remain in the coat for a much longer period, particularly if the host has a thick coat or heavily feathered legs. In such cases microscopic examination of coat brushings or surface adhesive tape strips may confirm diagnosis. In the majority of cases such an approach is unrewarding and steps must be taken to identify the source of infestation and extract the mites from it for microscopic examination. Identification of forage mites is extremely time consuming. There are over 60000 known species of mite, of which only a relatively small number have so far been confirmed as being capable of causing skin disease in the horse and other animals.



Treatment and control

Infested animals at pasture should be moved immediately, either onto another pasture or into a yard or stable. Palliative treatment may be required, either as topical steroid or short-acting systemic steroid. Fipronil spray has been demonstrated to be effective against trombiculid mites. In the case of harvest mite infestation, animals may usually be returned to the original pasture after 8 wk but, as infestation is likely to remain from year to year, animals should be removed from the pasture at the same time the following years. Populations of other forage mites in vegetation tend to show transitory fluctuations related to growth of herbage and microclimatic factors and it is usually safe to return animals to the original pasture after a break of 2 mo.


Survival and multiplication of forage mites within stored hay, straw and cereals is dependent on the microclimatic conditions within bales, stacks or stores. Many are capable of undergoing massive increase in numbers when in store over winter and when humidity and temperature are favorable. Frequently such massive increases in number will be confined to one small group of bales within a stack. Infested material must be removed and replaced by material from a different source. If this is done, then the chance of recurrence of symptoms is low. If the infestation is in the bedding then this may be replaced with shavings or the animals turned out to grass.



PEDICULOSIS





Clinical signs and diagnosis

The symptoms and cutaneous changes associated with louse infestation in the horse show marked individual variation unrelated to parasite numbers. Many animals with a comparatively heavy louse burden present with minimal skin changes and irritation, while other animals with very low levels of infestation present with severe pruritus, patchy alopecia, erythema and excoriation.


Louse infestation is common in the horse and should always be suspected in cases of skin irritation. Isolation of lice can be difficult when numbers are low, as adult lice are relatively small and, being either pale pink or gray, merge easily into the coat. A long and careful search of the coat under strong light is often essential to find the parasite. On some occasions its presence is more easily recognized by the identification of eggs or egg cases firmly attached to hairs on the trunk and neck, or the base of the mane and tail.




FLY-RELATED DERMATOSES



Fly bites and fly worry


Lesions resulting from the bites of tabanids (Tabanus, Haematopota and Chrysops spp.), stable flies (Stomoxys calcitrans), and black flies (Simulium spp.) are crusted papules, which fade within a few days. Mosquito bites result in papules without crusts and also fade in a few hours to days. Considerable “worry” can result from large numbers of biting insects, and large swarms of black flies can kill cattle and horses.


Hypersensitivity reactions to these insects have been postulated but not demonstrated. Face flies (Musca autumnalis), head flies (Hydrotaea irritans), and house flies (Musca domestica) do not bite but feed on moist secretions in wounds or near mucocutaneous junctions.


Diagnosis of the problem is by fly identification. Fly bites should be distinguished from other types of cutaneous nodules such as early dermatophytosis, urticaria, equine eosinophilic (collagenolytic) granuloma and sarcoids (q.v.).




Myiasis



Etiology

Primary myiasis due to screwworm infestation of healthy tissue of wounds, dermatitis lesions, ulcerated neoplasms or granulomas, and mucocutaneous junctions occurs in the Americas (Central and South primarily), Africa and Asia. The genera Cochliomyia and Chrysomyia are involved. The adult fly deposits eggs in the moist areas. When the larvae hatch, they burrow into the healthy subcutaneous tissue causing liquefaction and enlargement of the lesion. They drop out in 3–6 days to pupate.


Secondary myiasis (blowfly strike) is not common in horses but may occur. The genera involved are Lucilia, Calliphora, Phormia, and Cordylobia. These flies lay their eggs in decomposing tissue of wounds and macerated skin lesions as well as in carcasses. Larvae feed on the decomposing matter and secrete enzymes, causing wound enlargement.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on The skin

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