Bacterial diseases

6 Bacterial diseases

Bacterial skin diseases are a common practice problem although fortunately there are relatively few clinical diseases associated with them and relatively few bacterial species that are regular causes of disease. An example of the confusion is provided by staphylococcal (bacterial) folliculitis (pyoderma); the commonest clinical presentation is circular crusting painful dermatitis that is very easily confused with some early forms of dermatophytosis. This means that while the diseases may be recognizable to an experienced clinician, there may be some confusing overlapping clinical presentations. Whilst there are some organisms such as various staphylococcal organisms that are regarded as pathogenic, there are many commensal organisms that may in certain circumstances cause disease. The former can cause disease, which is local usually but occasionally more generalized, in the absence of any overt immune compromise. The latter often require some local or generalized immunocompromise before disease develops.

Skin damage, which may be trivial or more obvious, and compromised local or general immunity can predispose to skin infections. The clinical objectives have to be to identify the organisms involved, to establish whether they are primary or secondary and then to control and eliminate the infection. In this way the primary skin diseases might rapidly resolve if a specific treatment option was available and convenient. There may be significant benefit in controlling secondary skin infection also. For example, an old horse with known pituitary pars intermedia dysfunction (Cushing’s disease) might have a serious skin infection involving a normally commensal bacterial organism or may have serious disease from a pathogenic species. Controlling the infection(s) will certainly help but treating the underlying condition should be the main objective – provided of course this can be done at all.

The various different bacteria that are involved in skin infections often have a different location within the skin. Thus Staphylococcus spp. usually cause folliculitis (i.e. they remain, at least at first, in the follicles) but deeper extension results in a nodular condition commonly referred to as ‘botryomycosis’. The misleading name gives the impression of a fungal lesion but this simply reflects the clinical similarity to Actinomyces (and other fungal granulomatous) infections. Corynebacterium pseudotuberculosis causes a deeper pyoderma and this can extend rapidly in lymphatic vessels and so results in a condition known as ulcerative lymphangitis.

There are some very serious bacterial skin diseases that are either potentially zoonotic (including multi-resistant/methicillin-resistant Staphylococcus aureus (Yasuda et al 2000, Cuny et al 2006) and Burkholderia (Pseudomonas/Malleomyces) mallei) or are life-threatening to the horse. An example of life-threatening bacterial disease is the anaerobic clostridial infections that affect horses in several different ways. Tetanus can develop from deep infection with C. tetani even in an apparently trivial skin or foot injury and several gas-forming clostridial species can cause massive necrotizing damage to the skin and subcutaneous tissue. Of course these diseases also illustrate the point that the specific circumstances of a wound may dictate the species of bacteria that can survive therein. Clostridia require anaerobic conditions and some other species require either acidic or alkaline environments.

In order to treat many of the bacterial skin diseases (whether primary or secondary) it is important to know the species involved and where possible its antibiotic sensitivity profile. There are few antibiotics that are available for horses and so careful/rational use is essential. Diagnostic bacteriology may be helpful but it can also be misleading – the underlying primary disease can be immunological, for example, and the detection of potentially pathogenic bacteria on the diseased skin may not be the most important aspect of the disease. Diagnostic investigations for bacterial disease are well established but it is important that a truly representative culture is achieved and therefore a presumptive diagnosis should be made and the sampling adjusted to take account of where the bacteria might best be identified. Deep swabbing from the site of a biopsy or culture of a whole biopsy specimen may be indicated. There may be some wish to surgically prepare the site before biopsy or swabbing but this will clearly have a very significant effect on the outcome of any attempt to culture the whole spectrum of bacteria involved (see p. 64).

If a diagnosis of bacterial skin disease is made and treatment applied it should resolve! If it does not, there are several reasons that can be considered.

1. The delivery of the drug to the site of infection is not effective either because the drug is not administered appropriately or because the site precludes delivery.

2. The drug is wrong for the bacterial species concerned or the wrong dose or route is being used.

3. There has been a change in the bacterial species or the sensitivity of the cultured organism.

4. The diagnosis is wrong.

In spite of the theoretical benefits of antibacterial medications, many skin diseases do not respond fully or in some cases at all. If treatment fails then a full review of the case should be carried out.

Some bacterial diseases such as glanders and sporotrichosis are very serious and statutory control measures may be required to eliminate the conditions.

Abscess

Profile

An abscess is a circumscribed collection of inflammatory debris (pus) developing from acute or chronic localized infection and/or tissue destruction. Abscesses are relatively common in horses in particular locations – the foot is possibly the commonest site. Frequently in the early stages they are accompanied by pain, heat, swelling and perilesional inflammation. Latterly the inflammation becomes liquefied and the abscess becomes fluctuant. Then the overlying skin thins and finally ruptures, discharging the inflammatory debris.

Dermal abscesses may be infected or sterile – infection is not a prerequisite for abscessation but it is the most common cause. Infected abscesses can be caused by a variety of organisms; the more common organisms involved are Streptococcus equi, Staphylococcus spp. and Corynebacterium pseudotuberculosis. Streptococcus equi equi infection (strangles) often manifests as discharging abscesses associated with lymph nodes around the head (submandibular and parotid in particular). Systemic (haematogenous) dissemination of infective emboli (septicaemia/pyaemia) is a potential component of severe infections with Streptococcus equi equi (bastard strangles) and Rhodococcus equi and cutaneous abscess can be found with both infections. Abscesses due to Clostridium spp. are often associated with intramuscular injections and particularly with non-antibiotic and mildly irritant drugs. Sterile abscess usually develops in response to non-infective tissue necrosis. It can also develop if infection is controlled but inflammation and inflammatory debris persist.

The infecting organism usually gains access through skin wounds but can be disseminated into the skin through the blood and/or the lymphatic system or as extensions from infected foci in adjacent subdermal tissues. Ectoparasites may be responsible for transmission of infection into the skin, for example in some cutaneous abscesses such as those due to Corynebacterium pseudotuberculosis in the pectoral and chest regions and widespread small abscesses due to Histoplasma farciminosum (epizootic lymphangitis) (Welsh 1990).

Key points: Abscess

1. Cutaneous and subcutaneous abscesses are a regular clinical presentation. The foot is the most common anatomic site.Sterile abscesses also occur. Some remain localized, others are less defined.

2. Focal or diffuse swelling with signs of inflammation. Size varies. Single and multiple abscesses are encountered.Characteristic early painful focus that becomes softer and then bursts with purulent discharge. Purulent discharges tend to cause skin inflammation below the abscess site.

3. Diagnosis is based on clinical examination and ultrasound examination. Aspiration can help to confirm the diagnosis and the infective organism.

4. Treatment involves establishing drainage of the abscess site.Antibiotics may be appropriate in some sites and the tetanus status of the animal should always be considered.

5. In most cutaneous sites the prognosis is good but this may be varied when the underlying cause is difficult to resolve. Multiple abscesses can occur and when they are at different stages of development different approaches may need to be taken.

In immunocompromised horses (such as Cushing’s disease cases) oral abscesses are particularly common from even minor infections of small abrasions.

Clinical signs

Clinical signs vary widely with location, number and cause. For example, an abscess within the hoof capsule causes extreme pain while a superficial abscess on the skin may be only painful on palpation.

There is a characteristic progression from a hard, painful nodule surrounded by an area of acute inflammation with heat and swelling (Fig. 6.1) to a softer texture with skin thinning in one site (usually). The fluid may fluctuate on palpation in the later stages.

Figure 6.1 This large, hot swelling developed over some days following an intramuscular injection. Initially it was firm and painful on palpation. After several days a thinner area of skin was identified and under ultrasound guidance an incision was made into the abscess.

The overlying skin becomes thinner and eventually bursts. The accumulated debris/pus is usually fluid in consistency at the stage of bursting and if in a suitable site it may drain under gravity. In some cases there is no fluid pus and the site is made up of necrotic tissue that is far more difficult to remove.

Purulent discharges can cause significant skin damage (Fig. 6.2).

Figure 6.2 (A) This donkey had a large abscess on the shoulder. (B) The discharge caused extensive dermatitis down the skin.

Multiple interlinking cutaneous abscessation involving relatively small individual abscesses is a feature of epizootic lymphangitis (see Fig. 7.20), glanders (see p. 161), sporotrichosis (see p. 178) and ulcerative lymphangitis (see p. 155) amongst some other conditions. The causative organisms and the epidemiological situation may help to establish the cause and the likely outcome.

Oral and facial abscesses are common in horses suffering from Cushing’s disease (hyperadrenocorticism).

Clostridial abscess is characteristically associated with gas formation and has a less localized character. Pain and local swelling are often severe and there may be extensive sloughing of the skin and underlying necrotic muscle, etc. (Fig. 6.3). The affected horse is invariably very ill until the infection is controlled.

Figure 6.3 Clostridial infection of an injection site resulted in extensive skin necrosis and a prolonged recovery time with extensive scarring

Slide courtesy of Mark Cubberley.

Chronic (cold) and sterile abscesses are usually well walled-off and may or may not be attached to the skin and there may be a relatively small discharging sinus tract leading to a much larger subcutaneous sterile (or infected) necrotic abscess. This circumstance quite commonly occurs with periapical abscessation of the mandibular and premolar maxillary cheek teeth. In the case of the maxillary teeth the skin sinus may be obvious over the apices of the teeth on the face. Discharging sinuses on the lower jaw may be much more difficult to identify (Fig. CD6 • 1) .

Purpura haemorrhagica (see p. 275) can be a sequel to prolonged abscessation (particularly that due to Streptococcus equi equi, S. equi zooepidemicus) whether or not the abscesses are satisfactorily treated and/or drained.

Note

• The purulent discharge from most abscesses is laden with bacteria and this can be a significant source of infection for other animals and, indeed, other sites of injury on the affected horse.

• Horses suspected of having either strangles or Wyoming strangles should be isolated before and after abscesses are surgically drained. Discharging abscesses also attract flies (both surface feeders and myiasis species).

• The discharges and swabs, instruments etc., from the serious diseases such as strangles (Streptococcus equi), epizootic lymphangitis (Histoplasma farciminosum), glanders (Burkholderia (Pseudomonas/Malleomyces) mallei), sporotrichosis (Sporothrix schenckii) and Corynebacterium pseudotuberculosis must be effectively disinfected.

Cheek abscess

Profile

The specific development of single abscess or more usually multiple, small, interlinked abscesses on the side of the cheeks is a recognized entity. Acute abscess formation at or around the lateral commissure of the mouth is typical. It is possibly caused by dirty bits, dental damage to internal mucosa of the cheek or penetrating wounds due to barley or oat awns etc.

Key points: Cheek abscess

1. A specific entity affecting the cheeks as a result of either abrasion or penetrating foreign bodies from outside or within the mouth. Usually streptococcal organisms such S. equi var.zooepidemicus are involved.

2. Multiple, painful focal abscessation with small discharging sinus tracts and a generalized local painful swelling (suggestive of cellulitis) is presented. Abscessation seldom develops into a single focal mass and is more often a series of smaller linked tracts.

3. Diagnosis is based on clinical grounds and on culture from the discharging purulent material.

4. Treatment with antibiotics and irrigation is indicated.Antibiotic selection will depend on culture results. Most are streptococcal, however, and penicillin is usually a reasonable first choice approach. The cause must be addressed if it can be identified

5. The prognosis is good but some local deformity, alopecia and loss of pigmentation may occur.

Clinical signs

Acute, painful, hot swelling of the commissure of the mouth or adjacent cheek area is typically encountered. Multiple small interlinking sinus tracts with small areas of exudation over pointing abscess can be seen (Fig. 6.5).

Figure 6.5 Cheek abscess. Note hair loss overlying the abscess.

Hair loss (alopecia), pigmentary changes and scarring of the skin overlying the abscess are common. Although oral lesions heal well for the most part, chronic ulceration at the site of cheek abscesses is also common. A few cases are slow to heal and then foreign bodies or repeated traumatic insults should be suspected. A careful examination of all tack and a dental examination must be performed.

Note

Oral abscessation (possibly involving the cheeks) is a common secondary feature of pituitary pars intermedia dysfunction (PPID/Cushing’s disease/ pituitary-based hyperadrenocorticism) in horses over 15 years of age. Unless this is addressed, the affected area may not heal at all.

Differential diagnosis

• Bit injury in the mouth: usually this is focal and has an obvious cause.

• Buccal laceration from teeth inside the commissure: multiple buccal and possibly lingual ulceration usually present.

• Foreign bodies: these are difficult to identify.

• Tumours (ulcerated melanoma, squamous cell carcinoma or sarcoid in particular): usually these are firmer and better defined and are seldom patently infected.

Diagnostic confirmation

• Cuts or wounds inside the mouth adjacent to the abscess.

• Typical clinical features of heat, pain and swelling with an interconnected multifocal inflammatory nature.

• Culture of discharges and swabs taken from within the lesions usually identify Streptococcus spp. bacteria.

• Ultrasonography can help to differentiate this from tumours and other inflammatory and non-inflammatory disorders.

Treatment

Further damage or irritation in the area should be avoided and where possible any causative factor (tack/bit or dental conditions) should be addressed. Working horses may be amenable to use of a bitless bridle, at least in the short term to allow the area to heal.

Alternating hot and cold fomentation will hasten development of the abscess, but it may be impossible to apply these principles to the lesion if the area is very painful and handling is resented. Sedation and careful drainage can be helpful.

Surgical lancing and drainage when the abscesses ‘point’ sufficiently is the treatment of choice. Early use of antibiotics should probably be avoided. Once the condition is in a suitable state (or if the inflammation is having a systemic effect on the horse), antibiotics should be administered. Penicillin is usually the first choice but the culture results may have a bearing on the final choice. Once healing starts it usually progresses rapidly.

Bacterial folliculitis and furunculosis (acne/superficial pyoderma)

Profile

Folliculitis is inflammation of the hair follicle with accumulation of inflammatory cells within the follicle lumen. Most commonly it is caused by bacteria involving coagulase-positive Staphylococcus spp. (S. aureus, S. intermedius and S. hyicus) (Chiers et al 2003) and Streptococcus species are isolated. Multi-(methicillin)-resistant Staphylococcus aureus (MRSA) has recently been identified from healthy horses (Yasuda et al 2000) and whilst the significance of this to horses has yet to be established it is becoming more of a concern to veterinarians.

Key points: Bacterial folliculitis and furunculosis

1. A relatively common occurrence – often with Staphylococcus spp. or Streptococcus spp. bacteria. Most cases appear to relate to hygiene or cutaneous immunity problems. Exposed, hairless (glabrous) skin surfaces are most often involved, e.g.the sheath/groin and the sides of the face, but the saddle region and body skin can be seriously infected.

2. A painful localized region in which papules can be ‘ felt ’ in the skin that develops into an inflamed, exudative and oedematous area is typical; these lesions are often circular and exudative in the early stages. Some develop into isolated individual pustules while others coalesce into abscesses or furunculosis.

3. Diagnosis is usually simple but the changes are not pathognomonic for any particular bacterial species; early lesions can easily be confused with various forms of dermatophytosis.Pain usually indicates Staphylococcus spp. bacteria but other species may be involved.

4. Treatment is sometimes problematic and recurrences are common. Local warm washes with antiseptic shampoos such as chlorhexidine and povidone-iodine, systemic antibiotics and oral potassium iodide are all helpful. Skin and stable hygiene are important. Treatment courses of several months may be required.

5. The prognosis is fair but some cases take a long time to resolve and the healing lesions are sometimes alopecic and/ or have permanently altered hair colour and quality.

As the infective process proceeds, degeneration of the hair follicle leads to infection of the surrounding dermis and subcutis. This is called furunculosis. The condition is usually related to areas of rubbing due to dirty or ill-fitting harness, rugs or saddle cloths. Most cases occur in late spring or early summer coincidental with hair shedding, humidity and increasing work with dirty or poorly maintained tack. Poorly groomed horses have a higher tendency to develop the condition. Horses in poor condition, or immunocompromised animals or those on medications likely to be immunosuppressive also seem more likely to be affected (Inokuma et al 2003).

Dermatophilus congolensis is a common cause of bacterial dermatitis and folliculitis and is considered in its own right (see p. 156). More rarely Corynebacterium pseudotuberculosis is responsible and this too is considered in its own section (see p. 155). Fungi including dermatophytosis and cutaneous mycetoma/fungal granuloma can also be involved (see p. 183).

Clinical signs

Usually a rapid development of small (2–5 mm diameter), painful papules is reported – in the early stages these may be more easily felt than seen unless the hair is closely clipped (Fig. 6.6). The first sign may be the presence of roughly circular areas (2–6 cm diameter) of erect hairs with a small matted crust around the base.

Figure 6.6 Bacterial folliculitis due to Staphylococcus intermedius infection.

The so-called truncal form is restricted to the body trunk and is commonly called Canadian horse pox (a considerable misnomer), contagious pustular dermatitis or contagious acne, and is associated with poor tack hygiene in early spring in particular. This form may represent a deeper penetration of the Staphylococcus spp. infection and is very similar to infection with C. pseudotuberculosis, the cause of ulcerative lymphangitis on the limbs and neck.

Palpation is usually resented and pain is usually marked. The papules develop an ulcerated central area with extensive local oedema and exudation (Fig. 6.7).

Figure 6.7 (A, B) This 9-year-old Shire-cross gelding developed a very painful folliculitis in the sheath area. There was a great deal of inflammatory swelling and severe resentment to palpation.

Where multiple infections coalesce locally, it becomes a carbuncle or boil; these occur in horses with saddle rash/scab (Staphylococcus aureus, S. intermedius), heat rash, cheek abscess (see Fig. 6.5) and ‘pigeon-breast’ (Wyoming strangles due to Corynebacterium pseudotuberculosis).

More diffuse coalescing folliculitis leads to a furunculosis and focal areas of necrosis, especially on the distal limbs. This may be a result of an inappropriate response to bacterial toxins or a hypersensitivity response (Fig. CD6 • 3A–C) .

There may be evidence of enlarged cutaneous veins and lymphatics leading from the affected sites (Fig. 6.8).

Figure 6.8 This was one of several young Thoroughbred horses in a racing yard that, over a period of some weeks, developed very painful cutaneous inflammatory foci associated with Staphylococcus aureus infection. Note the prominent lymphatics leading away from the sites. Careful attention to hygiene, warm water antibacterial washes, and a prolonged course of potassium iodide orally gradually resolved the problem.

Figure courtesy of Chris Riggs.

There is seldom pruritus and in uncomplicated cases there are no systemic signs of illness.

Healing lesions become flat and alopecic and may develop leukoderma and leukotrichia (white saddle or girth marks may arise from this as well as other causes including persistent trauma).

Some cases of dermatophilosis are indistinguishable from this condition and indeed could be grouped in the same category of folliculitis. This is particularly so for the common Dermatophilus congolensis infection that involves the pastern and cannon regions (mud fever) and the back (rain scald) (see also p. 156).

Some superficial lesions fail to heal and these may become deeply infected (furunculosis) with sinus tracts and discharging sinuses. Chronic granulation tissue diffusely infiltrated with microabscesses and sinus tracts is often classified as botryomycosis (pseudomycetoma) on the skin (see Fig. 6.10), or as scirrhous cord at the site of non-healing castration wounds (Fig. CD6 • 4A and B) .

A staphylococcal infection of the palmar/plantar pastern region (see p. 153)is a recognized entity in the pastern dermatitis complex (see p. 471). Although this condition is caused by the same group of bacteria it is a recognizably different syndrome – usually poor hygiene is not the main instigation but skin maceration due to continued wetting and a primary superficial infection may be involved.

Differential diagnosis

• Dermatophilosis: this can be difficult to differentiate without bacteriological cultures and skin biopsies but it usually affects the dorsum of the back and the lower limb regions.

• Other bacterial pyoderma (e.g. Streptococcus equi equi– strangles).

• Fungal folliculitis: these are usually much more long-standing, slower progressing and isolated and are seldom painful; fungal organisms can be identified in skin biopsies.

• Dermatophytosis: this seldom shows significant pain and exudate but secondarily infected dermatophytosis lesions can be similar; the fungal organisms can be identified in skin scrapings and hair plucks.

• Onchocerciasis: this condition induces a more diffuse change in most cases and is seasonal and regionally restricted; the microfilariae or adult parasites can usually be identified directly or in biopsies; these lesions can be secondarily infected as well.

• Generalized granulomatous disease (enteritis/sarcoidosis) multi-organ involvement.

• Pemphigus foliaceus: generalized focal or diffuse junctional exfoliative disorder.

• Greasy heel syndrome: complex of conditions resulting in a chronic verrucose thickening of the pastern with pseudo-tumour formation and granulation, infection and ulceration.

Diagnostic confirmation

• Clinical features and pain are usually characteristic but mixed infections are common.

• Biopsy is usually avoided if possible but a fine biopsy punch (3 mm) may be useful, particularly for culture.

• Bacteriological culture and sensitivity are important but deep biopsy may be needed for significant culture (Fig.CD6 • 5A and B) .

Treatment

Three aspects must be considered during the management of a case of pyoderma. These are:

1. Control of the specific lesions on the affected horse. The affected areas should be clipped and washed with warm water and an antiseptic such as chlorhexidine or povidone-iodine surgical scrub solution. This should be left on the skin for some 5–10 minutes before being rinsed off thoroughly, again with warm water. Washes are useful also for controlling the spread of the organisms responsible (Fig. CD6 • 5C and D) .

Proprietary antiseptic shampoos are available (usually containing chlorhexidine, povidone-iodine or hexachlorophene) but human equivalents should probably not be used.

Open ulcerated pustules and abscesses should be managed carefully and hydrosurgical debridement is an effective mechanism for the removal of much of the bacterial content of a focus of infection. Clearly this would only apply to limited areas.

Parenteral antibiotics are useful adjunctive therapy. Culture and sensitivity testing should always dictate the selection but the most common antibacterial used is potentiated sulphonamide because it is usefully effective and can be maintained for weeks if necessary by oral dosing for up to 2 weeks (White 1988).

Many isolates are resistant to parenteral penicillin but where they are shown to be sensitive this can be useful, at least initially. Enrofloxacine can be effective also but should not be used in horses under 2 years of age. Although vancomycin and other potent drugs in that class would probably be effective (Orsini et al 2005), their use represents irresponsible antibiotic use except in absolutely exceptional circumstances.

Note

Donkeys and mules are known to eliminate the drugs faster than horses so either the dose interval or the dose itself may need to be adjusted.

1. Topical dressing with silver sulfadiazine or where feasible silver wound dressings should always be considered. Modern adhesive dressings such as adhesive hydrocolloid (DuoDerm, Convatec, UK) are very useful, even on the body trunk, and can be combined with hydrofibre silver dressings (Aquacel Ag, Convatec, UK) to provide an excellent local antibacterial effect.

2. Prevention of further outbreaks on the affected animal. This usually involves significant improvements in the overall hygiene and the health status of the horse. However, even in the best management systems with the healthiest horses, recurrences are to be expected. Turning out into sunshine (but avoidance of wetting by rain) is helpful and where this cannot be done, ultraviolet and infrared lamps can be helpful. Continued washing is to be avoided as far as possible but may be unavoidable. Recurrences are nevertheless common.

3. Control of the environmental contamination and prevention of spread to other horses in contact. All tack and rug contact with the affected skin should be prevented. Washing and sterilizing of all tack, grooming equipment, rugs, etc. is strongly advised Tertiary amine disinfectants have strong antibacterial properties without much risk of damage to tack or other equipment. Fumigation of all tack with formaldehyde gas is often the best approach to sterilization of the equipment but is more difficult (see p. 86).

Severe outbreaks in stables have been controlled with autogenous vaccines but reliance on this is not advisable. However, in the investigation of a widespread problem in a stable yard, hygiene is imperative. Regular cultures and if possible genetic studies on the organisms involved may allow the identification of a carrier animal but even then hygiene is the critical issue. Stable managers should always address hygiene as a priority – individualizing and regular sterilization of tack, grooming equipment, water and feeding bowels and buckets and in particular rugs and harness are important aspects of control.

Surgical procedures such as castration, prosthetic laryngoplasty, check ligament desmotomy in stables that are known to have a major Staphylococcus spp. problem must be considered very carefully. In the event that MRSA is identified in any stable, the local environmental health office should be advised. The human implications are considerable but it is not yet clear how much human infection derives from horses and how much equine infection is related to carrier humans.

Bacterial granuloma (botryomycosis/staphylococcal pseudomycetoma/deep pyoderma)

Profile

This is a bacterial pyogranulomatous disorder most often associated with skin injury such as laceration, puncture or surgery. Most lesions occur on the limb or lip/chin regions. The clinical appearance is due to the formation of miliary interlinking microabscesses from Staphylococcus spp. infection. In spite of the names botryomycosis and pseudomycetoma, there is no evidence of a fungal aetiology.

Clinical signs

A slow or non-healing wound with induration of the wound margins (Fig. 6.9) is typical. A chronic purulent discharge from one or more sinuses within the wound site is common.

Figure 6.9 Bacterial granuloma (botryomycosis/staphylococcal pseudomycetoma) showing the typical non-healing indurated margins. The lesion showed an intermittent slight purulent discharge.

Exuberant granulation tissue forming rosettes of complex granulation tissue and multiple microabscesses with sinuses may develop (hence the derivation botryo– meaning grape-like) (Fig. 6.10). Small yellow-white granules/grains may be visible in the purulent discharge. Tracking along lymphatic vessels sometimes occurs. It can reach large (or even enormous) proportions. The condition is seldom painful or pruritic. Affected horses do not often show systemic illness, but very large lesions can be debilitating.