Equine Canker

An article in The Horse October 29^th on equine canker refers to the fact that the disease has typically ‘frustrated veterinarians’ and refers to a new treatment with a chemotherapy drug.

I’ve collected dozens of photographs of examples of equine ‘canker’ and I have yet to find a case of canker in an otherwise 'normal' hoof.  It is a disease almost unknown in New Zealand where I live, despite some pretty horrendous hoof forms.  

So does the disease cause or result in the pathological hoof form, or vice versa? Are all cankers caused by the same thing? Is infection secondary to an inflammatory condition?

The drawing from Diseases of the Horse by H Caulton Reeks (below left) shows a type of warty growth reminiscent of epidermodysplasia verruciformis which is at its most extreme in the case of Dede Koswara, a Javanese fisherman whose hands are pictured below right. 

This is uncommon these days and there may be a genetic component to the condition in horses as there is in humans, which allows a virus that is normally managed easily by the immune system, to wreak havoc. However, we still have to ask, why would it affect the soft horn and soft tissue of the heel region of hoof?

There are many types of lesion that are described as canker. Most start in the frog but it can involve the solar and laminar coria in the heel region. There are types of lesion that obviously are just extreme degeneration of the frog but others in which the tissue has become hyperactive.  Just as proud flesh on a leg wound is the body's attempt to heal a lesion in an area that lacks underlying muscle and in which the dermis is under a lot of tension, it seems logical that the proliferative form of equine canker is a similar process.

It is necessary to look beyond the symptoms – to the causes  - which very often lie in a pre-existing pathological hoof form, combined with other factors such as stress and possibly diet. 

There are two broad types of hoof that are prone to canker and other degenerative diseases. The narrow, high heeled, inwardly contracted form as typified by the photo on the left below, and the splayed, vertically compressed form, as typified by the photo on the right below.

In both, the highly vascular tissue of the laminar and solar coria, and the less vascularised frog corium are subjected to abnormal persistent pressure and concussion and reduced flow of oxygen and nutrients. As a result, the tissue struggles to survive and in its weakened state, horn production cannot keep pace with the processes of exfoliation and pathogenic action. Contusions and infection exacerbate the situation. The discomfort from this makes the horse even less likely to load its hooves properly with a range of other consequences for overall health and well being.

So what is going on in these hooves that results in such degeneration? Although I have typified two broad types, there are many variations.

The narrow contracted hoof

• The long vertical heels and long high arched bars greatly inhibit or even prevent any normal lateral expansion of the heels. 
• The frog corium is permanently compressed inwards by the heels and bars. 
• The central frog sulcus becomes turned in on itself creating a cleft in which pathogens proliferate.
• The depth of the collateral groove creates a breeding ground for pathogens which eat away at the sides of the frog pad. 
• The arteries that supply the frog corium run along the top of the bars and the pressure from the ground onto the rigid arched bars may impact upon them. 
• The steepened HPA may mimic the arrangement of the foot bones that occurs naturally as the fetlock descends under load in which there is a reduction of blood flow in the hoof and at full pastern extension, flow is cut off entirely. 
• The lateral cartilages are deformed inwards.
• The solar and laminar coria in the heel region are placed under an abnormal persistent pressure. 
• The fibrocartilage in the DC cannot receive stimulation and degenerates. 

The splayed, compressed hoof

• The wall and bars receive load in too shallow a direction resulting in the hoof splaying/flaring. 
• The hoof lacks vertical depth and structure exposing all the volar surfaces to abnormal compressive and concussive force.
• The long but forward running and collapsed heels and bars, sit across and press onto the sole corium.  
• The unpigmented bar horn may fuse with sole horn sometimes overlaying almost the entire sole.
• The bars may also wedge sideways into the base of the frog. 
• Because the frog pad is exposed to abnormal persistent pressure, the digital cushion may displace.  
• Depending on how the heels deform, this sort of hoof may not be able to expand laterally, or it may expand too much resulting in strain on the back of the coffin joint. 
• The lateral cartilages are pulled under and deformed by the underrunning heels.
• The fibrocartilage in the DC cannot receive stimulation and degenerates. 

In both, the balance between the flexor and extensor muscles systems may be disrupted with consequences for correct limb flight and for the stay apparatus.

In either extreme, the horse is unable to either land or stand with comfort. It is a creature of movement; it evolved to be on its hooves for almost its entire existence. It is not anatomically, physiologically or psychologically equipped to be recumbent for any length of time. It needs healthy pain free feet to achieve a balanced frame and it needs a balanced frame to have healthy feet.

There may well be other contributory factors, and it seems logical that stress plays a significant role in the genesis of these conditions.  Diet may also be a contributory factor especially in the horse with weak, splayed feet.

Persistent and abnormal stress impacts on the immune system and a compromised immune system is less able to deal with opportunistic infection. A horse that is in constant pain, or is suffering from a dietary overload or deficiency, is feeling persistently anxious about some aspect of its management etc, may have chronic imbalances in the complex and delicately balanced chemical messengers of the neuro-endocrine and peptidergic nervous systems which are constantly feeding out and feeding back in response to stimuli.  We do not know enough about the way these act and interact and the different ways in which different animals may respond, to discount this as a factor.

There can be no better advice for any owner or vet or other professional involved with horses, than to look to the conditions in which the horse evolved and see how far and in what ways the management of an unwell horse differs from that.   (see post on the essential nature of the horse)

And as for the treatment of canker - as with everything, prevention is far better than cure.  Once there is a high level of damage, veterinary treatment may be necessary to restore something approaching the status quo but the advice I would give both owners and vets is:

• Find someone who knows how to trim to restore hoof form or, or learn yourself. In particular, understand the role of overgrown heels and bars in the genesis and perpetuation of the condition.
• Treat the damaged tissue with the mildest thing first, ie follow the old medical maxim - maximum dilution is the best solution.
• Maintain hygiene and pad / boot to get the horse out into the fresh air with a kind companion to lower stress levels and facilitate self-motivated movement.
• Do not nail or glue on shoes.
• Cut out high carb feed but do not starve; the horse needs to eat almost constantly - our job is to make sure that it eats the right stuff. 
• Feed good quality (spray free if possible) WHOLE food with some salt - iodised if you are in an iodine deficient area and with added selenium if in selenium deficient area.
• Remove possible sources of toxins - including top dressing feeds with supplements. 
• Leave a non-iodised well balanced mineral mix for the horse to help itself to.
• Cut branches of unsprayed, healthy, palatable trees and a range of broadleaf plants for the horse to eat if it wants to.