Do the diseases we’d almost forgotten about still exist today? Patrick Pollock FRCVS sheds new light on old problems
These marble-sized lumps are the work of the warble fly, whose correct name, Hypoderma, gives you an idea of its intent (from the Greek “hypo”, meaning under, and “derma”, meaning dermis or skin).
The fly lays eggs on a variety of animals – usually cattle, but also horses, sheep and occasionally humans. The eggs hatch and the larvae set off on their journey to their preferred site for development – usually the fat along each side of the dorsal spinous processes of the vertebrae or, for another species of the fly, the tissue around the oesophagus. They can migrate as far as the brain, causing severe and fatal neurological disease.
The larvae settle to form lumps under the skin, where they pupate and eat their way out as mature flies. The advent of modern insecticide treatments, such as the drug ivermectin, has greatly reduced the number of warble flies – the disease was last reported in the UK in 1990 and has been eradicated from much of Europe. Thankfully, warble lumps are rarely encountered in horses today.
While this hoof disease receives little attention in modern veterinary textbooks, canker is perplexing nonetheless. Correctly termed hypertrophic pododermatitis, it affects the same area of the foot as thrush. However, rather than leading to necrosis (death) of tissue, canker spreads through living tissue and causes the keratin-producing cells of the hoof horn to overgrow.
The heel bulbs and the crevices, or “sulci”, of the frog quickly fill with spongy, grey tissue that resembles cauliflower and produces caseous (cheesy) pus. The production of new, abnormal tissue led to the name canker, because the condition was mistakenly thought to be a form of cancer.
A possible cause is infection with bacteria known as anaerobes, which can grow without oxygen. The many different treatments suggest that no one method is very effective, although vets are united in the idea that there is no value in being conservative.
As much abnormal tissue as possible must be cut away, before the foot is dried out and treated with a topical treatment such as copper sulphate, the antimicrobial metronidazole, potassium permanganate or benzyl peroxide.
This affects the supraspinous bursa located on top of the dorsal spinous processes of the vertebrae, where inflammation may result in abscesses that discharge foul-smelling pus.
Although now rare in the UK, fistulous withers is commonly encountered in working horses, donkeys and mules in low- and middle income countries, where poverty can drive owners to work their animals with poor-quality, ill-fitting tack. A small wound or rub quickly develops infection, which can then extend into the deeper tissues and leave them exposed.
Because the affected area is on the top of the back, the fluid produced does not drain out well. Left untreated, infection can eat into the bone of the spinous processes.
An old name for cracks, mallenders develops in the skin at the back of the carpus (knee). Most common in heavier, feathered breeds, it can occur at other sites, including the front of the hock (where it is colloquially termed sallenders) and around the fetlocks, spreading to the lower legs in severe cases.
Mallenders is correctly termed hyperkeratosis, meaning “too much keratin”, the protein making up much of the connective tissue in skin, hair and hoof horn. Signs include thickened, greasy skin with scabs overlying cracks and small wounds, sometimes with secondary infection.
While there is no cure, hyperkeratosis can be managed by soaking the area and removing scabs, before applying emollient creams to soften the skin. In severe cases, clipping the hair is effective.
This ancient disease – recorded in veterinary texts from as early as 1731 – is similar to fistulous withers, but involves inflammation of the occipital bursa, a fluid-filled compartment at the base of the skull; this may result in a pus-filled abscess. Infection can extend to the nuchal ligament, which connects the skull to the body, causing intense pain and creating multiple fistulous (hollow) tracts.
“Poll evil” refers to the horse’s behaviour when objecting to a halter, bridle or other piece of tack, which in times past would have rendered him unfit for work. The condition can now be treated with anti-inflammatories and antimicrobials, often with surgical drainage of the affected area.
The classic poll evil infection was caused by Brucella bacteria, which are rarely implicated in the condition today. With any infection or swelling in the poll area, however, it is important to rule out the presence of Brucella. The bacteria are linked with brucellosis, a disease typically found in cattle that can also infect humans.
We now know that navicular is not one disease, but a group of conditions affecting different parts of the complicated anatomical structures in the back of the horse’s foot.
The terms navicular disease or syndrome, palmar or caudal heel pain and podotrochlosis are all used to describe the presentation; this is typically forelimb lameness with pain localised to the foot and, more specifically, to the area around and including the navicular bone.
An affected horse may “point” a painful foot.
As a fulcrum for the deep digital flexor tendon, the navicular bone is under extreme pressure. It was thought that this pressure restricted blood supply to the bone and eventually affected its density, but pain can arise from the bone or its cartilage, the deep flexor tendon or one of the many ligaments holding the bone in position.
With the advent of magnetic resonance imaging (MRI), surgeons could at last “see” into this hard-to-reach area, detecting inflammation and even investigating inside the navicular bursa.
Conformational traits such as upright pasterns, small feet or narrow, upright feet certainly predispose a horse to problems. A long toe and a low heel will also place constant stress on the navicular bone, even as the horse stands at rest.
On either side of the hoof, roughly level with the coronary band, sit the collateral cartilages, movable structures that are slightly springy to the touch. In years gone by, draught horses often sustained injuries and abrasions to the skin overlying the collateral cartilages, leading to a condition known as quittor, or gravelling.
Cartilage has no direct blood supply and takes its nourishment from surrounding tissue. Once infected, it develops multiple small abscesses containing dried-out pus. Since antimicrobials cannot penetrate these abscesses, successful treatment requires surgical removal of the affected cartilage – a process that is complicated by the proximity of the coffin joint.
Most veterinary practices still have a set of “quittor knives” for this purpose. Modern techniques such as MRI and ultrasound can be useful to determine the extent of infection, allowing surgeons to carefully plan excision of the infected tissue.
Technically laminitis, founder presents with marked foot pain and reluctance to bear weight. Laminitis is as common today as it ever was – maybe more so, due to the huge number of obese horses with equine metabolic syndrome at increased risk.
Inflammation around the laminae, the finger-like projections that attach the pedal bone to the hoof, results in separation of these structures. The pedal bone rotates and then sinks; if left unchecked, it will penetrate the sole. This painful condition may result in permanent lameness or the requirement for the horse to be euthanised.
Ref Horse & Hound; 9 July 2020