White Line Disease

Patrick Murray

Kerry Pride

Dr. Jeremy D. Hubert

Introduction

White line disease (WLD), also known as dew poisoning, hoof rot or yeast infection, is a descriptive term of a disease process that involves the separation of the hoof wall. There are multiple factors that have been incriminated as the cause of WLD, but none proven. The contributing factors are environmental, nutritional, mechanical, and infectious.

Anatomy

The hoof wall consists of three layers, the stratum externum, stratum medium, and the stratum internum. The stratum externum (external layer) is a thin layer of cells that gives the wall its smooth shiny appearance. The stratum medium (middle layer) consists of horn tubules and intertubular horn. It is this layer that makes up the bulk of the hoof and is the main supportive structure of the wall. In dark hooves this layer is pigmented, except for the deepest layers. The stratum internum is nonpigmented and attaches the hoof wall to the third phalanx or coffin bone. The white line is defined as the junction between the sole and the hoof wall, and this is where the initial insult of WLD is thought to occur. The disease process begins when a separation occurs in the nonpigmented horn at the junction between the deepest layers of the stratum medium and laminar horn of the stratum internum.

Etiology

Heat, humidity and excessive moisture are predisposing environmental factors for WLD. The disease occurs worldwide, but there is a greater geographic distribution in the southeastern United States.

Mineral, vitamin, and amino acid imbalances are nutritional factors, which may lead to WLD. In particular, a proper balance of biotin, lysine, methionine, selenium, zinc, and calcium: phosphorus ratio all contribute to hoof quality, growth rate and strength. Imbalances in these substances adversely affect hoof health and predispose to WLD.

Mechanical factors such as improper trimming, unbalanced feet, clubbed feet, hoof cracks and gravel all effect the weight bearing surface of the foot. This abnormal weight-bearing can alter the pull of the deep digital flexor tendon, which can cause the bond between the junction of the stratum medium and stratum lamellatum to weaken and separate.

Infectious causes vary from bacterial to fungal. Attempts to culture usually result in multiple bacteria being isolated. These organisms, often normally found in the soil, are usually not the initial cause, but instead grow secondary to an altered hoof state. This microbial or fungal growth adds further damage to the hoof wall laminae junction.

Presentation

The most common presentation for WLD is an unexpected wall separation observed at the time of hoof trimming. Mild lameness may or may not be present. The separation is usually seen at the toe and quarters as an undermining of the wall or a crumbling of the wall’s ground surface. The separated area becomes filled with a black to gray crumbly material, but it can progress to a black liquid discharge. One or more feet on the same horse can be involved and one or more horses on the farm may be involved. In early stages of WLD, soles may be tender to hoof testers, there is a flattening of the sole and poor hoof growth. Later stages of WLD where there is extensive hoof damage, the process can mimic laminitis both clinically and radiographically.

Treatment

Treatment for WLD is based on four principles: (1) Remove all undermined hoof wall; (2) Keeping the foot dry; (3) Prevent further infection; (4) and stabilize the remaining hoof wall.

Separated, diseased hoof wall must be removed in order to prevent further infection. The debridement must be complete until healthy hoof wall is identified. Debridement may have to be repeated every two weeks, or until a solid junction is encountered. Removal of the hoof wall allows air and light to enter and better for the area to dry. Thus organisms are less likely to grow.

This debrided area is then medicated with one of the following: betadine ointment, 2% iodine, copper sulfate, turpentine, or methiolate. These medications must be continuously applied until the hoof wall is completely grown out. Once topical medication is applied, the defect is packed with gauze and secured in place with duct tape or another type of water resistant tape. The packing needs to be changed at regular intervals. An alternative treatment is to apply a medicated Equilox ^TM once the area is debrided to a solid junction. The medicated Equilox, a polymethylmethacrylate, will not only help with the infection, but also gives stabilization to the foot. A method of keeping the foot dry is to apply an E-Z boot or similar rubber shoe.

The type of shoe used to stabilize the hoof wall will depend on the extent of damage to the hoof wall. If the defect is small the hoof is balanced and shod normally. If the defect is large a heart bar shoe is commonly used. The heart bar shoe supports the heel and transfers some of weight off the hoof wall and on to the frog. This treatment continues until the hoof is completely grown out.

Prevention

Prevention of WLD can be difficult, primarily due to the fact that an exact causative agent remains unknown. Maintaining proper and routine hoof care, such as regularly picking out feet and maintaining clean stalls, is an important aspect of prevention. Having one’s farrier examine each foot while shoeing is also important especially if one is in a predisposed geographical location.