Extra stress makes a hoof more susceptible to this fungus.

Warco van de Halhoeve was diagnosed with white line disease two years ago, and the promising young show jumper spent most of a year recovering.
   
“In his first year at grand prix he has won two World Cup [qualifiers] and was second in another,” said his rider, Ali Nilforushan of San Diego, Calif. “He’s an amazing, great horse.”
   
White line disease was not suspected at first, but it ended up sidelining him for eight months. “The horse wasn’t really lame, though he once in a while favored that foot. He’d be fine and then off a little,” said Nilforushan.
   
Warco van de Halhoeve’s hoof was cut almost all the way up to the coronet band to get rid of all the diseased tissue.
   
“Thank God the farrier was able to get all of it by the second time. We then had to wait for it to grow down,” said Nilforushan. “Every day we were squirting antibiotics into the hoof and keeping it dry. He couldn’t walk or do anything; he basically had to just stay in his stall for months.”
   
Nilforushan started riding the gelding again in March of 2006. “He’s 9 years old now, and this was his first year in the top level, so he’s just hitting his prime,” said Nilforushan, who purchased the 18-hand Belgian Warmblood as a 6-year-old. 
   
“Warco van de Halhoeve is by Darco, the same [sire] as [McLain Ward’s 2004 and 2008 Olympic horse] Sapphire. He is sound now and totally perfect,” said Nilforushan, who became the first rider from Iran to go to the Olympic Games, in 2000. His long-term goal is the 2010 World Equestrian Games (Ky.) and the 2012 Olympic Games (London).

Where Does It Comes From?

White line disease is a progressive infection and subsequent separation of the hoof wall—with the wall coming loose from the foot. This problem usually starts at the bottom of the hoof and travels upward as this area becomes hollowed out.
   
Scott Morrison, DVM, a farrier and veterinarian at Rood And Riddle Equine Hospital in Lexington, Ky., described white line disease as an ascending fungal infection of the white line. The technical term for this is onychomycosis, which means fungal infection of the nail, a similar condition in humans.
   
“Several studies have shown that the pathogen in white line disease is a fungus. It’s an opportunistic pathogen, which means the tissue must be somewhat damaged already in order for this fungus to invade. The fungus may become established in an old abscess tract or an area where the white line is stretched or flared due to a dish in the toe of the foot,” said Morrison.
   
A hoof that’s out of balance and under additional stress can be at risk.
   
“If there’s a flare or hoof capsule distortion, the white line may be stretched, and a small fissure or opening may occur. Fungi can invade those fissures in the white line and set up shop. The hoof wall is three layers, the stratum externum, the stratum medium, and stratum internum. The fungi usually attack the inner portion of the stratum medium—the thick, pigmented part—and the horn produced by the terminal papillae,” he said.
   
The bottom part of this layer can be seen as the white line—between hoof wall and sole—when you pick up the bare foot.
   
“The fungus becomes established there and digests the horn tissue of the stratum medium. Other areas can also be attacked if fungi get started in a toe crack or quarter crack or in some old nail tracks,” he said.
   
Any hole or break that goes through that area can allow fungi to enter. The severity can range from small cavities in the white line on the bottom of the foot—that you can see on the ground surface—to large hollow areas that extend all the way up to the coronary band.
   
“Sometimes you won’t see a cavity if the fungi entered through a small pinpoint opening higher in the wall. Then there may be a hollow area in the hoof wall with little or no sign at the ground surface. Or there may just be a small tract at the ground surface and a huge cavity higher up the wall. If it only invades the middle layer of the hoof wall, you’ll hear a hollow sound when the hoof is tapped; one of the old terms for this condition was hollow hoof,” said Morrison.
   
A lot of mild cases consist of small pockets that can be completely trimmed out when a farrier resets the shoes. “But it is important for farriers to realize that cavities that don’t trim completely out will need topical medical treatment to kill the fungus,” Morrison said.
   
There may be secondary complications, depending on how much wall is damaged.
   
“Some horses may actually have a displacement of the coffin bone if a lot of the inner hoof wall is damaged. The foot loses some of the attachment that binds the coffin bone to the hoof wall. It looks like laminitis but is not. With laminitis, the laminae fail and the bone drops, but with white line disease the hoof wall attachment between two epidermal layers fails.
   
“On radiographs it looks like laminitis since the bone will actually move or displace, depending on where the wall damage is. The bone may rotate (the front dropping down) or may sink to one side or the other, at the quarter,” said Morrison.
   
Morrison said he’s seen cases so severe that the horse had to be put down. “We’ve had coffin bones drop so much they detach from the hoof wall and penetrate the sole. Thus it is very important to catch it early,” he said.

White line disease has a characteristic appearance, with white, crumbly, chalky material.
   
“Some people, if they see a black, slimy appearance at the white line, will call that white line disease, but a black ooze is usually due to bacterial infection rather than fungal. This is an important distinction to make,” said Morrison, who emphasized that a bacterial infection would be treated differently.

Fighting The Fungus

The best way to treat white line disease is to trim out all diseased tissue. Trimming away the damaged hoof wall gets rid of most of the fungus. Then a topical medication can be applied to kill whatever fungi are left.
   
A study at Cornell University (N.Y.) a few years ago done by Mike Wildenstein, a farrier on staff there, looked at different things that could be used for killing the fungus. The most effective products are foot soak or topical gels that contain chlorine dioxide.
   
If the hoof wall attachments have been damaged to the extent that the coffin bone moves, the horse may need special shoeing for support. “This may range from frog support with a heart bar to a wall cast, in a severe case. The latter consists of a small fiberglass cast around the whole hoof wall to help stabilize the foot. Severity depends on how unstable the foot becomes and how much wall damage exists,” Morrison said.
   
If the problem is caught early it is not as difficult to treat.   

“It’s the ones that go too long that get a lot of instability and wall damage and are hard to treat. Sometimes you have to trim away so much wall that there’s nothing to nail to, nothing for attaching a shoe. Some of those horses have to stay in boots for awhile, or a wall cast to protect the foot until they grow more hoof,” said Morrison.
   
“A mild case can be treated by trimming it out and packing the area with a chlorine dioxide gel or using a soaking solution to eliminate it. A mild case can also be killed with other products like copper sulfate. I don’t like using iodine inside the hoof wall because it tends to damage the proteins of the hoof horn,” he said.
   
Iodine may also cause too much drying and flaking, making the hoof chalky and more susceptible to additional infection.

Risks For White Line Disease

Some horses seem more predisposed to this condition than others.
   
“This may be due to the hoof being out of balance—perhaps a conformational fault that causes the horse to load the foot abnormally. Some horses tend to get white line disease repeatedly, so once we get the problem cleared up I usually recommend that owners soak the feet in chlorine dioxide every couple months or so to make sure the horse won’t get it again,” said Morrison.
   
He added that the company that sells chlorine dioxide recommends disinfecting the horse’s stall, but he doesn’t think this is practical.
   
“The fungi that cause white line disease are in the soil, in wood, all over the horse’s environment,” he said. “The best preventative is just to keep the foot balanced and strong and catch any signs of infection early. There are several different species of fungi that can cause white line disease; they have the ability to break down the protein structure of the hoof wall.”
   
This opportunistic disease can also be transmitted from one horse to farriers’ tools or transmitted to human fingernails.
   
“When I work on a horse with white line disease I usually wipe my tools with a disinfectant or heat them with a propane torch to kill any pathogens. Metal tools can be safely heated hot enough to kill the fungi,” said Morrison.
   
Most severe cases are in horses that spend a lot of time in stalls. “I see a lot of Saddlebreds with white line disease and Quarter Horse halter horses,” said Morrison. “Some people feel that being in wet fields is a risk, but the worst cases occur in horses in stalls.
   
“The risk doesn’t seem so much environmental (like wet conditions) but more in horses that have hoof capsule defects and flares. If a horse has a lot of hoof and long toe, the feet often become dished. The normal hoof wall is designed to be worn away as fast as it grows—continuously produced and continuously worn away. The longer the hoof capsule just sits there without being worn away or trimmed, the more vulnerable it is to fungal infections,” he added.
   
In the unbalanced hoof there is more stress on the white line, stretching it and creating openings for pathogens to enter. 
   
“We also see white line disease in laminitis horses because they don’t grow much toe very quickly. The wall isn’t replaced with fresh, strong horn in a timely manner; the old horn just sits there and becomes exposed to fungal infections. Also, horses with laminitis tend to have a dished, turned-up toe and a stretched white line. There is not a healthy, tight junction between outer hoof wall and sole. There are a lot of fissures and separations, so these feet more readily get white line disease because this makes a perfect environment for the fungus to become established,” said Morrison.