Rain Rot

Rain rot is an interesting infection...

...its one of the few horse diseases out there that is going to bother you more than your horse!

The only painful part of the disease for your horse is when you remove the scabs (more on that later). The actual infection itself doesn't cause your horse any pain or irritation -- not even any itchiness.

Which is amazing, because just looking at it makes me itchy!

Rain scald.

This disease is also known by a few other names, including:

  • dew poisoning (when its on the legs)
  • rain scald
  • streptithricosis
  • mud fever

Throughout this page, I'll use the terms interchangeably...but I'm talking about only one disease.

What causes dew poisoning?

Dew poisoning is caused by a nasty little organism called dermatophilus congolensis...now isn't THAT is a mouthful?

Thankfully like a number of nasty little organisms that invade our horses' space, D. congolensis is pretty easy to keep at bay in most cases.

However, before you can successfully keep him at bay, you need to know a little bit more about him.

D. congolensis is a facultative anaerobe. All this means is that he uses oxygen to produce energy when it's available, but he's able to switch to fermentation when necessary and therefore is perfectly at home in an oxygen-deprived environment.

How is rain scald transmitted?

Rain scald, or rain rot, is transmitted a number of ways. It can be transmitted directly between horses, or it can be carried on items that are used on an infected horse then on another horse.

The most usual culprits for spreading rain scald (which is very contagious) are grooming tools. Prevention of this disease is just one reason its a good idea to have a set of grooming tools for each horse (besides, who doesn't need more grooming tools in lots of colors?!?).

Another common culprit is shared saddle blankets or pads. Or pretty much anything else that touches a horse's skin and is shared between horses.

One place that many owners wouldn't think about as spreading rain scald is "scratching posts"...any place a horse regularly itches himself where another horse might also itch.

So if you can't determine a source of infection, maybe look to your pastures and/or stalls, and see if there is a place (tree, post, etc) that most of the horses are scratching themselves.

It's important to keep in mind that horses can be carriers of this disease without showing symptoms...so a seemingly uninfected horse can still transmit it to another horse if he's simply carrying rain scald and not displaying symptoms.

Also important to note is that horses can re-infect themselves if the grooming tools and tack are not treated after every contact with the infected horse. Also treat any scratching areas such as trees or posts.

What does rain rot look like?

Good question, and I'm glad you asked. The two pictures on this page1 show what rain rot looks like from two different angles.

Rain rot.
As you notice, the hair looks rather dull and lifeless where the rain rot is. If you were to pull on those tufts of hair, they would come out very easily, and under them you would find pink skin, probably oozing a little bit.

It is important to get these scabs

off, as that allows air to get to the skin underneath them, which helps heal the infection.

If the infection gets more severe than the infection in the picture, the horse will start to lose hair. This is extremely unsightly, though again, it doesn't cause the horse any discomfort.

Once the scabs have been removed and the skin starts to heal, it will dry up and be grey in color.

How does rain rot get started?

Contrary to popular belief, a horse in any climate and conditions can acquire rain rot. However, like the name implies, it is most often found in warm, humid climates that get a lot of rain.

D. congolensis needs a moist environment to thrive and reproduce, thus why rain rot often occurs in these warm and humid climates.

This disease can be more common during the winter months (despite drier weather) due to the long hair coats that horses develop. The winter hair coat is great at trapping heat and sweat near the body, creating the perfect environment for D. congolensis.

Once your horse has picked up the organism, he doesn't necessarily become infected. In order to become infected, his skin must be compromised to allow the organism entrance. Usually this happens through a bug bite or a scratch or scrape.

How can I prevent streptothricosis?

Streptothricosis is most easily prevented by good horse-keeping practices. Horses that are generally kept dry and clean will usually not develop streptothricosis.

However, if you have re-curring issues with rain rot, it is wise to look into the underlying causes. The most common cause is a lack of nutrition...not lack of calories, lack of nutrition.

Check out Babs' Story to read the interesting story of one horse who proves that nutrition plays a much larger role than we often give it credit for.

How do I treat Dew Poisoning?

Dew poisoning is best treated by first giving the horse a bath with soap to loosen and remove the scabs. Some people choose to use a medicated shampoo, while others simply use any regular shampoo.

Let the soap sit on the horse for a few minutes to loosen the scabs. After a bath, the area will need to be treated with something to get rid of the organism.

You can use a number of things, including antiseptic ointment or other remedies that are available at most feed and tack stores...many people recommend the Micro-Tek products.


Rain rot, rain scald, dew poisoning, or streptithricosis...whatever you want to call it, the disease is fairly easy to prevent, yet fairly common in the horse world.

It is actually the number one skin condition that affects horses!

Knowledge of what causes it, how to treat and prevent it, and underlying causes can help you more effectively battle this disease and prevent it in your horses.



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1The horse in the two pictures appearing on this page belongs to Patty Sloan, and the pictures are hers and used with permission.

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