If you're looking for the cliff notes: as soon as you see a new lump take a picture of it so you have a record of when it first appeared. If you are worried about it, take some notes. Is it hard or soft, and does it pit? Is the horse sensitive if you touch it? Is it itchy? Is it everywhere or localised? Is there any discharge, and what colour? You might want to measure it so you can accurately gauge if it changes in size. Are there other symptoms, like lameness, lethargy, or a change in temperature? All of this is useful information for a later diagnosis.
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Horses can develop lumps for various reasons. Here are some common causes:
Warts: Warts are caused by several strains of papillomavirus and usually seen in horses less than three years old. This virus often takes advantage of skin that has been compromised or stressed by sunburn, minor injury, or insect bites. Typical signs of warts include single or multiple small growths/bumps on the skin surface. They can be grey, pink or white. Warts are most commonly found on the muzzle and lips, but can also be found elsewhere. They generally resolve on their own.
Teething Bumps (Eruption Cysts): These are found on the lower jaw and are common in young horses as their teeth come in. They are normal and they will resolve as the adult teeth erupt through the gums.
Sub-mandibular lymph-node abscess: When a lymph node abscesses, you’ll feel a round, hard lump, usually the size of a golf ball or ping-pong ball sitting under the skin. In some cases, the abscessed node enlarges and you may see soft swelling in the surrounding tissues. In others, it can be painful to the touch, or come to a head (much like a pimple) as it prepares to burst and drain. You can ask a vet to lance the abscess if you are worried about it.
Trauma: Injuries from kicks, falls, or collisions can cause swelling and lumps due to inflammation and pooling of blood. Hematomas: (Bruises) These are collections of blood outside of blood vessels, usually caused by trauma. A hematoma is blood, whereas a seroma is a clear fluid lump. Both usually resolve on their own.
Ventral edema: An accumulation of fluid under the skin right in the middle of the belly that’s settled to the lowest point of your horse’s body, due to gravity. The fluid most commonly originates from inflammation somewhere in the body, such as an allergic reaction or some kind of trauma. In some cases, fluid accumulates due to more serious causes such as heart failure or kidney or liver disease.
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Splints:
A ligament, located between the cannon bone and the splint bones, is quite elastic in young horses. As the horse ages, the ligament ossifies; that is, the ligament is replaced by bone and the three bones fuse. During ossification, there may be inflammation and pain. Jumping, running and working a horse during this time produces further irritation.
Splints usually occur in horses 2 to 5 years old. Most often it is the forelimbs that are affected. Splints rarely occur in the hind legs. In older horses, the splint bones are fused solidly to the cannon bone.
The majority of splint problems occur on the medial side (inside) of the forelimbs. The medial splint bone usually is the one affected because it has a flat surface next to the knee. The lateral (outer) splint bone has a more slanted surface. When the weight is transmitted to these bones, the medial splint bone probably bears more weight than the lateral splint bone. Therefore, the ligament between the medial splint bone and the cannon bone is subjected to more stress than the outer ligament.
Softer lumps on the horses legs can be windgalls, which are usually due to overworking the tendons. You can read more about that here.
Insect Bites: Bites from flies, mosquitoes, or other insects can cause localized swelling and lumps. Insect bites and hives often look similar.
Allergic Reactions: Horses can develop lumps (hives) due to allergic reactions to food, medications, or environmental factors. Take a note if you have changed your feed, or the brand of your feed, or if there are new types of grasses in your pasture. Be aware of types of trees that are in your pasture or on adjoining properties.
Infections: Bacterial, fungal, or viral infections can lead to lumps, often accompanied by other symptoms like fever or discharge.
 Abscesses: These are pockets of pus that form due to infection, often resulting in painful, swollen lumps. These can occur anywhere but are commonly on the coronet band from a hoof abscess.
Tumors: Both benign and malignant tumors can present as lumps on a horse’s body. Grey horses are far more likely to have tumours. Appaloosas and other horses with white eyes are more likely to have squamous cell carcinoma.Â
Squamous Cell Carcinomas (SCC): These tumors typically affect areas with squamous cells, such as the eyes, eyelids, and genitalia. SCC can be aggressive and may require surgical removal or other treatments.
Sarcoids are usually grey. Types of sarcoids include:
Occult Sarcoids: These are flat, hairless patches with a grey, scaly surface. They are often circular and can be mistaken for ringworm.
Verrucose Sarcoids: These appear as grey, scabby, or warty lesions. They can be irregular in shape and may cover large areas.
Nodular Sarcoids: These are solid nodules under the skin, varying in size. They are commonly found around the groin and eyelids.
Fibroblastic Sarcoids: These are aggressive, fleshy masses that can ulcerate and bleed easily. They often develop at wound sites.
Malevolent Sarcoids: These are rare and highly aggressive, spreading extensively through the skin.
Boney lumps: horses that develop swollen faces maybe due to "bighead", which is a calcium deficiency. You can read more about that here.Â
Ringbone: Bony growths that form usually around the pastern in response to cartilage breakdown and inflammation, often due to poor conformation or trauma.
Lumps can have many causes. While we obviously can't offer any specific solutions for all lumps, you can support your horse's immune function in general with Sound Advice Trace Mixes, which supply optimal zinc, making them better equipped to fight infection, or recover from trauma.
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