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The protein lumps and bumps that sometimes form on the neck and back skin of the horse are often eosinophilic granulomas. These are small, benign nodules that may occur as single bumps or develop in groups spread across large areas of the horse’s body.
Eosinophilic granulomas are generally asymptomatic (other than the presence of the bumps themselves) and do not cause discomfort or health issues for the horse. Why these bumps form is not completely understood, but the condition is associated with allergic reactions, insect bites, and other hypersensitivity factors.
Although the condition may resolve on its own, lumps that persist can become mineralized, making treatment more difficult and recurrence more likely. Treatments include surgical removal and the administration of corticosteroid medications.
Understanding the nature of these lumps and bumps and the available treatment options can help in managing the condition effectively. Read on to learn everything you need to know about equine skin lumps and bumps so you know what to do if you spot something unusual on your horse’s skin.
Protein Bumps on Horses
The protein lumps and bumps that appear on a horse’s skin are scientifically known as eosinophilic granulomas.
Eosinophilic granulomas are small, benign lumps that appear under the surface of the horse’s skin, typically on the back or the flanks of the horse. [1] They may appear as individual bumps or there may be many spread across the body. [2]
Other than the presence of the bumps themselves, eosinophilic granulomas usually do not cause symptoms and are slow growing. [2][3] Generally, horses that are affected by this condition are otherwise healthy. [2]
Eosinophilic granulomas are also referred to as: [1][3][4]
- Collagenolytic granulomas
- Nodular collagenolytic granulomas
- Nodular necrobiosis
- Nodular collagen necrosis
- Acute collagen necrosis
Veterinary research uses different terms to describe the condition. Although these names are still in use, it has been argued that they are not true synonyms and do not accurately reflect the nature of the eosinophilic granuloma complex because true collagen degeneration is rare in this condition. [1][5]
What is a Granuloma?
Granulomas are areas of inflammation that typically appear as small, firm nodules under the skin. They often form when there is a foreign stimulus in the body that the immune system cannot eliminate, such as certain bacteria or fungi. [3]
Some granulomas may form due to an immune response against dead or dying cells within the horse’s skin, even without an infectious agent present. [3]
When the immune system encounters something it cannot eliminate, it uses cells called macrophages to “wall off” the area to prevent spread of the abnormal stimulus. These cells accumulate around the stimulus, producing a local hard swelling or thickening of the tissue. The nodules are usually not harmful to the horse. [1][3]
Eosinophilic granulomas are a subtype and their triggering factor is not fully understood. Unlike granulomas related to infection as described above, this form of nodule is sterile, meaning it is not associated with any type of infectious pathogen. [3]
Protein and Granulomas
It is important to note that even though granulomas are known colloquially as protein lumps and bumps, they do not result from too much protein in the diet. The reference to protein is simply a passing acknowledgment that these bumps are made of dense skin tissue.
This terminology also helps differentiate granulomas from fatty cysts such as lipomas, which are soft, squishy, and move easily under the skin.
Although protein intake is unrelated to granulomas, it’s important for all horses to receive enough protein to meet their requirements, which vary based on age, breed, lifestyle, and type of work performed. If you are unsure whether your horse is getting an appropriate intake of dietary protein, consider speaking to a qualified equine nutritionist about your feed regimen.
Symptoms
The characteristic symptom of eosinophilic granulomas are one or more small lumps under the horse’s skin. [4] Some horses have a few of these bumps, and some have hundreds. [2]
These lumps and bumps vary in size and shape but tend to be approximately pea-sized. [4] They tend to be quite small, measuring approximately 3 to 6 millimeters in diameter. [1] In some cases they can grow quite large, even reaching 10 cm (4 in) in diameter. [2][3][5][6]
Eosinophilic granulomas are round, clearly defined, and firm to the touch. [2][4][5] The skin and hair tend to grow well over them. [2][4] Some of the lumps are filled with fluid. Others appear as thickened, raised skin. [2]
Some of the lumps are visible to the eye, and some are small enough that they are not visible but can be felt through the horse’s skin. [1] In some cases, eosinophilic granulomas that grow close together can merge to form a single, larger bump. [1] Typically, eosinophilic granulomas are not itchy or painful. [4]
They often appear in the saddle area, but can also affect other parts of the horse’s body such as the trunk, neck or flank. [1][4][7]
In some cases, especially when the bumps are persistent, mineralization can occur. This usually leads to a patch of white material at the center of the bump. [4]
Risk Factors
Eosinophilic granulomas are a common skin condition in horses. [4][5][6] There does not appear to be an increased risk due to age, sex, or breed considerations. [3][4][7]
Horses that have a tendency to develop allergic diseases are more likely to develop eosinophilic granulomas. Examples of associated conditions include: [2]
- Recurrent airway obstruction (heaves)
- Summer (sweet) itch
- Hives
- Dermatitis in response to common allergens
Horses that have recently been injected using a silicone-coated needle are more at risk of developing eosinophilic granulomas. [5] Horses whose hair is clipped are also more at risk. [5]
Eosinophilic granulomas tend to appear most often in the spring and summer, but there are reports of year round occurrences in some locations. [2][4][5][6]
Causes
The causes of eosinophilic granulomas are not fully understood at this time. [2][3][4][5][7] The condition is likely multifactorial, meaning it occurs when several factors come together. [2][4][5]
Hypersensitivity to insect bites is closely associated with this condition. [2][3][4][5][7] However, some reports of outbreaks during the winter months raise questions about this association. [5] Atopy or allergic skin disease is also considered a predisposing factor. [2]
The causes of allergies are not fully understood, but they are known to result from an inappropriate immune system response to harmless stimuli. Given the immune system’s involvement in the development of granulomas, there may be a pathogenic connection between allergies, autoimmune disorders, and eosinophilic granulomas. More research is needed to identify how these conditions are linked.
In some cases, eosinophilic granulomas are associated with injection administration. [3] This is particularly true when standard silicone-coated needles are used. [5][6] A hypersensitivity to silicone is considered likely in these cases as these same horses do not develop eosinophilic granulomas when non-coated needles are used. [5]
Since eosinophilic granulomas appear on the areas where the saddle or other tack usually sits, some researchers suggest this condition may be related to damage to the skin from wear and tear. [1][6]
Skin damage in horses may be related to: [1]
- Skin trauma
- Collagen damage
- Fibroblast (cells involved in building new skin tissue) disruption
The presence of free hair shafts in some biopsies along with a history of body clipping has raised the question of a causal link between grooming practices and eosinophilic granulomas. [5]
Severity
Eosiniphilic granulomas are benign, meaning they do not pose any harm to the horse in most cases. [1] Some horses only have a few lumps and bumps, others develop large numbers of them. [2]
Eosinophilic granulomas are soft to begin with. [5] If left untreated, long-standing inflammation affects deeper tissues and the bumps begin to harden over the course of weeks or months. [3] This is due to mineralization that occurs as the end-result of tissue death.
When the lumps and bumps become mineralized, they begin to feel gritty to the touch. [1] Mineralization may also be noticeable as a whitened central patch within the bump, known as the necrotic core. [4][5]
Individual bumps may burst or be rubbed open. When this happens, sometimes a little fluid or coarse material is released. [1] Open skin is at risk of infection, therefore wound care for open eosinophilic granulomas is recommended.
Eosinophilic granulomas tend to be persistent and often become chronic. [4] Whether the lumps and bumps resolve on their own or are brought into remission through treatment, they are likely to return. [4]
In rare cases, peripheral blood eosinophilia develops. [5] In this condition, the granuloma(s) lead to elevated white blood cell counts.
Diagnosis
Diagnosis of eosinophilic granulomas is usually based on the presence of the characteristic skin lumps and bumps during physical examination by a veterinarian. [3][4]
Differentiation between eosinophilic granulomas and other similar-looking conditions is based on examination of the tissues under a microscope (biopsy). [1]
Tools used to extract material for examination include: [3][4][6][7]
- Needle aspiration: Withdrawing and testing fluid from the center of the bump
- Skin biopsy: Extraction of a bit of skin usually performed with a skin punch
- Skin scrapings: Collection of skin cells by scraping the surface of the skin
- Acetate tape impressions: Collection of skin cells by pressing tape to the surface of the skin
Treatment
Since eosinophilic granulomas are neither painful nor itchy, and do not cause health issues for the horse, they may safely be left to resolve on their own in most cases. [3][4][5] Resolution may take anywhere between 3 to 6 months. [5]
It might be helpful to adjust saddle padding to protect the bumps from too much wear and tear during recovery. [4]
Unfortunately, some bumps may become mineralized, making them harder to treat. [4]
In cases where treatment is preferred or necessary, surgical removal of the eosinophilic granulomas may be performed, especially when only a few are present. [3][4][5][6][7] Even large numbers of eosinophilic granulomas may be removed without too much risk to the horse. [1] This is typically accomplished with a biopsy punch. [1]
Another possible treatment is administration of corticosteroid drugs. [3][4][6][7] Corticosteroids are a naturally occurring class of steroid hormones that can be used as anti-inflammatory and immunosuppressive agents to treat various conditions.
Single lumps or those that occur in small enough numbers can be treated by injecting a very small amount of steroid directly into it. [1][3] Alternatively, an oral steroid can be administered when there are too many bumps to effectively inject each one individually. [2][3][6]
Oral steroids can also be used to control further outbreaks if individual injections have resolved earlier cases of eosinophilic granuloma but recurrence has occurred. [1]
Treatment with steroids is less effective when the granulomas are mineralized. [2][4] In these cases, surgery is required. [2][5]
Prognosis
Eosinophillic granulomas are not harmful to the horse. [1]
Even in cases where surgery or drug therapy has resolved an outbreak of eosinophilic granulomas, recurrence is common. [4] Treatment tends to be equally successful in resolving new outbreaks. [2]
Recurrence is especially likely for lumps and bumps that are mineralized as these tend to be particularly persistent. [4]
The prognosis for full recovery is fair to guarded. [4] It is likely that the lumps and bumps will come back even in cases where the condition appears to have been resolved. [4]
Treatment with corticosteroids is less effective for chronic and mineralized lesions. [4] Horses that undergo steroid treatments may have a higher risk of developing laminitis. [4]
Prevention
Since the causes of eosinophilic granulomas are not fully understood, prevention strategies primarily aim to avoid the associated risk factors. Risk factors and sensitivities to avoid include:
- Insect hypersensitivity: The implementation of a careful and effective insect control program is recommended to prevent bug bites, especially during the spring and summer months.
- Injection sensitivity: The use of uncoated stainless-steel needles is recommended to avoid allergic reactions to silicone.
- Clipping: Avoiding clipping the coat of horses that are prone to this condition is also advised.
Summary
The small, round protein lumps and bumps found on the back and neck of horses are technically known as eosinophilic granulomas. These skin bumps are not harmful to the horse but can become mineralized over time, making them harder to treat.
- Eosinophilic granulomas are not itchy or painful to the horse and do not impact overall wellbeing.
- The causes of this condition are not fully understood but may be related to hypersensitivity to bug bites, silicone, or other allergens.
- Diagnosis is based on the presence of characteristic lumps and bumps and the confirmed with microscopic analysis of the tissues.
- Treatments include waiting for the condition to run its course, surgery, or the use of injected or oral corticosteroids.
- The prognosis for the horse’s overall health is good. The prognosis for remission from the lumps and bumps is guarded as the condition tends to recur.
References
- Pilsworth, R. C. and Knottenbelt, D. C. Nodular Collagen Necrosis (Collagenolytic Granuloma). Equine Veterinary Education. 2005.
- Mathison, P. Eosinophilic Nodular Dermatoses. Veterinary Clinics of North America: Equine Practice. 1995. View Summary
- Costa, L. R. R. Granulomatous Diseases. Equine Clinical Immunology. 1st ed. Wiley. 2016.
- Littlewood, J. D. et al. Practical Equine Dermatology. 1st ed. Wiley. 2022.
- Scott, D. W. and Miller, W. H. Jr. Miscellaneous Skin Diseases. Equine Dermatology. 2003.
- Wobeser, B. K. Skin Diseases in Horses. Veterinary Clinics of North America: Equine Practice. 2015. doi: 10.1016/j.cveq.2015.04.007. View Summary
- Stannard, A. Nodular Diseases. Veterinary Dermatology. 2000. View Summary
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