Warts in Horses (Equine Papillomatosis): Transmission, Risk Factors & Treatment

Warts in horses, also known as equine papillomatosis, are small, benign growths caused by equine papillomaviruses (EPVs). These growths primarily affect young horses under the age of three, as their developing immune systems are more susceptible to the virus. ^[1]

EPVs target the epithelial cells of the horse’s skin, leading to lesions with a rough, cauliflower-like texture. Typically gray or flesh-colored, warts commonly appear on the muzzle, lips, or around the eyes.

Although EPV is highly contagious, spreading through direct contact or contaminated equipment, the condition is generally self-limiting and resolves without medical intervention. As the horse’s immune system matures, it usually mounts an effective response, leading to resolution of the warts.

Most horses recover fully within a few months without the need for extensive treatment. Robust management practices, including good hygiene and isolating affected horses, can help control the spread of papillomavirus and reduce the risk of new infections.

Symptoms of Warts in Horses

Equine warts typically appear as small, rough lesions on the skin ranging from a few millimeters to about 2 centimeters in diameter. They often have a cauliflower-like texture and are usually gray, white or flesh-colored. ^[1][2]

In horses, warts are most commonly found on the muzzle, lips, around the eyes, and occasionally on the ears, pasterns or genital area. In some cases, warts may cause mild discomfort or itching, especially if located in areas prone to friction. ^[3][4]

While warts are generally harmless and typically resolve spontaneously within 1 to 9 months, their presence can still be a cosmetic concern. Additionally, if located in sensitive areas, the warts may interfere with the horse’s daily activities. ^[5][6]

Risk Factors

Warts are common in horses, especially among younger animals. Understanding the risk factors associated with equine papillomatosis can help in managing and preventing the occurrence of warts.

Identified risk factors include: ^[1][5]

• Age: Young horses, particularly those under 3 years old, are more likely to develop warts as their immune systems are under-developed. Older horses are less commonly affected but may experience persistent infections.
• Health Status: Horses with weakened or compromised immune systems may be at higher risk for developing warts.
• Shared Space and Equipment: Warts are contagious and can spread through direct contact with infected horses, and through contaminated surfaces, such as stalls, feeders, and grooming tools.
• Pasture or Grazing Area: Grazing in contaminated areas increases the risk of developing equine papillomavirus (EPV), as the virus can persist in the environment where horses graze. This is why warts are sometimes referred to as “grass warts.”

Causes

Papillomaviruses (PVs) are believed to be among the oldest viruses in existence, with a long history of co-evolution alongside their host species. These viruses are known for causing papillomas, or warts, in a variety of animals, including humans. ^[6][7][8]

In horses, warts are caused by a species-specific subtype of virus referred to as equine papillomaviruses (EPVs).

Transmission

EPVs specifically target the epithelial cells of a horse’s skin, leading to the formation of benign growths, or warts. The virus enters the skin through small cuts, abrasions, or other openings, infecting the epithelial cells and causing them to proliferate abnormally, forming warts. ^[4]

The infection is highly contagious and spreads through direct contact with an infected horse or indirectly via contaminated objects such as grooming tools, tack, blankets, or other equipment. ^[1][2]

Further, when a horse grazes in areas where an infected horse has previously been, it may come into contact with the virus shed from sloughed skin cells (keratinocytes) that remain in the environment. ^[1][2]

Nine distinct EPVs have been identified, each contributing to the development of papillomas in horses. Young horses, especially those under three years of age, are most commonly affected as their immune systems are still developing and may not be fully effective in combating the virus. ^[3]

Equine Congenital Papilloma

Equine congenital papilloma is a rare type of wart that is present at birth, developing on the skin of foals and, in rare cases, on the placenta. These congenital lesions rarely resolve spontaneously and usually require surgical excision for treatment. ^[6]

Although generally benign, their persistence can lead to complications if left untreated. While this form of warts is not fully understood, it is believed that congenital papillomas are transmitted from the dam to the foal, potentially during or shortly after birth. ^[1][9]

Surgical removal is considered curative, and early intervention is recommended to prevent potential issues as the foal matures. The occurrence of these lesions on the placenta is particularly uncommon, adding complexity to their diagnosis and management.

Zoonosis

Humans do not get warts from EPVs. The strains of papillomaviruses that cause warts in horses are specific to equines and do not infect humans. ^[3][10]

Human warts are caused by different types of human papillomaviruses (HPVs), which are distinct from EPVs. The risk of cross-species transmission between horses and humans is extremely low. However, maintaining good hygiene practices when handling animals with warts is always recommended to avoid any potential risk of contamination to other horses.

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Diagnosis

Veterinarians typically diagnose warts in horses through a combination of physical examination, medical history, and the characteristic appearance of the growths. ^[1][2]

During the physical exam, the veterinarian assesses the wart’s physical characteristics, including:

• Size
• Shape
• Location
• Texture

The horse’s age and exposure history can also provide important clues, as warts are most commonly seen in younger horses with developing immune systems. ^[1][2]

Laboratory Analysis

For a definitive diagnosis, particularly when growths are atypical or there is concern, they might be something other than warts, a biopsy or sample of the growth can be collected. This sample is then sent to a laboratory for histopathological examination. ^[1]

Lab analysis can confirm the presence of EPV and help rule out other conditions such as sarcoids, dermatophyte infections, or other skin lesions that may resemble warts. In some cases, molecular testing like PCR may be employed to detect viral DNA, though this method is uncommon. ^[6][16]

By examining a small tissue sample under a microscope, veterinarians can accurately identify the presence of EPV and distinguish it from other conditions. This procedure provides a clear and reliable diagnosis, enabling appropriate treatment and management of the condition.

Differential Diagnosis

When diagnosing warts caused by EPV in horses, it’s important to differentiate them from other conditions that may present with similar skin lesions. Here are some key differential diagnoses to consider:

Sarcoids

Sarcoids and warts may appear similar, but they are distinct conditions with no relationship to each other. Sarcoids are aggressive fibroblastic tumors that frequently appear on the head, neck, legs, or underside and commonly develop at sites of previous injury or scarring. ^[1][6]

Unlike EPV warts, which typically resolve on their own, sarcoids often recur after removal and do not usually regress spontaneously. Sarcoids are commonly associated with Bovine papillomavirus (BPV) DNA, whereas EPV warts typically do not show this association. ^[11][12]

Hives (Urticaria)

Hives, or urticaria, are raised, smooth, and often itchy bumps that can appear anywhere on the horse’s body. They usually result from an allergic reaction to food, insect bites, or environmental factors. ^[13]

Unlike EPV warts, which are rough and cauliflower-like, hives are typically smooth and can vary in color from pink to red. Hives often appear in clusters and may fluctuate in size. They usually resolve once the allergen is identified and removed or with treatment using antihistamines.

Pemphigus Foliaceus

Pemphigus foliaceus, an uncommon autoimmune condition, leads to the formation of pustules and crusts that typically begin on the face, ears, and around the eyes, and may spread to other areas. Unlike EPV, the lesions are characterized by widespread crusting and the presence of pustules. ^[1][14]

Horse Poxvirus

Horse pox is an extremely rare condition that is almost extinct today. It is caused by a poxvirus and presents with rounded, raised papules that may develop into crusty sores. Unlike EPV warts, horse pox lesions are smoother and can be accompanied by systemic symptoms such as fever. ^[1][15]

Treatment

Warts are typically managed based on their severity and location. In most cases, these lesions resolve on their own within 4 to 12 weeks as the horse’s immune system naturally clears the infection. Veterinarians often recommend watchful waiting unless the warts cause discomfort or interfere with the horse’s daily activities. ^[1]

For more severe cases or congenital warts that do not resolve on their own, surgical excision or cryosurgery (freezing the wart) may be recommended. However, treatment is generally avoided for warts in the ear due to the risk of severe irritation and complicated future management. ^[1]

Ultimately, the approach to treating warts in horses is individualized, with many cases requiring minimal or no intervention.

Autogenous Vaccine

An autogenous vaccine is a specialized vaccine created using viral DNA extracted from warts on the affected horse’s body.

To prepare this vaccine, a sample is taken from EPV warts, inactivated (typically with formalin), and then re-administered to the same horse. ^[16][17]

The goal of this vaccine is to stimulate the horse’s immune system to recognize and fight the specific EPV strain causing the warts. Anecdotally, these vaccines may be effective in treating warts, with better results typically seen in young horses and less effectiveness in older horses. Further research is needed to fully understand their efficacy. ^[1]

Prognosis

The prognosis for warts in horses is generally positive. Most horses recover fully within 3 to 6 months as their immune system fights off the virus. Once recovered, horses typically develop sufficient immunity to the virus within 3 to 4 months, which usually protects them from recurrence of warts from the same viral strain. ^[5]

In cases of EPV where the affected horse is fully mature, the infection may persist for a longer period. In rare cases, warts may reappear, or new warts may develop if the horse is exposed to a different strain of the virus.

Overall, warts are usually not a serious concern and resolve on their own without treatment. ^[1]

Prevention

Preventing the spread of EPV and the development of warts requires a combination of vigilant management practices and maintaining a healthy environment.

General strategies include:

• Controlling exposure
• Maintaining hygiene
• Supporting a healthy immune system

Controlling Exposure

To reduce the risk of young horses contracting EPV, proactive measures are essential. Quarantining new arrivals before introducing them to the herd can help prevent the spread of the virus. ^[1][18]

Additionally, keeping young horses away from those showing signs of warts or known to have been infected with EPV is critical. These precautions can significantly reduce the risk of transmission and protect more vulnerable, younger horses.

Maintaining Hygiene

Practicing good hygiene and biosecurity are vital for preventing EPV. Using dedicated equipment for each horse, including grooming tools, tack, and other items, is a cornerstone of avoiding cross-contamination.

Regular cleaning and disinfection of stalls, feeders, and water troughs helps reduce the risk of viral transmission in shared spaces. Horses should not be turned out on paddocks known to be contaminated. These practices are essential for maintaining a healthy environment and minimizing the risk of an EPV transmission. ^[1]

Immune System Support

Providing a balanced diet is critical for young horses to ensure they receive the necessary nutrients for robust immune system development.

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In addition to proper nutrition, managing the horse’s environment and routine to minimize stress is important. Reducing stress helps maintain a strong immune response, offering further protection against infections like EPVs. ^[19]

Summary

Warts in horses, known as equine papillomatosis, are benign growths caused by equine papillomaviruses (EPVs), primarily affecting young horses under three years old.

• EPVs are highly contagious, spreading through direct contact or contaminated equipment.
• Warts are typically small, rough, and asymptomatic, appearing on the muzzle, lips and around the eyes.
• Warts often resolve spontaneously within 1 to 9 months as the horse’s immune system matures. In persistent cases, surgical removal may be needed.
• Preventive measures include good hygiene, isolating affected horses, and providing proper nutrition to reduce the risk of EPV spread.