Bovine Papillomavirus in Horses

Bovine papillomavirus (BPV) is a viral infection that plays a crucial role in the development of sarcoids, the most common type of skin tumour in horses. Although the virus primarily affects cattle, it can also infect horses, donkeys, and mules, causing chronic skin growths that can be difficult to treat and manage. Understanding the nature of BPV, how it spreads, and its effects on horses is essential for every owner who wants to protect their animals’ health and wellbeing.

What Is Bovine Papillomavirus?

Bovine papillomavirus is part of the Papillomaviridae family, a group of DNA viruses that cause warts and tumours in various animal species. There are several types of BPV, but types 1 and 2 are particularly important in horses.

While BPV infection in cattle usually causes benign warts that resolve naturally, in horses it can lead to persistent, locally invasive skin tumours known as sarcoids. Unlike viral warts in cattle, sarcoids do not usually regress on their own and often require veterinary treatment.

Types of Bovine Papillomavirus That Affect Horses

There are many strains of BPV, but only two have been definitively linked to equine disease:

• BPV Type 1 (BPV-1): Commonly associated with fibroblastic and nodular sarcoids.
• BPV Type 2 (BPV-2): Frequently linked to more aggressive forms of sarcoids, including fibroblastic and malevolent types.

Some studies also suggest that BPV Type 13 may occasionally be involved in sarcoid formation, but this is less well understood.

How Horses Become Infected

BPV does not naturally occur in horses, so infection typically happens through cross-species transmission from cattle or contaminated environments. Once the virus enters the horse’s skin, usually through a cut, abrasion, or insect bite, it can cause infected fibroblasts to multiply uncontrollably, forming sarcoids.

Common Routes of Transmission

1. Flies and Insects
   Biting flies, such as stable flies or horseflies, can transfer BPV between horses and cattle by carrying viral particles on their mouthparts.

2. Direct Contact
   Horses may become infected by rubbing against contaminated surfaces, grooming tools, fencing, or other horses that carry the virus.

3. Wounds and Skin Damage
   Areas of broken skin are especially vulnerable, as they allow the virus to penetrate and establish infection in the deeper layers of tissue.

4. Environmental Contamination
   The virus can survive for extended periods in the environment, especially on organic material such as scabs or skin debris.

5. Maternal or Genetic Factors
   Although BPV itself is not inherited, some horses may have a genetic susceptibility to infection or tumour formation due to immune system differences.

The Link Between BPV and Sarcoids

Sarcoids are the most well-known manifestation of BPV infection in horses. The virus does not spread internally or cause systemic illness, but it alters the function of skin cells, causing them to behave abnormally. Once established, the virus integrates its DNA into the host cells, driving continuous, uncontrolled cell division.

How BPV Causes Sarcoids

• BPV infects fibroblasts (connective tissue cells) beneath the skin.
• The viral DNA incorporates itself into the horse’s own DNA.
• This triggers overproduction of growth factors that lead to tumour formation.
• The horse’s immune system often fails to eliminate the virus, allowing the sarcoid to persist or spread locally.

This process can result in different types of sarcoids, from small flat lesions (occult) to large, ulcerated growths (fibroblastic or malevolent).

Clinical Signs of BPV Infection in Horses

Because BPV infection in horses is not visible until tumours form, owners usually notice sarcoids rather than the virus itself. Common signs include:

• Hairless patches with thickened, scaly skin
• Firm nodules under the skin
• Warty or cauliflower-like growths
• Fleshy, ulcerated lumps that may bleed
• Lesions that attract flies or interfere with tack
• Persistent lumps that do not heal or worsen over time

Sarcoids are most often found on the head, ears, chest, groin, and legs, but they can occur anywhere on the body.

Diagnosis

A definitive diagnosis of BPV-related sarcoids requires veterinary examination. Although the appearance is often characteristic, laboratory tests can confirm BPV infection.

Diagnostic Methods

• Visual examination: Experienced veterinarians can usually identify sarcoids by sight.
• PCR testing: Detects BPV DNA in tissue samples.
• Biopsy and histopathology: Microscopic examination of tumour cells to confirm diagnosis.
• Immunohistochemistry: Identifies viral proteins within the tissue.

Caution is advised before taking a biopsy as disturbing a sarcoid can cause rapid growth or spread, so samples should only be taken when immediate treatment is planned.

Treatment Options

There is no cure for BPV infection itself. Instead, treatment focuses on removing or controlling the sarcoids that result from infection. Each case must be assessed individually, taking into account the type, size, and location of lesions.

Common Treatments

• Surgical removal: Effective for small or isolated sarcoids, though recurrence is possible.
• Cryotherapy: Freezing the tumour with liquid nitrogen to kill cells.
• Laser surgery: Vaporises the sarcoid with minimal bleeding.
• Topical treatments:
  • Liverpool Cream (AW4-LUDES) – cytotoxic cream applied by vets.
  • Imiquimod (Aldara) – immune-stimulating cream for smaller lesions.
• Chemotherapy: Direct injection or implantation of drugs such as cisplatin.
• Immunotherapy: Boosts the horse’s immune response to fight infected cells.
• Radiotherapy: Very effective for delicate areas like eyelids, but expensive and limited to specialist centres.

Because BPV infection is lifelong, sarcoids may return even after successful treatment. Early intervention gives the best chance of long-term control.

Prevention and Management

1. Fly Control

Use fly repellents, fly masks, and rugs during warm months to reduce insect-borne transmission.

2. Avoid Cross-Contamination

Do not share grooming tools, tack, or rugs between horses, especially those with known sarcoids.

3. Wound Care

Clean and treat cuts promptly to prevent viral entry through damaged skin.

4. Separate from Cattle

If possible, keep horses away from cattle or areas where cattle congregate, particularly if those animals have warts.

5. Support Immune Health

Provide a balanced diet, maintain low stress levels, and manage parasites effectively to help the horse’s immune system control the infection.

6. Regular Monitoring

Inspect your horse frequently for new or changing lumps. Photographing lesions monthly can help track progress or recurrence.

Research and Vaccination

Scientists are actively studying BPV and its relationship with equine sarcoids to develop better treatments and possible vaccines. Experimental vaccines targeting BPV antigens have shown promising early results, but no commercial vaccine is yet available for horses.

Immunotherapy using BPV-derived components or DNA vaccines may become a valuable tool in the future, particularly for horses with recurrent or resistant sarcoids.

Prognosis

The prognosis for horses infected with BPV varies widely. Some horses develop only a few small sarcoids that remain stable for years, while others suffer from multiple, aggressive tumours that interfere with daily comfort or performance. With careful management and veterinary care, most horses can live normal lives, but ongoing vigilance is essential.

Conclusion

Bovine papillomavirus is the hidden culprit behind most equine sarcoids, a condition that can cause persistent skin tumours and long-term management challenges. Though the virus originates in cattle, it can infect horses through minor wounds or insect bites and remains in the skin indefinitely.

While there is currently no cure for BPV infection, understanding its transmission and effects enables horse owners to take proactive measures. Good fly control, proper wound care, hygiene, and early veterinary intervention are the best defences against BPV-related disease. With regular monitoring and the right treatment strategy, affected horses can continue to live healthy, active, and comfortable lives.

As research continues, new therapies and vaccines may soon offer even better outcomes for horses affected by this fascinating yet frustrating virus.