Can Sarcoids Be Left Untreated?

Short answer

Yes—sarcoids can sometimes be left untreated, but only with active monitoring and informed judgement. In many cases, doing nothing initially is reasonable. In others, it allows the condition to become significantly more difficult to manage.

Why this is not a simple yes-or-no decision

Equine sarcoids are highly variable in their behaviour. Some remain unchanged for long periods, while others progress unpredictably, particularly after disturbance.

They are:

• Non-metastatic (they do not spread internally)

• Locally invasive

• Influenced by immune response, trauma, and environment

Because of this, the decision to leave a sarcoid untreated is not passive—it is a strategic choice based on risk assessment.

What the evidence shows

Clinical observations and published reviews (Knottenbelt, 2005; Martens et al., 2001; Nasir & Reid, 1999) indicate that:

• Some sarcoids remain static for months or years

• A minority may show spontaneous regression

• Many eventually progress, particularly after trauma or irritation

There is no reliable way to predict which path an individual sarcoid will take.

When leaving a sarcoid untreated may be appropriate

1. The sarcoid is small and stable

Occult and verrucose sarcoids often:

• Remain flat or slow-growing

• Cause minimal disruption

• Show little short-term change

In these cases, immediate intervention may not improve outcomes and may introduce unnecessary risk.

2. The location is low-risk

Sarcoids located away from:

• Tack areas

• Eyes

• Limbs or joints

…are less likely to interfere with function. These lesions may be suitable for observation, particularly if they are not changing.

3. There is no sign of irritation or trauma

Stable sarcoids that are:

• Not being rubbed

• Not exposed to frequent fly activity

• Not previously treated

…are less likely to become activated in the short term.

The risks of leaving sarcoids untreated

While observation can be appropriate, it carries important risks.

1. Unpredictable progression

A key feature of sarcoids is that they can change behaviour without warning.

Triggers for progression may include:

• Minor trauma

• Fly irritation

• Immune fluctuations

• Attempted treatment

A previously stable lesion can become:

• Larger

• More aggressive

• More difficult to treat

2. Conversion to more aggressive forms

Sarcoids are classified into types (occult, verrucose, nodular, fibroblastic), and these can change over time.

Less aggressive lesions may:

• Transform into fibroblastic sarcoids

• Become ulcerated or fleshy

• Show more invasive growth patterns

This progression is often associated with disturbance or irritation.

3. Missed opportunity for early intervention

Early-stage sarcoids are often:

• Easier to manage

• More responsive to treatment

• Less invasive

Delaying intervention in a case that is likely to progress can result in:

• More complex treatment later

• Higher recurrence risk

• Greater impact on the horse

The concept of “watchful waiting”

Leaving a sarcoid untreated should not mean ignoring it.

A more appropriate approach is:

Active monitoring (watchful waiting)

This involves:

• Regular visual checks

• Tracking size, shape, and texture

• Noting any changes in behaviour

• Avoiding unnecessary disturbance

Photographic records can be particularly useful for detecting subtle changes over time.

When observation is no longer appropriate

You should reconsider a “leave it alone” approach if:

• The sarcoid increases in size

• It becomes raised, fleshy, or ulcerated

• There is bleeding or discharge

• It begins to interfere with tack or movement

• Multiple lesions start to appear

These changes suggest increasing biological activity and may justify intervention.

A common misconception

A frequent assumption is:

“If I leave it alone, it won’t get worse.”

The evidence does not support this as a reliable strategy.

While some sarcoids remain stable, others progress—and there is no consistent way to predict which will do so.

Equally, another misconception is:

“All sarcoids must be treated immediately.”

This is also inaccurate and can lead to unnecessary or poorly chosen interventions.

The role of immune response

There is increasing recognition that host factors play a role in sarcoid behaviour.

Research suggests:

• Immune response to BPV infection influences tumour persistence

• Some horses may contain lesions effectively

• Others show ongoing susceptibility

This variability partly explains why some sarcoids remain static while others progress.

Practical takeaway

Sarcoids can be left untreated in certain cases, particularly when they are:

• Small

• Stable

• Located in low-risk areas

However, this should always involve:

• Ongoing monitoring

• Awareness of risk factors

• Willingness to intervene if the situation changes

The key is not choosing between “treat” or “ignore,” but recognising that management decisions may need to evolve over time.

References (for credibility positioning)

• Knottenbelt, D.C. (2005). A suggested clinical classification for the equine sarcoid. Clinical Techniques in Equine Practice.

• Martens, A. et al. (2001). Bovine papillomavirus DNA in equine sarcoids. Journal of General Virology.

• Nasir, L., & Reid, S.W.J. (1999). Bovine papillomaviral gene expression in equine sarcoids. Veterinary Journal.