Looking Beyond the Slogans: Three Questions About PZP and Long‑Term Fertility

What the Research on PZP Actually Shows—and Why Salt River Needs Answers

For years, I’ve followed the scientific debate over porcine zona pellucida (PZP) and its use in managing wild horse populations. As someone who has spent more than a decade observing and documenting the Salt River herd, it has been impossible not to.

Over the past several weeks, however, I decided to go back to the original scientific literature and read it again from beginning to end—not summaries, not advocacy pieces, but the published studies themselves.

Public discussions about PZP often reduce a complex body of research to simple slogans.

One side describes PZP as a safe, reversible contraceptive that has been studied for decades.

The other argues that repeated treatment can leave mares infertile for years—or even permanently.

After working through the published research, I found myself coming back to three questions.

1. Is PZP reversible?

The short answer is yes—and maybe.

The published research clearly shows that PZP can be reversible. Mares treated for one or two consecutive years generally returned to fertility after treatment ended.

On Assateague Island, every mare treated for one or two consecutive years eventually returned to fertility. In another carefully controlled study, mares receiving a standard primer-and-booster course resumed normal ovarian cycling within ten months after treatment. Those same mares did not become pregnant while the vaccine was active, demonstrating that the contraceptive worked as intended.

If those were the only studies available, the answer would simply be yes.

But once researchers began following mares that received repeated or long‑term treatment, the picture changed.

2. Does the number of treatments matter?

The short  answer is yes—definitely.

Across every long-term study I reviewed, one pattern was remarkably consistent: longer or more intensive treatment was associated with longer delays in return to fertility.

This means that the simple statement “PZP is reversible” is no longer an adequate description of what the scientific literature shows.

Any discussion of reversibility also has to ask: “After how many treatments?”

For example:

• After three consecutive years of treatment on Assateague Island, 69% of mares eventually returned to fertility, with recovery occurring one to four years after treatment ended (Kirkpatrick & Turner, 2002).
• In Przewalski’s horses, only 45% of mares treated for three consecutive years later produced a foal, and those that did averaged 6.7 years after their final treatment (Feh, 2012).
• After three consecutive years of treatment, researchers documented suppressed ovarian function, including absent ovulation and significantly reduced estrogen production. The researchers concluded that repeated treatment can interfere with normal ovarian function (Kirkpatrick et. al., 1992).

Longer-term studies reported similar overall trends (Nunez et. al., 2017; National Academics 2013 report).

Some mares treated for four or five consecutive years eventually became pregnant, although recovery sometimes required many years.

By contrast, none of the mares treated for seven consecutive years had returned to fertility during the observation period reported by the researchers, although some had resumed ovulating.

A detailed long-term study of the Shackleford Banks herd found that mares receiving four or more lifetime treatments showed no statistically detectable improvement in fertility during the years they were followed after treatment ended. The same study also found population-level associations with later foaling dates and more frequent changes in social bands.

The details differ from study to study.

The overall pattern does not.

Longer or more intensive treatment was consistently associated with delayed return to fertility and, in some mares, no return to fertility during the periods researchers were able to observe.

3. What do we actually know—and what don’t we know?

Perhaps the biggest surprise was not what researchers know.

It was what they continue to investigate.

The published studies consistently report prolonged infertility after repeated treatment in some mares.

They also report delayed return to fertility, changes in ovarian function, later foaling dates, and changes in social behavior in some populations.

Researchers have proposed biological explanations for these findings, including altered follicular development and ovarian dysfunction, but these mechanisms are still being investigated.

What the literature does establish is that treatment history matters. The important finding is not that every mare experiences the same outcome. The important finding is that repeated treatment does not produce the same outcome. Duration of treatment changes the pattern of recovery.

Age, individual immune response, vaccine formulation, treatment schedule, genetics, nutrition, and the length of time researchers were able to follow individual mares all influence the outcomes researchers observed.

That uncertainty is not a weakness in the science.

It is simply an honest description of what has—and has not—been demonstrated.

What This Means for the Salt River Horses

The studies summarized above were conducted on other wild and feral horse populations—not on the Salt River herd. That distinction matters. But the research raises questions we cannot answer for Salt River because the necessary information has never been made public.

We know PZP has been used on Salt River mares since at least 2019–2020. We know foal production has declined, and we know the herd is aging.

What we do not know is just as important:

• which mares have been darted
• how many lifetime treatments each mare has received
• how many consecutive years individual mares have been treated
• how many treated mares have returned to fertility
• whether enough fertile mares remain to sustain a self‑reproducing herd

These are not academic details. They go directly to the future of the Salt River horses.

Calls for additional scientific studies—genetics, habitat, range conditions, population dynamics—are all worthwhile. But in SRT’s view, the most urgent need is a comprehensive assessment of mare fertility before any additional horses are permanently removed from the herd.

The published literature makes one point unmistakably clear: treatment history matters. Without knowing which mares have been treated, for how long, and whether they have recovered, we cannot know whether the herd’s long‑term reproductive capacity has been affected.

There is also a broader lesson.

History is full of examples where people intervened in natural systems with confidence, only to discover years later that the long‑term consequences were more complex than expected. The scientific literature on repeated PZP treatment raises legitimate questions about long‑term fertility—questions that have not yet been answered for Salt River.

In SRT’s view, that uncertainty should not be ignored.

First, there should be no removals until a comprehensive, independent assessment of mare fertility is completed. Before any horse is removed, we must know how many mares remain fertile, how many have received repeated treatments, how many have returned to fertility, and whether the patterns documented in other horse populations are occurring here.

Second, there should be a moratorium on further PZP administration until independent scientists have full access to all darting records and can verify which mares have been treated, how often, and with what outcomes. The management group’s internal records are not a substitute for independent scientific review.

Because these are public horses, the data that determine their future should be publicly accessible. A full accounting of treatment history and fertility outcomes is essential for responsible management. Self‑reporting by the management group is not enough. Decisions about the future of a public herd must be based on information that is independently reviewed and publicly available.

The Salt River horses deserve management based on evidence. The public deserves transparency.