The old hypothesis — that PZP merely blocks sperm attachment — has been disproved.

Kaur & Prabha (2014) found that the infertility brought on by PZP is ” … a consequence of ovarian dystrophy rather than inhibition of sperm-oocyte interaction.” They reported that PZP’s antibodies induce ovarian dystrophy, oophoritis (inflammation of the ovaries), destruction of oocytes in all growing follicles, and depletion of resting follicles.

Despite all the hype about PZP being non-hormonal, the manufacturer himself knew that it had an adverse hormonal effect — significantly-lowered estrogen. In 1992, he reported that ” … three consecutive years of PZP treatment may interfere with normal ovarian function as shown by markedly depressed oestrogen secretion.” Thus, PZP is an endocrine disruptor.

Worse yet, Sacco et al. (1981) found that PZP antibodies are transferred from mother to young via the placenta and milk. The transferred antibodies cross-react with and bind to the zonae pellucidae of female offspring. This is bad news because BLM regularly administers PZP to pregnant and lactating mares, who transfer the destructive antibodies to their filly-foals. Thus, the fillies get their first treatment with PZP in utero, while nursing, or both.

Nettles (1997) found an association between PZP and stillbirths. In 2015, the International Society for the Protection of Mustangs and Burros reported that 7 mares previously treated with PZP, when taken off it, were able to get pregnant. However, 6 of those 7 mares — that is, 86 percent — produced foals that were stillborn. All other ISPMB mares that had not been previously injected with PZP successfully birthed healthy foals. Environmental and other conditions were identical. The only variable was PZP. Meanwhile, over on the East Coast, the Corolla herd, long-managed with PZP, has recently experienced birth defects among its newborns.

Gray & Cameron (2010) questioned the supposed benefit of PZP-sterilized mares living much longer than their normal life expectancy, and and Knight & Rubenstein (2014) warned of unintended consequences of PZP’s ironic effect of extended longevity. Ultra-elderly mares take up scarce slots within AML-restricted herds. They consume resources but no longer contribute to the gene-pool. It is detrimental to a population’s genetic viability to carry significant numbers of sterile herd-members way-beyond their normal life-span. Meanwhile, those few foals that are born have to be removed to achieve AML because they’re more adoptable.

Ransom et al. (2013) conducted a longitudinal study of three herds currently being managed by PZP — Little Book Cliffs, McCullough Peaks, and Pryor Mountain. They found that the the birthing season lasted 341 days — nearly year-round — which puts the life of mares and foals in jeopardy. Nature designed the equine birthing-season to occur in Spring, not year-round, and certainly not in the dead of Winter.

Ransom et al. also found that, after suspension of PZP, there was a delay lasting 411.3 days (1.13 years) per each year-of-treatment before mares recovered their fertility. They warned: “Humans are increasingly attempting to manage the planet’s wildlife and habitats with new tools that are often not fully understood. The transient nature of the immunocontraceptive PZP can manifest into extraordinary persistence of infertility with repeated vaccinations, and ultimately can alter birth phenology in horses. This persistence may be of benefit for managing overabundant wildlife, but also suggests caution for use in small refugia or breeding facilities maintained for repatriation of rare species.”

PZP’s manufacturer conceded that it could take up to eight years to recover fertility after just three consecutive PZP treatments.

The study on PZP by Knight & Rubenstein (2014) found that ” … three or more consecutive years of treatment or administration of the first dose before sexual maturity may have triggered infertility in some mares.” These findings are particularly troubling. They suggest that, actually, only two consecutive PZP-treatments may be reversible. Except, that is, in the case of fillies who have not yet reached puberty — they could be sterilized by just one injection.

PZP’s manufacturer was quoted describing PZP as “so safe it is boring.” Independent research shows otherwise — that PZP is a powerful hormone disruptor that could sterilize a female with just one injection. If staff and volunteers believe that PZP is boringly safe, they will be less likely to protect themselves adequately from this dangerous pesticide. Indeed, many of the volunteers are women and, therefore, at risk. Accidental self-injection with PZP could cause them to suffer diseased ovaries and depressed estrogen-levels — in addition to infertility and, potentially, sterility. Consider the magnitude of the risk — the PZP-in-question is a horse-size dose.

~ Marybeth Devlin, Wild Horse Advocate

Protect Mustangs is an organization who protects and preserves native and wild horses.