Fact-Checking Misinformation on Wildlife Fertility Control

The wildlife fertility control vaccine PZP is supported by nearly 40 organizations as a humane way to manage wild horses and reduce the use of inhumane roundups. However, some opponents spread misinformation about PZP via social media and websites. This misinformation impedes productive, science-based discussions about reducing wild horse roundups and ensuring humane treatment of herds.

Addressing Misinformation About PZP

Here is a look at some of the inaccurate claims about PZP that have appeared online, countered by factual replies from longtime fertility control researcher Allen Rutberg, PhD, director of the Center for Animals and Public Policy for the Cummings School of Veterinary Medicine at Tufts University.

Misinformation: PZP Causes Ovarian Dystrophy

MISINFORMATION: 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.

FACT: Kaur & Prabha (2014) cite no studies on PZP in horses. The effect described is drawn from a 1984 study (Skinner et al.) of the effect of PZP in laboratory rabbits. Species differ in their responses to PZP.

Misinformation: PZP is an Endocrine Disruptor

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 estrogen secretion.” Thus, PZP is an endocrine disruptor.

FACT: Yes, PZP may reduce estrogen production. So do winter, pregnancy, lactation, and human birth control pills. Reduced estrogen production does not imply harm.

Misinformation: PZP Antibodies Transfer to Offspring

MISINFORMATION: 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.

FACT: Sacco did his experiment on mice. The experiment was only possible because PZP did not work as a contraceptive in mice. (If it did, there’d be no pregnant mice to do the test in.)

Receiving anti-PZP antibodies (“passive immunity”) is different from receiving a PZP vaccine (“active immunity”). The antibodies that babies receive in their mother’s colostrum convey no long-term immunity; kids still need to get their vaccinations. In the case of Sacco’s mice, the anti-PZP antibodies vanished after about 100 days.

Big picture: hundreds of offspring of PZP-treated mares have grown up and had offspring of their own, and these have had offspring of their own, too.

Misinformation: PZP Linked to Stillbirths

MISINFORMATION: Nettles (1997) found an association between PZP and stillbirths.

FACT: Nettles is citing a 1983 study (Gulyas et al.) in which caged monkeys received up to 9 injections of a preparation of PZP that the authors concede was visibly contaminated. The authors attribute all major side effects to the contamination.

The monkeys were also repeatedly anesthetized for blood sampling, one dying under anesthesia. What a “normal” rate of stillbirths would be under those experimental conditions is not established.

Misinformation: Extended Longevity of PZP-Treated Mares

MISINFORMATION: Gray & Cameron (2010) questioned the supposed benefit of PZP-sterilized mares living much longer than their normal life expectancy, 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.

FACT: Except for the attempt to portray the principal side effect of PZP -- a longer healthier life for treated mares -- as a vice, these comments have nothing to do with PZP itself. Issues of long-term population viability need to be addressed in a management plan, as the National Park Service has done.

Misinformation: Year-Round Birthing Season

MISINFORMATION: 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 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.

FACT: The same paper found no effect of PZP treatment on foal survival. At only one of the three sites did treated mares give birth past September, and that was a total of 3 births over 7 years. Breeding season length was longer for treated mares at one site, longer for untreated mares at one site, and identical for treated and untreated mares at one site.

Misinformation: Delayed Fertility Recovery

MISINFORMATION: 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.”

FACT: The increased delay with repeated treatments was the same as that reported by Kirkpatrick et al. at Assateague. The rest is rhetoric that again highlights the importance of good management plans.

Misinformation: Infertility from Early PZP Treatment

MISINFORMATION: 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.

FACT: This is speculation. Ransom et al (2013) cited above found that age of first inoculation did not influence time to first parturition. (We haven’t published this but of the 22 2-year olds we treated with PZP-22 at Cedar Mountain HMA in February 2012, 16 had already had at least one foal by 2015, i.e., within three years.)