Fact-Checking Misinformation on Wildlife Fertility Control

The wildlife fertility control vaccinePZPissupported by nearly 40 organizationsas a humane way to manage wild horses and reduce the use of inhumane roundups. However, some opponents spread misinformation aboutPZPvia 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 aboutPZPthat have appeared online, countered by factual replies from longtimefertility controlresearcher 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 byPZPis “a consequence of ovarian dystrophy rather than inhibition of sperm-oocyte interaction.” They reported thatPZP’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 onPZPin horses. The effect described is drawn from a 1984 study (Skinner et al.) of the effect ofPZPin laboratory rabbits. Species differ in their responses toPZP.

Misinformation: PZP is an Endocrine Disruptor

Despite all the hype aboutPZPbeing non-hormonal, the manufacturer himself knew that it had an adverse hormonal effect — significantly-lowered estrogen. In 1992, he reported that “three consecutive years ofPZPtreatment may interfere with normal ovarian functionas shown by markedly depressed estrogen secretion.” Thus,PZP is an endocrine disruptor.

FACT: Yes,PZPmay 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 thatPZPantibodies 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 administersPZPto pregnant and lactating mares, who transfer the destructive antibodies to their filly-foals. Thus, the fillies get their first treatment withPZPin utero, while nursing, or both.

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

Receiving anti-PZPantibodies (“passive immunity”) is different from receiving aPZPvaccine (“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-PZPantibodies vanished after about 100 days.

Big picture: hundreds of offspring ofPZP-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 betweenPZPand stillbirths.

FACT: Nettles is citing a 1983 study (Gulyas et al.) in which caged monkeys received up to 9 injections of a preparation ofPZPthat 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 ofPZP-sterilized mares living much longer than their normal life expectancy, and Knight & Rubenstein (2014) warned of unintended consequences ofPZP’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 ofPZP-- a longer healthier life for treated mares -- as a vice, these comments have nothing to do withPZPitself. 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 byPZP— Little Book Cliffs, McCullough Peaks, and Pryor Mountain. They found that thebirthing 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 ofPZPtreatment 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 ofPZP, 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 immunocontraceptivePZPcan 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 onPZPby 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 consecutivePZP-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 withPZP-22 atCedar Mountain HMAin February 2012, 16 had already had at least one foal by 2015, i.e., within three years.)