The Low-Down on PZP
We recently put up an Instagram post with some quick facts on PZP and asked if we should do a follow-up blog. There were a lot of positive responses so here it is! We’re going to cover what PZP is, how it works (yes, science but we’ll keep it simple), and its uses for population control. For starters, if you missed our post, here it is:
The Science Behind PZP
First thing’s first: what the heck is a zona pellucida?! If you’re a science/veterinary nerd like me, you may already know this has to do with reproduction. But before we dive into the nitty gritty science FUN we need a few definitions:
Ova (plural), Ovum (singular): Female reproductive cell (i.e., eggs)
Oocytes: a cell in an ovary which can form an ovum
Follicle Cells: contain a bunch of immature eggs
Oogenesis: the production/development of an ovum after it has been fertilized (also a v fun word to say)
So back to the zona pellucida: this is a membrane surrounding all mammalian ova and embryos. So what does it do, you ask? This is where it gets a little science-y but also where those definitions will come in handy: the zona pellucida supports communication between oocytes and follicle cells during oogenesis. Specifically, it protects oocytes, eggs, and embryos during development, and regulates interactions between ovulated eggs and free-swimming sperm. Now that we know what it is, let’s learn how it works to prevent pregnancy in horses: the zona pellucida has specific protein receptors, one of which attaches to sperm. When PZP is injected into a mare, it tells her immune system to produce antibodies against the vaccine. These antibodies attach to the sperm receptor and block fertilization. Basically, PZP tricks the mare’s immune system into making antibodies that attach to her eggs and prevent sperm from entering the egg (SO cool, right?!) (1).
Ok so now that you know HOW it works, I bet you’re wondering IF it works (spoiler alert: it does). The best example? A herd of horses off the coast of Virginia and Maryland (Misty of Chincoteague, anyone?!). The Assateague wild horse herd has been exclusively managed with PZP since 1988. The National Park Service manages this herd and has been able to keep the population below 125 horses. They don’t use round-ups or adoptions to keep populations at a sustainable level, just PZP (2). Seems like a great solution, right? But how much does it cost?
Assateague wild horses. Photo by National Parks Service.
Cost of PZP
Ok so before I started researching, I imagined this was going to be a really quick, easy answer but it’s a little more complicated than that. There are two main PZP vaccines available: ZonaStat-H and PZP-22. ZonaStat-H is $26/dose. The actual cost of administering the dose via darting by the BLM (friendly reminder: the BLM stands for Bureau of Land Management, the branch of the government responsible for managing wild horses) is about $500/mare (3). This covers the cost of the PZP and a dart (about $30), plus the time and expenses for the darter (vehicle, etc.). Alternatively, the BLM can capture, treat, and release mares for $2,500/horse. The reason for this higher cost is because “BLM gathers a wild horse by helicopter, transfers it to a holding pen, injects a primer dose of PZP, holds and feeds the horse for 2-4 weeks, delivers a booster dose of PZP, then releases the horse back to the range” (4).
PZP-22 costs about $300/dose and is a single dose, unlike ZonaStat-H. Sounds great, but its efficacy is kind of all over the place. Regardless, it is (so far) the best option for a one-treatment vaccine.
So just as a quick aside: there are other fertility treatment options including different kinds of vaccines, IUDs, (Intrauterine Devices), and surgical sterilization (spaying mares and castrating stallions). Through studies supported by the BLM, research has shown that these are often ineffective and/or impractical. I could write a whole other blog on these options but let’s stick to PZP for now :)
Administering PZP
In captive horses, PZP is delivered like any other vaccine: via injection. For wild horses, the answer is remote darting. Herds need to be tracked and monitored closely to ensure the correct mares are vaccinated. In some HMAs (another friendly reminder: HMA = Herd Management Area, land where wild horses live) volunteers work closely with the BLM to help with this process. A great example is the Spring Creek Basin HMA in Colorado. Louisa was actually able to (pre-COVID) meet with and interview TJ Holmes, one of the volunteer darters at this HMA. She had this to say about wild horses: “The mustang issue to me is not an overpopulation problem, it’s a reproduction issue”.
I want to circle back to the Assateague herd: this HMA has had really great results but it is not a good example of a typical HMA in the West; what I mean is that the Assateague herd has an AML (Appropriate Management Level - the number of horses that can live on the land sustainably) of 120 horses across 8,850 acres. The Jackson Mountains HMA in Nevada, for example, has a similar AML of 170 horses but the acreage is much higher, about 283,000 acres (3). But let’s look at Spring Creek Basin: as of March 1, 2020, the estimated population was 69 horses, only four more than the high end of the AML for this area. That may not sound like a success, but compare that to the Diamond Hills North herd area: the high AML is 37 horses. Their current population: 620 (5). The Diamond Hills complex does use PZP so this got me wondering why the Spring Creek Basin herd was so much closer to their AML than the Diamond Hills herd. I couldn’t figure it out from my research but I did discover that the BLM is using PZP on a lot more herds than I had previously thought. In my research I also stumbled across a bill amendment that was passed in 2020, requiring the BLM to allocate $11 million to implement PZP to manage wild horse numbers (6). I’ve still got lots of questions (and I’m sure you do too!) but I’m hoping our documentary will be able to answer them. Stay tuned ;)
Wild horses Hollywood, Alegre and Houdini under a rainbow in Spring Creek Basin. Photo by TJ Holmes.
PZP as a Solution
So, onto the big question: is PZP the answer to the wild horse problem? I’m not an expert (I am FAR from it. I’ve realized this especially after writing and researching for this blog) so I’m going to give you a big, fat I don’t know. I don’t think it is the singular solution to fixing the problem. However, I believe it will play a vital role when combined with other management solutions. Each HMA is different and I think one of the keys to solving the problem is recognizing that we need to manage them individually, rather than on a broad level. But that’s just my personal opinion.
I have to say, I learned a LOT from researching for and writing this blog (it took me WEEKS to write this one!!). I’m SURE I missed stuff and we’d love to see your comments, especially if you have some hands-on experience with PZP! Hopefully You learned a little something, I know I certainly did. Let’s keep the conversation going :)
xoxo, Katherine & the wild women 💕
Works Cited
Wassarman P;Chen J;Cohen N;Litscher E;Liu C;Qi H;Williams Z; “Structure and Function of the Mammalian Egg Zona Pellucida.” The Journal of Experimental Zoology, U.S. National Library of Medicine, pubmed.ncbi.nlm.nih.gov/10497324/#:~:text=The zona pellucida supports communication,sperm during and following fertilization.
“February 19, 2010 – PZP & Assateague Island.” PryorWild, 19 Feb. 2010, pryorwild.wordpress.com/2010/02/19/february-19-2010-pzp-assateague-island/#:~:text=The wild horses of Assateague Island National Seashore are managed,the population at that time.
Kane, Albert J. “Currently Available Contraceptives and Sterilization Techniques for Wild Horses and Burros.” BLM.gov, Bureau of Land Management, 2017, www.blm.gov/sites/blm.gov/files/wildhorse_2017AdvisoryBoard_Kane.pdf.
“PZP.” Wild Horses of America Foundation, 26 Oct. 2017, wildhorses.org/pzp/.
Bureau of Land Management. “Wild Horse and Burro Population Estimates as of March 1, 2020.” BLM.gov, Bureau of Land Management, 2020, www.blm.gov/sites/blm.gov/files/wildhorse_2020_HAHMA_Stats_508.pdf.
“WILD HORSE AMENDMENT CLEARS RULES COMMITTEE; HEADED TO HOUSE FLOOR.” Congressman David Schweikert, 23 July 2020, schweikert.house.gov/media-center/in-the-news/wild-horse-amendment-clears-rules-committee-headed-house-floor#:~:text=The amendment, “Requires the Bureau,to manage wild horse populations.”