January 21, 2010 – PZP’s Effects, part 1: Injection Site Reactions & Reproductive Organs

Research on PZP has not only focused on desired effects; it has also focused on undesired side-effects. These effects have been the subject of numerous studies that date from the early 1990’s until fairly recently. Today we will be focusing on two different physiologic effects that are relevant to studies on the use of PZP in wild horses: injection site reactions and effects on the reproductive organs.

Injection Site Reactions

Injection site reactions are often generalized as being abscesses. However, there are a number of different reactions. A recent study (Roelle, J.E. and Ransom J.I., 2009) defined four types of possible reactions:

  • Abscesses: Open sores, possible with drainage, at the injection site.
  • Nodules: Circular, raised areas at the injection site.
  • Swelling: A raised area, that is not as defined as a nodule, at the injection site.
  • Stiffness: A case where the natural movement of the leg associated with the injection site has been affected.

By defining this range of possible reactions, it is possible to make numerous observations following injections so that injection site reactions can be properly quantified. Though the aforementioned study will be a primary focus on this topic, other earlier studies have presented data on injection site reactions. During the first recorded use of PZP on the wild horses of Assateague Island, it was found that abscesses resulted from 4.3% of injections with field darts (Kirkpatrick, J.F., Liu, I.K.M., and Turner, J.W., 1990). A later discussion of the use of PZP on Assateague Island indicated .8% of injections with field darts resulted in abscesses (Lyda, R., Hall, R., and Kirkpatrick, J.F., 2005).

The aforementioned study (Roelle, J.E. and Ransom J.I., 2009) involved analysis of observations made on three different wild horse herds: the Pryor Mountain Wild Horse Range, Little Book Cliffs Wild Horse Range, and McCullough Peaks Herd Management Area. In the McCullough Peaks, observations were made on 36 horses that received 72 injections by hand or with jab sticks. In the Pryors and Little Book Cliffs, observations were made on 90 horses that received 314 injections from 2001-2007. Though injections by hand and with jab sticks did occur in these two herds, field darts were primarily used to deliver the PZP. Throughout the study, a total of 1,440 observations were made on horses treated in the Pryors and Little Book Cliffs. Not as many observations were done in the McCullough Peaks, but this was not a huge problem given that injection site reactions were found to be rare when the PZP was injected by hand or with jab sticks.

Abscesses only were found following injections with field darts, though they were rare. In the Pryors and Little Book Cliffs, eight abscesses resulted from 278 injections. Three abscesses were observed in the Pryors, and the other five were in the Little Book Cliffs. It should be noted that four of the Little Book Cliffs abscesses were associated with one mare. Most of the abscesses in the two herds were gone within 90 days. Following is a photograph of an abscess on a Pryor mare:

Similarly to abscesses, injections by hand or with jab sticks only resulted in one nodule. Nodules were more common than abscesses; about 25% of treated mares in the Little Book Cliffs and Pryors had them. The nodules are not fully understood; but it is probable that they are small granulomas, which are basically small areas of scar tissue. In this way, they are not unlike the scars that resulted from smallpox vaccinations. The nodules were found to last for varying amounts of time; some disappeared in less than a month while some lasted up to five years. Following is a photograph of a small nodule on a Pryor mare:

Swelling was found in a small percentage of injections by hand or with jab sticks. It was also found following injections with field darts. In the Little Book Cliffs, 11% of treated horses exhibited swelling while 33% of the Pryor horses had swelling. Swelling typically lasted less than a month, and no swelling lasted over three months. Stiffness was observed in 1.4% of treated horses in the Little Book Cliffs and 11.7% of horses in the Pryors. Stiffness never lasted longer than a month.

The study was designed to determine the relationship between injection site reactions and adjuvants, delivery methods, and age. As can be seen, delivery methods seemed to be related to injection site reactions; field darting produced more reactions than injections by hand or with jab sticks. Even with this, though, significant injection site reactions were rare; and most reactions went away relatively quickly. There was no strong evidence that any of the other variables were associated with injection site reactions. There was no indication that any injection site reactions significantly weakened the treated horses. Results from the study are also consistent with those of the earlier studies described above.

Effects on Reproductive Organs

Given that PZP’s function is to disrupt reproduction, it is with good reason that studies have taken place to understand how the reproductive organs, such as the ovaries, are affected by PZP treatments. It is possible to monitor the ovarian function of horses by analyzing relevant substances in urine samples. A 1992 study determined that Assateague Island horses treated for four straight years had normally functioning ovaries (Kirkpatrick, J.F., Liu, I.K.M., Turner, J.W., Naugle, R., and Keiper, R., 1992). A later study  found that horses treated for five straight years also had normally functioning ovaries (Kirkpatrick, J.F., Naugle, R., Liu, I.K.M., Bernoco, M., and Turner, J.W., 1995). This is a topic we will be returning to later while discussing the reversibility of PZP.


Kirkpatrick, J.F., Liu, I.K.M., and Turner, J.W., 1990, Remotely delivered immunocontraception in feral horses: Wildlife Society Bulletin, v. 18, p. 326-330

Kirkpatrick, J.F., Liu, I.K.M., Turner, J.W., Naugle, R., and Keiper, R., 1992, Long-term effects of porcine zonae pellucidae immunocontraception on ovarian function of feral horses (Equus caballus): Journal of Reproduction and Fertility, v. 94, p. 437-444

Kirkpatrick, J.F., Naugle, R., Liu, I.K.M., Bernoco, M., and Turner, J.W., 1995, Effects of seven consecutive years of porcine zona pellucida contraception on ovarian function in feral mares: Biology of Reproduction Monograph Series 1, Equine Reproduction VI: 411-418

Lyda, R., Hall, R., and Kirkpatrick, J.F., 2005, A comparison of Freund’s Complete and Freund’s Modified adjuvants used with a contraceptive vaccine in wild horses: Journal of Zoo and Wildlife Medicine, v. 36, p. 610-616

Roelle, J.E. and Ransom J.I., 2009, Injection-Site Reactions in Wild Horses (Equus caballus) Receiving an Immunocontraceptive Vaccine: United States Geological Survey, Scientific Investigations Report 2009-5038

Published in: on January 21, 2010 at 11:08 pm  Comments (4)  

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4 CommentsLeave a comment

  1. Thank you for this information. There are many mis information sites out there and this is a highly emotional topic for many of us. One blog I read stated that PZP could be administered in feed to deer to limit reproduciton. If that were possible it would mean these gathers weren’t needed to dart the horses but I can see that that particular blogger was mis informed! Tahnks for this!

    This is off topic but I have been wondering if you knew what had happened to the foal with the big bite out of his neck probably from a cougar attack. When you posted his photo you said he was healthy despite the awful wound, but he didn’t seem to be one of the horses rounded up in September. Did he die of the wound or ?…

    • Betty,
      Thanks for your post. It is true that PZP works well on deer. However, I haven’t heard of any way PZP can be administered in feed. Because PZP is a simple protein, it would be broken down in the digestive system. This is why PZP can’t pass through the food chain like other contraceptives. I will look into this more, but I am fairly certain there are areas where this type of thing is working.

      Heritage’s foal seemed to be doing well, I don’t know what ended up happening. He was there one week and gone the next. With a wound of that type, there are a number of different reasons he could have died. His mother’s age (2) is also something we should consider. I’ve also wondered if the neck injury was the result of a bite from another horse; it was about the right size in about the right place for one. Just for your information, too, that harem (Lakota) was not gathered.

      Thank you very much for your comment and questions! It always make me happy to know people think about all of the horses, even the lesser known ones like Heritage and her colt.

  2. Matt, did they ever decide if Phoenix’s wound was a case of an extreme reaction to PZP or not? If not, what did they figure it was?

    All this info is very interesting. Thank you for your time and effort to compile it and make it quite understandable.

    Do you know if Tony and Lone Wolf are still around? Lone Wolf was so thin—but so goes the cycle of life.


    • I will check on conclusions on Phoenix’s wound before I post my memories of what was found.

      Tony and Lone Wolf are still around; we’ve had some sightings of them lately on Sykes Ridge. The weather has thus far not been too bad this winter, but we have been getting a pretty good amount of snow this weekend. We still have a ways to go before spring too. I really hate the thought of poor Lone Wolf having to suffer.

      It’s good hearing from you, hope things are well!

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