January 19, 2010 – PZP’s Efficacy & Population Effect

In past posts, I have presented information on the technical aspects of PZP. From this, it can be seen why PZP should be able to work in the management of wild horse populations. It can also be seen that it is feasible to use it on wild horses. Now, though, we need to think about how it actually works on wild horses. Today I will be discussing an important requirement for PZP, and any wildlife contraceptive, which is efficacy. That is, what percentage of a treated group of mares will experience a reduction in foaling due to PZP? Also, I will be discussing the demographics of populations managed through fertility control. The data for today’s topics mainly originated from Assateague Island. This is because that population has been managed exclusively with PZP for many years, and it is also a very well-studied herd.


Efficacy is a term that, in this case, defines how well PZP works. It is expressed as a percentage. For example, if the efficacy of PZP was found to be 40%, then that would mean that 40% of mares treated with PZP would not get pregnant while 60% would. An early study performed on Assateague Island (Kirkpatrick, J.F., Liu, I.K.M., Turner, J.W., Naugle R., and Keiper, R., 1992) compared mares treated with PZP to a similar control group that had not been treated. The study found that PZP was over 90% effective. This was a relatively small study that involved ten treated mares. It would thus be of interest to have data from multiple years and mares. A fairly recent study (Kirkpatrick J.F. and Turner A., 2008) did just this; it analyzed the results of managing the Assateague Island herd with PZP for 12 years. It was found that PZP was 92% to 100% effective. Similar results have been found in calculating efficacy for other herds treated with PZP; efficacy is at least 90% to 95% effective. These figures on efficacy reflect analyses of many different wild horses in many different areas. Data is collected wherever PZP is used per the stipulations of its status as an Investigational New Animal Drug; this raw data allows for efficacy to be determined.

Population Effect

A primary goal of wild horse management is to keep herds at sizes consistent with the carrying capacity of the areas they live in. Often, populations are managed by removing horses periodically. This basically involves the herd growing to a certain size, removing a certain number of individuals to reach a desired size (Appropriate Management Level), and then repeating this cycle by removing more individuals after the herd has grown over a number of years. PZP works a little differently.  The aforementioned study (Kirkpatrick J.F. and Turner A., 2008) provides an in-depth analysis of the ways in which the Assateague Island herd’s population demographics shifted following management with PZP.

An important concept to first understand is that PZP does not provide instant results like removing horses does. This is because there is a lag time before PZP’s effects start to emerge. Here is an example to illustrate this:

The Pryor mare Sacajawea foaled in May of 2007. Presumably, she got pregnant again a few weeks later. In July of 2007, she received her first PZP treatment. However, she was already pregnant. Thus, Sacajawea foaled again in May of 2008. She did not foal in 2009, though. There is a good possibility that this was due to her not getting pregnant in 2008, which was in turn likely due to the PZP she received in 2007. Thus, if a mare is treated while pregnant on year 1, the effects of PZP can emerge on year 3. (This concept also alludes to PZP’s lack of effects on unborn foals, we’ll talk about this later.) Alternatively, if a mare is treated before she gets pregnant, then PZP’s effects can be seen in year 2. This is how it works on Assateague Island, the mares there are treated in the spring before they get pregnant. Whether there is a one year or two year lag time, there is a lag time that should be known about. When this lag time is combined with the requirement to treat new mares each year while also re-treating mares, you can see that it should take a number of years before PZP is able to fully function throughout a population.

Knowing this, let’s take a look at what happened on Assateague Island. Following is a chart from the aforementioned study (Kirkpatrick J.F. and Turner A., 2008). It shows the Assateague herd’s population size over time (the line) along with each year’s foal population (the bars). This chart has population data from 1976 to 2007. I shaded in the years 1994 to 2007 green as the first management-level use of PZP started in 1994.

There are a few non-PZP related things to note about what this chart is showing. The population rose from just over 40 individuals in 1976 to 166 in 1994. The changes in population in the late 1980’s are due to some contraceptive studies and some anomalous deaths of horses due to illness and other natural factors. Around 20 to 30 foals were born each year in the years prior to the use of PZP there.

We’ll probably come back to this again, but here is a summary of what the management goals for Assateague Island are: The National Park Service wanted the 1994 population (166 horses) to stabilize. They then wanted the population to go down to 150 and then to stabilize it at 120. They also wanted to do this only with PZP; they did not want to remove any horses to achieve their goals.

Management with PZP on Assateague Island started in 1994 and was in full swing by 1995. Notice how fast the population reacted back then. After only a couple of years, there was a noticeable decrease in the foal crop; and the population stopped growing so quickly. Thus, the PZP caused the population size to stabilize rather quickly. Unlike removing horses, though, the population did not quickly decrease. This relates to PZP’s requirement of a few years before it starts to fully affect a population. The population started to decrease in the mid-2000’s; this continued on through the 2007. Thus, the National Park Service was essentially able to reach their final goals that were laid out in the mid-1990’s.

Why did it take a number of years for the population to start to decrease? This had to do with PZP increasing the health of mares significantly. We’ll have a more in-depth discussion about this in a later post, but the Assateague mares can now live into their 30’s. Prior to this, it was fairly rare for Assateague mares to even live 20 years. Thus, the lag time before there was a reduction in the population size was also part of the population transitioning to a new age structure. Following is a chart showing the age-class structure for the Assateague Island herd at different points in time.

The population in 1990 was very characteristic of wild horse populations. Most of the population was young, and the population of each age cohort got smaller as they got older. (This is basically what the Pryor Mountain wild horses’ age-class structure looks like right now.) This shape of age-class structure is indicative of a population that is expanding. Also, notice that the oldest horse on this chart is 20 years old. By 2000, the population was stabilizing; the large cohorts of horses born in the early 90’s were moving through the population while they were being replaced with smaller and more stable populations. Notice what ages of horses there were by then; there were multiple over the age of 20. These were mostly mares that had become very healthy after being treated with PZP. By 2007, the population’s age-class structure was looking quite similar to that of a stable population. Thus, PZP can lead to a change in the age-class structure of a population.


What did it take to manage the Assateague Island herd for the desired goals? PZP is a tool that requires a well-designed plan for maximum effectiveness. On Assateague Island, a varying number of mares are treated each year. From 1994 to 2006, the average percentage of mares treated annually was 65%, though there was a range of 48% to 76% of the mares being treated. (We will go into more specifics on this in a future post.) This isn’t necessarily how every population should be managed; each manager may have different goals that will require different strategies. The Assateague Island strategy simply provides a great example of how PZP can be used to exclusively manage a wild horse population.

It is hard to deny that PZP can play a significant role in the management of wild horses given the Assateague example. However, it is worth understanding the other ways that PZP can affect wild horses. There is a variety of information available regarding the physiologic and behavioral effects of PZP. Tomorrow we will start learning about different effects by examining research that has been done.


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: 437-444

Kirkpatrick, J.F. and Turner, A., 2008, Achieving population goals in a long-lived wildlife species (Equus caballus) with contraception: Wildlife Research, v. 35, p. 513-519

Published in: on January 19, 2010 at 7:33 pm  Leave a Comment  

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