Is contraception an effective management tool for deer?
Is contraception an effective management tool for deer?
When faced with issues of human-deer conflict, many people feel it is important to look for a way to manage the situation while preserving the lives of the local deer. In our own West Seneca town survey (2021), approximately 1/4 of residents opposed lethal means of deer control, and another 18% preferred to exhaust non-lethal options before attempting to manage deer through some sort of controlled hunt. When considering non-lethal options for managing deer populations, contraception is often suggested. However, contraception options for deer are complex, and the solution is not as simple as one might imagine. So, is contraception an effective management tool for suburban deer? Let's start with the "short answer" and clear up some common misconceptions.
Is contraception an effective management tool for suburban deer populations?
The short answer is that contraception for deer is often an unrealistic approach to controlling suburban deer populations due to its high cost and relatively low effectiveness in comparison with lethal control measures. In addition, while it is a nonlethal method, it is not without stress and risk to the animals. People often confuse contraception with vaccination methods, such as rabies bait drops, where a drug can be orally administered. There is currently no oral contraceptive for deer. This means that to administer a contraceptive program will involve extensive capture and handling of deer, often repeatedly over time. This can be extremely stressful for deer and may even result in death in some cases. Click here to read more about capture myopathy, a cause of death due to stress from capture and handling in deer.
When considering a contraception program for deer, a town must consider all the pros and cons of the methods available. If you are interested in learning more about how deer contraception programs work, what they cost, and how effective they are, please read on for a detailed look at the topic.
What contraception options are available for White-tailed Deer?
There are currently three methods that may be used for reducing deer reproduction: surgical sterilization, Porcine Zona pellucida (PZP), and GonaCon.
Surgical Sterilization
Overview: Although vasectomy (sterilization of males) is generally a simpler surgery than sterilization of females, efforts to control deer populations in this manner must focus on females. Sterilization of male deer is unlikely to be effective, since a single buck can mate with and impregnate a large number of females. This means that nearly all males in a population would need to be treated, which is rarely possible (Merrill et al., 2003). Thus, control of deer population growth is more practically attained by managing fertility of females (Porter et al. 2004). The most commonly used methods of surgical sterilization for female deer are tubal ligation and ovariectomy (removal of the ovaries). Performing these procedures requires the deer to be captured, either by using a live trap or by darting with a tranquilizer. Deer are anesthetized and moved to a clean location for surgery. Ear tags, and often a radio collar, are attached for identification and tracking. Deer are returned to the capture site immediately after surgery (MacLean et al., 2006; Boulanger and Curtis, 2016). For detailed information about the surgical process, click the links to MacLean et al., 2006 and Boulanger and Curtis, 2016 in the references section below.
Effectiveness:
A Cornell University study on the effectiveness of surgical sterilization of female deer found that pregnancy was prevented in 100% of deer that received ovariectomies and 96% of deer that received tubal ligations. Follow-up monitoring using remote cameras showed a 38% reduction in the numbers of adult females detected, and a 79% reduction in the number of fawns. The ratio of adult females to fawns was reduced from 0.7:1 to 0.2:1 over a period of 3 years. However, the number of adult male deer detected by the cameras increased by 873% over this same time period, and the overall population size did not appear to decrease. One possible explanation for the increase in males in the study area is that tubal ligation, which was used on some of the female deer in the study, results in prolonged estrus cycling through late winter or early spring, which may have attracted additional males to the area. Computer models of deer populations treated with surgical sterilization have indicated that the method has the potential to reduce population sizes, but these models assume a closed population (meaning one with no movement of animals in or out). It is still unclear if this type of population reduction can be achieved under natural conditions, where animals are dispersing between populations (Boulanger and Curtis, 2016). Research on deer movement behavior suggests that the dispersal of female deer may be particularly important in the long-term success of deer management (Porter et al., 2004). Surgical sterilization does reduce the numbers of fawns produced, and may be most useful to increase the effectiveness of lethal control measures (Boulanger and Curtis, 2016).
Cost: Surgical sterilization costs are approximately $1000-$1200 per deer, including capture time, veterinarian time, and surgical drugs (Boulanger et al., 2012; Koblinsky, 2016).
Summary of Pros and Cons:
Pros:
Ovariectomy has a nearly 100% success rate in preventing pregnancy, and eliminates estrus in treated animals
The treatment is accomplished in a single capture event
Surgical sterilization reduces the number of fawns produced, and may allow for more rapid population reduction when implemented in conjunction with lethal control measures
Cons:
Surgical sterilization is expensive
The procedure is stressful and invasive. Deer must be captured, transported, and extensively handled. There is a risk of mortality due to the surgery itself and to capture myopathy. The mortality risk reported in studies of the was considered low, but was still notable. MacLean et al. (2006) reported a project-related morality rate of 6.1% (11 deaths out of of 181 animals). Four of these animals died in traps before ever being handled, two died during restraint, four died within several days after capture from capture myopathy, and one died during surgery. Similarly, Boulanger and Curtis (2016) reported 5% mortality in all animals captured (4 out of 120 deer), and 3% mortality during surgery (3 out of 93 animals).
Tubal ligation, which is a less expensive and easier surgery than ovariectomy, results in a prolonged estrus cycle which may draw additional males into the area, cause increased stress and reduced winter survival, and increase deer movement, resulting in higher deer-vehicle collisions (Boulanger et al., 2014)
Surgical sterilization alone does not appear to sufficiently reduce deer populations to alleviate issues of human-deer conflict. This is likely due to the effects of deer dispersing into the area from nearby populations.
Deer that have been tranquilized must be marked to indicate that they cannot be used as a food source (National Park Service, 2012). This may be important as some towns neighboring West Seneca do have deer hunting.
Porcine Zona Pellucida (PZP):
Overview: PZP is an injectable contraceptive vaccine that causes female deer to produce antibodies that bind to the the zona pellucida, a protein envelope surrounding the egg. This prevents fertilization from occurring (Rutberg, 2014). There are currently several different formulations of the PZP vaccine that are available for experimental use. In the United States, the most common of these is the brand name Zonastat-D, which was approved by the US Environmental Protection Agency (EPA) in 2017 for use by government agencies, Native American tribes, and the Humane Society of the Unites States (NYSDEC, 2021). A second version of the vaccine, called SpayVac, is under consideration for approval by the US EPA (SpayVac, 2022). All PZP vaccines work in the same way, using proteins from the membrane surrounding the eggs of pigs to generate an immune response in deer that prevents fertilization (Science and Conservation Center, 2022).
PZP is typically administered as a hand-injection or remote injection by dart. However, for the Zonastat-D formulation currently in use in New York, the initial injection must be followed by a booster given 2-6 weeks later, and an annual booster must be administered each year to maintain the contraceptive's effectiveness (Science and Conservation Center, 2022). This means that the treated deer need to be marked for identification, requiring capture for at least the first administration of the vaccine. A single-dose PZP vaccine is in development, and shows promise in providing pregnancy reduction for two breeding seasons. This vaccine is not yet approved for use, but may eventually reduce the cost of treatment and stress on animal since boosters would only be needed every other year (Rutberg et al., 2013).
Effectiveness: If administered correctly (including boosters), PZP can reduce fawning rates by 80-90%. However, a high number of females need to be treated to achieve stabilization or reduction of the deer population. Although computer models have indicated that under ideal conditions vaccinating 80% of the females in a population may produce overall population size reductions in a reasonable time frame, field work under natural conditions suggests this number is loser to 95%. Reaching this level of vaccination is time consuming, expensive, and may not be feasible in all areas (Curtis, 2020). A study of PZP conducted by researchers from Tufts University found that suburban deer populations at three field sites were stabilized and modestly reduced (20-50%) through PZP treatment Typically, population stabilization is rapid, but population reduction occurs more slowly (5-10% per year). Although high rates of female immigration could overwhelm efforts to control populations, the effect of deer movement varies, so the importance of dispersal to population reduction efforts are location-specific (Rutberg, 2014).
Cost: The costs of immunocontraception are considerably lower than those of surgical sterilization. However, anytime deer must be captured and tagged, there will be costs associated with those activities. The cost of PZP treatment may vary depending on the accessibility of the deer and how much time it takes to capture them for initial treatment. On average, administering an initial dose of PZP costs about $500 per deer (including capture). If subsequent boosters are administered by dart, this lowers the cost of boosters to approximately $100 per deer. Overall, achieving rapid population stabilization and slow decline for about 300 deer would require approximately $40,000 for both the first and second years, and lower amounts after (Rutberg, 2014). Obviously, this total will vary based on how many deer a municipality needs to treat to achieve the desired goals.
Summary of Pros and Cons:
Pros:
PZP has the ability to stabilize a population, and even to achieve a slow population decline, without the need for lethal control
PZP treatment is less expensive than surgical sterilization
Cons:
PZP treatment requires a considerable amount of time and effort, and may be expensive in comparison to lethal control
Deer must be captured and tagged to ensure repeated boosters can be administered. This process is stressful and can induce capture myopathy.
As with tubal ligation, female deer may experience a prolonged estrus cycle which may draw additional males into the area, cause increased stress and reduced winter survival, and increase deer movement, resulting in higher deer-vehicle collisions (Boulanger et al., 2014)
Because of the high percentage of females that must be vaccinated to achieve population reduction, PZP treatment alone often does not sufficiently reduce deer populations to alleviate issues of human-deer conflict. Lethal control measures may be used in conjunction with PZP to achieve population reduction goals more quickly.
Deer that have been tranquilized must be marked to indicate that they cannot be used as a food source (National Park Service, 2012). This may be important as some towns neighboring West Seneca do have deer hunting.
GonaCon:
Overview: GonaCon is another immunocontraceptive vaccine that has been approved for use in female deer over 1 year old by the EPA and US Department of Agriculture. Like PZP, GonaCon works by stimulating an immune response. However, GonaCon differs from PZP in what the antibodies produced actually target. Rather than simply preventing fertilization, GonaCon's antibodies target a hormone call Gonadotropin-Releasing Hormone (Gn-RH). This hormone's role in the body is to stimulate the production of other sex hormones (such as estrogen). By binding to Gn-RH, the antibodies produced in response to the GonaCon injection prevent the production of sex hormones, effectively "switching off" all reproductive activity (USDA, 2010). GonaCon is typically administered using only a single injection, which prevents pregnancy for at least two years (Gionfriddo et al., 2009). However, there is some evidence that a second injection may provide increased effectiveness (Walker et al., 2021). GonaCon must be administered by hand injection into the deer's muscle tissue, so capture is necessary to treat deer with this vaccine (USDA, 2010). Field studies have demonstrated that the vaccine is considerably more effective when administered by hand injection than by dart (Evans et al., 2015).
Effectiveness: Studies on the effectiveness of GonaCon indicate that it slightly less effective than PZP in preventing pregnancy in deer. Gionfriddo et al. (2009) found that after receiving a single injection of GonaCon, 88% of treated deer did not become pregnant during the first year and 47% did not become pregnant during the second year after treatment. Walker at al. (2021) found that 67% of deer that received only a single dose were pregnant the following year, and only 14% of deer that received two doses were pregnant. However, the Walker et al. study also found that the deer population had increased after vaccination, indicating that GonaCon vaccination alone may not be sufficient to achieve population reduction.
Cost: As with PZP, the cost of Gonacon treatment may vary depending on the accessibility of the deer and the amount of time required to capture and vaccinate them. Reported costs range from several hundred dollars per deer (USDA, 2010) to over $2000 per deer (Walker et al., 2021). By contrast, the reported costs of culling deer average $500-$600 per deer for sharpshooting (Hilltop Conservancy, 2012; Walker et al. 2021), and may be lower if community volunteers are used to carry out the cull.
Summary of Pros and Cons:
Pros:
GonaCon has the ability to stabilize a population, and even to achieve a slow population decline, without the need for lethal control
GonaCon treatment is less expensive than surgical sterilization
GonaCon eliminates all reproductive activity, so there is no estrus cycle for treated deer. This eliminates concerns about attracting males or inducing extended movement periods and increased deer-vehicle collisions
Cons:
GonaCon treatment requires a considerable amount of time and effort, and may be expensive in comparison to lethal control
Deer must be captured and sedated every 2 years to administer the injection. This process is stressful and can induce capture myopathy.
Because of the high percentage of females that must be vaccinated to achieve population reduction, GonaCon treatment alone often does not sufficiently reduce deer populations to alleviate issues of human-deer conflict. Lethal control measures may be used in conjunction with PZP to achieve population reduction goals more quickly.
Deer that have been tranquilized must be marked to indicate that they cannot be used as a food source (National Park Service, 2012). This may be important as some towns neighboring West Seneca do have deer hunting.
*All immunocontraceptive treatments are considered experimental in New York State, and may only be performed under a research permit. This means that towns who wish to use these methods must partner with a university or other research organization to collect data on the effectiveness of the method (NYSDEC, 2018)
Why is there no oral contraceptive option?
The quest for an effective method of contraception in deer has been underway since at least the 1970s. Early attempts focused on the use of oral hormone therapy, such as is used in the human oral contraceptive pill. Oral contraception was ruled out as an effective option because, although it could be effective if administered correctly, it was found to require daily oral exposure to prevent pregnancy. In addition, it had to be administered at very high doses, and deer were found to avoid eating food containing the hormone (Matchke, 1977; Warren, 2000). Immunocontraceptives, like PZP and GonaCon, are proteins that are broken down when taken orally, making them ineffective as potential oral contraceptives (Science and Conservation Center, n.d.; USDA, 2017). Research into an oral immunocontraceptive vaccine is ongoing, but the such a vaccine (and its oral delivery system) would need to be species-specific. There are considerable challenges yet to be overcome in terms of both research needs and regulatory approval for this to occur, and currently oral contraception appears for deer appears to be unlikely for the foreseeable future (Kirkpatrick et al., 2011).
References:
Boulanger, J.R., P. D. Curtis, E.G. Cooch, and A.J. DeNicola. 2012. Sterilization as an alternative deer control technique: a review. Human-Wildlife Interactions 6(2): 273-282. Accessed at https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1141&context=hwi#:~:text=Surgical%20sterilization%20is%20scale%2Dlimited,of%20immigration%20can%20be%20minimized
Boulanger, J. R., Curtis, P. D., and Blossey, B. 2014. An integrated approach for managing white-tailed deer in suburban environments: the Cornell University study. Cornell University Cooperative Extension and Northeast Wildlife Damage Research and Outreach Cooperative. Accessed at https://deeradvisor.dnr.cornell.edu/sites/default/files/resources/IntegratedApproachForManagingWTDeerInSuburbanEnvironments-28ax086.pdf
Boulanger, J.R. and P.D. Curtis. 2016. Efficacy of Surgical Sterilization for Managing Overabundant Suburban White-Tailed Deer. Wildlife Society Bulletin 40(4): 727-735. Accessed at https://www.wildlifefertilitycontrol.org/wp-content/uploads/2016/12/Boulanger-and-Curtis-WSB-2016.pdf
Curtis, P.D. 2020. After decades of suburban deer research and management in the eastern United States: where do we go from here? Human-Wildlife Interactions 14(1): 111-128. Accessed at https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1627&context=hwi
Evans, C.S., A.J. DeNicola, J.D. Eisemann, D.C. Eckery, and R.J. Warren. 2015. Administering GonaConTM to white-tailed deer via hand-injection versus syringe-dart. Human-Wildlife Interactions 9(2): 265-272. Accessed at https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1098&context=hwi
Gionfriddo, J.P., J.D. Eisemann, K.J. Sullivan, R.S. Healey, L.A. Miller, K.A. Fagerstone, R.M. Engeman, and C.I. Yoder. 2009. Field test of a single-injection gonadotrophin-releasing hormone immunocontraceptive vaccine in female white-tailed deer. Wildlife Research 36: 177-184. Accessed at https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1915&context=icwdm_usdanwrc
Hilltop Conservancy, 2012. Deer Population Control Methods – Cost & Effectiveness Comparison. Accessed at https://www.hilltopconservancy.org/wp-content/uploads/2018/11/Deer_control-methods-cost-comparison-v10.pdf
Kirkpatrick, J.F., R.O. Lyda, and K.M. Frank. 2011. Contraceptive Vaccines for Wildlife: A Review. American Journal of Reproductive Immunology 66: 40-50. Accessed at https://returntofreedom.org/wp-content/uploads/Contraceptive-vaccines-wildlife-2011.pdf
Koblinsky, D. 2016. WSB study: Sterilizing female deer doesn’t reduce deer populations. Accessed at https://wildlife.org/wsb-study-sterilizing-female-deer-doesnt-reduce-deer-populations/
MacLean, R.A., N.E. Mathews, D.M. Grove, E.S. Frank, and J. Paul-Murphy. 2006. Surgical technique for tubal ligation in White-tailed Deer (Odocoileus virginianus). Journal of Zoo and Wildlife Medicine 37(3): 354-360. Accessed at https://pubmed.ncbi.nlm.nih.gov/17319135/
Matchke, G.H. 1977. Microencapsulated Diethylstilbestrol as an Oral Contraceptive in White-Tailed Deer. Journal of Wildlife Management 41(1): 87-91. Accessed at https://www.jstor.org/stable/pdf/3800095.pdf?casa_token=5zwI2tYXkOcAAAAA:DRBExKwcRZJA7rLW6GNiKkNVDqGYE4AcLTAwmqNExA-5ivLQLWtwLuUzjE5c-ped8rOvCAK7fu4SLd9ggYeS6M7ClAOGr43udBPf7qrK2dmvxnWhi6xx
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