Managing rat populations with hormonal interventions

Photo: Carlos Aranguiz/Adobe Stock

Rats have long been a ubiquitous part of life in New York, New York (NYC). They thrive in dense urban environments, taking advantage of readily available food in garbage and shelter in subway tunnels, parks, and vacant lots. Their presence is not just an inconvenience but a significant public health risk.¹

Rats are prolific breeders. One pair can potentially produce 15,000 descendants in a year, and they are known carriers of diseases like leptospirosis and salmonella, with their waste contaminating food sources.^1,2 Efforts to control the population have largely focused on eradication through lethal means, but these methods have proven to be temporary solutions. According to exterminators and pest control experts, rats are intelligent, adaptable creatures that quickly learn to avoid traps and poisons, leading to persistent infestations.¹

Traditional methods of rat control, such as snap traps, glue boards, and poisons, are ineffective in the long run and also raise ethical concerns. These methods often cause rats to die slow and painful deaths, sparking criticism from animal rights organizations and lawmakers who argue for more humane approaches. Glue traps, for example, can also catch small animals like birds and squirrels, leaving them to die from dehydration or starvation. Rat poison, on the other hand, causes internal bleeding, leading to a prolonged and painful death.¹

Recently, the idea of using birth control to curb the rat population has gained renewed attention. Contraceptives offer a more humane, ethical, and potentially effective solution to managing a city's rat problem compared to traditional methods such as poisons and traps.

The contraceptive being piloted in NYC is 4-vinylcyclohexene diepoxide (VCD), an EPA-registered² product that has shown promise in reducing rat populations in other cities. VCD (ContraPest; SenesTech) works by targeting the reproductive systems of both male and female rats. In female rats, the contraceptive disrupts ovarian function, making it difficult for them to conceive. In male rats, it interferes with sperm production, reducing their fertility.¹ The contraceptive pellets, made from fat and salt, are more appealing to rats than digging through trash.³ The hope is that by reducing the reproductive capacity of the rat population, the overall number of rats will decrease over time.

Recent research has explored the use of fertility control in Rattus rattus (the black rat), specifically focusing on the combination of quinestrol and levonorgestrel. Investigators captured live black rats and fed them bait with varying hormone concentrations to assess fertility impacts. The researcher monitored the rats for bait acceptance, physiological changes, and reproductive outcomes.⁴

Results from this study indicated that the physiological impact on reproductive organs was evident, particularly in females, where uterine edema (inflammation and swelling) was observed. In males, the baits led to a reduction in the weight of sex organs and a decline in sperm motility and count.⁴

One key finding was that the fertility control baits effectively reduced pregnancy rates in females. When females consumed the baits containing the contraceptive, none of them became pregnant, regardless of whether the male partner had also consumed the bait. This suggests that the treatment had a direct and significant effect on female fertility, making it a promising option for population control in R rattus.⁴

Despite the promise of contraceptives, challenges and criticisms exist. Some pest control experts argue that while birth control may reduce the population, it is not a silver bullet. Rats are highly adaptable and intelligent creatures, and there is a concern that they may eventually learn to avoid the contraceptive pellets, just as they have learned to avoid traps and poisons. Additionally, the use of contraceptives requires constant monitoring and maintenance to ensure that the pellets are being consumed and are effective.1

Another potential issue is the cost and logistics of implementing a widespread birth control program. Distributing contraceptive pellets across a city as large and densely populated as New York would require significant resources and coordination. The question of whether the contraceptive method will be as effective in all areas of the city remains, given the different environmental conditions and rat behaviors in various neighborhoods.¹

Despite these challenges, contraceptives like VCD offer a more humane and potentially effective solution compared to traditional methods. Although factors such as rat adaptability and logistics still pose hurdles, fertility control shows promise for long-term, ethical population management.

Ava Landry is a 2026 PharmD candidate at the University of Connecticut in Storrs, where she is studying veterinary pharmacy.

References

1. Fitzsimmons EG. The next frontier in New York’s war on rats: birth control. The New York Times. April 10, 2024. Accessed October 5, 2024. https://www.nytimes.com/2024/04/10/nyregion/nyc-rat-contraception-birth-control.html?unlocked_article_code=1.O04.fIxb.9o4oxNJtSjPh&smid=url-share.
2. ContraPest approved for general use in New York State. News release. SenesTech. February 6, 2023. Accessed October 25, 2024. https://www.prnewswire.com/news-releases/contrapest-approved-for-general-use-in-new-york-state-301739051.html
3. Khan M, Attanasio C. To lower its rat population, NYC considers birth control as a humane option. PBS. April 14, 2024. Accessed October 5, 2024. https://www.pbs.org/newshour/science/to-lower-its-rat-population-nyc-considers-birth-control-as-a-humane-option.
4. Selemani M, Makundi RH, Massawe AW, Mhamphi G, Mulungu LS, Belmain SR. Impact of contraceptive hormones on the reproductive potential of male and female commensal black rats (Rattus rattus). Integr Zool. 2022;17(6):991-1001. doi:10.1111/1749-4877.12563