During the 10th International Conference on Wilflife Fertility Control, held in Barcelona at the end of April by the Botstiber Institute https://wildlifefertilitycontrol.org/ , many speakers focused on rodents. A presentation by Professor Steven Belmain, University of Greenwich (United Kingdom), Natural Resources Institute, was devoted to their impact especially in Africa and the possibilities for ecologically and ethically sustainable intervention.
Professor Belmain, let’s start with the enormous scale of the problem. Worldwide hundreds of millions of rodents are killed annually in agricultural and urban settings – to prevent crop loss, food contamination, and the spread of disease to humans and livestock. Anticoagulant rodenticides cause a slow, painful, and often inhumane death …
Conventional methods like anticoagulant rodenticides are also facing increasing restrictions in many countries due to concerns about environmental accumulation and the development of resistance. Without effective and sustainable alternatives, there is a risk of increasing zoonotic spillover, potenzial new pandemics, and greater economic losses for farmers. Fertility control is emerging as a promising alternative to lethal methods. Rodents, with their prolific breeding and short lifespans, present unique challenges and opportunities for this approach compared to larger mammals. The application of contraceptives in larger settings like cropping systems or urban pest control is considered a more socially acceptable, humane, and environmentally sustainable option.
What is the possible alternative?
The primary goal in these contexts is to reduce rodent density and maintain populations below socio-economic damage thresholds. A combination of the synthetic hormones levonorgestrel and quinestrol, known as EP1, has a long history of safe and successful use in limiting reproduction in humans and various wildlife species such as marsupials like kangaroo, and primates in zoos. Evidence now shows this combination is also effective against several key rodents species, including cosmopolitan pests like the black rat (Rattus rattus), the common mouse (Mus musculus) as well as regionally significant pests such as the multimammate rat (Mastomys natalensis) which is the main rodent pest acfoss Sub-Saharan Africa. Field studies comparing the contraceptive EP1 with an anticoagulant rodenticide have shown comparable reductions in rodent populations. A single administration at the start of the cropping season in agricultural areas can significantly reduce populations from increasing, reducing crop damage.
The oral contraceptive EP-1 (combination of the synthetic hormones levonorgestrel and quinestrol), which is used in China and in Africa, is it administered on a large scale in those countries?
EP1 is registered for use in Tanzania, but there is no business manufacturing and selling it. However, it is used through government channels in response to outbreaks. The Tanzanian government supplies poisons to farmers when rodent outbreaks are expected – not all the time and everywhere, but in areas of high risk in some years. Through this established process they have been also trying to deliver fertility control bait instead. I don’t have any data on how widely it is being used there though, and it is ad hoc. The same is true in China – it is not commercialised and sold, but is used through government channels to help reduce populations of Mongolian gerbils at risk of outbreaking. EP1 has been used elsewhere in Zambia and South Africa, but here it has been experimental use only and not yet registered in these countries. This is something I am trying to promote and hopefully we will get to a point where it can be used officially in South Africa.
Why EP1 is it not used massively? Is there a cost problem for the contraceptive bait?
Cost of production is comparable to rodenticides. The hormones are produced at large scale for human birth control pills, implants and HRT therapies, so they are cheap to buy and where the main costs are the food bait ingredients to make a rodent bait. This is the same as for an anticoagulant poison where the cost is really the food bait ingredients. The problem is that commercial pest control companies have no interest in manufacturing – for them it is much easier to just continue producing rodenticides.
Are there other obstacles too?
Although EP1 works, regulatory hurdles in Europe and the USA will likely prevent EP1 being registered because of the already recognised issue of estrogenic contamination of the environment (through our human waste and livestock waste which floods the environment with estrogens). So officials will most likely not allow a product that may contribute to this, even if only in a very small way. Countries in Africa do not have these issues with high estrogen waste and have seen EP1 as reducing environmental contamination with anticoagulant poisons, so are more willing to let them be used to reduce non-target poisoning. Through research I have ongoing in South Africa we plan to investigate these further issues.
How do you get the bait to be eaten by mice and only by them?
There are two ways, one is to put the bait directly in the rodent burrows. This is called burrow baiting and widely used in delivery of poisons, so we simply do the same with our contraceptive bait. We also use small segments of bamboo and/or segments of plastic pipe as a bait station. The segments are placed out across the cropping area with the bait placed in the middle of the segment. This stops birds and larger animals getting at the bait and we can show this is the case with camera traps.
What about using plant extracts for a contraceptive bait?
Neem has been tested in laboratory studies and it can reduce fertility. But it needs to be delivered over a long time, and it is super bitter tasting so it is not easy to get rodents to eat it. Bitterness is a problem with many natural products being tested for fertility effects. There are natural products which are being used. One sold in the USA contains cotton seed oil, which contains the compound gossypol. Gossypol has fertility control effects, but it also causes heart failure, liver toxicity, and many other toxic effects. Another one, also sold in the USA, contains triptolide. This comes from a plant called the Thunder God Vine (Tripterygium wilfordii). This has reproductive effects, but also many other known serious toxicities. As both products are supposed to be delivered long-term to rodents, cumulative toxicity to the rodents eating the bait is likely, and some observations by pest controllers suggest this is the actual mode of action, instead of reducing reproduction. There is no published evidence from field trials on these products available in the USA showing their mode of action is limiting reproduction. Ongoing trials in New York City are aiming to collect more efficacy and mode of action data on one of these products.
