Norbormide – The Old New Solution for Rat Control

Scientists in New Zealand are working on a reformulation of norbormide, a rodenticide that originally entered the market over 60 years ago. 

Regulatory bodies are focusing on two significant issues regarding the continued use of rodent baits: firstly there is the issue of non-target poisoning, where a pet or native animal eats the rodent bait; and secondly the issue of secondary poisoning, where predators ingest a rodent or other animal that has eaten rodent bait. Accordingly, there is significant pressure to develop rodent management techniques that minimise or eliminate these risks.

Enter, norbormide, a fast-acting rodenticide that doesn’t bioaccumulate, with specific activity on Rattus species. It may sound too good to be true, but it is not without its efficacy challenges. However, researchers in New Zealand have been working on solutions to overcome these issues and it appears new commercial products are on the horizon.

Norbormide is an old active, with its rodenticide potential discovered by accident in 1964. It was under development as an anti-rheumatic drug and during testing on animals, although it had no effect on house mice or cats, it was found to have a lethal effect on Rattus species. It works by vasoconstriction of the peripheral blood vessels, likely leading to cardiac damage, although the exact processes leading up to mortality are unclear. Symptoms appear within 10-15 minutes after ingestion, with mortality typically in 3-4 hours.

However, although norbormide baits were launched back in the 1960s, performance in the field was poor due to its poor palatability leading to ingestion of sub-lethal quantities of bait. Due to its fast action, rats eating a sub-lethal dose quickly demonstrated bait shyness. With the arrival of anticoagulants in the 1970s, norbormide products were phased out.

 

Norbormide
Norbormide was marketed as a species-selective rodenticide
in the 1960s and 70s (photo credit: eBay)

 

However, with the increasing regulatory focus on the impact of anticoagulant rodenticides, the potential for norbormide has been revisited. The key challenge is to overcome its poor palatability. Some groups are looking at encapsulation of norbormide, others are looking at analogues of norbormide, which are both more palatable and also slower acting, allowing the ingestion of a lethal dose before the rats detect a problem. However, a research group comprising Invasive Pest Control Ltd, with toxicology support from Lincoln University, wildlife and production application expertise from Boffa Miskell and expertise in synthetic chemistry from the University of Auckland, has taken a different approach. Led by Dr Lee Shapiro (Boffa Miskell, main picture above) and Professor Charlie Eason (Lincoln University), the group focused on improving the palatability of norbormide by removing impurities through a different synthesis route, resulting in a more palatable bait formulation.

Although this new formulation overcame the taste aversion issue, it was still important that the rats ingested a lethal dose in one sitting to avoid receiving a sub-lethal dose, resulting in bait avoidance. With the focus of the group being to eliminate rats from the New Zealand environment as part of the Predator Free 2050 program, the control methods did not have the same time pressures that would exist for rat control in a commercial account. As a result, they pre-baited rats using bait without norbormide before presenting the norbormide bait, to avoid initial bait shyness issues and maximise intake and therefore mortality.

After initial laboratory trials were successful, the researchers carried out trials on poultry farms targeting Norway rats. The norbormide paste bait delivered 100% control on three different poultry farms (and 96% control in a fourth farm). The next stage of the development was to test the bait on the ship rat. Not only is this the dominant rat species in New Zealand, but it is less susceptible to norbormide than the Norway rat (ship rat LD50 is 52 mg/ kg, Norway rat LD50 is 10 mg/kg). This means that palatability is even more important for ship rat control to ensure they eat sufficient bait for a lethal dose.

 

Close-up of two rats in a chicken coop
Rats cause problems in farming and can devastate native wildlife, especially birds

 

Two trials were carried out in native forest blocks fenced off from surrounding farmland. Bait stations containing norbormide-free bait were deployed in both areas for approximately two months for the pre-baiting period, before being replaced with the 1% norbormide paste formulation for a two-week baiting period. Using chew cards to monitor rat presence at the end of the trial, it was concluded that 100% control had been achieved (no marks on the chew cards).

This norbormide paste formulation is now in registration in New Zealand and the team is working on a solid bait formulation. Work continues to try and fully understand the norbormide mode of action, with the hope that norbormide variants can be developed that also target other pest rodents, such as the house mouse.

 

Lee, S et al. (2022). Continuing Field Efficacy of Norbormide against both Rattus rattus (Ship Rats) and Rattus norvegicus (Norway Rats). Proceedings, 30th Vertebrate Pest Conference.

Fusi F, et al. (2024). The Enigma of Norbormide, a Rattus-Selective Toxicant. Cells. 6;13(9):788. doi: 10.3390/ cells13090788. PMID: 38727324; PMCID: PMC11083043.

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