Methods
Anticoagulant-resistant mice: why bait is no longer enough
A station eaten every week, and fresh droppings all the same.
The scene has become commonplace on mouse sites, and it has a name: resistance.
Provided that is really what it is. A failed baiting programme has four possible causes, only one of which is genetic. Here is how to tell them apart, and what to do when bait is no longer enough.
What an anticoagulant does, and why it is slow
Anticoagulant rodenticides block the recycling of vitamin K in the liver by inhibiting an enzyme called VKORC1. Without that recycling, the animal stops producing clotting factors and dies of internal haemorrhage.
This mechanism explains the delay: it takes several days, and above all several feeds, before the accumulated dose becomes lethal. That same delay is what stops the rodent associating the bait with feeling ill, and it makes the whole approach depend on one thing: that the animal comes back to eat at the same spot several times.
With mice, that is already a difficulty in itself. Unlike rats, they nibble very small amounts spread over dozens of points during the night, and they take part of their water from their food. A bait station is, for them, only one source among many.
Resistance is not a habit, it is a gene
Anticoagulant resistance is not acquired through exposure, as is sometimes said. It comes from mutations in the VKORC1 gene, which alter the target enzyme: the molecule binds less well, and far higher doses are needed for the same effect. These mutations are passed on to the offspring.
In house mice the phenomenon is particularly widespread in Western Europe. Part of the population carries a version of the gene inherited through hybridisation with a neighbouring species, the Algerian mouse (Mus spretus), which confers strong tolerance. In other words: some mice are born resistant, before they have ever met a bait.
Selection pressure does the rest. With every treatment, the most tolerant individuals survive and breed. On a site treated with anticoagulants for years, the population left is precisely the one the product does not kill.
Resistance or simple failure? Telling them apart
Bait untouched. This is not resistance. It is either neophobia or food competition: in a store room or a kitchen, the mice have better on offer. A station left intact for three weeks mainly says the station is badly placed or pointless.
Bait eaten, activity unchanged. This is the signal that should alert you. The animals feed, come back, and the signs stay at the same level week after week. If the placement is right and enough time has passed, resistance becomes the most likely explanation.
Activity falls, then picks up again. Often recolonisation: the problem is no longer the product but how well the building is sealed and the pressure from outside.
To settle it, measure rather than estimate: clean an area, recount 48 to 72 hours later, and follow the trend from one visit to the next. That is the method described in our article on sizing up a population. In the laboratory, confirmation comes from a blood clotting test or from sequencing the VKORC1 gene, but in the field the activity curve is enough to decide.
Why switching molecule settles very little
The usual reflex is to move to a stronger anticoagulant. Sometimes it works, often it disappoints: resistance is cross-resistance. A population tolerant to a first-generation molecule is frequently tolerant, to varying degrees, to several second-generation molecules, since they all target the same enzyme.
Escalating mainly has three consequences: it selects resistant individuals a little further, it increases the risk to predators that eat poisoned rodents, and it prolongs a treatment that regulation frames ever more strictly, permanent baiting now having to be justified.
There is finally a more mundane problem: even when the product works, no one sees it. The animals die inside wall cavities, the client has no proof, and the end-of-visit conversation rests on your word.
What works when bait is no longer enough
Close the entry points. A mouse gets through a 6 to 7 mm gap. Cable runs, ducts, door bottoms, joints: as long as those stay open, the site refills whatever you do.
Remove the resources. Storage on shelves rather than on the floor, closed bins, cleaning of the warm areas behind appliances. A mouse with a choice does not go for bait.
Read the site before setting anything. Droppings, smears, routes along the walls: the signs tell you where the animal goes, and therefore where to place equipment. True for a bait, truer still for a trap.
Move to mechanical trapping. A trap asks nothing of the animal's genetics. A mouse resistant to anticoagulants is still a mouse that runs along walls and goes through an opening at floor level. Resistance becomes irrelevant.
Where Mirabox comes in
Mirabox was designed for these situations: no biocide, so no possible resistance and no delay in action. The trap is set on the identified travel routes and intercepts the animal as it moves naturally, through two entrances at floor level, with nothing to climb.
Captures come from the first night and continue between visits, with no resetting. The result is shown and counted, where a partly eaten block proved nothing.
The equipment is reserved for pest control companies, because a trap is only worth anything inside a method: diagnosis, exclusion, hygiene, follow-up.
Key points
Anticoagulant resistance is genetic, inherited and already established in part of the mouse population. It is corrected neither by the dose nor by changing molecule.
When bait is eaten without activity falling, the right decision is not to persist: it is to change lever. Close the entry points, remove the easy food, read the routes and capture mechanically. The result then becomes visible, measurable and defensible in front of the client.