Researchers have identified a troubling new development in some urban rodent populations: genetic changes that may make them much harder to control with common poisons.
Rats have long been among the most resilient animals living alongside people, having spread across the world by traveling on human ships and settling nearly everywhere humans do.
Their success is not accidental. They breed rapidly, move with speed and agility, and are adaptable enough to navigate unfamiliar surroundings and solve problems as they encounter them.
Because of those traits, rats have often displaced native wildlife in places they invade. Their link to disease has also made them a persistent source of concern for people.
For centuries, rat catchers have played a key role in protecting food supplies and limiting health risks. Now, however, certain rodent populations in several US cities appear to be even more difficult to control than usual.
Scientists say this is due to the discovery that some groups of rats and mice have built up resistance to certain poisons commonly used in pest control.
A recent Rutgers University study examined 290 rodents collected from urban areas in New York, New Jersey, Pennsylvania and Washington, DC, including 147 house mice and 143 Norway rats. The researchers found that 84 percent carried at least one mutation in the Vkorc1 gene, which is associated with resistance to anticoagulant rodenticides, the most widely used class of rat poison in the United States.
The findings suggested that house mice showed especially notable genetic changes, while Norway rats were less likely to carry the same resistance-linked variants.
Lead researcher Jin-Jia Yu said pest control professionals had told the team that rodent control seemed to be getting harder in some places, even when standard methods were being used. The study was designed to test whether those field observations had a genetic explanation.
Rodent infestations can become a major public health issue when populations grow unchecked, as the animals can spread disease through contact with their bodies as well as their urine and faeces.
Researchers said in the Pest Management Science journal:
“The house mouse and the Norway rat are globally distributed commensal rodent species,” researchers said the Pest Management Science journal, adding: “They cause substantial economic losses by damaging furniture and buildings and pose serious public health risks through the transmission of zoonotic diseases.”
Poison is not the only method available for controlling rat numbers, with traps also widely used.
Still, rats are famously clever, and they can sometimes escape traps or learn to steer clear of them altogether.
Rodenticides generally fall into two main categories: anticoagulants, which prevent blood from clotting and can lead to fatal internal bleeding, and neurotoxins, which trigger seizures. In both cases, it may take several days for the poison to kill the animal.
The new findings fit a wider pattern seen in urban rats over the past few years. Scientists have shown that brown rats in North America spread rapidly along ports and shipping routes, while other studies have documented rodenticide resistance in rat and mouse populations in Europe, Asia and the United States. Together, that research suggests the problem is not just that cities have more rodents, but that some of those rodents are adapting to survive the tools used against them.
In New York City, some locals have turned to a more direct approach, moving through neighbourhoods with dogs and shaking or kicking trash cans so the animals rush out and are caught.