Why are rats becoming immune to poison and rodenticides?

When you imagine a rat, you picture a small rodent, but what if you were to encounter a rat as large as a cat? For some cities, this is no fanciful dream, but their worst nightmare comes true.

Rats, when left to their own devices, can do a lot of damage to buildings and can spread disease to humans and wildlife. A natural scavenger they will eat anything, and everything put in their path, so the easiest way to control them has always been to provide them with food laced in poison. This has always been an effective measure, however over the past few years there have been reports that the rat has started to fight back, becoming immune to conventional poison and making us vulnerable to the vermin population. But is this true? Are rats really becoming resilient to rodenticides?different types of mouse poison

What is rat poison and how does it work?

Rat poisons and rodenticides work effectively due to toxic ingredients such as anticoagulants. Designed to thin the blood and prevent it from clotting, these odorless pellets are appealing to pests but once consumed, the outcome proves fatal.

Poison has always been regarded as a successful form of pest control, most recently having been used to eradicate entire islands of rats including one off the coast of South Georgia and Howadax Island in Alaska.  Once overrun by rodents, these invasive predators were sending the resident bird populations into decline and altering the native ecosystem dramatically. Now completely rat-free, these islands are restored and their wildlife is thriving once more.

Types of rat found in America

Rats are not native to the Americas, having originally arrived in the 1800s from Asia and then later from Europe via shipping passages. In total there are three species of rats commonly found across America and these include:

Norway Rat
Norway Rat: Often referred to as the brown rat, these nocturnal creatures like to live outside, burrowing holes and scampering into small spaces. Stocky in stature they can grow up to 16 inches long whilst their tails are short and stumpy and usually covered in scales. Their shaggy fur can be either a brown or grey color and they have two sharp yellow incisor teeth for chewing through cables, wood, and other hard materials.

black rat
Black Rat. Also known as the Roof Rat is typically found along the east and south coast of America. These rats are black or dark brown in color and easily distinguishable against the Norway rat due to their long tail, big ears, and round eyes. Their fur is smooth and sleek and will leave a thin grease mark along walls and surfaces. These rats like home comforts, preferring to live indoors. Making their nests up near the eaves, in the roof, or inside your attic space. You can often hear them scratching and clawing before you see them or any evidence of their existence.

Wood Rat
Wood Rat. These rats are not as common as Norway or Roof rats but can be found in northern parts of Florida. Although they are roughly the same size as a Norway rat, you can tell them apart thanks to their hamster-like features, fine fur, and fluffy tails. A pest in more ways than one, they not only enter your house to steal food but have been known to take other small items away.

Rats may sound like super cute furry friends, but regard them as your foe, because once they inhabit your house, business, or city, they are a destructive force to be reckoned with.

Cities overrun by rats

Across America, cities are becoming overrun with rats. In New York City alone, 2 million rats reside side by side with citizens and commuters, gorging on an abundance of leftover food and shelter in the steaming sewers and dilapidated buildings. And they are not alone. Philadelphia and Boston have also been ranked as two of the most rat-infested cities in the States with Houston, Texas, and Chicago reporting an increase in rat sightings.

Experts are now claiming that our hot summers and mild winters are also to blame, encouraging rats to breed at a startling rate.  Appreciating the need to control these pests, rat poison is a common method of choice. But does it still work, or are some rats simply becoming resistant to it?

are super rats immune to poison

The super rat comes to town

I have dealt with countless rodent invasions and have yet to come across a case I couldn’t exterminate. That said, over the years there have been more and more reports of rats resisting poison and requiring alternative methods to be introduced.

So, are rats starting to fight back and become immune to certain types of poison? In the endless fight between man and rat, it appears that the rodent is winning the day, not only becoming resistant to off-the-shelf poisons but feeding off them to gain strength and increase in size. These rats are being dubbed ‘Super rats’.

The term ‘super rat’ became popular in Great Britain where it was highly reported that up to 75% of rats were unaffected following toxic treatment. The discovery of a mutant gene meant that these rats; just like humans exposed to a constant course of antibiotics; had built up immunity to the rodenticides used. This required a newer, more potent version of previous poisons to be created, however, there is concern that over time these ‘super rats’ will become unstoppable.

We are lucky in the United States and so far, these ‘super rats’ have yet to invade our shores. What we are seeing, however, are these disease-ridden rodents growing in number and ballooning in size. As the government struggles to devise a successful strategy to control our rat-infested cities, the situation is only set to get worse.

Ineffective rat poison

Often people blame the failure of poison on rats developing resistance to anticoagulants but this is very rarely the case. It is far more likely to be due to the administration of the poison than rats gaining immunity to it. You must always make sure that you follow the instructions correctly, or else your efforts could prove futile.

Always make sure that:-

  • you have successfully located all areas of the infestation
  • you put down an adequate amount of bait and poison
  • you wait for the required amount of time to allow rodents to start feeding. Although they are curious animals they tend to only feed on trustworthy sources, so it may take a while for them to be tempted enough to take the bait
  • you give the bait time to become effective. Poison works over a short period of time and so results will not be immediate
  • you do everything you can to prevent a re-infestation

    Rat poisoned by toxic bait
    Credit: sever180, Deposit Photos

Pest Control

There is no such thing as the perfect rodent deterrent. It simply depends on what you are dealing with, the surrounding environment, and the sheer scale of the problem. It is important to note, that one method does not fit all and in cases where rats are resisting the regular forms of pest control, other techniques may have to be applied.

There are various forms of pest control, although poison still remains the more effective. For your average rat infestation rat pellets can be laid within holes close to the nest or via bait boxes. Cats (link to can cats get rid of rats) can also be great rat-catchers as well as a constant deterrent and offer a more humane way of getting rid of rats.

In order to ensure that your property remains rat-free, it is important that you keep it clean. You should always wash down surfaces where food has been prepared, keep leftovers well out of reach, and store groceries within glass or plastic containers.  Never leave trash bags on the ground but in locked outbuildings and if you have pets, make sure that their food bowls are clean and not left unattended.

For more advice, take a look at my blog on ‘How to stop a rat infestation’ and the ‘ultimate guide to getting rid of rats 2019


It is very rare that if you are using poison as your chosen method of pest control, that you will not see the desired effects. Whilst some rats in Europe may be building up an immunity to rodenticides I firmly believe that in the United States, this is not the case. Poison still remains the most effective form of pest control, but only when used correctly.

There is speculation within the media that ‘super rats’ are taking over our cities, and whilst they may be increasing in both size and numbers, this situation should not be blamed on the resistance to poison. Instead, as our population continues to grow, we create a more urban mess for our furry foes to thrive. As the government struggles to get a grip on this rat epidemic, it is left to each individual city to find a way to fight back, with poison still being a rodent’s number 1 nemeses.

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