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On Common Ground: Rodenticides, an unnecessary poison

Dr. Duane Tom, DVM Director of Animal Care, California Wildlife Center
10:00 am PDT June 27, 2017

Anti-Coagulant Rodenticides (ACRs) first came into use during the early 1950s. 

The first compound to be used in the control of rodent populations was warfarin. Over time, as rodents became more resistant this poison, variations were developed which were faster acting and more toxic.

People often reach for these toxins when they have problems with rats and mice around their property, but these toxins also poison wildlife. They can kill as wildlife eat newly poisoned rodents as well also build up in those wild animals’ systems over time as they continually feed on them until it reaches a poisonous level.

Anticoagulant rodenticides kill by depleting the body’s supply of Vitamin K. Vitamin K is essential to clotting blood. All animals, including humans, continuously have small areas of bleeding which occur under normal situations throughout their life. The body takes care of it without us ever knowing it. 

However, as the ACRs deplete the body’s available Vitamin K, bleeding is no longer controlled. Impacted animals might cough up blood, vomit blood, or pass blood in their feces or urine. Oftentimes, it will not be as noticeable as they bleed into their chest cavity or abdomen. All vertebrates are affected: hawks, owls, bobcats, coyotes, foxes, mountain lions, skunks, raccoons — the list goes on and on. Parents may take poisoned meals to their babies and end up killing their own young with these toxins. It’s a very horrific way to die.

When our pets or children get sick from eating ACRs, we often know what might have happened or we can run tests to figure it out and treat them. But with wildlife, there is nobody keeping a close eye on them and they will almost always die unknown. If by chance they are found and admitted to a wildlife rehabilitation facility, it is often too late to help them.  

Recent legislature has placed more severe restrictions on the use of these ACRs and if you see them being used, it’s always appropriate to make sure they are being used in a legal manner. 

However, as ACRs have become more controlled, newer toxins such as Bromethalin and Cholecalciferol are being used and unfortunately these have no antidotes. 

While the use of rodenticides may provide a short-term solution to people’s rodent problems, it actually makes it worse in the long run and causes harm to the wildlife important to the ecosystem that we live in.

Prey animals, such as rodents, tend to maintain a population the environment can support in terms of food, water and shelter. While poisoning rodents may result in a temporary decline, it may also decrease predator populations due to scarcity of food. 

Prey animals, such as rodents, are more robust reproducers than predators and will re-establish populations to the supporting environment faster than predator populations. 

The result is a rodent population now poorly controlled naturally, thus upsetting the balance of the ecosystem. 

A far better and maintainable solution is to decrease the carrying capacity of rodent populations by controlling their food, water and shelter sources. 

Do not leave food out that rodents can also eat. 

Eliminate areas of free standing water. 

Make sure tool sheds and other sheltered areas are rodent proof. 

There is no good long-term reason for using any form of rodenticide.  By controlling their populations through natural means, we can help preserve the natural ecosystem health of all wildlife that lives around us.

On Common Ground is a monthly column written by various California Wildlife Center employees. CWC, a nonprofit located in Calabasas, cares for injured wildlife in Malibu and beyond.