Starr is a typical Labrador retriever, which means he loves to eat and isn’t picky about the menu. So it was no surprise that when he discovered a block of rat poison, Starr chowed down. After 35 years in practice, calls about rat poison ingestion usually don’t fluster me. In the past, most commercial rodenticides (i.e. products that kill rodents) were anticoagulants, that is, chemicals that interfere with blood clotting. Warfarin was the first on the market, and the most common, but there are many similar products. Some, called “multiple dose” products, require repeated feedings by the rodents to be lethal. Others, called “single dose,” take just one meal. But with all of them, varmints eat the bait, then several days later hemorrhage and die. Warfarin is still actually used in human medicine, known by the brand name Coumadin, along with other similar drugs, to prevent blood clots in a variety of conditions, from heart attacks to deep vein thrombosis. People taking warfarin are carefully monitored and advised to avoid activities that increase risk of bleeding or injury.
Dogs eating rat bait, however, take no such precautions. Luckily, treating dogs for anticoagulant rodenticide ingestion is pretty straightforward, as long as we start within a few hours of them eating it. Induce vomiting. Give activated charcoal to minimize absorption of any product not barfed up. Treat with the antidote, vitamin K, orally for anywhere from two to six weeks, depending on the exact product eaten. As long as we had the package label and knew how much and when the dog ate it, we could calculate whether it was a dangerous dose or not. “But he doesn’t look sick,” owners would say. Then we would explain, again and again, that with anticoagulant rodenticides, clinical signs of bleeding often don’t develop until three to five days post-ingestion. Once bleeding starts, it’s far harder to treat, requiring multiple transfusions and 24-hour intensive care, and at this point, it is usually fatal. That’s why many veterinarians do the whole routine — induce vomiting, administer charcoal, prescribe vitamin K — regardless of the amount ingested, just in case a dog has eaten more than an owner reported. Over the years, I have treated hundreds of cases of anticoagulant rodenticide ingestion, and thankfully have only seen one fatality.
But Starr hadn’t eaten the old standard anticoagulant rodenticide. He had eaten bromethalin. In 1985, bromethalin was developed and released in response to increasing resistance to warfarin-like products in rodents. It has slowly gained in popularity, and over the past few years has taken over the majority of the market. Bromethalin is a highly potent chemical, providing a lethal dose to rodents in a single feeding. Death occurs within a day or two. The mechanism of action requires a class in biochemistry to follow, but for you science nerds out there, here it is. It uncouples oxidative phosphorylation in the mitochondria of the central nervous system, leading to decreased production of ATP. This then leads to increased cranial pressure, damage to nerve axons, and inhibition of neural transmission, resulting in paralysis, convulsions, and death.
Signs can occur within hours to days, depending on the amount ingested. Low doses can cause hind limb incoordination, depression, vomiting, and loss of appetite. Over one to two weeks, this can progress to paralysis, seizures, and coma. Dogs may occasionally recover, with aggressive extended treatment, but may have permanent neurological damage. With high-dose exposure, signs generally begin within two to 12 hours, and include hyperexcitability, severe muscle twitching and spasms, seizures, heat stroke, and depression; it virtually always ends with death. And here’s the kicker. There’s no antidote.
Starr’s owners had the poison package. Perfect. How much did he eat? Three-quarters of an ounce of bait. What was the active ingredient? Bromethalin. What was the concentration? 2.48 milligrams per ounce. How much did Starr weigh? Forty kilograms. Next I looked up the lowest reported lethal dose, as well as the lowest reported dose to cause clinical signs in dogs, and did the math. Starr would have to eat 16 milligrams of bromethalin to get sick, and 40 milligrams to risk death. He had eaten less than two. Because he is a very big dog, who ate a very small amount, we could relax. Still, erring on the side of caution, we tried to make him throw up the bait, but Starr … well, let’s just say he has a strong stomach. He refused to vomit, no matter what we tried. Since the math confirmed he was not in any real danger, we dosed him with activated charcoal for good measure and sent him home. Had he been a smaller dog, or eaten a larger amount, we would have had to do whatever necessary to empty his stomach.
Cats are even more sensitive to bromethalin than dogs. Luckily, they are less likely than their canine counterparts to eat bait, but it is something cat owners should think about. Most sources say that “relay toxicosis” is unlikely, that is, toxicity from eating a dead or dying poisoned rodent, but individuals such as barn cats, who might repeatedly be exposed by eating poisoned rodents on a regular basis, may be at risk, at least theoretically.
It’s easy for owners to get confused about what product their pet has been exposed to. Many rodenticides have similar names, such as the anticoagulants brodifacoum and bromadiolone. Don’t rely on brand names. Companies may market different chemicals under the same familiar labels. Nor can you tell by color, size, or shape. The active ingredient in last year’s waxy green square of bait may not be the same as the active ingredient in this year’s waxy green square of bait. It is crucial to verify the active ingredient by reading the label. Even better, avoid exposure in the first place. Use pet-proof devices when putting out rodenticides. Know what you are using. Save the packaging in case of emergency. Call your veterinarian immediately if the Starr in your house eats the bait.