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Rat Poison

Rats are prime suspects in the vectors of several diseases communicable to humans, and gravitate to areas of dense human populations in search of food and water sources.

Rats survive by scavenging, meaning that they subsist by searching out and eating dead plant or animal materials that are readily available in their environment. When humans are less than conscientious in discarding of food waste, populations of rats will be drawn into closer contact. An abundance of discarded food will create increasing reproduction rates in the rodents, compounding the problem.

Rat infestations are generally dealt with via the use of rat poisons and traps. Rat poisons or rodenticides are a group of chemicals known to be fatal to rodents. In general most of the toxic rat poison ingredients do not cause immediate death to the animal; depending on the class of rodenticide utilized, the term of effectiveness can be from a few days to a couple of weeks.

Rats are anatomically and neurologically incapable of emesis (vomiting). Vomiting is a coordinated muscular response to toxic substances which forcefully expels stomach contents back up into the esophagus and out of the mouth. The rat esophagus is long and there is a strong barrier between stomach and esophagus which makes forceful expulsion impossible. The rat neural network lacks the connections that coordinate the diverse muscle systems (abdomen, throat, shoulders) involved in emesis. Rats are capable of regurgitation which is a passive means of material moving out of the stomach and into the throat, whereas vomiting is characterized as an active process due to unified action of muscle groups.

Because rats lack the ability to forcefully expel a contaminated or poisonous substance once ingested, they have adapted other means to avoid such materials. The rat’s first line of defense is to “sample” food in small amounts, if a small portion doesn’t make them sick, they will return and eat more of it. They also have keen senses of taste and smell—and can detect harmful substances with these markers in minute amounts. They are also capable of observing the cause and effect of one of their number dying quickly after eating a particular material, resulting in the colony learning to avoid that food in future.

Types of Rodenticides

Warfarin, also known as Coumadin and by several other brand names, is recognized by most people as a human medicine instrumental in controlling the formation of blood clots in people who are prone to them. In fact, Warfarin was first developed in the 1940s as a rat poison, only later was its beneficial medical use discovered.The best rat poison has historically been that which is odorless, tasteless and which does not have a nauseating or immediately lethal effect. This has resulted in the development of several classes of rodenticides, the most popular and enduring of which are the anticoagulant based rat poisons.

Anticoagulant rat poisons suppress the animal’s ability to metabolize Vitamin K, which is vital in the formation of blood clots. Blood clotting (when not pathologic) is beneficial; limiting the amount of blood lost from an injury and paving the way for the healing process to begin. Suppression of vitamin K synthesis results in “thin” blood unable to clot which in turn causes diffuse internal hemorrhage. In rats this process can take between seven to fourteen days to end in the desired lethal outcome.

One of the shortcomings of this class of rat poisons is that the extended death trajectory allows the animals to return to various haunts to die, many of them inaccessible to humans. Most rats killed by this method returned to their nests—which are usually in walls or roof rafters, subfloors or foundations. The unexpected consequence is structures contaminated with the odors of decomposition; sometimes for months after the poison is administered.

The first wave of anti-coagulant rodenticides were low dose; meaning they contained the rat poison in small amounts which required several feedings over many days for the animal’s body to reach lethal levels. So called “second generation” anti-coagulants were later introduced which were promised to be one dose effective and shorten the time leading to death. These rat poisons contained high doses of the anti-coagulant agent. This had the drawback of making them even more prone to secondary poisoning occurrences than the first generation poisons.

What is “Secondary Poisoning”?

Secondary poisoning refers to the exposure of non-target animals and people to the effects of the anti-coagulant ingredients of the rat poison. Wildlife and pets can consume rat carcasses—or the poison bait itself– which retain high levels of the poisons and in turn be sickened and die. Human children, especially toddlers, may encounter the rodenticide and being at that stage of human development where everything must go into the mouth to be tasted, accidentally ingest the poisons.

Secondary poisoning led to the further development of other classes of rat poison, including the use of metal phosphides; Vitamin D2 and D3 based rodenticides (which result in calcium overload affecting internal organs), Bromethalin (chemical compound which causes brain swelling) as well as sulfamide derivatives, which induce lethal seizures. None of these, with the possible exception of the phosphides, which convert to phosphine gas when in contact with rat stomach acids, are free from the stigma of secondary poisoning. Even phosphides when used as a fumigant rat poison, can be lethal if pets or wildlife are exposed to the gas.

Aside from direct exposure or ingestion, it is also possible for rat poisons to permeate soil or contaminate water sources, and thereby create another avenue for pets, wildlife and children to be exposed to their toxic effects.

How quickly does rat poison work?

The length of time a rodenticide takes to kill the rat depends on the type and class of rat poison used.

As already noted, anti-coagulant based poisons take anywhere from a few days to two weeks to work, largely dependent on which generation of anti-coagulant poison is used.

Vitamin D derivative rodenticides work by increasing calcium levels in the blood of the rat. This hypercalcemia leads to heart and kidney failure as organs and blood vessels mineralize. One feeding by the rat is usually all that is required, with death occurring between two to four days after ingestion.

Zinc Phosphide rat poison kills by means of phosphine gas being released when the substance comes into contact with the stomach acids of the rat. It can take anywhere from one to two days for the effect to occur.

Bromethalin developed in 1985 as a solution against anti-coagulant resistant rat colonies is highly toxic, designed to kill in one feeding. A neurotoxin, it attacks the central nervous system resulting in convulsions, paralysis and death. This rodenticide usually leads to a lethal outcome within 24 to 36 hours after ingestion.

To avoid the chances of accidental secondary poisoning, all rodenticides should be used only in conjunction with tamper resistant bait traps that enclose the rat poison away from the reach of pets and young children. Traps are best placed along the rat’s usual path of travel between nest and food sources.

Recognizing rodenticide side effects

The side effects of rat poison are variable, depending on which class of rodenticide is involved and whether the affected is human or animal.

Anti-coagulant side effects in humans are usually related to the inability of blood to clot after coming into contact with the rat poison. Rat poison in humans will usually require a few days for symptoms to manifest. Depending on the level of exposure (and the age and body size of the victim) this side effect may be relatively minor or may result in massive internal hemorrhage. Signs include increased bruising, nosebleeds, blood in urine or feces, bloody gums, dizziness, and fatigue. Digestive disruptions including blood tinged diarrhea and vomiting are common.

Like humans, the side effects of anti-coagulant rat poison in dogs and cats take time to manifest; they also depend on the length of exposure to the poison. A single large dose may be less toxic than several repeated ingestions due to the relatively quick excretion of the one dose in the animal’s feces as opposed to repeated, prolonged exposure.

The age and general health of the animal are factors as well. In cats onset of signs and symptoms are usually delayed to their characteristically lower exertion levels compared to dogs. Some of the most common symptoms are depression, lethargy, non-specific fevers, weakness, pale gums, confusion, cold extremities and high heart rates. Sadly, some pets will exhibit no symptoms at all, and be found dead after a sudden collapse, usually because a severe internal hemorrhage into a large body cavity, the brain or the heart occurs.

Secondary anti-coagulant poisoning has been found to be treatable in both pets and humans with doses of Vitamin K1. The length of time that the antidote must be taken can amount to several weeks for both animals and humans.

Other classes of rat poisons will present with other symptoms.

Bromethalin rat poison exposure will present with excitability, running fits, depression, seizures, and severe muscle tremors, especially in dogs and usually within 8 to 10 hours of ingestion. Also included in aftereffects: loss of ability to bark, loss of appetite, and lethargy. No antidote currently exists, but symptoms are treated with corticosteroids.

The vitamin D based rat poisons ergocalciferol and cholecalciferol work by increasing calcium absorption, eventually leading to calcium crystals building up in blood vessels and organs. Depression, lethargy and anorexia are three of the earliest symptoms to present; kidney failure due to mineralization generally occurs well into the poisoning process. Difficulty in breathing and symptoms of heart failure may also be observed.

Phosphide rat poisons (most often zinc phosphide) cause similar side effects in humans and animals. Tightness in the chest (in humans), difficulty breathing and cough; fluid in the lungs and irregular heartbeat may all be in evidence. Phosphides also cause liver failure which is detectable by yellowing of the skin and the whites of the eyes. In later stages, coma resulting from toxin build up in the brain (caused by liver failure) can occur. Fortunately phosphides tend not to build up in rat tissue, reducing the chance of secondary poisoning somewhat.

Where can I buy rat poison?

Rodenticides are widely available for purchase. Chain home improvement stores like Lowes or Home Depot generally carry a wide range of rat poisons as well as traps and bait stations. Big Box retailers such as Wal-Mart are also sources for rodenticides. Hardware stores, Feed and Grain stores as well as Farmer Co-ops also often have rat poisons for sale. Online sources abound as well.

The Environmental Protection Agency of the United States, in an effort to cut down on the secondary poisoning of wildlife, pets and children placed restrictions on certain classes of rodenticides, their sale and distribution.

In May 2008 restrictions were announced which were to be phased in to become fully effective in 2011.

These are the restrictions that affect the purchase of rat poison by consumers for personal/home use:

• 2nd generation anti-coagulant rodenticides will no longer be available for consumer purchase. Only lower dose first generation anti-coagulant rat poisons will now be available for home use.

• Only farmers, livestock owners and certified pest control technicians will be able to purchase rat poison in bulk. No bag or container of rat poison in excess of 8 pounds will be sold to the consumer market

• Loose pellets will no longer be available for home use. Nor will pastes, powders, blocks or other poisons which are not contained in tamper resistant packaging.

• Rat poisons are to be kept above ground and only in EPA approved tamper resistant bait stations.

These restrictions were resisted by rodenticide maker Reckitt Benckiser, Inc. which sells poison rat baits under the brand name “d-Con”. Reckitt Benckiser previously launched a legal appeal with three other rodenticide manufacturers, refusing to implement the new regulations.

As of January 2013 Reckitt Benckiser has found itself facing an “EPA order of cancellation” as the result of their failure to comply to the new EPA standards. This order will prohibit the sale of twelve of the company’s d-Con products. Until the cancellation is fully implemented, the affected d-Con products will remain available for purchase.

This still leaves at least 30 compliant rat poisons available to the consumer market, and provides better protections—even if they are not perfect—for wildlife, pets and children against secondary poisoning by rodenticides.


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