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General classification of pesticides: rodenticides

by Wan Zainal Azman Wan Abdullah
The Sun, April 26, 1997

RATS AND MICE COMPETE WITH humans for food. In some developing countries. It can cause starvation. Rodents are also hosts for human diseases, including plague, endemic rickettsiosis, leishmaniasis, spirochaetosis, tularaemia, leptospirosis, tick-born encephalitis and listeriosis. Rats also occasionally bite people. In addition, they cause other damage, mainly by gnawing.

As far as possible, rodent populations should be controlled by limiting their access to food and shelter. Individual animals or small groups may be removed conveniently by trapping. However, there will always be a need for poisons in rodent control.

Rat poison or rodenticide is any product commercially available and designed expressly to kill rodents, mice, squirrels, gophers and other small animals. The chemicals used range from highly toxic one with the ability to kill on injection of a one-time dose to less toxic ones requiring repeated ingestion over a period of time.

Most rodenticides produce their toxic or lethal effects in humans mainly by ingestion of a large enough single doe. The same "acute" or single-dose effectiveness considered advantageous in killing rodents makes a substance more dangerous in inadvertent or delinerate human ingestion.

The various single-dose rodenticides exert their toxic effects on target organs in the body. The toxic effects in humans are the same or very similar to the effects in rodents which are, after all, mammals, albiet much smaller. In this case, young children are most commonly associated with rodenticide ingestion. In addition, suicidal patients, victims of attempted homicide, exerminators, intoxicated, psychiatric or impaired geriatric patients are all at risk. Finally, the patient who accidentally ingests a rodenticide placed in a container commonly used for an edible product illustrates the danger of transferring toxic substances to other containers.

Rodenticides may be classified in several ways but the most medically useful of classifying them is by toxicity.

Highly toxic rodenticides

Highly toxic rodenticides are those substances with a single dose LD50 of less than 50mg/kg body weight. This group includes thallium, sodium monofluoroacetate, strychnine, zinc phosphide, yellow phosphorus and arsenic. Some of these compounds have largely been abandoned because of serious human toxicity.

  • Thallium
Thallium is an odourless; tasteless compound that can be absorbed through unbroken skin and can cause death secondarily. There is no known effectiveness antidote for thallium. In sublethal doses, thallium can cause complete loss of hair, paresthesias, nausea, vomiting and abdominal pain, pulmonary oedema and bronchopneumonia.
  • Sodium monofluoroacetate
Sodium monofluoroacetate is a white, odourless, tasteless, water-soluble salt that looks like flour or baking soda. Unlike thallium, sodium monofluoroacetate cannot be absorbed through unbroken skin. However, it is toxic when ingested, inhaled in dust or absorbed through open wounds.Toxic effects in humans, extrapolated from observed effects in rhesus monkeys, might be nausea and apprehension followed by cardiac arrhythmias, seizures and coma. Death may result from ventricular tachycardia and fibrillation or respiratory failure secondary to pulmonary oedema or bronchopneumonia.
  • Strychnine
Strychnine is a central nervous system stimulant that causes painful recurrent motor seizures. It is rapidly absorbed from the gastrointesstinal tract and nasal mucosa but not from the skin. Symptoms include nausea and vomiting, diaphoresis, blurred vision and severe symmetric extensor muscle spasms during which patients are conscious.
  • Zinc phosphide
Zinc phosphide is another highly toxic rodenticide in which its colour, "rotten fish" odour and taste make it unattractive to other animals but attractive to other animals but attractive to rats. Usually mixed with a tartar emetic, it nevertheless is highly toxic because it releases phosphine gas on contact with water or weak acids.

Poisoned patients manifest hypotension, dyspnea, pulmonary oedema, circulatory collapse, vomiting, cardiac arrhythmias, convulsions and coma, renal damage, leukopenia and death in four days to two weeks.

  • Yellow phosphorus
Yellow phosphorus is highly poisonous compares with the relatively non- toxic red phosphorus. When used as a rodenticide, yellow phosphorus is usually mixed with molasses or peanut butter and spread on bread as bait for rodents or roaches. For obvious reasons, it is occasionally ingested by children.

Yellow phosphorus is most toxic to the gastrointestinal tract and liver. Ingestion is usually followed by vomiting which is said to be luminescent and have a garlicky odour. Delirium, coma and death from cardiovascular collapse may ensue.

  • Arsenic
Arsenic is a white, crystalline powder that causes dysphagia, muscle cramps, convulsions, vomiting and bloody diarrhoea. Death can occur due to circulatory failure.
Moderately toxic rodenticides
Among the moderately toxic rodenticides, those with LD50 of more than 500 mg/kg body weight are alpha-naphthyl-thiourea (ANTU) and DDT.

Patients who ingest large quantities of ANTU may develop dypnea, rales and cyanosis (secondary to pulmonary oedema or pleural effusions) and hypothermia.

Poisoning from exposure to DDT can result in symptoms such as vomiting, tremors and convulsions. How much exposure is required to cause severe illness or even death is however, not certain.

Low toxicity rodenticides

Low toxicity rodenticides are those with LD50 between 500 and 5,000 mg/kg body weight and include red squill, norbomide and anticoagulant, warfarin-type rodenticides, which are the most commonly used rodenticides today.

  • Red squill
Red squill contains several compounds with chemical and pharmacological properties similar to those of digitalis glycosides. Because of its emetic properties, poor gastrointestinal absorption and decreased potency (compared to that of digitalis), red squill has seldom been associated with human toxicity.
  • Norbomide
Norbomide is an irreversible smooth muscle constrictor. It causes widespread ischaemic necrosis and death in rats but does not appear to affect other animals or humans, presumably due to the presence of a specific smooth muscle norbomide receptor found only in rats.
  • Anticoagulant
Anticoagulant rodenticides belong to a group that exerts a blood-thinning effect. It has been used for many decades and are readily available as over- the counter preparations.

Technically, these may be divided into two types: hydroxiycoumarin compounds are made up of warfarin, difenacoum, bromadiolone and brodifacoum while the indandiones cover a wide range of chemicals including diphacinone, pindone and chlorphacinone. Aside from warfarin, the others are said to be longer-acting. They produce a more potent and prolonged anticoagulant effect and they are often referred to as superwarfarins.

The toxicity of anticoagulant rodenticides depends entirely on repeated exposure to relatively small doses. This is true for both rodents and humans. An adult would have to ingest 50g of an ordinary warfarin rat bait to receive the equivalent 12.5 mg daily warfarin dose. On the other hand, daily ingestion for six days of small quantities has produced severe illness in an attempted suicide and a single ingestion of one of the superwarfarin rodenticides such as difenacoum and brodifacoum may result in marked anticoagulant effects for up to seven weeks.

Haemorrhage is the most frequent encountered complication from anticoagulant poisoning. The effect seen in cases of acute ingestion, however, depends very much on whether warfarin or superwarfarin rodenticides have been ingested. A one-time ingestion of warfarin usually does not lead to any bleeding problems. Otherwise, ingestion of warfarin rodenticides has to be done repeatedly over a periods of days before bleeding can occur.

Warfarin poisoning can cause spontaneous bleeding, usually from the nose, gums as well as the gastrointestinal and urinary tracts. Haemorrhage into the skin and brain can also occur, throuh this is less common. Ingestion of superwarfarin, on the other hand, can produce a prolonged coagulopathy in humans even with a single ingestion. This is thought to be associated with the firmer binding capability of superwarfarins to the lipophilic sites of the liver.

The writer is a Science Officer at the National Poison Centre, Universiti Sains Malaysia, 11800 Minden, Penang.

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