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.
- Sodium monofluoroacetate
- Zinc phosphide
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 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.
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
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.