Acute-acting substances

None of the acute rodenticides is now widely used. Indeed, with the exception of alphachloralose none is currently registered in Europe as a biocide, although more are available in North America. Prior to 1950 all vertebrate pesticides were non-anticoagulants, most of them acute or quick-acting, but after the introduction of warfarin and the other anticoagulants the importance of these compounds was reduced. After the emergence of anticoagulant resistance in some populations of rodents and the discovery of residues of the second-generation anticoagulants in wildlife, interest in non-anticoagulants, or at least less persistent “low residue” vertebrate pesticides, has revived and more new acute substances have been investigated. Most acute rodenticides, for example norbormide, thallium sulphate, strychnine and red squill, are either no longer available, no longer registered for use, or, where they are available and registered, are not recommended because of a number of adverse characteristics. In particular, many of these substances lack antidotes and reliable efficacy due to the development of bait shyness.

Alphachloralose is a narcotic with a rapid effect. It slows a number of essential metabolic processes, including brain activity, heart rate and respiration, inducing hypothermia and eventual death. It is most effective against small rodents such as house mice in cold or cool conditions. Alphachloralose is most often used in baits containing 2-4% of the active material for mouse control. In a number of countries there is some use of this compound for controlling bird pests and clearly because of its toxicity to birds it must be used with care when applied in baits for control of mice.

Zinc phosphide was first used as a rodenticide in 1911 in Italy. It is an effective acute rodenticide and was the most widely used rodenticide worldwide until the introduction of anticoagulant compounds in the 1940s and 1950’s. It is still used as a rodenticide in the USA, Australia, the Asia-Pacific region, Europe and China. Its use in Europe has become limited to field rodents in crop protection. Elsewhere it still remains the toxin of choice for use in some situations, for example mouse plagues in Australia, and can be rapidly broadcast from ground spreaders or aircraft. Zinc phosphide is a fast-acting compound, with clinical signs first appearing from 15 minutes to 4 hours after intake and, following a lethal dose, death generally occurring in 3–12 hours. The emetic action of the zinc portion reduces the toxicity of zinc phosphide to some non-target species; however, rats lack a vomiting reflex. Death is mediated by a combination of cardiac failure and respiratory failure.

Sodium fluoroacetate (1080) was first prepared in Belgium in 1896 but was not seriously investigated as a pesticide until the 1940s, when shortages of other acute rodenticides such as strychnine and red squill necessitated the development of other toxicants. Sodium fluoroacetate occurs naturally at lethal concentrations in poisonous plants. The toxin is formulated into baits to kill a range of introduced mammalian pests including rodents. The period between the time fluoroacetate is consumed and the appearance of symptoms of poisoning in mammals is between 0.5 and 3 hours, and animals receiving a lethal dose mostly die within 24 hours. Inhibition of energy production in the tricarboxylic acid (Krebs) cycle results in death from heart or respiratory failure.