CAPITULO IV. CASO PRÁCTICO
4.1 DEFINICIÓN E IMPORTNCIA DE LA PLANEACIÓN FISCAL
Legislation in the UK allows the chemical control of rats and certain species of mice, voles, shrews and moles in addition to a few species of birds that may cause damage to crops, or are considered a public nuisance. Such species include pigeons (Columbia livia), rooks (Corvus frugilegus), jackdaws (Corvus monedula) and magpies (Pica pica). Measures to control birds and mammals in the UK often need to take into account the sensitivity frequently associated with their deployment, and sentimental affinities for furry or feathery warm-blooded animals. This image may be contrasted with the damage and suffering that can result from the activities of species such as rats. Rats consume growing crops and crop produce in storage. They excrete on food, may carry microorganisms such as those responsible for bubonic plague and Weils disease, can burrow into embankments and drains, chew through cables and invade houses and dwellings.
Surveys conducted in the UK in the late 1990s showed that populations of the principal species – the brown rat – were increasing.
Chemicals used to control rats, mice and other vertebrate pests are generally toxic to other mammals and vertebrate species such as birds. The selective action of these chemicals is achieved entirely through their placement. Aluminium phosphide is a synthetic substance of high toxicity that may be used to kill moles as well as rabbits through the release of phosphine gas. Sodium cyanide may be placed into the burrows of rabbits where, in the presence of moist soil, release of the lethal gas hydrogen cyanide occurs. In the UK the naturally occurring compound strychnine, extracted and concentrated from Strychnos spp., was formerly used to kill moles; zinc phosphide is used in baits to control mammalian and avian pests such as squirrels, moles and rats. Animals that ingest a lethal dose of these chemicals usually die within a few hours. The risks to health from many of the above chemicals are such that the use and storage of many in the UK is governed by the Poisons Act (1972) and the deployment of these compounds is generally restricted to licensed pest operators.
Some compounds are normally formulated as baits and placed in positions accessible to the target species but which present a low risk of exposure or contact with non-target species. The narcotic alphachloralose may be mixed with grain and used to control mice and rats in buildings, as well as birds such
Table 5.1 Toxicity of alphachloralose
Species
Oral LD50of alphachloralose (mg/kg body weight)
Rat (Rattus norvegicus) 190–300
Mouse (Mus musculus) 300–400
Mallard (Anas platyrhynchos) 42
Starling (Sturnus vulgaris) 75
Canada goose (Branta canadensis) 54 From Timm (1994)
as pigeons. However, this compound has been misused, notably to kill raptor birds through illegal treatment of animal carcasses that may then be ingested by hawks and other birds of prey (9.10). Alphachloralose is toxic to some species of birds at low dose (Table 5.1).
Somewhat less toxic although slower acting rodenticides (Figure 5.2) were introduced in the late 1940s, with the most widely used being the hydroxycoumarin derivative warfarin. Warfarin is an anticoagulant, effectively preventing blood clotting, and is used medicinally at specified dosages for this purpose. At low doses of around 1–10 mg per patient, it can slow down thrombin formation in blood and hence reduce the risk of clotting and arterial blockage.
The concentration used in bait for pest control is around 0.005% with a lethal dose for the rat being around 1–3 mg/kg body weight.
Warfarin interferes with the regeneration of vitamin K (phylloquinone) that is involved in the synthesis of coagulant proteins such as prothrombin, essential for blood clotting. Inhibition results in a decrease in vitamin K and a consequent inability of the blood to clot. Specific inhibition of a reductase enzyme (KO reductase) that is involved in the recycling of vitamin K in the liver occurs.
After ingestion, death results from extensive internal bleeding. The process is slow, and animals that have ingested warfarin generally take several days to die.
The more powerful so-called second-generation anticoagulants were intro-duced in the late 1970s. Most of these are also hydroxycoumarin derivatives, including bromadiolone, difenacoum and brodifacoum, and may control both warfarin-resistant and warfarin-sensitive rodents. Their potency may be linked to strong lipophilicity and thus absorption to membranes, the site of the target KO reductase enzyme (WHO, 1995). The second-generation compounds are used at similar dose rates to warfarin, at about 0.005% to 0.001% by weight of bait. Rats that have ingested these compounds die more quickly than those that have ingested warfarin, and with some species such as mice and voles, a
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single feed may be sufficient to impart a lethal dose. Other second-generation compounds include the indanedione derivatives diphacinone and chlorophaci-none, which also interfere with the regeneration of vitamin K, but in a slightly different manner to that of the hydroxycoumarin rodenticides. Although the coumarin-based rodenticides can be toxic to human beings, even at low dose, they are considered among the safest of all pesticides since, unless a very high dose is consumed, their action may be reversed by administration of vitamin K. Indeed some are marketed to the general public.
The effectiveness at very low doses of anticoagulants has led to concerns being expressed about risks to non-target vertebrates. Indeed the oral LD50of warfarin in terms of milligrams ingested per kilogram of body weight in species such as pigs and cats is close to that for rats. Cattle are somewhat less suscep-tible (Table 5.2). Studies in the UK have shown that barn owls (Tyto alba) may be killed by relatively low doses of second-generation rodenticides. In Finland, where second-generation rodenticides are used extensively against voles (Microtus and Arvicola spp.), placement in vole burrows markedly
Table 5.2 Mammalian toxicity of warfarin
Species
Oral LD50of warfarin (mg/kg body weight)
Rat 1–3
Pig 3
Cat 5–50
Dog 20–50
Cattle 200
From Timm (1994)
reduced the risks to non-target species. These studies also showed that birds of the corvid family (such as magpies – Pica pica) likely to prey/consume voles were highly tolerant of second-generation anticoagulants compared to barn owls in the UK. However, these studies also showed no ill-effects on other potential scavengers of dead (treated) voles, such as dogs and cats (Myllymaki, 1996).
Control of rodents by chemical means involves both the design and presen-tation of baits. The palatibility and particularly the taste and smell of the bait must be acceptable to the test species and baits should be placed in a situation inaccessible to non-target animals, particularly cats, dogs and farm stock. Bait stations have now been designed that prevent access of non-target vertebrates.
One of the problems of rodent control is bait shyness, often associated with absorption of a sublethal dose and consequent avoidance of the bait. The prin-cipal method of overcoming bait shyness is to prebait with an acceptable but untreated sample of bait to familiarise animals with the baits, bait containers and bait locations before introduction of treated bait. Once a regular feeding pattern has been established, the rodents may well ingest a lethal dose at one visit at the bait location (Dennis, 1991).
Exposure at a given site for 3–6 weeks will normally control rodent infes-tations. However, a major problem with warfarin and other so-called first-generation anticoagulants is the development of resistance in rat populations.
The resistance is expressed as a reduced affinity for warfarin to bind to the target enzyme, KO reductase. However, in many cases this resistance does not extend to second-generation rodenticides. Whereas warfarin only inhibits KO-susceptible strains of rat, brodifacoum produces a decrease in plasma prothrombin in both warfarin-susceptible and warfarin-resistant individuals (WHO, 1995).