Acquired resistance occurs as a result of genetic changes due to mutation, or to the acquisition of genetic material, which confers a stable and heritable decrease in susceptibility to one or more rodenticides.

Behavioural resistance is a phenomenon which is the result of a change in behaviour which confers an increased probability of individual animals, or populations, surviving applications of rodenticides or other treatment mechanisms, such as trapping. The behaviour may be sometimes related to a reluctance to take rodenticidal baits or to approach and enter rodent control equipment, such as bait boxes and traps. Few published scientific studies have been conducted on behavioural resistance and none has so far confirmed a genetical element. However, it is postulated that a significant part of resistance in the L120Q focus of anticoagulant resistance in central southern England has a behavioural component.

Blood clotting response (BCR) test is a simple and quick non-lethal method to determine susceptibility or resistance to anticoagulants. A dose of an anticoagulant is delivered, usually either by gavage or injection, which is known to impair blood clotting in a given percentage of the susceptible population. If the blood continues to clot in a significantly greater proportion of the animals tested than expected, the sample is said to be resistant.

Cross-resistance occurs when an individual possesses resistance to one compound which confers on it resistance to one or more other compounds – usually these compounds are of a related chemical type. For example, it is generally considered that resistance to one of the first-generation anticoagulants confers resistance to at least some of the other first-generation compounds.

Co-resistance occurs when an individual possessed more than one type of resistance mechanism. There are few examples of this phenomenon among rodents but it is comparatively common in insects.

Ecotoxicology is the study of the toxicants within ecological systems.

Enzyme, a complex organic molecule, usually a protein, that speeds up (or catalyses) a chemical reaction in an animal or plant.

First-generation anticoagulant, one of the series of rodenticide active substances invented, mainly during the 1950s and 60s, the first of which was warfarin. The most commonly used of these compounds are chlorophacinone and diphacinone (indane-diones) and coumachlor, coumatetralyl, and warfarin (hydroxycoumarins). (See second-generation anticoagulant.)

Gene, a discrete piece of genetic material, usually a series of nucleotides at a specific location in the DNA, responsible for a specific hereditary trait. Genes undergo mutation when the sequence of nucleotides changes. Genes may exist in alternative forms called alleles.

Genome is the entire sets of genes and other genetic material in the cells of an animal or plant. The genome is situated within a set of chromosomes that are found in almost all mammalian cells.

Heterozygous animals possess two different copies of the same gene, one obtained from the father and the other from the mother. Usually, one of the copies is dominant and one recessive so that the dominant copy determines the nature of the relevant trait. (See homozygous.)

Homozygous animals possess two similar copies of the same gene one, obtained from the father and the other from the mother. (See heterozygous.)

Integrated pest management (often abbreviated to IPM) is a term used where a set of complementary control techniques, and subsequent integration of appropriate measures that discourage the development of pest populations and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment.

Intrinsic resistance occurs in animals which have an ability to survive doses of a chemical substance which are normally fatal to other animals, either of the same or of a different species. For example, normal (i.e. not anticoagulant-resistant) house mice are able to survive doses of anticoagulants that are frequently fatal to Norway rats. This is because the species is ‘intrinsically’ less susceptible to anticoagulants. This is not ‘resistance’ in the true sense but it plays an important part in the application of anticoagulants against these species.

Mutation occurs when the sequence of nucleotides changes in a gene. A mutation may result in a demonstrable change to the expression of the gene (see below) - a ‘mis-sense mutation’. If the mutation has no demonstrable effect on the expression of the gene is a ‘silent mutation’.

Phenotype is the entire composition and outward expression of an individual’s genetic traits, as seen in its physical and biochemical characteristics. For example the phenotypic expression of a resistance mutation is an ability to survive anticoagulant applications.

Resistance is a term which has several current definitions. The definition used by RRAC is that of Greaves (1994) as follows: Anticoagulant resistance is a major loss of efficacy in practical conditions where the anticoagulant has been applied correctly, the loss of efficacy being due to the presence of a strain of rodent with a heritable and commensurately reduced sensitivity to the anticoagulant. This definition has three important aspects: 1) a measurable loss of efficacy apparent to practitioners, 2) correct application and 3) an heritable basis. It may be called practical resistance. An alternative definition recently used by the European Commission is: A heritable decrease in susceptibility of a lack of susceptibility of an organism to a particular treatment with an agent under a particular set of conditions. This definition lacks the requirement that resistance should have a practical impact. The term ‘technical resistance’ is used to refer to resistance in which a consistent and measurable change of susceptibility is seen which falls short of having practical impact.

Resistance factor is an expression used to describe the degree or severity of resistance. The resistance factor is calculated, for a specific dose percentile (usually the 50th, 90th, 95th or 99th percentile), from the quotient of the doses required to kill (or to have an effect) in susceptible and resistant animals respectively. For example if the LD50 of a compound is 2.5 for susceptible rodents and is 25.0 in resistant rodents, the resistance factor is 10. The term ‘resistance ratio’ is used interchangeably with this term.

Single nucleotide polymorphism (or SNP pronounced snip) occurs when a single nucleotide in the DNA sequence differs among individuals within a species

Second-generation anticoagulant, one of the series of rodenticide active substances invented, mainly during the 1970s and 80s, in response to the development of resistance to compounds of the first-generation. The five second-generation anticoagulants are (in order of their chronological introduction) difenacoum, bromadiolone, brodifacoum, flocoumafen and difethialone. Difenacoum and bromadiolone are sometimes called ‘multi-feed’ compounds because rodents usually require more than one feed for a lethal effect. The other three compounds are called ‘single feed’ because often (but not invariably) one feed is sufficient for lethality. Resistance now occurs among rats and mice to difenacoum and bromadiolone but no practical resistance has been observed in the other three ‘single-feed’ compounds. (See first-generation anticoagulant.)

Tolerance is a term sometime heard in a discussion of resistance. It has no generally-agreed definition. Physiological tolerance to a chemical compound may be acquired by ingestion of progressively larger doses. However, the term is also used to describe individuals that are, within the normal distribution of differences in susceptibility, at the end of the distribution which is less susceptible. Therefore, tolerance may develop in a population of rodents when poor application practice, perhaps the use of insufficient quantities of bait, results in the removal of the most susceptible animals and the survival of the least susceptible.

Susceptible is a relative term to describe animals that are capable of being controlled with a rodenticide active substance – thus the term is often used as the opposite of resistant. Susceptible strains and populations of rodents are those that do not contain individuals which carry resistance mutations, or the frequency of occurrence of mutation is so low that it cannot be detected but normal experimental procedures.