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Rat control 1: impact on farm

Course: Rat control | Last Updates: 7th October 2015

Dr. Alan Buckle
Biography >>

Rat behaviour

The Norway or brown rat (Rattus norvegicus) is the main UK rural rat. They have no special affinity to Norway, nor are they uniformly brown.

They will infest any premises, refuse tips, farms, rural dwellings and watercourses that provide food and shelter.

Rat populations have been rising steadily during the past 30 years, with premises in rural areas twice as likely to be infested as urban dwellings. Warmer winters have allowed rats to survive and to breed for longer periods. In some localities there may also be control difficulties due to resistance to the early anti-coagulant rodenticides, such as warfarin.

Another possible cause of higher rat numbers in rural areas is the fact that stubble burning is no longer practiced and this leaves food in the fields for rats, such as spilled cereal grains.

Rats will travel to find food, water, shelter and a mate. This might be more than two miles a night in the countryside and at certain times of the year when immediate food supplies are not available or weather conditions are harsh.

The degree of movement can be an important factor in the spread of problems associated with rats – namely health problems, contamination and wastage and damage to property, equipment and materials.

The demand for high quality food and concern for the health and safety of farm employees and livestock make these problems highly significant and enforce the need for proper control.

The problems

1. Health

Contact with rodent urine and faeces are the most frequent causes of disease transmission from rats to both humans and livestock.


Recent research shows that rats caught on UK farms carry a surprising number of disease-causing organisms. And new more sensitive tests for leptospirosis and toxoplasmosis have shown infection rates of these particular diseases to be much higher than expected in both humans and animals.

The results of a recent survey of the diseases commonly carried by rats in the UK (see left) reveal the dangers to humans and livestock.

A high proportion carried the protozoa that cause cryptosporidiosis – an intestinal illness in humans – and the organism responsible for toxoplasmosis.

This is a common, usually mild, infection but it is a serious risk to an unborn child if contracted by the mother during pregnancy. 14% carried bacteria-causing leptospirosis (Weil's disease), which is a notifiable disease that causes flu-like symptoms in humans and is fatal if not treated promptly.

Diseases transmitted by rats may also have a significant impact on animal production. Effects on milk yield, fertility and growth rates are often not directly attributed to rat-transmitted diseases, only because screening for them is not as comprehensive as it could be.

2. Contamination and waste

A Norway rat can pass 12 to 16ml of urine and produce 50 droppings in a 24-hour period. This will damage and spoil food in store and render it unfit for sale.

The value of the contaminated food is always far greater than the cost of the food actually eaten. A study showed that 70% of a tonne of wheat had been spoiled by 10 to 26 rats during a 12 to 28 week period, although only 4.4% had been eaten.

3. Damage to property and materials

Rats can cause considerable damage to property and buildings. On farms, damage to the electrical wiring of vehicles and equipment is a common cause of breakdown and the need for expensive repairs.

It is estimated that 50% of farm fires are caused by rats chewing electric cables.

Based on today's prices, the damage on farm caused by rats is estimated to cost the UK farming industry £14 to £28 million a year.

Survey results: Diseases commonly carried by rats




Tape worm and round worms

Capillaria spp



Toxocara cati



Hymenolepis nana

Rat tapeworm



Leptospira spp



Listeria spp



Yersinia enterocolitica



Pasteurella spp



Pseudomonas spp



Coxiella burnetii (antibodies)

Q Fever


Salmonella spp




Cryptospoiridium parvum



Toxoplasma gondii



Virus (antibodies)




Source: JP Webster, DW Macdonald

Controlling rodents

The main weapons used for rat control are baits, carrying one of the second-generation anticoagulant rodenticides. These are used widely because of their practicality, effectiveness and excellent safety record.

The main active ingredients of rodent baits used in the UK are bromadiolone and difenacoum, although brodifacoum, difethialone and flocoumafen are also available for use indoors.

Impact on wildlife

Because rodenticides are so widely used in the countryside, low-level residues of some of the commonly used products are found in UK wildlife. Surveys have shown residues in red kites, barn owls, kestrels and foxes.


Monitoring in the 1990s revealed that 40% of barn owls carried traces of rodenticides which they had ingested via rats, or other rodents, treated with rodenticides. Most recent surveys have shown that more than 80% of barn owls, kestrels and red kites now carry residues of anticoagulants.

Rodenticide residues find their way into wildlife via two main routes: wildlife directly consuming baits that are not properly covered to prevent them from doing so (primary exposure) or consuming dead or dying rodents that themselves have eaten the bait (secondary exposure).

In most cases it is unlikely that these low residues have adverse effects, either on the individual animals that carry them or on wildlife populations, but rodenticide usage must always be carried out in a way that minimises any exposure to wildlife.

Wildlife monitoring

Reviewing the evidence, the six members of CRRU – all of who are involved in the manufacture or marketing of anticoagulant rodenticides in the UK – believe that if rodenticides are used sensibly they create no significant threat to wildlife.

The government monitors pesticide use and wildlife casualties through the Wildlife Incident Investigation Scheme. The scheme shows that very few incidents are caused if rodenticides are properly used.

However, monitoring low-level residues in wildlife is also essential and therefore CRRU is one of several agencies that assists with the funding of the Predatory Birds Monitoring Scheme which sets out to monitor pesticides in birds of prey which fall short of lethal levels (see http://www.pbms.ceh.ac.uk).

CRRU seven-point code:


  • Have a planned approach
  • Record quantity of bait used and where it is placed
  • Use enough baiting points
  • Collect and dispose of rodent bodies


  • Leave bait exposed to non-target animals and birds
  • Fail to inspect bait regularly
  • Leave bait down at the end of the treatment

* CRRU has developed a seven-point code to promote effective and responsible rodenticide use among all those working in the countryside, such as farmers and gamekeepers.

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