Think Wildlife: The Problems With Rats (Campaign for Responsible Rodenticide Use)

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THE PROBLEM WITH RATS

Rats - a rising problem
Main cuplrit - the Norway rat
Health Problems
Contamination and Wastage
Damage to property and materials

Rats - a rising problem
Evidence shows that the rat population in the UK has been steadily increasing, with marked rises in infestations in domestic premises in both urban and rural areas. A 1993 survey showed a 39% increase in infested domestic premises over the previous 20 years. Subsequent surveys have shown a continuing increase in infestation levels. There are now probably more than a million houses in the UK infested with rats.

Domestic premises in rural areas appear to be more at risk from infestations. Research found that they were twice as likely to be infested with rats, at 7.8%, than those in urban environments.

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Main culprit - the Norway rat
Almost all the rats found in rural areas are Norway or brown rats (Rattus norvegicus). This is in spite of the fact that they are not always brown and they have no particular affinity with Norway.

Unlike the black or ship rat, which is thought to have caused the spread of the Black Death in the Middle Ages, the now more common Norway rat is a recent immigrant into Europe, having first been identified in Denmark at the beginning of the 18th century.

It is now the dominant species in the UK and is often found living in sewers and drains. It will infest both urban and rural environments - domestic and commercial premises, refuse tips, waterways, warehouses, farms and other rural enterprises.

Rats do not normally move great distances, but this is not always so on farms. Where it is necessary to find food, water or safe shelter, they have been found to travel more than two kilometres in a night.

This degree of movement can be an important factor in the spread of the problems associated with rat infestations.

Problems caused by rats can be divided into three main areas:
• Health problems
• Contamination and wastage
• Damage to property and materials

These problems are fundamental to the need to control populations both on farms and in rural areas. None of these problems can be ignored.

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Health problems
Rats have a role in the spread of many human pathogens, heightened by to their close association with humans.

Contamination of stored produce and animal feed with rodent urine and faeces may transmit disease to both humans and livestock. Recent research has shown that rats caught on farms carry a surprising number of disease causing organisms. These include cryptosporidiosis (63%), toxoplasmosis (35%), leptospirosis (14%) and listeriosis (11%). The full results are shown in Table 1.

Cryptosporidiosis is a particular concern because rats can be implicated in the contamination of cattle foodstuffs that in turn leads to cattle slurry contaminating watercourses from which drinking water is extracted.

Cryptosporidiosis causes intestinal illness in humans and several other mammals. Symptoms are abdominal cramps and watery diarrhoea. In healthy people these symptoms last only one to two weeks. There is currently no cure.

In 2001 there were more than 4400 reported human cases of cryptosporidiosis in the UK.

Toxoplasmosis is a parasitic disease caused by a protozoan - a single-celled animal. It is a common illness in both animals and humans throughout the world. In human cases it can cause spontaneous abortion and birth defects. In animals, Toxoplasma is an important cause of ovine abortion.

In 2001, 129 laboratory-confirmed cases of toxoplasmosis were reported. Serological surveys indicate that voluntary laboratory reports underestimate the level of infection.

Leptospirosis is a bacterial disease that affects humans and animals. It is a notifiable disease in humans throughout the UK and causes a wide range of symptoms ranging from flu-like fevers, headaches, chills and vomiting.

Photomicrograph of liver tissue revealing the presence of Leptospira bacteria.

In the cattle and pig industries it causes major economic losses through its adverse effects on the reproductive performance of these animals. In the dairy industry it can adversely affect milk yields and fertility.

In 2001 there were 48 confirmed reports of leptospirosis among humans in the UK. Leptospirosis may be fatal if not treated promptly.

Listeriosis causes symptoms ranging from mild, flu-like illnesses to severe, life-threatening infections such as septicaemia and meningo-encephalitis. Those at highest risk are pregnant women, the elderly and the immuno-compromised.

In 2001, 153 cases of human listeriosis were diagnosed in the UK.

Food safety is paramount and the focus nowadays starts on the farm and ends on the fork. It is more important than ever to ensure that there is no contamination at any stage of the food chain. Farm assurance schemes specify that approved rodent control programmes are effectively carried out.

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Contamination and Wastage
There are economic reasons why rats need to be controlled.

In a 24-hour period, a Norway rat can pass 12 to 16 ml of urine and produce 50 droppings. This will damage and spoil food in store. The cost of the contaminated food is considered to be greater than the cost of the food eaten.

This is borne out by a study that claimed 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 actually been eaten. The associated costs are highly significant.

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Damage to property and materials
Rats can cause considerable damage to property and buildings. It has been estimated that the most significant economic damage on farms comes from the gnawing of electrical cables; around 50% of farm fires are caused by this.

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

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