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Generally brownish grey in colour (across the back and grey underneath).  Colours may vary as black forms have been found.

The Brown rat has an average weight of 335g.  It is 200-270 mm in length (head and body) and the length of the tail is 150-200 mm.  The common rat has small ears and eyes and its body is of heavy build.

Life Cycle
Common Rat colonies typically develop from a pair or a single pregnant female.  Rodents can breed even more efficiently than rabbits and size able infestations can develop very quickly.  Conditions that suit a rapid population increase are a good supply of food and water, suitable temperatures and an undisturbed cover.  With these ideal conditions breeding will continue throughout the year.

Rats and mice are capable of reproducing from the young age of three months.  Pair bonds are not formed and mating is carried out on an opportunistic and promiscuous basis.  Mating is brief and can take place with a number of males.  After mating and conception there is a relatively short period of pregnancy (gestation) 21-24 days.  The average size of the Common Rat litter is between 6-11.  The weaning period is 3 – 4 weeks.

Unlike most mammals rats do not have to wait until the original litter is weaned and the female has stopped giving milk before coming back into oestrus.  This means that the female rat may be willing to mate and can conceive again soon after the original litter is born.

Rats have been known to live as long as two years or more in a laboratory.  How ever in natural conditions it is thought that less than 5% of rats survive more than 12 months.  Females tend to live longer than males.

Brown rats are omnivorous but prefer starch and protein-rich foods, such as cereals.  Their diet includes meat, fish, vegetables, weeds, earthworms, crustaceans, nuts and fruit.  They sometimes catch food to return to later.

How to spot for signs of a rat infestation
The most reliable way to tell if there is an infestation present is with an actual sighting of the identified pest.  If Common Rats are seen frequently during the day this may indicate that a large infestation is present as rats are nocturnal by nature.

Rats produce a stale odour.  This will be noticed when the infestation has been present for a while in an indoor area.  The odour can remain present for a little time after the rodents have been controlled.

Common rats can produce up to 40 droppings a day; they are on average about 12 mm in length and taper to a point at both ends.  Fresh droppings are a tell tale sign of rodents present.

Rats tend to take the same path when traveling, which in turn creates a run that is visible to the naked eye.  Outdoors the trails of Common Rats can appear as continuous depressions in the grass or other vegetation.  Eventually the rat runs will turn into well-worn dusty paths that lead from their burrows to their food and water supply.

Outdoor burrows are usually very easy to spot and are often situated on sloping ground, such as the banks of ditches or covered areas like tree roots.  The entrance holes are generally 70-120 mm in diameter.  The soil around the burrow will be trampled if it is well used and cobwebs will be present if unused.

Damage to goods
Unfortunately, often the first sign of rodent activity is the discovery of partially eaten, spilled or hoarded food, damaged packaging material and other signs of gnawing.  Damage is a frequent consequence of indoor rat infestations, gnawing of electric cables and wires in the loft area is a common problem.  Rats can be distinguished as their incisor teeth marks are about 3 mm wide.

Why are rats a pest?
Common Rats are generally considered a pest because of their adverse effects upon the human population.  The most important of these are the ability to transmit diseases to man and his livestock and the economic damage they may cause to foodstuffs and structures.  Rodents whether being seen or heard can be very distressing and causing abhorrence and fear to some.  On these facts alone rodent control is justified.

Weil’s Disease
This is a type of leptospirosis associated with rats called Weil’s disease.  The causal organism is the bacterial Leptospira icterohaemorrhagiae that has been found in the kidneys of up to 50% of the individuals in sampled rat populations.  It is shed in the urine of infected rats that do not appear to suffer harm.

Wet environments are necessary for the survival of the bacterium outside the rat’s body.  It is usually transmitted to humans by contact with contaminated water or moist soil or by contact with rats.  The excreted organism enters the body through cuts and abrasions, or through the mucus membrane of the nose and mouth.  Weil’s disease used to occur commonly amongst agricultural workers and those who worked in damp places such as sewers.  More recently, there has been a rise in the number of cases amongst those engaged in recreational water activities (swimmers and water-skiers).

Weil’s disease has been a notifiable disease for some years, and can range in severity from mild flu-like symptoms to jaundice, renal failure and death.  Public education is very important in its prevention, and the issue of warning cards to those at particular risk should be encouraged.

Another disease of particular significance in this country is salmonellosis caused by bacteria of the salmonella group.  It ranks as one of the most wide spread animal-borne diseases.  Infection in humans commonly occurs from the contamination of food and drink with rodent excreta, or as a result of passive transmission of the bacteria by rodents.  Salmonellosis is a type of food poisoning and symptoms can include acute gastro-enteritis accompanied by headache, fever and vomiting.

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