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Rats and Public Health


Norway ratNorway Rats (Rattus norvegicus, also known as brown rats, sewer rats, wharf rats, or water rats) are notorious public health pests. Their infamy dates back at least to the 14th Century A.D., when plague carried by fleas carried by rats killed perhaps as many as one out of four members of the world's then-known population. But awareness of the connection between rats and disease may go back a lot further than that.

In the Old Testament book of 1 Samuel, the Philistines were afflicted with "tumors" and their land infested with rats after they captured the Ark of the Covenant. The "tumors" could very well have been the bubos associated with bubonic plague, which is transmitted by rat fleas. As a sacrifice to the God of Israel, the Philistines returned the Ark along with a sacrifice of five gold "tumors" and five gold rats, indicating some awareness of the relationship between the rats and the disease. (1 Sam. 5,6)

In addition to plague, rats have also been implicated in the transmission of salmonella, Lyme disease, and typhus, making rats one of the deadliest disease vectors in the natural world.

In addition to carrying diseases, rats also contaminate our surroundings with their urine, feces, and parasites; cause untold damage and fires through their gnawing; and can inflict painful, disabling bites with their razor-sharp teeth.

Norway Rat Biology

Norway rats are brownish or gray in color and can grow to lengths exceeding 18 inches, including the tail. They weigh between seven and twelve ounces on average, but can sometimes grow to more than a pound.

Norway rats often excavate burrows in the ground in fields, under sheds or buildings, at the base of trees, and under rocks. But they also make themselves at home in abandoned buildings, basements, garbage dumps, sewers, around garbage dumpsters, and pretty much anywhere else they can find food, water, and relative seclusion.

Norway rats are also very good swimmers, have good balance, can climb well when need be, and are comfortable on ledges, pipes, and other narrow horizontal surfaces.

Norway rats have excellent senses of hearing, smell, touch, and taste, which makes it difficult to develop rodenticides that are palatable to them. Their vision, however, does not seem to be very sharp beyond a few feet, and they are believed to be color-blind.

Like most rodents, Norway rats have impressive reproductive potential. They reach sexual maturity at the age of three months, on average, and adult females come into heat every four or five days. The gestation period lasts about 23 days, and there are an average of six to twelve young in a litter. Females may have as many as six litters in a year, successfully weaning about 20 to 30 young every year, on average.

Rat Behavior

Rats usually live in packs, although there's little evidence of sufficient cooperation or division of labor to characterize them as truly social animals.

Adult male rats are territorial and will often chase away other males with aggressive posturing, shreaking, and other behaviors. Much has been written about Alpha, Beta, and Gamma rats, which supposedly represent three levels of dominance of males in a rat pack (with Alpha's being the most dominant). Because much of this research was conducted under artificial conditions, it's questionable how well it actually represents wild rat behavior. In the wild, territoriality and dominance among males seems more a function of age and size than of any particular "personality" traits.

Norway rats are also xenophobic, which means that they are cautious about new objects in their environment and tend to avoid them for several days. This has important implications for rat control because as rats typically will avoid traps or bait stations for several days after they are placed, and will permanently avoid them if their first contact resulted in sickness or narrowly escaping disaster in a trap.

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