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Elderly rats occasionally suffer strokes, often affecting balance and coordination, and rats of all ages very occasionally develop an inner-ear infection which causes loss of balance resembling stroke. One way to distinguish them is that the acute phase of inner-ear infection only lasts about a day, after which the animal regains at least some of its bearings. It usually takes rather longer - say three or four days - to show noticeable improvement after a stroke.
Initially the animal will usually make the problem worse by panicking and will crash about, falling over things and often rolling over and over. Inner-ear infection may be helped by a combination of antibiotic and steroids, but this isn't really neccessary as the acute phase is normally so short. Stroke is untreatable. Even without treatment, however, nearly all rats recover very well from either condition. They rapidly learn to compensate for the problem and get about at least a bit, and then improve gradually over a period of weeks. Until they regain their balance they should be kept in a plain single-storey cage with nothing they can fall off or over. Many will be left with a permanent slight head-tilt, but this has no obvious ill-effects - apart from causing the rat to rotate as it wriggles, which makes it very hard to hold!
During the acute phase they may lose the ability to chew and need to be fed liquid feed (Complan, Readybrek, baby-food etc.) from a dropper or needle-less syringe: this may also be the case with very old and/or dying rats.
Inner-ear disease is caused by a mycoplasma which nearly all rats carry, and which normally just generates the occasional runny nose. Vey rarely, especially in the humid weather of spring and autumn, this same organism can invade the central nervous system and cause encephalitis.
Treating with antibiotic and steroids might help, but in my experience this condition is always fatal. It shows itself as progressive slowing down and floppiness, usually accompanied by minor epileptic fits (e.g. paddling with the hands; turning round and round with the nose on the base of the tail). Eventually the animal becomes so paralysed it has to be fed soft food from a syringe.
From infection to death takes two or three weeks in Norway rats. The one good thing about this condition is that it doesn't seem to be painful: the victim appears quite happy and relaxed and so can usually be allowed to drift off naturally. However some do become unable to swallow at all, and have to be put down.
Stroke can cause other symptoms apart from loss of balance - partial paralysis, loss of coordination and/or strength etc.. All would be marked by a sudden onset, usually followed by a slow recovery (assuming the animal survives at all).
Stroke and inner-ear disease both come on quite suddenly and violently, and then gradually improve. Symptoms which resemble stroke - loss of balance, head-tilt, circling, limpness, partial paralysis etc. - but which begin slowly and progressively worsen may be due to encephalitis, or to a genetic fault (if the rat is young), or to a tumour of the brain or pituitary gland. Brain tumours can also cause demented aggression. The incidence of different types of tumours is strongly strain-related, and these particular tumours seem to be more common in American rats than in British ones.
Older rats sometimes lose some or all of the use of their hindlegs. Often this is due to progressive neurological failure, called degenerative myelopathy: in some cases this is a sign of impending death, but in others the degeneration has no effect on lifespan. Partial paralysis can also occur as a result of stroke, arthritis or a slipped disc. Rats who have had severe strokes may also be left with a tendency to fall over a lot.
Rats being so low-slung, these problems are not nearly so difficult for them as they would be for a human. A rat that falls over only has a couple of inches to fall from, and will just roll over and keep on going. Rats who lose the use of their hind-legs just hoick themselves up onto their elbows and lurch along at remarkable speed: their quality of life is generally fine and there is no need to put them down, but they may need regular bathing around the stomach and rear-end, to prevent skin-irritation due to being unable to lift themselves clear of their own urine.