Like everything else here in the Pet Emergencies section, this is not good. Most of the reasons for a pet to be dragging a leg are painful. Best to get help soon....
Translated, this means that the pet has "thrown a blood clot".
Cats that have heart disease can develop a blood clot in their heart. If this blood clot breaks free, it is sent out of the aorta and travels down this large "superhighway" of an artery. It will often lodge right where the aorta splits into the two main arteries that go down the back legs.
It is entirely possible for your cat to have heart disease, and for you to not know it! Some of the heart disease "types" are not identifiable by cardiac auscultation (when your vet listens to your cat with a stethoscope).
These cats are acutely (suddenly) and severely painful. They are often actually screaming in pain, and are unable to use one or both hind legs. Unfortunately, the long term prognosis for these cats is very poor...
GET TO YOUR VET NOW
Clots can also be thrown to the front legs (usually just one), and will cause front leg pain and lameness. Dogs can also do this, although it seems to be much less common.
For some reason, people think things like this can't happen to their pets.... Well, they can, and do.
Pets hit by cars, gunshot wounds, caught in traps, caught in automobile fan belts, caught in the garage door, mauled by another animal, and abused by a human can all have broken backs, broken legs, and mangled muscles and ligaments. Obviously, this would cause the cat or dog to be not using or dragging a leg or legs.
GET TO YOUR VET NOW
Translated, the pet has "blown a disk" in their back or neck. Signs of this can range from limping on one leg, to complete paralysis of all 4 legs - it just depends on where the disk is, and how badly it is "squishing" the spinal cord.
In severe cases, there are only hours to respond to this problem with any hope of regaining limb function.
GET TO YOUR VET NOW
Boy, I don't know about other vets out there, but I see a lot of pets with cancer.... Bone cancer would be the main one that would cause a pet to be dragging a leg, and it would be seen more commonly in dogs than in cats.
When the cancer first starts, it will generally cause a lameness (limping). The cancer makes a weak spot in the bone. These patients will break their legs at the site of the cancer. It is at this point that they begin carrying the limb rather than using it. The prognosis for these dogs is very poor...
The earlier the diagnosis and treatment, the better the chances of providing your pet with the best quality of life that you can. Even if you can't afford "cancer treatment", pain medications can make their remaining lives better.
This is the one thing that owners seem to check for if their pet is limping. I hear "I didn't find anything stuck in his foot Doc" with some regularity!
Occasionally an owner will find a foreign body, and remove it. As long as no infection sets in, and you removed ALL of the foreign body, and the foreign body didn't damage any nerves or tendons etc., then you fixed it! Good job!
Sometimes though, the thorn/glass shard/metal shard or whatever gets jammed up under the skin, where you can't see it. Infection and pain follow, which your dog or cat will show as a lameness with swelling and excessive licking of the area. Your vet will need to take care of this one.
So, your nerves carry the orders from your brain to your muscles, which then allow you to move. If the nerve is damaged - bruised, or overly stretched, or cut - then those orders can't get through.
Nerve damage to the front side of a limb will make the pet unable to bring the leg forward - they will be dragging a leg.
Bruised nerves will recover, stretched nerves will recover if not overly stretched, and cut nerves don't heal. Many times only time will tell you if the nerve will be able to heal or not.
The actual information that your nerves carry comes in electrical, chemical, and physical forms... It's complex. But the point is that a failure in any of those "forms" will cause muscle weakness or failure.
It may lead to a generalized muscle weakness, where it would appear as if all limbs were not working well. Metabolic, infectious, or toxic events can cause this.
Or it may be a more localized issue, which would be more common with a structural process, like a pinched nerve or a fibrocartilagenous embolism (kind of a "stroke" in the spinal cord).
The odds are very good that you are not going to know which of these things is causing your dog or cat to be dragging a leg.
The odds are also very good that you are not going to be able to do anything about it at home.
Go to your vet, and let them help you to help your pet!
Page last updated 2/11/13
Love + Knowledge = Healthy Happy Pets.