Although relatively rare (fortunately), Rabies is probably the most important infectious disease we can vaccinate for-at least from a legal and human health standpoint.
Rabies can affect all warm-blooded animal including humans. It is nearly worldwide in distribution. There are some areas of the world (mostly island nations) that are rabies free. These include Hawaii, Great Britain, Japan and several others. Traveling to or moving to these areas with your pet requires extensive preparation as there are very detailed protocols in place to make sure these areas maintain their rabies free status.
The rabies virus is spread via wounds, most commonly a bite wound. From there is replicates in the muscle cells and then penetrates local nerve cells. It then follows the nerves to the brain. Average time to reach the brain is 20-30 days but can be up to one year depending on how far from the brain the bite wound was. Prior to reaching the brain, the animal is only incubating the virus and is not considered infections or able to transmit the virus. Two to three days after reaching the brain the virus is present in all body secretions, including saliva, and clinical signs begin to occur. Once clinical signs begin the animal is able to transmit disease.
There are three stages of infection. The first stage is the Prodromal stage and is characterized by a change in behavior, personality or the sound of the voice. The second stage is the Excitative ("mad dog") stage where the animal shows no fear and suffers from hallucinations. This would be the classic "Old Yeller" presentation. The third stage is the Paralytic stage where the drooling occurs because of the paralysis of the larynx. Ultimately the animal dies because the muscles that control breathing are paralyzed. Symptoms can vary, and be hard to recognize, especially if the full history is unknown. There can also be atypical presentations so rabies should be a differential for any unvaccinated animal showing neurologic signs. Especially if exposure to wildlife is known.
Once clinical signs begin, death occurs in approximately 10 days or less. There is no treatment and the disease is considered uniformly fatal. There are one or two reports of a person surviving but the treatments used on them have not worked on subsequent cases.
So, why not run a test on suspected cases? Warning-the next part is a little graphic.
The only way to test for rabies is to cut off the suspected animals head and submit the brain for testing. Naturally, this is not ideal for most pets. So, what happens in real life?
If an animal has bitten, or is showing signs consistent with rabies then it can be quarantined for 10 days. Because we know death occurs within 10 days of becoming infectious, if an animal remains alive and well for the 10 days we know it was either not exhibiting signs of rabies, or was not able to transmit the virus. Remember, it could still be incubating the virus, meaning that the person it bit will be fine, but the pet will ultimately succumb once the virus reaches the brain. Sometimes an owner elects euthanasia and testing anyway, depending on the situation. If a pet is known to have been bitten by a wild animal, then vaccination followed by a 6 month quarantine may be suggested or even required, especially of the animal has subsequently bitten someone.
Prevention is aimed at routine vaccination and limiting exposure to wildlife. Dogs and cats are vaccinated at 12 weeks of age, then again in 1 year. After that local laws apply. Three year vaccines are accepted in most areas although some only require vaccines yearly or every two years. There is a vaccine specifically designed for cats which requires annual administration. The most common wildlife to carry rabies in the US are skunk, bat, fox, raccoon, and coyote.
Laws regarding bite wounds are also very regional. However, the rights of the owner are significantly better if the pet is current on it's rabies vaccine.
More information about rabies can be found on the CDC's Rabies page as well as their page on post-exposure protocols.