How Rabies Works

Rabies is a viral disease that attacks the brain and spinal cord, or central nervous system (CNS). It's part of the Rhabdoviridae family of viruses, under the genus Lyssavirus. The virus itself, like all members of Rhabdoviridae, is shaped like a bullet. Upon entering the body, it makes its way to the spinal cord via the peripheral nervous system's afferent nerves (nerves that carry impulses toward the CNS). Once the virus gets into the spinal cord, it's quickly sent up to the brain, where it begins replicating itself inside the mind's nerve cells, destroying them in the process.

After it reaches the brain, the virus typically travels through the efferent nerves (nerves that carry impulses away from the CNS) to the salivary glands, which often causes increased salivation, or foaming at the mouth. It's important for the virus to do this, as this saliva is its principle method of transmission into new hosts. After hitting the salivary glands, the virus continues its way down throughout the rest of the body.

As you may have guessed from the way it operates, rabies is anything but an average disease. Though there are actually several different strains of the virus, there are only two real physical variations. The most common is the encephalitic, or "furious," form of rabies. This is the mad-dog, foaming-at-the-mouth version, usually highlighted by increased agitation and aggression, disorientation and hallucinations. This is the form most people imagine when they think of rabies. The other form, the paralytic or "dumb" form, is more peaceful, but no less deadly. With this version, the victim initially appears weary and lethargic.

Both of these forms take place during rabies' acute stage, the point at which the virus has successfully infiltrated the body, symptoms have occurred and all hope for recovery is gone. As the virus makes its way throughout the body, lethargy soon turns to partial or almost total paralysis, then to coma and death.

It's actually not uncommon for symptoms of both rabies forms to appear in a single case. If the person or animal doesn't die during the furious form, the virus will develop into the dumb form. Paralysis will occur, and the animal or person will slip into a coma and die, usually from respiratory paralysis.

Despite rabies' ferocity, it's a picky disease. It's found exclusively in mammals, but even with them, it's still pretty selective. For example, rabies is rarely seen in mice, hamsters, rabbits or squirrels. These creatures are perfectly capable of contracting the virus (Pasteur experimented extensively with rabbits while researching his cure), but rarely do [source: Cohn]. This is probably because animals of this size are unlikely to survive the kind of attack that results in rabies. However, not all small mammals are so fortunate; woodchucks, bats and groundhogs are all common vectors for the disease.

Also, there is still some uncertainty of the virus's effects in regards to host size. Rabies typically runs its course over a period of a few months in most animals, yet in humans the virus may lie dormant for months, even years, before making itself known.

Rabies' selection and treatment of hosts aren't the virus's only unusual traits. Although the disease is easily transferred through both saliva and brain matter, the blood, urine and feces of a carrier pose no threat. Since few animals or people are inclined to go out looking for suspiciously acting creatures' brain matter, outside of saliva, the virus is largely non-transferable. However, rabies is a uniquely adaptive disease, and there are other ways that it has been spread.

Perhaps the most frightening potential method of rabies transmission is through the air (aerosol transmission). It's extremely rare; in fact, there is only one documented case of it happening outside of a laboratory environment. It occurred in a cave that is believed to have housed tens of millions of infected bats [source: Merck Vet]. The virus became airborne through the oral and nasal discharges of the rabid animals, infecting several people who entered the cave. But again, this method of transmission is almost unheard of. In fact, the CDC states that it takes "extraordinary circumstances" for aerosol transmission of the rabies virus to occur [source: CDC].

Although it's also extremely rare, rabies has been spread between humans. It's a very unusual occurrence, and usually only happens through a transfer of tissue in hospitals where unwitting rabid organ donors pass on their infections. This has happened several times with cornea transplants, for example [source: CDC].

Although rabies certainly does affect humans, it's largely a disease that exists and spreads among animals. Most people are knowledgeable enough about the disease to be wary of an animal that's foaming at the mouth, but there are many other symptoms an infected creature may exhibit. In fact, some rabid animals, including those who have contracted the "dumb" form of the disease, never foam at the mouth. Almost all cases of rabies in humans are contracted from an infected animal, so it's important to know what to look for.

Rabid animals often:

Rabies typically infects a variety of animals that vary by region. In the United States, raccoons are the most common carriers of the disease, but bats more often transfer it to humans [source: CDC]. Skunks and foxes are also frequent vectors for the virus, but it has been found in everything from woodchucks to chimpanzees. A good rule of thumb: If an animal is exhibiting unusual behavior, try to avoid it and contact your local animal control office as soon as possible.

Unfortunately, it's often difficult to tell if an animal is just acting strangely or actually poses a real threat. A dog may turn around in circles and snap at the air just because it feels like it, not because it's rabid.

Rabies is a disease that affects the nervous tissues, meaning that it's not found in blood, feces or urine. As you can probably imagine, this makes it quite difficult to test animals for the virus. To complicate matters further, potential testing outlets, such as saliva, don't yield results as quickly or as accurately as brain matter. Therefore, an animal's head must be removed, post-mortem, in order to gauge its infection status accurately. For more than 40 years, this has been the only accepted rabies testing method for animals in the United States [source: CDC].

Believe it or not, if you suspect a small wild animal of having the disease, you'll often be encouraged to try and capture it yourself, especially if it's a bat. But don't worry, no one is going to expect you to wrestle with a frothing coyote or raccoon. If the offender can't be safely apprehended, Animal Control will usually attempt to capture it later. Then, it's off with the animal's head. After the creature has been decapitated, its head is placed on ice for preservation and sent to a laboratory where it can be tested.

Unvaccinated pets that have a run-in with a potentially rabid animal are either euthanized or placed under strict, isolated observance for 10 days. If the animal makes it through this probationary period without incident, it's given a clean bill of health and allowed to return to life as normal.

To help ease the wild rabies threat in the United States, some state governments have begun distributing rabies vaccine bait in highly affected areas. The bait is essentially a high-dose oral version of the vaccine. It's primarily aimed at attracting raccoons (and is safe for contact with most mammals, including dogs and people).

Even though the vast majority of rabies cases are contained in the animal kingdom, the disease still kills tens of thousands of people worldwide every year. Check out the next page to learn which animal poses the greatest danger to humans: bats or dogs.

Thanks to movies like "Old Yeller," most people are aware of at least some of the signs to watch for in a rabid beast. Though indicators like a foaming mouth and increased aggression from a previously friendly pooch are good things to be aware of, in many parts of the world, cases of dogs with rabies are much less frequent than they used to be. Today, many experts say that the real rabies threat comes from bats. Has the stereotypical harbinger of rabies, the mad dog, become antiquated? Are we in imminent danger from throngs of rabid, flying pests?

It's complicated, but in short, the answer to both questions is no. "Old Yeller" still provides an important message, but in the new millennium, the majority of rabies cases in the United States and other developed nations come from bats.

Most states in the U.S. now have laws mandating the regular vaccination of dogs and cats. Over the years, these laws have helped to significantly reduce the prominence of the rabies virus in man's best friend. In 2006, for example, only 0.011 percent of all rabies cases in the U.S. occurred in dogs [source: CDC].

That same year, bats made up an astounding 24 percent of all U.S. wildlife rabies cases. They are also to blame for two out of the three rabies-related deaths in 2006 [source: Blanton]. In fact, bats can take the blame for 28 out of the 37 confirmed cases of rabies in the U.S. since 1995. Not one came from an American dog [source: CDC].

In many ways, bats' success in taking the American-rabies crown makes sense. Bats have the ability to fly, making it easy for them to enter the upper level of a house or apartment. Their claws and teeth are small, so it's not unusual for people to wake up to find a bat in the room and not even be aware that they've been attacked.

Anyone who comes in contact with a bat should probably seek medical treatment. Even if there are no marks or puncture wounds, the virus still may be transferred. Both of the 2006 rabies fatalities that had encounters with bats believed the animals did them no harm [source: CDC].

Despite bats' potential to spread the disease, Americans and citizens of other developed nations are actually quite fortunate these flying mammals pose their greatest rabies risk. While it's expected that a few people in First World nations will contract the disease from bats, tens of thousands will do the same from dogs in other areas of the globe.

Between 30,000 and 70,000 people every year contract rabies [source: McPhee]. It's a staggering number and means, on average, that a person dies of rabies about once every 10 minutes. But this plight isn't spread equally. Things are much more optimistic for First World nations than they are in developing countries. The United States, for example, has an average of only two or three human rabies cases per year, thereby contributing less than 1/10,000th of the total number of fatalities of the global epidemic [source: CDC].

Not surprisingly, the countries where rabid dogs are a problem are the same ones that have high rabies mortality rates. The majority of annual human rabies deaths occur in developing nations, where vaccination programs are limited or nonexistent. The World Health Organization (WHO) cites rabies as one of the "most neglected among today's neglected global health problems" [source: WHO].

Ironically, these are the very same countries and people who can't afford to fight the disease. The New York Times estimates that the annual global cost of rabies is approximately $583 million [source: Judson]. That may sound like a lot, but it's really not that much money when considered on a global scale. What it really comes down to is the $40 cost of each vaccine. Unfortunately, for those living in the areas that need the greatest help, such as in the rural impoverished areas of Africa and Asia, coming up with $40 for each shot is next to impossible. (Rabies vaccines are much more expensive in developed countries because the series of shots contains a dose of immunoglobulin, which protects the body until it has created antibodies.)

Unfortunately, even if the shots were free, the vaccination schedule would still pose a problem for many rabies victims in the rural areas of afflicted developing countries. Frequent trips to a clinic to receive treatment are time consuming and often force victims to miss much-needed work. Organizations like RITA (Rabies in the Americas) and RabiesFreeWorld are doing their part to help eradicate the disease, but, despite the relatively low cost associated with doing so, there is still a long way to go.

As mentioned previously, rabies is usually considered to be 100 percent fatal for both animals and people. Vaccinations are available both pre-emptively and immediately after exposure, but if nothing is done before the virus hits your central nervous system, you will almost certainly die. Only one person has survived the acute stage of rabies without vaccinations, and she's considered a "medical marvel" [source: Fox]. In other words, she was very, very lucky. In both developed nations and the Third World, untreated rabies is still considered a death sentence.

Rabies isn't a virus you want to mess around with. If you think you've been exposed to the disease, remember to seek medical treatment before symptoms show up, preferably within hours of the exposure taking place. Even if you're unsure, a few shots in the arm is better than death.

The symptoms of rabies in humans are somewhat different than those typically shown by animals. The disease is found in most places throughout the world, so regardless of whether you live in upstate New York or sub-Saharan Africa, it's not a bad idea to know what symptoms to look for.

Common rabies symptoms in humans are:

As with animals, it's important to remember that the occurrence of these symptoms doesn't necessarily mean an individual has rabies. There are many causes for a fever and cough, for example, and rabies usually isn't one of them. If someone avoids treatment and develops an intense fear of water after being attacked by a foaming dog, then it might be time to start worrying. But for those who seek prompt treatment after an exposure -- or never come near a rabid animal at all -- a fever is just a fever and a cough is just a cough.

However, if you or someone you know has been attacked by what you fear may be a rabid animal, thoroughly wash the wounds with soap and water. Then, contact your local animal control office. You will probably also want to contact your state or county health department for the proper procedure and guidelines for your area. If you are outside the U.S., wash the wound and try to get to a local hospital or emergency clinic for treatment and advice.

Effective rabies prevention really just comes down to knowing what to look for. Good judgment and common sense are paramount to staying rabies-free. Rabies is a deadly disease, but most people go their entire lives without running into a rabid animal, so it can be avoided. However, if treatment becomes necessary, the rabies vaccine is still the principal, and only, line of defense.

Pasteur's vaccine relied on an attenuated, or weakened, form of the rabies virus gathered from the spinal cords of infected animals. By applying a series of vaccinations of increasingly potent forms of the virus over several days, Pasteur found he could inoculate animals to the disease. By slowly introducing the virus, Pasteur allowed the body to build up a resistance, so by the time the disease reached the central nervous system the body had developed an immunity to it [source: Cohn].

The vaccine we use today follows the same concept but is much less dangerous. Pasteur developed a live-attenuated vaccine, or an alive but weakened form of the virus. Today, we use an inactivated vaccine, or vaccine that is already dead. Immunity can still be built up using a dead virus, though it often doesn't last as long as a live-attenuated vaccine. There are also fewer side effects with an inactivated vaccine.

Both people and pets can be pre-emptively vaccinated for rabies. For pets in the U.S., it's often a crime not to do so, though that is not yet true in all states. Some people with a high risk for contracting the disease are often encouraged to be vaccinated before they become exposed. Veterinarians, spelunkers, some lab technicians and individuals who frequently travel to rabies hot spots around the globe often receive regular rabies shots as a precautionary measure.

Now that you're well-versed in all things rabies, check out the links on the next page to learn more about other gruesome but compelling sicknesses and diseases such as the bubonic plague and avian flu.