What is rabies?
Rabies basic stats
    The rabies disease is caused by the rabies virus, from the Rhabdovirus family of viruses. Other diseases in this family include the bovine ephemeral fever virus and the vesicular stomatitis virus. Rabies is a single-stranded, encapsulated RNA virus that looks like a bullet under electron microscopy. 

    All mammals can get the disease, but some are more likely to contract it than others. Carnivores and bats are the two types of mammals most likely to become infected. In North America, skunks and raccoons serve as reservoir animals and are the most likely wild carnivores to be infected with rabies. Pet dogs are among the at-risk species for rabies.
What does rabies do to animals?
    The rabies virus causes a severe viral encephalomyelitis (inflammation of the brain and spinal cord). This disease is ultimately fatal, with no available cure.

How is rabies transmitted?
    The saliva of an infected animal contains millions of copies of the rabies virus, each of which can potentially be used to spread the disease. In order to cause disease, the virus must gain access to the dog’s body. The most common mode of transmission is via a bite wound, which allows saliva containing the virus to get past the skin’s natural defenses. It is also possible for the virus-laden saliva to enter through cuts or cracks in the skin or mucous membranes (eyes, mouth etc.).
                                                                            
    The rabies virus is not particularly well suited to life outside of a host. It does not survive for more than a few hours in the environment and cannot tolerate temperatures greater than 50 C. Its viral envelope means that most disinfectants can kill the virus on surfaces. It is highly unlikely that your dog could contract the virus from such activities as lying on the lawn, sniffing hydrants, or licking statuary (as illustrated by the hound pictured here at the left).


   
One of the reasons why rabies makes so many people nervous is due to its long and variable incubation period. This refers to the amount of time inbetween infection and the first detectable symptoms. Most infected dogs begin to show symptoms between 21 and 80 days, although times may be longer or shorter than these. During the incubation period, the dog will appear normal, but may be able to shed the virus and spread it to others.


   
After infection, the virus uses peripheral nerves to ascend to the spinal cord. From there, the virus can travel up to the brain. Once in the brain, the virus can travel to the salivary glands around the animal’s mouth, thus allowing for spread to other animals via infected saliva. For more details on exactly how the virus accomplishes these movements, see the next section below.

Figure 1: Blue arrows show the movement of virus throught the body

How does the virus infect a cell?
    The surface of a normal healthy cell is covered in receptors that the cell needs for everyday functions. Viruses, including rabies, have adapted themselves to use these receptors to access a cell. The virus is drawn inside and uses the cell to make more copies of the virus. Rabies is an RNA virus, which means that it cannot immediately start making copies. It must first use an enzyme called RNA polymerase to copy its genome. This new molecule can act as mRNA (a protein-building blueprint) for the virus. Now the infected cell will use this mRNA to create the proteins the virus needs to replicate. Ultimately the cell creates and packages proteins into new virus. The new viruses exit the cell and spread to other nearby cells.
What are the symptoms of rabies?
    Unlike some other diseases, rabies does not have any definitive signs or symptoms. A dog infected with rabies will show symptoms that indicate a problem with the central nervous system (CNS), but unfortunately rabies is not the only condition that can cause these. Listed below are some examples of CNS signs.

Examples:
Behaviour changes (dog is more/less friendly than usual etc)
Trouble eating or swallowing
Increased salivation
Difficulty walking
Changes in posture
Progressive paralysis



    An infected dog may show any combination of symptoms and with many varying degrees of severity. For ease of discussion rabies is often split into two forms: “furious” and “paralytic”, depending upon what type of symptoms they display.

“Furious” form
      
    This form encompasses the symptoms most commonly associated with rabies; extreme behaviour changes and unpredictable anger. Dogs with this form may bite without apparent provocation, may harm themselves, and will follow and attempt to bite both animate (ex: a moving hand) and inanimate (ex: moving sticks) objects.

“Paralytic” form

    This form typically has more paralytic symptoms than the first type. Dogs may have trouble swallowing due to paralysis of the throat and masseter facial muscles. They may also drop their lower jaw.


   
The splitting of symptoms into different forms can unfortunately be misleading. Dogs may display symptoms from both forms, one only, or may even start out in one and progress to the other. Ultimately the outcomes are all the same. Rabies is a fatal disease with progression to coma and ultimately death from widespread paralysis.

How can rabies be diagnosed?

    At this time, rabies can only be diagnosed post-mortem (after the animal has died). The brain is sent to a special lab that is equipped to handle this potentially dangerous material. The most common test run is an immunofluorescence test. Tissue samples are stained with a special material that contains antibodies for the rabies virus. If the virus is present the antibodies will grab on and stick even after washing. Any areas not containing virus will be unable to hold onto the stain during washing. A special fluorescent microscope is used to examine the tissue. Any stain, and thus any virus, present will glow. The picture at the left shows a positive test result (the green blobs represent areas where stain clings to rabies virus).

   

    Another test is to examine brain tissue for Negri bodies. Negri bodies are the special name given to rabies inclusion bodies (small dots in a cell created due to viral-induced functional changes within the cell). They show up black when exposed to a special stain.