Rabies

Rabies is an acute, viral encephalomyelitis that is invariably fatal once clinical signs appear.  It primarily affects carnivores and bats, although other species may be affected.

 
A diagramatic representation of the Rabies Virus
http://www.vendian.org/howbig/UnstableURLs/virus_rabies_orig.jpg

Rabies is caused by lyssaviruses in the Rhabdovirus family.  There are many reservoir species for the rabies virus, that vary depending on the geographical location.  Skunks, raccoons, bats and foxes are know to transmit the virus to domestic dogs and cats.  Although rabies is known to be spread between domestic dogs, there is no known transmission between cats.  However, cats are the most commonly infected domestic species in North America and the most common domestic source of human infection.

The transmission of the rabies virus is almost invariably through the saliva of an infected animal, almost always via penetrating bite wounds.  Rarely, the virus has been spread from the saliva, salivary glands or brain through open wounds or intact mucous membranes.  The saliva usually becomes infectious shortly before or concurrently with the onset of clinical signs.  However, in skunks, viral shedding has been reported up to 8 days prior to the onset of clinical signs.

The incubation period of rabies is variable, but generally quite long.  In dogs, the average incubation period is 21-80 days, but it may be considerably longer. 

Following infection with the rabies virus, it typically remains in the for a considerable length of time.  The virus then travels through peripheral nerves to the spinal cord, and from the spinal cord to the brain.  After reaching the brain, the virus descends peripheral nerves to reach the salivary glands.  Once the virus is established in the salivary glands, it can be transmitted via the saliva, and will be detectable in the brain.

Aerosol spread of the virus is incredibly rare, as it requires a very high concentration of the virus to be present in a large volume of aerosolized saliva and direct attachment of the virus to the olfactory nerve endings.  Spread through the blood does not occur.

 

 
A dog displaying the aggression associated with the "furious form" of rabies.
http://www.affordablehousinginstitute.org/blogs/us/rabid_dog_small.jpg

The clinical signs associated with rabies are rarely definitive and are often consistent with a general central nervous system (CNS) disturbance.  The most reliable diagnostic signs are sudden and unexplainable behavioral changes and a rapid onset of a progressive paralysis. 

The behavioral changes include anorexia, irritability, anxiety, apprehension, hyperexcitability, unexplained aggressiveness and the animals may seek out solitude.  In wild species, affected animals may lose their fear of humans, and nocturnal animals may be observed to be out in the daytime.  In addition, the animals may experience a loss of coordination.

The clinical disease can be divided into three phases:

1. Prodromal Period
2. Excitative Period
3. Paralytic/Endstage

The prodromal phase is characterized by vague CNS signs and typically lasts 1-3 days.  The signs rapidly intensify in the Excitative period and behavioral changes will become apparent.  Once clinical signs are seen, the disease is almost invariably fatal.  With progression into the onset of paralysis, the animal enters endstage.

These phases do not occur in all affected animals.  Some animals die acutely with minimal clinical signs.

In some infected animals, aggression becomes incredibly pronounced.  This clinical manifestation of the disease is commonly know as the “Furious Form” of rabies.  There is rarely evidence of early paralysis in these animals, however, the animal becomes very irritable and the slightest provocation may result in vicious attacks.  The animal will appear alert and anxious and will often have dilated pupils.  These animals will lose fear of other animals and humans.  Animals affected by the “Furious Form” of rabies will often chew on bars, fences and attempt to bite moving objects.  They often swallow foreign objects, and are easily provoked to viciously attack other animals or people.  The disease will progress to incoordination and seizures and finally the animal will succumb to paralysis and death.

Another common manifestation of rabies is the “Paralytic Form”.  In this form of the disease, the throat and masticatory muscles become paralyzed resulting in an inability to swallow.  The bottom jaw is often dropped and accompanied by profuse salivation.  These animals are rarely vicious and may not attempt to bite.  The paralysis will rapidly spread throughout the body and the animal will often enter a comatose state and die within a few hours.

Although the infection rates of rabies are quite low in domestic animals in North America, this is attributable to the high incidence of effective vaccination of our pets.  Vaccination is recommended every three years, after an initial series of two vaccines, one year apart (Vaccination Program for Cats or Vaccination Program for Dogs).

Rabies can be spread from infected wild or domestic animals to human beings.  However, pre-exposure vaccination is only recommended in humans with high risk groups such as veterinary staff, animal control officers and people living in regions where canine rabies is endemic or epizootic.