Rabies

Rabies: Overview

Descriptions of rabies go back thousands of years as rabies has classically been one of the most feared infections of all time.  It is caused by a rhabdovirus which is relatively unstable in the environment, requiring fresh contact with mucous membranes to establish infection.

Causes and Development; Contributing Risk Factors

In most cases, disease is transmitted via bite wound.  Virus present in the infected animal's saliva enters the victim's tissues during the bite.  The virus attaches to the local muscle cells for a couple of days before penetrating to local nerves and beginning its slow ascent to the brain.  Once within nervous tissue, the virus is not accessible to the immune system and may safely proceed, though the journey is slow taking up to one year (average time between bite and detectable virus in the brain is 20-30 days).

The virus ultimately reaches the brain and in two to three days more is evident in all body secretions including the saliva.  At this point, the disease becomes transmissible and symptoms begin.

The stages of Rabies infection:
  • Prodromal Stage (First 36 hours after symptoms have started) – A change in personality is noted.  Friendly animals become shy etc.  The larynx begins to spasm and a voice change may be noted.
  • Excitative Stage (Next 2-3 days) – Classically, this would be the "mad dog" stage.  The human or animal has no fear and suffers from hallucinations.  The larynx is paralyzed resulting in an inability to swallow thus drooling and "foaming at the mouth" result.
  • Paralytic / Dumb Stage (Next 2 days) – Weakness/paralysis sets in.  The victim dies when the intercostal muscles (which control breathing) are paralyzed.  It is from animals in this stage where most human exposure occurs.  There is no treatment for animals or humans once clinical signs appear.
Once the virus has been released to body secretions, it is again accessible to the immune system; however, the patient dies before an adequate immune response is mounted.

The virus is carried mainly by wildlife species.  Humans and domestic animals are not generally exposed unless conditions promoting contact with wildlife occur.  In California, the chief reservoir species for rabies is the skunk.  In other areas raccoons, bats, and foxes are also important.  Rodents and birds are considered resistant.  Transmission is usually achieved via bite wound; however, humans have been infected by inhaling aerosolized bat urine while visiting bat-infested caves.  Recently, a human was infected via corneal transplant from a deceased wildlife ranger.  Not every bite wound from a rabid animal is infectious.  Whether or not infections sets in depends on the number of viral particles entering the wound (how much saliva contacted the victim, whether the bite was through clothing etc.)

Treatment and Prevention

A fresh bite wound should be washed out with water quickly as this may wash out viral particles.  The time it takes for the virus to reach the brain depends on the amount of virus present in addition to the proximity of the wound to the head.

If possible, the head of the biting animal is submitted to the health department for fluorescent antibody testing for the rabies virus.  This process takes a matter of hours so that any bite victims can know right away if they will require rabies treatment.  If the biting animal is a pet, its vaccination status should be confirmed as soon as possible.

Hyperimmune (antibody rich) serum is flushed into the wound in hope of inactivating the virus before it may penetrate to the nerves.  The patient receives an injection daily for 14 days of rabies vaccine.  In this way, when the virus comes out in secretions, a strong immune response is waiting to put down the infection.

Anyone working with animals should consider being vaccinated against rabies.

Rabies