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Clinical Signs of Rabies

What to look for: Clinical signs of rabies can vary significantly, so the following description should be regarded as very general.  The only positive way to diagnose rabies is with laboratory tests.  The disease is commonly divided into two main phases/presentations – Dumb Rabies and Furious Rabies.   The symptoms of the disease are caused mainly by inflammation of the brain.  Swelling and inflammation puts pressure on various areas of the brain, resulting in diverse clinical signs.  The most common sign of rabies is acute behavioral changes and unexplained progressive paralysis.  Nocturnal animals may be abnormally active during the day.  Some animals may die suddenly with no apparent clinical signs (15).

Figure 1- Caricature of  an infected man, or a man without his dentures ...? http://www.freddypharkas.com/tmdbohah/images/rabies.png

Furious Rabies: Animals often present with fever, irritability, anxiety, restlessness, salivation, muscle pain, difficulty swallowing, and dilated pupils.  They may have convulsions and/or attempt to bite, kick, or otherwise attack anything, including rocks, grass, and other inanimate objects.  The attacks are often vicious and completely without provocation, and they may be directed towards other animals, people, or even themselves.  This is part of an excitement stage lasting a few days, and is followed by progressive paralysis, coma, and death (14).  Animals affected by this form of rabies often roam large distances and lose caution of people or other animals.  They may also attempt to attack to the point of self-injury.  Puppies or kittens may appear excessively playful but bite when petted.  Horses may show severe agitation combined with signs easily confused as colic, which progress to severe aggression (15).

Dumb Rabies:  This form of rabies may have more subtle signs and is characterized by an apparently calm and quiet nature.  It often begins with paralysis of the masseter (a major jaw muscle) and throat muscles.   This results in profuse salivation and difficulty swallowing, possibly with dropping of the jaw.  This is followed by a quiet progression towards paralysis and death, without the aggression seen in furious rabies (15).  Affected wildlife may appear excessively tame and quiet or sick; therefore caution should ALWAYS be used when approaching sick or quiet wild animals.  If you are concerned for an animal’s welfare please contact the appropriate authorities for your area and avoid approaching the animal yourself.

Figure 2 - Furious Rabies in a dog                                               Figure 3 - Rabies case in a human

my.opera.com/jonimueller/homes/blog/rabid-dog.jpg

http://www.freddypharkas.com/tmdbohah/images/rabies.png

Usually, there will be alternating episodes of paralytic and excitable rabies or the animal may not exhibit any clinical signs at all. The animal succumbs to death when the paralysis spreads to other parts of the body. 

Rabies in humans manifests itself differently than in animals. At first, symptoms are mild. The infection usually starts with a fever, cough and sore throat. As the virus spreads via the nervous tissue, symptoms progress to more severe symptoms like hallucinations, encephalitis or seizures. Excitable rabies progresses more rapidly and is characterized by hydrophobia or aerophobia, hyperactivity.  There may also be a lack of neurological signs and bizarre behavior, muscle spasms, convulsions and cardiorespiratory arrest. It is almost always fatal, though with aggressive treatment before the virus reaches the central nervous system, it may be prevented. (4)(5). Paralytic rabies in humans is less notable but the final outcome is the same. With paralytic rabies there is flaccid paralysis with pain moving up the affected muscle.  Death is due to respiratory paralysis. 

Unfortunately, there exists no definitive test to diagnose rabies in humans before clinical signs appear.  If rabies is suspected, an attempt may be made to isolate rabies virus specific antibodies using a skin biopsy from the neck or a sample of cerebral spinal fluid, however these tests tend to be insensitive.  The virus lives intracellularly and is therefore protected from the body’s immunological response and difficult to detect since it doesn’t initiate an antibody formation until late into the infection.  (4)

Note: Be aware that it often takes a long time (anywhere from two weeks to up to six months) for an exposed animal to start showing clinical signs.  The length of time it takes to develop clinical signs can vary significantly with each animal and the area of entry of the virus.  For example, a bite wound to the head means that the virus has a shorter distance to travel to get to the brain than with a bite wound to the rear leg.  To cause clinical signs, rabies must first enter nerves and travel their length up to the brain.  It then travels on to the salivary glands, where it is available for spread to other animals.  This does NOT mean that animals not showing obvious clinical signs are not infective.  An animal may not be perceptibly ill, but can still potentially pass rabies on.

Possible Differential Diagnoses:  similar clinical signs may be seen with distemper, hepatitis, listeriosis, tetanus, botulism, some parasitic diseases, plant or chemical toxins (13) as well as almost any other central nervous system diseases (Porcine Enteroviral Encephalomyelitis, Bovine Spongiform Encephalopathy, Chronic Wasting Disease, Scrapie and Equine Encephalomyelitis are just a few examples).  Note that this is not an exhaustive list of differentials.  Whenever neurological symptoms are seen in an animal it is common to assume rabies as a possibility, and to take precautionary measures until proven otherwise.  This is especially true if the history of the animal is vague.