Introduction

Rabies is a neurological killer that has evolved a fool-proof technique of transmission, and it cleverly evades the species barrier to present a potent threat to mammalian life. While the simplicity of the virus insures its transmission, it also contributes to its weakness: its monoclonal antigenicity means that a single vaccination covers all strains of the disease. Though rabies is considered endemic in parts of the developed and undeveloped world, vaccination schemes have rendered the disease controllable to a satisfactory degree. Nonetheless, infection is still largely fatal and the disease should not be taken lightly.

Morphology

Large, enveloped, negative-sense RNA virus
Bullet-shaped with short glycoprotein spikes

Types and Subtypes

Two Genera:

Lyssaviruses: 7 genotypes
1. Genotype 1 is classical rabies
2. Genotypes 2-7 more limited in distribution
3. Genotype 4 infects insectivorous bats
Vesiculoviruses are all exotic to the UK:
1. Vesicular Stomatitis Virus
2. Ephemeral Fever
3. Fish Rhabdoviruses, such as viral hemorrhagic syndrome and infectious haematopoetic necrosis virus

Virulence and Pathogenesis

Rabies is moderately resistant: it can survive well in dark places at low temperatures for several days
Infection occurs through a penetrating bite in which virus is transmitted via saliva
Incubation period is variable but can be long (10 days to 12 months), and increases with distance of the bite to the CNS
- 80% show signs within 4 months
Primary replication of the virus occurs in the muscle
The virus gains access to nervous tissue and travels toward the brain, where it continues to replicate
It then migrates down cranial nerves to infect salivary glands and the cornea
Virus is shed in saliva and tears and can be shed BEFORE the onset of classical signs
The onset of clinical signs in dogs and cats includes:
- Behavioral changes
- Pyrexia
- Salivation
- Dilation of pupils
Two forms of the virus exist:
- Furious form:
  - Common to cats and dogs
  - Abnormal aggression, salivation, and attack without provocation
  - Disordered wandering
  - Incoordination, convulsions, coma and death within 3-10 days
- Dumb form
  - May or may not follow Furious form
  - Common to ruminants and horses
  - Ruminal tympany, tenesmus, diarrhoea
  - Paralysis of lower jaw, drooling saliva, tremors and progressive paralysis
  - In dogs, a change in voice may be observed (bellowing)
Human infection is characterized by hydrophobia

Epidemiology

Rabies is a NOTIFIABLE zoonosis
Rabies is found worldwide, though currently considered exotic to the UK and Australia

Host Range:

All mammals are susceptible
- Very susceptible: foxes, jackals, wolves
- Susceptible (10,000x virus necessary to infect): cats, rodents, bats, skunks, cattle, mongooses
- Moderately susceptible (100,000x virus necessary to infect): dogs, sheep, goats, horses, primates
Isolates can show to some species specificity in their reservoir host

Reservoir species by region:

Region	Reservoir Species
Europe	Red fox
Russia, Turkey, Middle East	Dog, Raccoon
USA	Raccoon, Fox, Skunk
Africa	Dog, Jackal, Mongoose
Asia	Dog, Mongoose
South America	Dog, Vampire bat
Worldwide	Insectivorous Bats

The current position of DEFRA holds bat lyssaviruses to be present in the UK
Human infection and death an occur after infection with bat rabies
- Nocturnal bat bites account for 50% of human cases in the USA
Humans develop disease but excrete little virus
Immune recovered animals are rare, but are not shown to be infections

Diagnosis

Rabies Control

Antigenicity allows a single antigenic type to vaccinate

Rhabdoviridae

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