Rabies

Syn: Dropped Jaw, Hydrophobia, Lyssa

• It is an acute, infectious viral disease of all warm-blooded animals and characterized by abnormal behavior, nervous disturbances, impairment of consciousness, ascending paralysis and death.
• Disease is invariably fatal.
• Present name rabies is derived from Latin name, meaning raging, furious, savage or madness.
• Hydrophobia is used specifically for rabies in man.

Etiology:

• Rhabdovirus or Lyssa virus of Rhabdoviridae family
• It is RNA virus, bullet shaped and measures about 180-250 nm in length by 75 nm in diameter.
• Nucleocapsid core is formed by RNA and 3 other proteins, i.e. nucleoprotein, glycoprotein and non-structural protein
• It is of two types; Street virus and Fixed virus
• Street virus: It produces natural disease in dogs and other animals. It produces Negri bodies and has affinity for salivary gland. It is not used in vaccine production.
• Fixed virus: When street virus is serially passages by intra-cerebral route, the virus becomes fixed. It donot produce negribodies. Virus is absent in salivary glands and saliva. It is used in vaccine preparation.

Epidemiology:

• Disease is distributed globally with major impact in Asia and Africa.
• In Nepal, rabies is reported to kill around 500 animals and up to 32 human beings in recent years.
• As a prophylactic measure, around 30,000 livestock and 300,000 humans get vaccinated each year in Nepal.
• Dogs are responsible for the spread of rabies, which is true in 95% of the cases in regions like Asia and Africa.
• Dogs alone are responsible for 99% of disease transmission in rabies endemic regions.
• the transmission and maintenance of rabies occur in two interrelated cycles: the urban cycle, which involves domesticated and stray dogs; and a sylvatic cycle, that involves wildlife like foxes, wolves, jackals, mongoose, raccoons, skunks and bats.
• Around 59,000 human deaths are estimated every year across the world due to rabies, out of which 45% prevails in South Asian Association for Regional Cooperation (SAARC) countries.
• Rabies is a major burden in Asia, mostly in the rural and underprivileged regions, with an estimated over 35,000 human deaths per year.
• In India, more than 15,000 people die each year due to rabies infection.
• In the last five years, around 20,000 to 40,000 animal bites were reported in Nepal.

Transmission:

• Under natural condition, it is commonly occurring by bite of rabid animals; usually carnivorous animals.
• 90% of human cases occur due to bite of rabid dog.
• By inhalation or aerosol route
• Ingestion of infected material like raw meat, raw milk
• Scratch wound contamination by virus through infected animals.
• Through mucus membrane of mouth, conjunctiva, anus and genitalia.
• Man, to man transmission is possible; if corneal transplantation occurs.

Pathogenesis:

• Following bite, virus is deposited in depth of wound by infected saliva.
• Local replication of virus in epithelial cells.
• Virus invades CNS via nerve pathways. Virus moves centripetally to CNS via nerve pathway.
• After reaching brain, virus invade ganglion cells and spread centrifugally to peripheral nerves.
• Virus then reaches salivary gland, thymus, lymph node, taste bud, olfactory cell, oral and nasal secretion.
• Invading brain, they damage brain stem and medulla. Irritation of nerve cells cause increased excitability. Infected nerve cells undergo degeneration and there is paralysis of various muscles.
• Paralysis of jaw muscles prevent closing of mouth and dropped jaw. Paralysis of respiratory muscles causes asphyxia and death.

Clinical Signs:

• Incubation period varies from 7 days to several years.
• Clinical signs in animals is observed in 2 forms:
  • Furious form
  • Dumb or Paralytic form

Furious form:

It is divided into three stages:

a. Melancholic stage:

• Changed behavior of animals
• Dog goes to dark place and donot obey owner.
• Biting of inanimate object, excitability towards stranger
• Very sensitive to touch
• Appetite normal initially and later unable to take food.
• Excitability is much longer than dog in case of cattle.
• It lasts for 1-3 days

b. Maniacal/Irritative/Excitement stage:

• Excitement will be increased, voice will be changed, i.e. coarse voice due to paralysis of vocal cord
• Hydrophobia (fear of water)
• Dog become furious and tries to chew inanimate object
• Drooling of saliva
• Animal walks aimlessly and bites other animals and human
• Eyes becomes congested, pupil dilated and anxious look

c. Paralytic stage:

• Animal become less vicious, complete loss of voice due to complete paralysis of vocal cord
• Inability to swallow due to paralysis of tongue and masseter muscle.
• Dropped jaw
• Emaciation
• Tongue dry with copper like discoloration
• Paralysis of hind quarter then ascending paralysis

Dumb form:

• Excitability and other symptoms is absent
• Paralysis of lower jaw, tongue, pharynx, and hind quarter
• Hanging of jaw and unable to bite
• Unable to close mouth
• Coma and death
• Clinical course of disease is 1-7 days

Clinical Findings in Cattle:

• Affected cattle and buffalo appears incoordinated.
• Shows loss of appetite and sudden fall of milk yield
• Trembling or twitching of ears
• Paralysis of muscles of deglutition with excessive salivation and grinding of teeth
• Tend to slobber and have difficulty in drinking water
• Animal often shows signs simulating choke
• Cattle will bellow incessantly in a characteristic low-pitched voice due to vocal cord paralysis
• Increased sexual excitement may be noted.

Diagnosis:

• On the basis of clinical findings
• Histopathology: Presence of Negri bodies in neurons of hippocampus gyri by setler’s stain.
• Animal inoculation: 5% suspension of brain, salivary gland, saliva with volume of 0.03 ml inoculated intracerebrally into 2-3 weeks old mouse. Symptoms appear in 5^th-7^th
• Serological test: CFT and FAT

Treatment and Control:

• No specific treatment. Once the clinical signs appear, disease is always nearly fatal (100%).
• Notification of suspected case and euthanasia of suspected dog with clinical signs and dogs bitten by suspected rabid animals.
• In case of animal bite; hold animals at least 10 days for observation. If signs develop, they should be killed and brain sample should be sent to lab for examination.
• Vaccination:
  • Preexposure: veterinary β-propiolactin antirabies vaccine (20% suspension of sheep brain infected with fixed virus); Dose; cat @ 3ml, SC, at 3 months and then yearly. Dog @5 ml, SC at 3 months and then yearly
  • Novivac (cell culture vaccine) @ 1ml, SC, at months and then yearly
  • Multidose 5% suspension of sheep brain
  • Rabipur vaccine @1ml, SC or IP at 0,3,7,14,21 or 28 and 90 days (for vets and animal holders).