Rabies

Overview

Rabies is a highly fatal, preventable viral disease transmitted by the saliva of infected bats and certain other infected mammals especially carnivores. The causative agent of rabies is an RNA virus of the Rhabdoviridae family. It causes inflammation of the brain or spinal cord. Rabies can be prevented by vaccinating pets, staying away from canine wild animals such as skunks, raccoons, mongooses, foxes, jackals, and wild dogs, and seeking medical care in case of potential exposure immediately. Rabies is usually fatal by the time symptoms appear, so one should seek medical help without waiting for symptoms to appear.

Rabies causes an estimated 59,000 deaths a year worldwide. In the US dogs are required to be vaccinated by law, so the dogs account for less than 1%  of rabid animals. Most cases of rabies in the US are due to wild animals and bats. In other parts of the world, 99% of rabies occurs due to dog bites.

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The virus has a marked affinity to nerves and salivary glands. After entering the human body the virus replicates in muscles around the site of a wound. It penetrates the nerve endings and travels to the spinal cord and then to the brain where it proliferates and then from central nervous system it spread to other parts of body through nerves.

Signs and Symptoms

The incubation period usually is 1-3 months but it can vary from a week to several years. This period may be as short as four days or more than six years, depending on the location and severity of the wound and the amount of virus introduced. A bite on the head, neck, or face has a shorter incubation period. The closer the bite is to the brain, the sooner the disease will appear.

Two clinical varieties are recognized.

  • Furious rabies (classical or encephalitic variety) which occurs in 80 % of human cases
  • Dumb rabies  (paralytic variety) which occurs in 20% of cases, cause paralysis as a dominant symptom.

Furious rabies

Pain or tingling develops at the site of a wound. Initial symptoms are nonspecific flu-like such as fever, headache, and feeling of being unwell, anorexia, nausea and vomiting, excessive salivation. Encephalitis develops after a few days causing restlessness, anxiety, confusion, insomnia, hallucinations, uncontrolled excitement, rigid neck muscles, and involuntary muscle twitching.

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Hyperexcitability is a characteristic feature of this form of rabies and is precipitated by auditory or visual stimuli. Hydrophobia ( fear of water) is present in 50 % of patients and is due to painful spasms of throat muscles on attempting to eat and drink and even at having thought of it. Aerophobia (fear of air) is pathognomonic of rabies. Patients develop seizures, respiratory paralysis, irregular heartbeats, and death usually occur within 3-14 days.

Paralytic from or dumb rabies

In paralytic form or Dumb rabies which occurs in 20 % of cases ascending paralysis leading to quadriplegia develops without hydrophobia and excessive excitement. This type of rabies commonly occurs after a bite from an infected bat.

Diagnosis of rabies

Diagnosis of rabies is generally made clinically. Rabies is suspected in an individual with unexplained encephalitis (inflammation of the brain) or ascending paralysis and a history of an animal bite or exposure to bats. Diagnosis of rabies is performed in the laboratory by the following tests

  • Direct fluorescent antibody testing of a skin biopsy from the nape of the neck 
  • polymerase chain reaction (PCR) test of  cerebrospinal fluid, saliva, or tissue samples
  • CT scan, MRI and EEG are normal or nonspecific

Treatment

Treatment is supportive. The patient is heavily sedated and nursed in a quiet, darkened room. The patient may need cardiovascular or respiratory support. Once symptoms appear, death usually occurs within 3-14 days. Few patients have survived after getting rabies. Many of these received immunoprophylaxis before the symptoms have appeared. Evidence shows that giving rabies vaccine and immunoglobulins after the symptoms have appeared may cause more deterioration of the disease.

Prevention of rabies

Exposure is considered when an infected animal bites, scratches, or licks abraided skin or if saliva from an infected animal comes into contact with the eyes, mouth, or nose. Wild animals such as skunks, raccoons, mongoose foxes, jackals, and most other carnivores are regarded as rabid and immunoprophylaxis is given immediately. If the animal is available, it is euthanized and tested for rabies virus. The vaccination is not given if the test for rabies virus is found negative. Bat bites are often difficult to detect, so contact with bats is an absolute indication for rabies immunoglobulins and vaccines.

Dogs and cats are held for observation for 10 days, if any sign of rabies is observed or the animal escaped then immunoprophylaxis are given immediately. Vaccination is not recommended if during 10 days of observation they remain healthy. If any sign of rabies is observed then the animal is euthanized and diagnosis is confirmed in the laboratory.

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In patients, laboratory tests may show antibodies but they appear late. The virus can also be isolated from saliva and skin biopsy, however, it is too late by the time diagnosis is confirmed, so better to start postexposure prophylaxis immediately without waiting for a confirmed diagnosis.

Pre-exposure prophylaxis

Human diploid cell vaccine (HDCV) is safe and recommended for high-risk people such as animal handlers, laboratory workers, veterinarians, and travelers to rabies endemic areas. Two or three 1ml doses of rabies vaccine are given intramuscularly one each at 0, 7, and between days 21and 28. The vaccine usually gives lifelong protection. Immunity may decrease over time, so antibody titer is checked every 6 months for continuous exposure, and every two years if the exposure are frequent. A booster dose is given, if the antibody titer is lower than a certain level.

Read more: How to recognise a rabid animal

Postexposure prophylaxis

Post-exposure prophylaxis involves washing the wound with soap and water for at least 15 minutes and applying povidone-iodine or benzalkonium chloride to kill the virus. Deep wounds are adequately debrided, flushed with moderate pressure and usually left open.

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Postexposure prophylaxis is given with rabies vaccine and and rabies immunoglobulins. Human rabies immunoglobulin (HRIG) is given 20IU/kg around the site of the bite. If injection volume is too much for the site such as bite around the nose or face then the rest of the dose is given by intramuscular injection (Deltoid muscle is preferred).

The rabies vaccine (human diploid cell vaccine- HDCV or purified chick embryo cell vaccine- PCECV) is given in a series of four 1 ml injections on the days 0, 3, 7, and day 14. The rabies vaccine is 100% effective if given early, and still has a chance of success if delivery is delayed. For immunocompromised individual a 5th dose is given on day 28. Rarely a serious systemic or neuroparalytic reaction occurs. Antibody titer is checked if there are enough antibodies to decide for further vaccination to continue or stop.

People who have previously been vaccinated against rabies do not need to receive the immunoglobulins—only the two doses of rabies vaccine are given on days 0 and 3.

July 14,2022

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