Rabies virus
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| Rabies Virus |
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| Scientific classification |
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Description and significance
The Rabies virus is a member of the order, Mononegavirales, which encompasses viruses containing non-segmented, negative-stranded RNA genomes. The term originates from the Latin, “rabies,” which means “madness.” This may have been derived from the Sanskrit word, rabhas, “to do violence.” The virus has a distinct “bullet” shape, placing it in the Rhabdoviridae family, which includes at least three genera of viruses that infect animals, such as Lyssavirus, Ephemerovirus, and Vesiculovirus. The rabies virus belongs to the Lyssavirus genus (Greek word, lyssa, is derived from “lud” or “violent.”) which also includes the Aravan virus, Australian bat Lyssavirus, Duvenhage virus, European bat virus 1 and 2, Lagos bat virus, and Mokola virus. Lyssaviruses have helical symmetry and are about 180 nm in length, with a cross-sectional diameter of 75 nm. The most prevalent strain of rabies is Rabies Virus (RABV; genotype 1), which is the type that infects both bats in the Americas and terrestrial animals throughout the world. Rabies is a zoonotic disease, meaning it is transmitted from animals to humans. The virus infects domestic and wild animals, and is transmitted to humans through close contact with infected saliva, usually through bites or scratches. This viral, neuroinvasive disease causes acute encephalitis (brain inflammation) in warm-blooded animals. The virus also enters the salivary glands in high concentrations, leading to further transmission. Once the signs and symptoms of the virus arise, there is no treatment and the disease is usually fatal. The disease is prevalent in nearly every continent of the world, with over 55,000 human deaths a year, 95% of which occur in Africa and Asia.
Genome structure
Rabies is an RNA virus. The genome encodes 5 proteins: nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G), and polymerase (L). The genome is non-segmented, linear antisense RNA. RNA-dependent RNA transcriptase and some structural proteins are present in the nucleocapsid.
Virus structure
The virus has a bullet-like shape; one end is rounded while the other is planar or concave. The virus’s lipoprotein envelope contains about 400 spikes on its surface, which are composed of glycoprotein (G). Within the envelope lies the membrane or matrix (M) protein layer, which may be invaginated at the planar end. The Glycoprotein spikes do not cover the planar end. The virion’s core consists of helically arranged ribonucleoprotein, which consists of protein and RNA.
Ecology (Hosts, Transmission, and Life Cycle)
Rabies is a zoonotic disease and is transmitted by mammals, usually as a result of an infected animal bite. A neuroinvasive disease, it causes severe encephalitis in warm-blooded animals. Multiple variants of the virus have been identified worldwide, each of which are linked to a single wild animal host, termed a “reservoir of infection” for a specific geographic region. For instance, in British Columbia, Canada, insectivorous bats (adapted to feeding on insects) are the only reservoir of rabies. However, other potential hosts exist, including terrestrial mammals, such as domestic dogs and cats, raccoons, skunks, foxes, wolves, and coyotes, though the virus is not maintained in these populations. Contrary to popular belief, rodents, such as mice and squirrels, are seldom infected. Unlike many rabid mammals, bats may get sick and die before showing symptoms common to other mammals. There are three known stages of rabies in animals. The first stage, the prodromal stage, lasts one to three days and marks the beginning of behavioral changes, such as loss of fear or unusual friendliness. The second, excitative stage lasts three to four days and is characterized as “furious rabies,” with symptoms including exceptional aggression and unprovoked attacks. The third, paralytic stage is caused by motor neuron damage. Here, prevalent symptoms include difficulty with coordination, paralysis, seizures, signs of choking or difficulty swallowing food or water, and frothing at the mouth. Other abnormal behaviors include depression, odd vocalizations, appearance of nocturnal animals during the day, and, in carnivores, evidence of having attacked porcupines. Death usually results from respiratory arrest. Also in the U.S., the CDC reports that the majority of rabies cases occur in bats, raccoons, skunks, and foxes. In fact, currently, over 90% of annual cases reported to the CDC, occur in wildlife, whereas, before 1960, the majority of cases were in domestic animals. Today, domestic animals account for less than 10% of reported rabies cases. Of these animals, dogs, cats, and cattle are reported most often to be rabid. In addition to bites by infected animals, rabies can be transmitted by infected animal licks or saliva that come into contact with the mucous membranes of the eyes, nose, lips, or skin wounds. The virus is inactivated by external factors, such as exposure to direct sunlight, ultraviolet radiation, chemicals, detergents, and strong acids and bases, and thus, cannot thrive in the environment. From the wound of entry, the rabies virus quickly travels along neural pathways first to the peripheral nervous system and then, via nerves, to the central nervous system (brain and spinal chord). At this point, the virus is not immediately evident within the host and vaccination may lead to cell-mediated immunity, which will prevent the symptoms of rabies in the host. Once the virus reaches the brain, it replicates and then spreads to other organs of the body via the nervous system. At this prodromal stage, symptoms appear and treatment is ineffective. The salivary glands in the mouth and cheeks receive a high concentration of the virus, thereby facilitating further viral transmission when the diseased animal bites another animal.
Pathology
In humans, early symptoms of rabies are nonspecific and include fever, headache, and overall lethargy. As the disease progresses, neurological symptoms emerge, including anxiety, insomnia, confusion, violent movements, partial paralysis, agitation, hallucinations, hyper-salivation, difficulty swallowing, and hydrophobia (fear of water), which is also another name for the virus. If left untreated, rabies is always fatal in humans and death can occur within days of symptom onset. In the final stages, symptoms include periods of mania and lethargy in the patient, and eventually, coma. Death may result from respiratory insufficiency, brain swelling, or pneumonia. This usually occurs two to ten days after the first symptoms, and the few humans known to have survived the disease all suffered from severe brain damage. The period between infection and the first flu-like symptoms normally occur between two and twelve weeks, but may even last up to two years! Transmission between humans is very rare though there are documented cases associated with transplant surgery. Even more rarely, transmission may occur through bites, kisses, and sexual relations. Rabies is responsible for about 55,000 human deaths a year, mostly in Africa and Asia. Only six people are reported to have survived symptomatic rabies, while two survivors received no rabies-specific treatment either before or after the onset of illness.