Rabies Virus

What is the rabies virus? The rabies virus (RABV) is a single-stranded RNA virus that serves as the pathogen responsible for causing rabies. Rabies is a zoonotic infectious disease that affects both humans and animals. Commonly known as "hydrophobia," this deadly disease currently has no effective treatment or antiviral drugs available. Once clinical symptoms appear after infection, the fatality rate is 100%. It remains the infectious disease with the highest mortality rate worldwide. In China, rabies is classified as a Class B notifiable infectious disease under the country’s Law on Prevention and Control of Infectious Diseases, and according to the "National List of Animal Pathogenic Microorganisms," it belongs to Group II viruses. However, timely vaccination—either before or after exposure—can significantly reduce the incidence of rabies, effectively achieving preventive measures. According to national standards, for canine populations, the basic-level vaccination program is considered successful only if the antibody positivity rate among immunized dogs reaches at least 70%.

Rabies virus belongs to the order Mononegavirales, family Rhabdoviridae, and genus Lyssavirus. Rabies virus particles are bullet-shaped, with one end rounded and the other flattened and concave, measuring 100–300 nm in length and about 75 nm in diameter. The virus features a non-segmented, single-stranded negative-sense RNA genome and contains five structural proteins: nucleoprotein N, phosphoprotein P, matrix protein M, glycoprotein G, and RNA-dependent RNA polymerase L. The viral particle consists of two main components: an envelope and a nucleocapsid. The genomic RNA, along with the tightly packed outer layers of N, P, and L proteins, forms the functional nucleocapsid, which is responsible for transcription and translation. On the surface of the lipid envelope, G protein spikes—arranged as trimers—are embedded, serving as the virus's neutralizing antigen and the site of interaction with host cell receptors. Meanwhile, protein M resides between the inner nucleocapsid and the outer envelope, acting as a crucial linker that connects these two distinct structural elements.

Survival Time and Resistance

● Rabies virus is not resistant to high temperatures—virus in suspension loses its infectivity after being exposed to 56°C for 30–60 minutes, or to 100°C for just 2 minutes. Meanwhile, rabies virus within brain tissue can remain viable for 7–10 days under normal temperature and autolytic conditions, and it can be preserved for 2–3 weeks at 4°C. Rabies virus is relatively stable at pH 7.2–8.0, but becomes easily inactivated above pH 8.0. The virus exhibits weak resistance to heat, ultraviolet light, sunlight, and dry conditions; its ability to survive outside the body is extremely limited, rapidly losing activity shortly after being removed from its host environment—typically within just a few minutes. As a result, rabies is generally not transmitted via indirect routes. Notably, the rabies virus is highly sensitive to lipid-soluble agents such as soapy water, chloroform, and acetone, as well as to ethanol, hydrogen peroxide, potassium permanganate, iodine-based compounds, and quaternary ammonium compounds (e.g., benzalkonium bromide). A 1:500 dilution of quaternary ammonium disinfectants, along with 45%–70% ethanol, 1% soapy water, and 5%–7% iodine solutions, can all effectively inactivate the virus within one minute. However, it is notably resistant to Lysol solutions.

● Currently, there are seven genotypes of rabies virus, with the classic rabies virus belonging to genotype 1 (RABV). Wild strains of this virus naturally infect a wide variety of animals and humans worldwide. The remaining six genotypes are rabies-related viruses: Lagos bat virus (genotype 2), Mokola virus (genotype 3), Duvenhage virus (genotype 4), European bat lyssaviruses 1 and 2 (genotypes 5 and 6), and the recently identified Australian bat genotype 7. Among these, genotypes 2, 3, and 4 are predominantly found in the wild. Genotypes 2 through 6 have been reported exclusively in Africa and Europe, with genotypes 5 and 6 being particularly common in European bat populations.

Rabies is widely distributed across the globe, with human cases reported on every continent except Antarctica. Currently, 99% of human rabies cases occur in developing countries, primarily concentrated in Asia, Africa, and Latin America, as well as the Caribbean region. Each year, rabies claims approximately 60,000 lives in more than 150 countries and territories—many of whom are children. Over 95% of these fatalities happen in Asia and Africa, with China and India being the most affected nations. Notably, Asia leads the world in the number of rabies cases, surpassing even India.

Animal-origin infection

● Rabies has natural reservoir hosts in the wild, including carnivores and bats. Animals such as foxes, wolves, jackals, ferret-badgers, raccoon dogs, skunks, raccoons, mongooses, and bats all serve as natural reservoirs of the rabies virus, capable of becoming infectious sources and subsequently transmitting the virus to domestic animals like pigs, cattle, sheep, and horses. Among these host species, bats stand out as particularly unique: because bat exposures often involve tiny, barely noticeable bites or injuries, the risk of exposure is significantly heightened. Both the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) classify bat exposures as severe, mandating that they be treated as Category III exposures. Susceptible animal groups to rabies primarily include canids, felids, and bats.

● Poultry, fish, insects, lizards, turtles, and snakes do not contract or transmit the rabies virus. Extensive testing of both wild and domesticated rodents has revealed that these animals are rarely infected with rabies, and sporadic cases of rabies virus spillover into them remain isolated incidents. This clearly indicates that rodents are not reservoir hosts for rabies and do not play a role in the disease’s transmission or outbreak. The U.S. CDC also notes that rodents—particularly small species such as chipmunks, squirrels, mice, rats, guinea pigs, gerbils, and hamsters—and lagomorphs (including rabbits and hares) are extremely unlikely to contract rabies, and no evidence has been found linking these animals to human rabies cases.

Modes of transmission

● The transmission of rabies viruses occurs through contact between infectious saliva and broken skin or mucous membranes—typically via bites, but also through scratches or licks. Transmission can also happen by consuming carcasses or infected animals; however, aerosol transmission is limited to rare cases involving viruses at very high concentrations.

Neurotropism is a key characteristic of natural rabies virus infection, as viral replication occurs almost exclusively within neurons. Initially, when the virus enters through a wound, it does not enter the bloodstream—typically, rabies virus is undetectable in blood—but instead replicates in the muscle tissue at the site of the bite. From there, it invades the peripheral nervous system via the motor neuron’s neuromuscular junction and axon, utilizing retrograde axonal transport to reach the central nervous system. Once inside the CNS, the virus spreads along neuronal pathways, ultimately disseminating widely throughout the body.

Rabies in dogs and humans typically manifests in two clinical forms: the furious form and the paralytic form. Symptoms in both humans and animals are somewhat similar. In dogs, classical rabies progresses through four stages: the incubation period, prodromal phase, excitation phase, and paralysis phase.

Prodromal phase

After a period of incubation, clinical symptoms typically begin with changes in behavioral patterns. This stage lasts 2 to 3 days in dogs, during which infected animals may exhibit markedly different behaviors. For instance, an aggressive dog might become friendly or timid, while a previously friendly or shy dog could turn aggressive. Other symptoms include a slight increase in body temperature, dilated pupils, the appearance of the nictitating membrane covering the eyes, and excessive salivation. In some cases, affected animals may progress directly from the prodromal phase to the paralysis stage. In humans, nonspecific symptoms such as malaise, loss of appetite, irritability, low-grade fever, headache, nausea, vomiting, cough, sore throat, and diarrhea may occur. Occasionally, patients might also experience abnormal sensations, sharp pain, or itching at the site where the bite healed. These symptoms usually persist for about 2 to 10 days.

Excitement Phase

During this stage, symptoms become more pronounced: the dog grows extremely aggressive, displaying unstable, erratic, and hyperactive behavior with heightened aggression. It may bite anything in its path. Additionally, laryngeal muscle paralysis leads to altered vocalization, while pharyngeal muscle spasms and paralysis result in speech difficulties, often causing excessive drooling. Even in the absence of paralysis caused by the disease, seizures can still be life-threatening. Meanwhile, humans may experience extreme agitation, autonomic dysfunction (such as excessive salivation, piloerection, arrhythmias, and terrifying spasms like hydrophobia—occurring in approximately 50–80% of patients)—as well as hallucinations and delusions, which may arise due to dysfunction in the limbic system and brainstem.

Paralysis Stage

When the excitatory phase is extremely short or absent, a paralytic phase ensues, characterized by progressive muscle incoordination, paralysis, coma, and ultimately, death. The most distinctive clinical sign in dogs is "mandibular drooping," caused by paralysis of the masseter muscles. Commonly heard is a choking sound—similar to a bone getting stuck in the throat—as the animal struggles to dislodge this "obstacle." In advanced stages, the animal may lose the ability to swallow and begin salivating profusely. As the condition progresses further, signs of paralysis first appear in the limbs, followed by the neck and head. Death typically results from cardiorespiratory failure. In humans, the flaccid or paralytic form arises due to spinal cord involvement, manifesting in four distinct types: flaccid paralysis, quadriplegia, transverse myelitis, and an ascending variant resembling Guillain-Barré syndrome. At either end of these two phases, coma may set in, often leading to death within a matter of days.

Currently, the main diagnostic techniques for rabies include virus isolation and identification, fluorescent antibody testing, enzyme-linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), and colloidal gold methods.

Virus isolation, identification, and diagnosis are relatively accurate, but they are time-consuming, involve complex procedures, and are not conducive to rapid detection.

Direct fluorescent antibody technology is the gold standard for diagnosing rabies in both animals and humans. This method boasts high sensitivity and specificity, but it is highly dependent on sample quality and the skill level of the operator, making it unsuitable for decayed or formalin-preserved samples.

The ELISA method is simple to perform, making it suitable for large-scale serological surveys. However, its detection sensitivity is limited, requiring a sufficient number of samples. This method is the most commonly used approach for monitoring vaccine antibody levels in dogs and is also well-suited for urban rabies epidemiological investigations.

The PCR method can directly detect the virus's genetic material, offering high sensitivity—but it comes with higher costs, a greater risk of contamination, and demands advanced technical skills from laboratory personnel. As such, it’s best suited for laboratory diagnostics. However, if an animal exhibits clear symptoms or has already died and is suspected of having rabies, this method can still be used for testing.

Currently, conventional colloidal gold methods have seen the introduction of related products both domestically and internationally. However, these methods suffer from low detection sensitivity, significant variations in product quality, and challenges in quality control. They can only provide qualitative results rather than quantitative measurements, making them suitable primarily as an auxiliary tool for disease diagnosis. While ideal for use in remote areas where timely sample delivery to laboratories is difficult, this method sees limited application in routine laboratory testing, often reserved instead for internal monitoring purposes.

Rabies is preventable and controllable, but not curable. 1. Sick dogs and other infected animals should be immediately euthanized, and their carcasses buried deeply or incinerated for disposal. 2. Regularly strengthen the management of dogs, cats, and other animals; ensure that domestic dogs receive routine vaccinations. 3. Individuals who have frequent exposure to the rabies virus, infected animals, or patients should undergo pre-exposure vaccination for preventive protection. 4. If bitten by an animal, promptly clean and treat the wound locally, and seek rabies vaccination as soon as possible. When necessary, administer passive immunization against rabies (such as rabies immune globulin or antirabies serum) without delay.

References

1. Document from the China Centers for Disease Control and Prevention, CDC-Transmission & Prevention Notice [2016] No. 10

2. Zandi M, Zandi S, Mohammadi R, et al. Biosensor as an alternative diagnostic method for rabies virus detection: A literature review. Biotechnology and Applied Biochemistry. 2021;1–6.

3. Christine R. Fisher¹, Daniel G. Streicker²³, and Matthias J. Schnell: The Spread and Evolution of Rabies Virus – Conquering New Frontiers

4. Biosensor as an alternative diagnostic method for rabies virus detection

5. Anil Kumar a, Sonam Bhatt, et al. Canine Rabies: Epidemiological Significance, Pathogenesis, Diagnosis, Prevention, and Public Health Issues