Dog infected with rabies
Table of Contents
Having a dog as a pet presents a myriad of benefits. One of them is having a companion reputed for being charismatic and loyal. Dogs, apparently, render a “cure” when melancholy “strikes“. However, there are repercussions to avoid or deal with when handling a dog. One of the most important concerns when domesticating a dog is preventing dog bites. Getting bitten by a dog is, in fact, how microbes could find their way through the skin. Dogs, inopportunely, can be agents of medically-important diseases like rabies.
Rabies is a viral disease that is almost always deadly. It can be acquired chiefly through a single bite by an infective dog. One could also get it when broken skin is exposed to infected saliva. Other potential routes include eyes, mouth, and nose. Nonetheless, not all dogs carry the virus causing the disease. Also, dogs are not the only ones that can transmit the rabies virus. Most warm-blooded vertebrates (e.g. monkeys, raccoons, cattle, cats, bats, etc.) can carry the virus and transmit it to a human host. The virus has further adapted, and hence, could grow as well as in cold-blooded vertebrates. However, because of the widespread domestication of dogs in human households dogs have consequently incited most rabies cases in humans.
Lyssavirus – the viral agent
The virus of rabies disease is a Lyssavirus, a type of RNA virus belonging to the family Rhabdoviridae, order Mononegavirales. It has a bullet shape. It carries a single negative-strand RNA as its genome, enough to code for proteins — namely, nucleoprotein, phosphoprotein, matrix protein, glycoprotein, and RNA polymerase — to establish within the host cell.
A coloured transmission electron micrograph of Australian bat lyssavirus (finger-like projections and the one that bud off from a host cell). Credit: Electron Microscopy Unit AAHL, CSIRO, CC 3.0 Unported license
In particular, the virus makes its way inside the host cell (e.g. muscle cell or nerve cell) through receptor binding and membrane fusion by way of endosome using its glycoprotein G. The virus transcribes its genome by its polymerase inside the endosome. Then, it fuses to the endosome to release its newly transcribed proteins and RNA into the cytosol.
The matrix protein regulates both transcription and replication of the virus. From transcribing, the polymerase shifts into replicating its genome. The nucleoprotein tightly binds to the newly replicated genome, thus, forming ribonucleoprotein complex. This, in turn, can now form new viruses.
The virus performs transcription and replication processes via a specialized inclusion body referred to as the Negri body. In fact, the presence of Negri bodies in the cytoplasm of the host cell indicates histological proof of Lyssavirus infection.
Rabies – two types
Early symptoms of rabies disease include fever, discomfort, and paraesthesia (burning sensation at the bite site). Eventually, the symptoms progress to behavioral changes when the virus spreads to the central nervous system.
Lyssavirus enters and hijacks muscle cells to replicate. From the muscle tissue, it travels to the nervous
system through the neuromuscular junctions. The virus enters the peripheral nervous
system directly and then spreads to the central nervous system where it can cause fatal inflammation in the brain and spinal cord.
Depending on the symptoms, the rabies may be described as “furious” or “paralytic“. The furious rabies — the more common form (80% ) — is characterized by hyperactivity, confusion, abnormal behavior, paranoia, terror, hallucinations, and hydrophobia (“fear of water“). The paralytic rabies, as the name implies, causes paralysis starting from the
site of bite (or entry). Both of these types may lead to coma and eventually to the death of the patient. However, patients with the furious type have higher risks, due to the likely cardio-respiratory arrest. Without an early and proper medical intervention, death may ensue typically two to ten days after these symptoms
Rabies – pathobiology
How rabies causes behavioural changes baffles scientists. In 1980s and 1990s, researchers explicated how the virus caused paralysis. Accordingly, the glycoprotein at the cell surface of the Lyssavirus competes against acetylcholine in terms of binding affinities to specific muscle receptors (e.g. nicotinic acetylcholine receptors). Lately, researchers conjectured that the virus could also be doing the same with the similar receptors found in the brain. Furthermore, they presumed that the interaction could have affected how the brain cells normally communicate, and thereby induced changes in the behavior of the host.
Recently, researchers from the Ohio State University College of Medicine and The Ohio State University Wexner Medical Center conducted a study aimed at identifying dog breeds and physical traits that pose a high risk of biting with severe injury. Their data could provide an empirical basis when deciding which dogs to own. Still, further studies on rabies are necessary since the disease is marked as fatal as soon as the clinical symptoms set in. Although vaccine-preventable, rabies, especially via a dog bite, remains a significant cause of annual deaths in humans, both young and old. Novel treatments and vaccines that are effective and economical could preclude death. At present, the staggering cost of treatment remains a major health-care restraint. Without the proper and early treatment, death from rabies, unfortunately, is almost always certain.
— written by Maria Victoria Gonzaga
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