Viruses come in various forms from different virus families. They are microscopically small but can severely infect a vast portion of the living world. This battle with viruses has been going on since the emergence of life. But due to those complex structures, the cure for most of these viruses has not yet been found. This complicated microscopic living-non living thing has caused major epidemics in history. Below are a few of those viruses:
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10 Deadliest Viruses in the World
Hantavirus is a single-stranded RNA virus in the family of Bunyaviridae. These viruses exclusively establish a persistent infection in the population of their specific rodents. It takes its name from the Hantan River in South Korea. And it was initially discovered in 1978 and has co-evolved for millions of years. Like other viruses, Hantaviruses enter the host cells by the interaction between the glycoproteins and other cell surface receptors. They reach the human body through contact with rodent urine, saliva, or feces. Once the viruses are engaged in the host cell, they rapidly multiply and spread throughout the body.
Hantaviruses are responsible for causing two types of serious illnesses: HFRS and HPS. This disease is characterized by numerous symptoms like fever, gastrointestinal complaints, sudden respiratory distress, and hypotension. The incubation period varies from a few days to several weeks after exposure and can, in some cases, take about 56 days. The fatality rate of HPV is 36% and 0.1 to 10% for HRPS. Cases of Hantavirus have been reported from all the continents. HFRS is usually reported from countries like China, the Korean peninsula, Russia, and Europe. And HPS has its highest incidences in Argentina, Chile, Brazil, North America, and Panama.
The diagnosis of this disease mainly relies on antibody detection through fluorescent or Enzyme assays. Still, in recent years, RT-PCR from patient blood is coming into use. Its treatment is mostly symptomatic as there is no treatment for the Hantavirus infection at present. Maintaining fluid balance, temperature, oxygen levels with the management of cardiac function is done. Ribavirin is the only drug used in severe Hantavirus infections.
Ebola virus disease is an acute viral hemorrhagic fever. It belongs to the Filoviridae family and has five strains, including Bundibugyo, Zaire, Sudan, Côte d’Ivoire (Taï Forest), and Reston. The first three have been responsible for the large outbreaks that have occurred in Africa. And the Reston has been only observed in animals in Asia but not in humans. Fruit bats of the Pteropodidae family have been recognized as the natural hosts of Ebola viruses, with other mammals being accidental hosts.
The virus spreads to people through direct contact with the blood, body fluids, and tissues of infected animals, such as fruit bats, chimpanzees, gorillas, monkeys, forest antelope, or porcupines. After the virus infects humans, it becomes contagious and spreads to other humans through infected body fluid contact. The virus is persistent even in recovered patients, in many cases. The virus remains in the areas that immunologically privileged sites. Ebola virus shields them from immune systems. The incubation period for this disease varies from two days to three weeks. Still, the person can spread the disease without developing the symptoms. The symptoms of Ebola include fatigue, muscle pain, vomiting, impaired kidney, and liver function. The average fatality rate is around 50% to 90%.
Unfortunately, there are no known medications developed, no accepted specific treatments, or approved medicine for this virus. And the diagnosis of this disease is relatively challenging because it is difficult to distinguish it from malaria and typhoid. Confirmation methods include antibody trapping methods, serum neutralization tests, and electron microscopy.
3. Lassa Fever Virus
Lassa virus is a single-stranded RNA virus, which belongs to the Arenaviridae family. This virus causes a type of viral hemorrhagic fever – Lassa Fever. And it was discovered about 40 years ago and is usually endemic in Africa, affecting more than one hundred thousand people annually. This virus affects rodents. And once a mouse is infected, it will excrete the virus throughout its lifetime. The virus is usually initially spread to people via contact with the urine, feces or other body fluids of an infected multimammate mouse. The spread among humans occurs via direct contact. The exposure to contaminated excrement is usually through respiratory or gastrointestinal tracts. It is also possible to be infected through broken skin or semen as well.
The symptoms of Lassa fever occur 1-3 weeks after the exposure. The majority of the symptoms are mild, like fever, weakness, and headache. In some cases, the patient might also progress towards more severe symptoms, including hemorrhaging, respiratory distress, swelling, and shock. Neurological problems like hearing loss, tremors, and encephalitis have also been reported. If it is left untreated, the patient is likely to die within two weeks due to multi-organ failure.
The most common diagnosis of Lassa fever is enzyme-linked immunosorbent serological assays. Reverse transcription can be used in the early stages, within ten days of first exposure. There are no approved vaccinations to prevent this virus. But the use of antiviral medication, ribavirin, has been observed as an effective treatment method if it is administered in the early stage. Various vaccinations are being developed, but none have passed all their required trials to be approved by the FDA.
4. Yellow Fever
Yellow fever virus (YFV) is a deadly mosquito-borne flavivirus, which causes yellow fever. This virus typically infects humans, some primates, and several types of mosquitoes. The primary carrier of this virus is Aedes aegypti, a kind of mosquito found in the tropical and subtropical ecotype. It is an RNA virus from the Flavivirus genus. This virus infects monocytes, macrophages, and dendritic cells by attaching themselves to receptors on the surface. The viral genome replicates itself inside the host in the endoplasmic reticulum and releases itself as infectious virions.
Clinically, most people with the Yellow fever virus are asymptomatic. But a few develop symptoms, including sudden fever, chills, myalgia, vomiting, and fatigue. The symptoms are visible after an incubation period of 3-6 days, and roughly only 15% develop these symptoms. Severe symptoms include a high fever with bleeding, renal failure, cardiovascular instability, and liver failure or jaundice. It may also include neurological complications like photophobia, agitation, cerebral edema, or coma.
As mild symptoms of this viral infection can cause an outbreak, any suspicion of this disease is treated seriously. A conformational test can be done by using the reverse transcription method or an enzyme-linked immunological assay.
An extremely effective vaccine has been developed to combat this disease. A single dose of this vaccine can confer sustained immunity for a lifetime. It produces active immunity within ten days, and the WHO has started an unprecedented initiative to immunize people against this disease and eradicate it in the long run.
Rhabdovirus is a virus that comes from the Rhabdoviridae family. This virus is a bullet-shaped virus enveloped in a fatty acid membrane. They are single helical stranded RNA viruses. There are eighteen recognized strains of this virus, which are known to cause diseases, including rabies encephalitis, vesicular diseases, and flu-like symptoms. These viruses enter the host body and replicate the negative-stranded RNA using the shuttering method of transcription. The main transmission route for this disease is biting. It is usually a carnivore mammal that transmits this virus to humans and causes a deadly disease known as rabies.
This is an infectious viral disease that is almost always fatal. It causes inflammation of the brain, and the early symptoms of this disease include fever and tingling sensation. However, as the disease progresses, violent movement, unreasonable excitement, fear of water, paralysis, and loss of consciousness are seen. Once the symptoms progress, the result is usually death. This happens because the virus reaches the central nervous system and damages it. The time taken for the symptoms to arise varies from one week to about a year. And it also depends on the distance the viruses have to travel.
The current diagnosing methods are not suitable to detect rabies before the rabies-specific clinical symptoms appear. In the early stages of this disease, it is usually confused with other diseases and aggressiveness. The only used method for confirmation of the disease is by using a fluorescent antibody test. And this method is also recommended by the WHO. If the disease is diagnosed within ten days of exposure, vaccinations will be able to prevent the disease without any risks. If the treatment is started after the symptoms arise, there are various methods like inducing coma and using antiviral medications, but none of them are useful.
Rotavirus is a double-stranded RNA and non-enveloped virus belonging to the Reoviridae family. There are nine species of Rotavirus recorded and are referred from alphabets A to H. The species that usually infect humans are strains A-E. Strains E and H cause infections in pigs and the remaining in birds and cats. The virus enters the cells and forms a vesicle, which disrupts the membrane. During infection, the virus produces protein while replicating to invade the cells of the host’s body. This virus is the main cause of diarrheal symptoms in children under the age of three. It is transmitted by a fecal-oral route, which is by contaminated hands, foods, or respiratory routes. It is highly contagious as the feces of an infected person has trillions of infectious particles and can transmit the infection to more than one hundred people.
The Rotavirus replicates in the gut and infects the villi of intestines, which affect its functioning. It also changes the calcium sensitivity of the cells and, therefore, causes diarrheal symptoms due to inflammation of the stomach and intestine. It can lead to severe diarrhea. Treatment of this virus involves non-specific methods like management of dehydration. Patients are usually administered with oral hydration therapy with zinc tablets to avoid dehydration. If it is not well-managed, the Rotavirus can cause other complications. As this is a viral infection, no antibiotics work, but recently four different vaccines can prevent this gastrointestinal infection.
7. Marburg Virus
Marburg virus is one of two viruses from the Filovirus family. It is a single-stranded RNA virus of negative polarity that appears in the shape of a shepherd’s crook with coils or branches. It is a zoonotic virus, which commonly selects fruit bats as its natural hosts. A cholesterol transporter protein named NPC1 is essential for infection with both the viruses of the filovirus family. As it enters the body, it mediates infection by binding the viral protein to the cholesterol transporter. The virus is known to cause viral hemorrhagic fever in humans and other primates.
This virus is transmitted by exposure to a species of fruit bats through direct contact with the blood, body fluids, and tissues. Initial human infection with this virus is a result of prolonged exposure to caves inhabited by bat colonies. Human to human transmission occurs via direct contact with body secretions and blood. This virus needs an incubation period of about five days. The initial symptoms of this infection are fever, chills, and myalgia, which later advance to maculopapular rash, nausea, vomiting, and chest pain. Symptoms become severe as the disease progresses and can cause jaundice, liver failure, massive bleeding, and multi-organ dysfunction.
The fatality of Marburg hemorrhagic fever is between 20-90% as the diagnosis in the early stages is usually confused with Malaria or Typhoid fever. Enzyme-linked assay and polymerase chain reactions are used as confirmatory tests of the Marburg virus. In spite of being able to confirm the disease, there are no known methods to treat it. Antiviral medications have proven to be ineffective while treating this disease. As there are no specific methods, supportive therapy is utilized to treat the symptoms. Supportive therapy in this disease includes balancing fluids and electrolytes in the body, maintaining blood pressure and oxygen, replacing lost blood, and treating other infections.
8. Zika Virus
Zika virus is a mosquito-borne virus. It is a member of the Flaviviridae family and is spread by a species of Aedes mosquito. This virus is related to other viral diseases like Yellow fever and Dengue and is known to occur usually in a narrow equatorial belt from Asia to Africa. This is a positive sense RNA that binds to the endosomal membrane of the host cell and initiates endocytosis.
This infection is spread to humans by the bite of an Aedes mosquito in the tropical region. This species of mosquito is active during early morning and late afternoon as the mosquito that transmits Dengue and Yellow fever. The first vertebrate host of this virus was monkeys and was in a mosquito-monkey-mosquito cycle. The first transmission to humans was recorded in 2007 as a pandemic. And human to human transmission occurs through sexual contact, blood transfusion, organ transplant, and also from mother to fetus.
The symptoms of Zika fever are seen usually after an incubation period of 3–14 days. The majority of the people suffering from this disease show no symptoms. There are only mild symptoms like mild fever, rash, joint pain, and red-eye, which usually lasts one week. Infection during pregnancy causes brain malformations and causes Guillain Barre Syndrome in adults by affecting the Schwann cells. After the onset of symptoms, it is diagnosed by testing blood, urine, saliva, or other bodily fluids.
Research is ongoing to develop drugs against this infection, but there are no available treatments as of now. The best way to prevent this disease is to protect oneself from mosquito bites and maintain a clean environment.
9. Human Immunodeficiency Virus
Human Immunodeficiency Virus (HIV) is a virus belonging to the genus lentivirus, which occurs in two different types: HIV-1 and HIV-2. Lentiviruses are responsible for long-duration illness and incubation period. These enveloped RNA viruses are inscribed into the host’s cell DNA. Once inscribed, these viruses can hide from detection by the host’s immune system for a period. This infected DNA can remain dormant as well as be released after rapid replication.
This virus is responsible for causing a disease known as Acquired Immunodeficiency Syndrome- AIDS, which is characterized by progressive failure of the body’s immune system. It infects the human immune systems cells like helper T cells and macrophages. As the level of T cells declines, immunity is lost, and the body becomes susceptible to various infections.
HIV is transmitted by contact with infected bodily fluids like blood, vaginal fluids, semen, or breastmilk. The virus has to get to the bloodstream via cuts or direct injection. This virus is not transmitted through bodily fluids like saliva, sweat, or tears. Infected patients will show flu-like symptoms in the early stages, which lasts for a long time. As the infection progresses, patients develop symptoms like diarrhea, weight loss, and repeated infections.
Diagnosing HIV in a body is done by blood tests. This test is not positive for a couple of months as antigens for the virus have not been developed yet. Without treatment, this disease will progress towards AIDS, but with antiretroviral therapies, one can manage a healthy life.
SARS coronavirus is a virus that is responsible for a disease that causes severe respiratory distress. It was discovered in 2003 in China. This single-stranded RNA virus belongs to the family of enveloped coronaviruses and is the largest RNA virus detected. This virus is believed to be an animal virus, mainly affecting bats, but the exact information for this prediction is still uncertain. Although the transmission route from animals to humans is unclear, this disease is spread between humans by person to person contact. When an infected person coughs or sneezes, millions of virions are released in the air. A healthy person can become infected if they inhale contaminated air within a three-meter radius from the infected person. It can also spread by contaminated hands or objects.
The symptoms of this disease occur after an incubation period of 2 to 10 days, including fever, chills, breathing problems, tiredness, and, in some cases, vomiting. The symptoms worsen as the disease progresses. People suspected of this disease are tested for SARS antibodies, blood counts, and a chest X-ray.
This disease has shown no response to antiviral medications like ribavirin. The only treatment options are to treat the symptoms of the disease and prevent pneumonia. In severe cases, the plasma from a recovered SARS patient is given. Still, the effectiveness of this method is not known.