Viral AgentsViruses

Viruses are submicroscopic infectious agents that replicate only inside the living cells of an organism. They vary in size from 10-200nm, even the largest is only 1/3rd size of a staphylococcus. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria.  When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of: (i) the genetic material, i.e. long molecules of DNA or RNA, but not both, that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures.  Viruses are unable replicate outside of living cells.

Potential Viral Agents

The letters in (brackets) represent the NATO military codename(s). The letters in [square brackets] represent the  National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) classification of organisms.

Eastern equine encephalitis virus (ZX) [Cat B] -

is the cause of eastern equine encephalitis (EEE), a rare illness in humans. The virus is transmitted to humans through the bite of an infected mosquito (Aedes, Coquillettidia, and Culex). Symptoms of infection typically appear 4 to 10 days after being bitten by an infected mosquito. The type of symptoms usually depend on the age of the person. People over age 50 and younger than age 15 are at greatest risk for developing severe disease. Severe cases of EEE infection begin with the sudden onset of headache, high fever, chills, and vomiting that may progress into disorientation, seizures, encephalitis (inflammation of the brain), and coma. Approximately a third of patients who develop EEE die, and many of those who survive have mild to severe brain damage.

Ebola virus - [Cat A]

is the cause of Ebola virus disease (EVD), formerly known as Ebola haemorrhagic fever. The virus is transmitted to people from wild animals and spreads in the human population through human-to-human transmission. The average EVD case fatality rate is around 50%.

Hantaviruses - [Cat C]

Hantaviruses are a family of viruses spread mainly by rodents and can cause varied disease syndromes.  In the hantaviruses in the Americas are known as 'New World' hantaviruses and may cause hantavirus pulmonary syndrome (HPS). Other hantaviruses, known as 'Old World' hantaviruses, are found mostly in Europe and Asia and may cause hemorrhagic fever with renal syndrome (HFRS)

Japanese encephalitis virus (AN) -

is a flavivirus related to dengue, yellow fever and West Nile viruses, and is spread by mosquitoes. Although symptomatic Japanese encephalitis (JE) is rare, the case-fatality rate among those with encephalitis can be as high as 30%. The incubation period is between 4-14 days. In children, gastrointestinal pain and vomiting may be the dominant initial symptoms. Severe disease is characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and ultimately death. The case-fatality rate can be as high as 30% among those with disease symptoms. Of those who survive, 20% to 30% suffer permanent intellectual, behavioural or neurological sequelae such as paralysis, recurrent seizures or the inability to speak.

Junin virus [Cat A] -

is the cause of Argentine hemorrhagic fever. The Junin virus is a member of the Arenaviridae family of viruses whose members are generally associated with rodent-transmitted diseases in humans. Other members of the family include: Lymphocytic choriomeningitis virus (LCMV), Machupo virus, Lassa virus, Guanarito virus, Sabia virus, Chapare virus and Lujo virus. The incubation period is 5-14 days (usually 7-12 days). Initial signs and symptoms may include fever, malaise, dizziness, myalgias, skin dysesthesia, oral ulcerations, lymphadenopathy, chest pain, back pain, abdominal pain, sore throat, headache, nausea, vomiting, cough, photophobia, conjunctival redness, facial flushing, and small axillary petechiae.

Lassa fever virus [Cat A] -

the cause of Lassa fever which is an animal-borne, or zoonotic, acute viral illness.The animal vector for Lassa virus is the multimammate rat (Mastomys natalensis). Signs and symptoms of Lassa fever typically occur 1 to 3 weeks after the patient comes into contact with the virus. For the majority of Lassa fever virus infections (approximately 80%), symptoms are mild and are undiagnosed. Mild symptoms include slight fever, general malaise and weakness, and headache. In 20% of infected individuals, however, disease may progress to more serious symptoms including hemorrhaging (in gums, eyes, or nose, as examples), respiratory distress, repeated vomiting, facial swelling, pain in the chest, back, and abdomen, and shock. Neurological problems have also been described, including hearing loss, tremors, and encephalitis. Death may occur within two weeks after symptom onset due to multi-organ failure, mortality being 1% overall, in the case of those requiring hospitalisation, mortality is as high as 50%, with deaths being due to multi-organ failure.

Machupo virus (VHF) -

is the cause of Bolivian hemorrhagic fever (BHF), also known as black typhus or Ordog Fever. The vector is Calomys callosus (large vesper mouse), a rodent indigenous to northern Bolivia. Infected animals are asymptomatic and shed the virus in excreta, thereby infecting humans. Evidence of person-to-person transmission of BHF exists but is believed to be rare. The infection has a slow onset with fever, malaise, headache and myalgia, very similar to Malaria symptoms. Petechiae  on the upper body and bleeding from the nose and gums are observed when the disease progresses to the hemorrhagic phase, usually within seven days of onset.  Severe hemorrhagic or neurologic symptoms are observed in about one third of patients. Neurologic symptoms involve tremors, delirium, and convulsions. The mortality rate is about 25%

Marburg virus  (MARV) [Cat A] -

causes a highly virulent disease, Marburg hemorrhagic fever (Marburg HF),  with a fatality ratio of up to 88%. It is in the same family as the virus that causes Ebola virus disease.  African green monkeys (Cercopithecus aethiops), and Rousettus bats are the natural hosts. After an incubation period of 5-10 days, symptom onset is sudden and marked by fever, chills, headache, and myalgia. Around the fifth day after the onset of symptoms, a maculopapular rash, most prominent on the trunk (chest, back, stomach), may occur. Nausea, vomiting, chest pain, a sore throat, abdominal pain, and diarrhea may then appear. Symptoms become increasingly severe and can include jaundice, inflammation of the pancreas, severe weight loss, delirium, shock, liver failure, massive hemorrhaging, and multi-organ dysfunction.

Nipah virus [Cat C] -

is a zoonotic virus (it is transmitted from animals to humans) and can also be transmitted through contaminated food or directly between people.  Fruit bats, also called flying foxes, are the animal reservoir for NiV in nature. In infected people, it causes a range of illnesses from asymptomatic (subclinical) infection to acute respiratory illness and fatal encephalitis. It is a member of the family Paramyxoviridae, genus Henipavirus. After exposure and an incubation period of 5 to 14 days, illness presents with 3-14 days of fever and headache, followed by drowsiness, disorientation and mental confusion. These signs and symptoms can progress to coma within 24-48 hours. Some patients have a respiratory illness during the early part of their infections, and half of the patients showing severe neurological signs showed also pulmonary signs. Even with hospitalisation mortality rates can exceed 40%.

Rift Valley fever virus (FA) -

causes Rift Valley fever (RVF) an acute, fever-causing viral disease most commonly observed in domesticated animals (such as cattle, buffalo, sheep, goats, and camels), with the ability to infect and cause illness in humans. The disease is rare in humans. Although RVF often causes severe illness in animals, most people with RVF have either no symptoms or a mild illness with fever, weakness, back pain, and dizziness. However, a small percentage (8-10%) of people with RVF develop much more severe symptoms, including eye disease, hemorrhage (excessive bleeding), and encephalitis (swelling of the brain). Human to human transmission has not been recorded.

Tickborne encephalitis viruses [Cat C] -

Tick-borne encephalitis (TBE) is an illness caused by a virus spread through tick bites. Tickborne encephalitis (TBE) virus is a single-stranded RNA virus that belongs to the genus Flavivirus. The median incubation period for TBE is 8 days (range, 4 to 28 days). The incubation period for milkborne exposure is usually shorter (3 to 4 days). TBE disease often presents with milder forms of the disease or a biphasic course. First phase: nonspecific febrile illness with headache, myalgia, and fatigue. Usually lasts for several days and may be followed by an afebrile and relatively asymptomatic period. Up to two-thirds of patients recover without any further illness. Second phase: central nervous system involvement resulting in aseptic meningitis, encephalitis, or myelitis. Findings include meningeal signs, altered mental status, cognitive dysfunction, ataxia, rigidity, seizures, tremors, cranial nerve palsies, and limb paresis. There are three sub-types: European, Siberian, and Far Eastern.

Tickborne hemorrhagic fever viruses [Cat C]

Crimean-Congo hemorrhagic fever is a severe tick-borne viral disease with the most extensive geographic distribution of the medically important tick-borne viral diseases.

Variola major (ZL) [Cat A] -

the cause of smallpox, an acute, highly contagious disease. It was one of the world's most devastating diseases known to humanity. Smallpox is considered one of the most serious bioterrorist threats. It was used as a biological weapon during the French and Indian Wars, (1754 to 1767) when British soldiers distributed smallpox-infected blankets to American Indians. In the 1980s, the Soviet Union developed variola as an aerosol biological weapon and produced tons of virus-laden material intended for release via intercontinental ballistic missiles.It was declared eradicated in 1980 following a global immunization campaign led by the World Health Organization. Smallpox is transmitted from person to person via infective droplets during close contact with infected symptomatic people. Smallpox has no cure and can be only prevented by vaccination.  The last naturally occurring case was diagnosed in October 1977. There have been several cases where the virus has escaped from laboratories.

Venezuelan equine encephalomyelitis virus (FX) [Cat B]  -

is a mosquito-borne viral pathogen that causes Venezuelan equine encephalitis or encephalomyelitis (VEE). Although primarily a disease of horses, it can cause disease in humans. The virus that causes VEE is transmitted primarily by mosquitoes that bite an infected animal and then bite and feed on another animal or human.  Healthy adults who become infected by the virus may experience flu-like symptoms, such as high fevers and headaches. People with weakened immune systems and the young and the elderly can become severely ill or die from this disease. During the Cold War, both the United States biological weapons program and the Soviet biological weapons program researched and weaponized VEE.

Western equine encephalitis virus (EV) [Cat B] -

is an Alphavirus, which is spread primarily by the bite of the Culex and Aedes species of mosquito, or possibly by small, wild mammals. Birds are a reservoir but not a primary vector for the virus. Most infections are subclinical but may present with a nonspecific viral syndrome consisting of fever, chills, malaise, and myalgias. While most patients recover spontaneously, some will progress to develop encephalitis. The neuroinvasive disease presents with confusion, somnolence, coma, and occasionally, death. Children who develop neurologic disease have a 30% chance of having a permanent disability such as seizures, spasticity, and cognitive or behavioral disorders. Mortality is low, about 4% with the elderly being more at risk.

Yellow fever virus (OJ or LU) [Cat C] -

the cause of  Yellow fever an acute viral haemorrhagic disease. The "yellow" in the name refers to the jaundice that affects some patients. Yellow fever virus is an RNA virus that belongs to the genus Flavivirus. It is related to West Nile, St. Louis encephalitis, and Japanese encephalitis viruses. Yellow fever virus is transmitted to people primarily through the bite of infected Aedes or Haemagogus species mosquitoes. Mosquitoes acquire the virus by feeding on infected primates and then can transmit the virus to other primates. People infected with yellow fever virus are infectious to mosquitoes (referred to as being 'viraemic') shortly before the onset of fever and up to 5 days after onset. The majority of people infected with yellow fever virus will either not have symptoms, or have mild symptoms and completely recover. For people who develop symptoms, the time from infection until illness is typically 3 to 6 days. Symptoms: sudden onset of fever, chills, severe headache, back pain, general body aches, nausea, vomiting, fatigue, weakness. Most people with the initial symptoms improve within one week, but for some people who recover, weakness and fatigue might last several months. A few people will develop a more severe form of the disease, in these cases (about 14%) people who have the initial symptoms, there will be a brief remission that may last only a few hours or for a day, followed by a more severe form of the disease. Severe symptoms include: high fever, yellow skin (jaundice), bleeding, shock, organ failure. Severe yellow fever disease can be fatal, among those who develop severe disease, 30-60% die.

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