Viral Agents
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.
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.
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 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)
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.
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.
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.
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%
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.
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%.
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.
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.
- European - The European subtype is associated with milder disease, a
case-fatality ratio of <2%, and neurologic sequelae in up to 30% of
patients.
- Siberian - The Siberian subtype has a case-fatality ratio of 6% to
8%, with rare reports of cases with slow or chronic progression over
months.
- Far Eastern - The Far Eastern subtype is often associated with a
more severe disease course, including a case-fatality ratio of 20%
to40% and higher rates of severe neurologic sequelae.
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.
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.
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.
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.
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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