[L. virus, poison]
A pathogen that is composed of nucleic acid within a protein shell but can grow and reproduce only after infecting a host cell. More than 400 types of viruses are known. All of them can attach to cell membranes, enter the cytoplasm, take over cellular functions, reproduce their parts, and assemble themselves into mature forms capable of infecting other cells.
Some of the most virulent diseases are caused by viruses, e.g., the hemorrhagic fever caused by Ebola virus. Viruses are also responsible for the common cold, childhood exanthems (such as chickenpox, measles, rubella), latent infections (such as herpes simplex), some cancers or lymphomas (such as Epstein-Barr virus), and diseases of all organ systems.
Viral architecture is very complex, but every virus contains at least a genome and a capsid. Most animal viruses are also surrounded by a lipid envelope (a bilayered membrane analogous to a cell membrane). This envelope may be parasitized from host cells; its chemical components are phospholipids and glycoproteins, and it is frequently dotted by spikes.
Viruses with lipid envelopes have a greater ability to adhere to cell membranes and to avoid destruction by the immune system. Both the capsid and the envelope are antigenic. Frequent mutations change some viral antigens so that the lymphocytes are unable to create an antibody that can neutralize the original antigen and its replacement. The common influenza viruses have antigens that mutate or combine readily, requiring new vaccines with each mutation. The body's primary immune defenses against viruses are cytotoxic T lymphocytes, interferons, and, to some extent, immunoglobulins; destruction of the virus often requires destruction of the host cell.
When viruses enter a cell, they may immediately trigger a disease process or may remain quiescent for years. They damage the host cell by blocking its normal protein synthesis and using its metabolic machinery for their own reproduction. New viruses are then released either by destroying their host cell or by forming small buds that break off and infect other cells.
VIRAL SHAPES ; SEE TABLE: Common Viral Characteristics
The 400 known viruses are classified in several ways: by genome core (RNA or DNA), host (animals, plants, or bacteria), method of reproduction (such as retrovirus), mode of transmission (such as enterovirus), and disease produced (such as hepatitis virus).
Antiviral drugs include such agents as acyclovir (for herpes simplex); oseltamivir and zanamivir (for influenza A); interferons (for chronic hepatitis B and C); ribavirin (for respiratory syncytial virus and chronic hepatitis C); and lamivudine (for HIV).
Common Viral Characteristics
|RNA||HIV, hepatitis A, polio, measles, mumps, rhinovirus, influenza|
|DNA||Herpesviruses, hepatitis B, adenoviruses, human papilloma viruses, cytomegalovirus|
|Humans||Measles, mumps, rubella, varicella-zoster, poliovirus|
|Humans and animals||Rabies, influenza, hantavirus, encephalitis virus|
|Plants||Tobacco mosaic virus, cowpea mosaic virus|
|Present||Herpesviruses, rabies, HIV|
|Absent||Rotavirus, Norwalk virus, adenovirus|
|Respiratory||Influenza, parainfluenza, hantavirus|
|Teratogenic||Varicella-zoster virus, cytomegalovirus, rubella|
|Neurological and fatal||Rabies|
|Paralytic encephalitic||Polio, many encephalitis viruses|
|Fulminant||Yellow fever, hantavirus, Ebola-Marburg|
|Cancer causing||Human T-cell lymphotrophic virus, hepatitis viruses, papillomavirus|
ABBR: AAV A genus in the parvovirus family whose members cannot replicate without the presence of another virus. AAVs are nonenveloped single-stranded DNA viruses used in gene therapy to insert copies of missing genes into host cells.
SYN: SEE: dependoparvovirus
A virus with reduced pathogenicity as a result of treatment or repeated passage through hosts.
A virus commonly found in macaques (Old World monkeys) but not in other primates. Handlers of macaques may be infected by bites or exposure to animal blood or body fluids. The virus is harmless to macaques or may cause only a herpetic rash in macaques, but in humans it often produces fatal infections of the brain and meninges.
SYN: SEE: cercopithecine herpesvirus 1; SEE: cercopithecine virus 1; SEE: herpesvirus B; SEE: herpesvirus simiae; SEE: monkey B virus
ABBR: BAV The type species of the southeast Asian genus Seadornavirus (family Reoviridae). It is transmitted to humans by the mosquito or tick bite and is implicated in some cases of encephalitis.
SEE: coltivirus; SEE: Reoviridae
A virus transmitted by mosquito bite that causes rash, fever, joint pain, and stiffness. It is found mostly in Australia and neighboring islands.
An orthomyxovirus that may be transmitted to humans by tick bite.
SEE: B virus.
A single-stranded RNA arenavirus identified in Bolivia in the eastern foothills of the Andes mountains. It causes fevers, headache, joint and muscle pain, vomiting, and a potentially deadly hemorrhagic fever. It is related to other South American arenaviruses, esp. the Sabiá, Guanarito, and Machupo viruses, all of which are carried by rodent vectors.
ABBR: CHIKV An Alphavirus that is typically found in the tropics and can be transmitted to humans by the bite of Aedes mosquitoes. After an incubation period of about a week, the virus produces high fevers, headache, nausea, vomiting, and severe joint pain, usually in the wrists or ankles.
A virus that is widespread in a community during a specific season or year.
coronavirus A family of enveloped, single-stranded RNA viruses that cause a variety of illnesses, including colds, and life-threatening MERS, SARS, and Covid-19.
A plant virus used in vaccine development to deliver antigens from pathogens and tumors. The virus does not infect animals and is therefore considered a safe vehicle for antigen display in humans and other species.
ABBR: CMV SEE: Cytomegalovirus.
A Flavivirus that causes meningoencephalitis. It is transmitted to humans by the bite of ticks infected with Powassan virus.
A virus particle that lacks certain essential factors and is therefore unable to replicate. Sometimes this can be overcome by the presence of a helper virus that provides the missing factor or factors.
SEE: hepatitis D
A virus such as the papilloma virus and the herpesviruses whose genome is DNA (deoxyribonucleic acid).
An HIV virus that enters a cell by binding to both the CXCR4 coreceptor and the CCR5 coreceptor.
SYN: SEE: mixed tropic virus
SEE: Epstein-Barr virus.
ABBR: echovirus An orphan virus that was accidentally discovered in human feces and is not known to be associated with a disease. Initially, 33 echovirus serotypes were designated, but numbers 10 and 28 have been reclassified. Various serotypes have been associated with aseptic meningitis, encephalitis, acute upper respiratory infection, enteritis, pleurodynia, and myocarditis.
SEE: Epstein-Barr virus
ABBR: EYAV A European tick-borne coltivirus implicated in some cases of meningitis and encephalitis.
A virus that causes infectious disease but is so small that it retains infectivity after passing through a filter of the Berkefeld type.
SEE: Berkefeld filter
A rabies virus stabilized and modified but only partially attenuated by serial passage through rabbits.
ABBR: GGYV A single-stranded RNA flavivirus transmitted by tick bite. It may cause fevers. It has been isolated in Macquarie Island, Australia.
SEE: Hepatitis G virus.
An arenavirus from the Tacaribe virus group that chronically infects rodents. It is the cause of sporadic outbreaks of Venezuelan hemorrhagic fever.
A phlebovirus identified in Missouri as a cause of an influenza-like illness. It causes fever, anorexia, diarrhea, fatigue, and malaise.
A virus that permits a defective virus present in the same cell to replicate.
SEE: defective virus
A Flavivirus that is found in blood in about 2% of blood donors and may be transmitted by injection, drug abuse, sexual contact, transfusions, and childbirth (from mother to infant). It is remotely related to hepatitis C virus. It causes chronic viremia but does not seem to cause hepatitis or liver damage.
SYN: SEE: GB virus type C; SEE: human pegivirus
ABBR: HSV-1, HSV-2 Either of two human DNA viruses (HSV-1 and -2) that cause repeated painful vesicular eruptions on the genitals and other mucosal surfaces and on the skin. After initial contact with the skin or mucous membranes, the virus migrates along nerve fibers to sensory ganglia, where it establishes a latent infection. Under a variety of stimuli, such as sexual contact, exposure to ultraviolet light, febrile illnesses, or emotional stress, it may reappear, traveling back to the site of initial contact although the vast majority of herpes simplex infections are neither recognized nor symptomatic. The rash caused by the infection has a red base, on which small blisters cluster. Herpetic rashes on the mouth or nose are called cold sores or fever blisters.
SYN: SEE: herpesvirus hominis
In immunosuppressed patients, the virus can cause a widely disseminated rash. Some infections with HSV may involve the brain and meninges; these typically cause fevers, headaches, altered mental status, seizures, or coma, requiring parenteral therapy with antiviral drugs. In newborns, infection involving the internal organs also may occur. Experienced ophthalmologists should manage ocular infection with HSVs. Health care providers are at risk for herpetic whitlow (finger infections) from contact with infected mucous membranes if gloves and meticulous hand hygiene are not used.
Acyclovir and related drugs, e.g., famciclovir, valacyclovir, may be used to treat outbreaks of HSV-1 and HSV-2 and are also effective in preventing recurrences of disease.
Standard precautions prevent spread of the virus. Prescribed antiviral agents and analgesics are administered. Their use is explained to the patient and instruction is given about adverse effects to report.
The patient with HSV-1 is instructed to avoid skin-to-skin contact with uninfected people when lesions are present or prodromal symptoms are felt. To decrease the discomfort from oral lesions, the patient is advised to use a soft toothbrush or sponge stick, a saline- or bicarbonate-based (not alcohol-based) mouthwash, and oral anesthetics such as viscous lidocaine, if necessary. He or she should eat soft foods. Use of lip balm with sunscreen reduces reactivation of oral lesions.
The patient with genital herpes should wash his or her hands carefully after using the bathroom. He or she also should avoid sexual intercourse during the active stage of the disease and should practice safe sex. A pregnant woman must be advised of the potential risk to the infant during vaginal delivery and to use cesarean delivery if she has an HSV outbreak when labor begins and her membranes have not ruptured. The patient with genital herpes may experience feelings of powerlessness. He or she requires assistance to identify coping mechanisms, strengths, and support resources; should be encouraged to voice feelings about perceived changes in sexuality and behavior; and should be provided with current information about the disease and treatment options. A referral is made for additional counseling as appropriate.
Caregivers with active oral or cutaneous lesions should avoid providing patient care.
ABBR: HIV A retrovirus of the subfamily lentivirus that causes AIDS (acquired immunodeficiency syndrome). The most common type of HIV is HIV-1, identified in 1984. HIV-2, first discovered in West Africa in 1986, causes a loss of immune function and the subsequent development of opportunistic infections identical to those associated with HIV-1 infections. The two types developed from separate strains of simian immunodeficiency virus. In the U.S., the number of those infected with HIV-2 is very small, but blood donations are screened for both types of HIV.
EFFECT OF HIV ON IMMUNE SYSTEM HIV contains several proteins: gp 120 protein around it and viral RNA and p24 protein inside. The gp 120 proteins attach to CD4+ receptors of T lymphocytes; HIV enters the cell and makes viral DNA; the enslaved host cell produces new viruses that bud, which destroy the host cell's membrane, causing cellular death and allowing the virus to leave to attack other CD4+ lymphocyte cells. SEE: acquired immunodeficiency syndrome
ABBR: HTLV-I A virus associated with adult T-cell leukemia.
ABBR: HTLV-II A virus associated with hairy cell leukemia.
ABBR: HTLV-III The former name for human immunodeficiency virus (HIV).
ABBR: HPIV SEE: Parainfluenza virus.
An RNA virus that infects the respiratory tract of humans (as well as birds, pigs, dogs, and horses), causing influenza. It is a roughly spherical virus. It is composed of eight segments, including a nucleoprotein (which folds its nucleic acids and helps them to be transcribed), a hemagglutinin (which helps it to enter cells), a neuraminidase (which helps it to bud out of infected cells), several transcriptases (which make copies of the viral RNA), a matrix protein (which supports the outer membrane), and several nonstructural proteins. The virus mutates frequently and causes annual disease outbreaks, some of which (pandemics) affect millions of people. It can be treated with antiviral drugs and prevented with annual vaccinations that target its frequently evolving antigens.
ABBR: INK A European arbovirus of the California family of bunyaviruses. It is transmitted to humans by the bite of infected mosquitoes, causing fevers, encephalitis, and meningitis.
A DNA papovavirus that causes progressive multifocal leukoencephalopathy in immunosuppressed patients. It is carried asymptomatically by a large percentage of the population.
An arenavirus that chronically infects rodents. It is the cause of sporadic outbreaks of Argentine hemorrhagic fever, a potentially lethal infection.
A group B Flavivirus that is one of the causative agents of encephalitis in Australia.
A single-stranded RNA virus transmitted to humans by tick bite. It causes a two-stage illness: fever followed by a brief remission, and then meningoencephalitis, hemorrhagic pneumonia, and hepatic and splenic disruption.
ABBR: LGT A tick-borne Flavivirus, responsible for encephalitis, primarily in mice. It causes mild disease in humans. Its genetic similarity to other tick-borne encephalitis viruses makes it a candidate for the development of encephalitis vaccines.
A virus that has the ability to infect the host, initially causes little or no evidence of illness but persists for the lifetime of the infected host, and may produce a clinically apparent disease later on by a specific triggering mechanism. This occurs with herpes simplex virus, which remains latent in sensory ganglia and is reactivated by trauma to the skin supplied by the distal sensory nerves associated with these ganglia. After reactivation, the virus may cause localized or generalized lesions in the affected area and the central nervous system.
Any virus that infects a cell and then lyses it.
A virus that ordinarily occurs in the host in a noninfective state but is activated and demonstrated by indirect methods.
ABBR: MAYV A mosquito-borne single-stranded RNA Alphavirus that can cause in humans infections that resemble dengue fever. The virus is found primarily in the Amazon. Symptoms of the disease include severe joint and muscle pain, fever and chills, ocular pain and headache, diarrhea, rash, and vomiting.
SEE: Dual tropic virus.
SEE: B virus.
A virus that is not surrounded by a lipid envelope.
A virus that reproduces in nerve tissue.
ABBR: NiV A member of the family of paramyxoviruses that can cause outbreaks of encephalitis and respiratory disease in humans. It is transmitted to humans from infected swine, e.g., in slaughterhouses.
ABBR: NLV A Calicivirus that causes more than half of the reported cases of epidemic viral gastroenteropathy. It commonly causes nausea, vomiting, and diarrhea. The incubation period ranges from 18 to 72 hr. Outbreaks are usually self-limited. Intestinal signs and symptoms last for 24 to 48 hr. Treatment, if required, is supportive and directed at maintaining hydration and electrolyte balance.
SYN: SEE: Norwalk agent
An arenavirus of the Tacaribe complex of viruses that normally infects rodents in the pampas of Argentina. It may cause a fatal hemorrhagic fever in humans.
A single-stranded RNA virus in the Flavivirus genus. It can be transmitted to people by the bite of infected ticks. Clinical symptoms of infection include hemorrhage, conjunctivitis, cough if the patient develops pneumonia, fevers, and meningoencephalitis.
SEE: Tumor virus.
An Alphavirus found in central Africa that causes epidemic fevers, joint pains, and swollen glands. The virus is transmitted to humans by mosquito bite.
One of several viruses that initially were not thought to be associated with human illness. This group includes the enteroviruses and rhinoviruses.
ABBR: PIV Any of a group of viruses that affects infants and young children. The viruses cause respiratory infections that may be mild or may progress to pneumonia. Most infections are so mild as to be clinically inapparent. Virtually all children in the U.S. have been infected by age 6.
SYN: SEE: human parainfluenza virus
ABBR: PUUV A Hantavirus that is the causative agent of nephropathia epidemica.
A rare Flavivirus that is transmitted by tick bite and occasionally causes encephalitis.
A virus whose genetic material has been recombined or reshuffled so that it contains new nucleic acid sequences, new antigenic structures, and new combinations of protein products.
ABBR: RSV A single-stranded RNA virus that is an important cause of upper and lower respiratory tract disease in infants, children, and the older population. When limited to the upper respiratory tract, RSV causes symptoms of the common cold. In the lower respiratory tract, it causes bronchiolitis, pneumonia, or respiratory distress and can be life-threatening. Respiratory syncytial virus is the most common cause of lower respiratory infections in infants and children under age 2. It is spread by physical contact, usually with infected nasal or oral secretions. In the U.S., its season begins in the fall and peaks in winter. In the tropics the infection occurs year round. About 90,000 young children are hospitalized with RSV infections each year in the U.S.
Three to five days following exposure to RSV, the patient typically develops an upper respiratory infection lasting 1 to 2 weeks with cough, mild to moderate nasal congestion, runny nose, and low-grade fever. If the infection spreads to the lower respiratory tract, symptoms worsen and may include wheezing and difficulty breathing. Infants and children with RSV pneumonia exhibit retractions; rapid grunting respirations, poor oxygenation, and respiratory distress. Vomiting, dehydration, and acidosis may occur.
Diagnosis is based on signs and symptoms and confirmed by isolating RSV from respiratory secretions (sputum or throat swabs). Immunofluorescence techniques, enzyme immunoassays, or rapid chromatographic immunoassays provide rapid identification of viral antigens for diagnosis.
Treatment is mainly supportive. Antibiotics are not effective. Acetaminophen or ibuprofen are given for pain or fever. Oxygen is administered if the patient’s SpO2 (oxygen saturation) falls below 92%. Bronchodilators, such as albuterol and epinephrine, are used to treat wheezing. In patients with severe RSV infections, noninvasive positive-pressure ventilation or intubation and mechanic ventilation are required. Intravenous fluids are administered as prescribed if the patient cannot take enough fluid orally. Nasopharyngeal suction may be needed to clear congestion (by bulb syringe for infants).
Strict adherence to infection control measures is important in preventing an outbreak in any facility. This includes using meticulous hand hygiene (the most important step in preventing RSV spread) before donning gloves for patient care, after removing gloves, and if any potentially contaminated surfaces have been touched. Standard and contact precautions should be observed for all patients with known or suspected RSV (gown, mask, and eye protection for direct contact with respiratory secretions or droplets). Protective coverings should be removed in this order: gloves (followed by hand hygiene), goggles or face shield, gown, and finally mask or respirator, discarding them in an infectious waste container in the patient’s room. The patient with RSV should be in a private room and dedicated equipment should be used in patient care, with terminal equipment disinfection by the appropriate agency facility. Room assignments should be arranged to avoid cross-contamination whenever possible. Individuals with symptoms of respiratory infection should be prevented from caring for or visiting pediatric, immunocompromised, or cardiac patients.
The administration of high doses of respiratory syncytial virus immune globulin is an effective means of preventing lower respiratory tract infection in infants and young children at high risk for contracting this disease. Palivizumab, a monoclonal antibody given intramuscularly, can prevent RSV disease in high-risk infants and children.
A phlebovirus that causes sporadic epidemics of hemorrhagic fever among humans and animals in Africa. It is transmitted by the bite of infected mosquitoes.
A virus such as the HIV, influenza virus, and polio virus whose genome is RNA (ribonucleic acid).
An Alphavirus transmitted by mosquito bite that causes fevers, rash, and epidemic arthritis in multiple joints. It is typically found in Australia and neighboring islands.
An arenavirus that causes Brazilian hemorrhagic fever. The reservoir for the virus is unknown. Ribavirin, which is effective against Lassa fever, may be effective against Brazilian hemorrhagic fever.
SEE: Toscana virus
senecavirus A ABBR: SVV A member of the picornavirus family. It causes vesicular diseases in swine and may be used as an oncolytic virus in cancer therapy.
ABBR: SDNV A California serogroup virus of the Bunyaviridae family. It causes a febrile encephalitis.
A family of HIV-like retroviruses that primarily infects African green monkeys, in which it produces an HIV/AIDS-like suppression of immunity.
An Alphavirus typically found in South Africa or Oceania and disseminated to humans by mosquitoes of the genus Culex. It can cause a transient febrile illness accompanied by a diffuse maculopapular rash and muscle and joint pains.
A virus that replicates and causes disease indolently.
SEE: slow virus infection
A rabies virus obtained from an infected animal rather than from a laboratory strain.
Simian virus 40, a member of the papovavirus family. The virus produces sarcomas after subcutaneous inoculation into newborn hamsters.
SEE: simian immunodeficiency virus
A group of viruses, originally identified in South America, that cause hemorrhagic fever in humans. They are members of the Arenavirus family and are typically found in rodents. One member of this group is the Sabiá virus.
A double-stranded DNA poxvirus that occasionally infects humans, causing a pustular, nodular rash, fever, headache, and other symptoms. It is related to other orthopoxviruses (which cause smallpox, monkeypox, and vaccinia).
ABBR: TAH A European arbovirus of the Bunyaviridae family. It is transmitted to humans by mosquito bite and causes fevers, respiratory illnesses, encephalitis, and meningitis.
A bunyavirus transmitted by insect bite, esp. the bite of the sandfly (Phlebotomus papatasi). The virus is endemic in Sicily, Cyprus, and elsewhere in the Mediterranean and may cause encephalitis, aseptic meningitis, or septicemia.
SYN: SEE: sandfly fever virus
ABBR: TTV A single-stranded DNA virus, found in recipients of blood transfusions, that colonizes in the liver. It is not known whether the virus causes liver disease, e.g., chronic hepatitis, or benignly colonizes the liver.
A virus that causes malignant neoplasms. Viruses suspected of causing tumors in humans include Epstein-Barr virus (associated with Burkitt lymphoma), hepatitis B virus (associated with hepatocellular carcinoma), papilloma virus (associated with carcinoma of the cervix), and human herpesvirus 8 (associated with Kaposi sarcoma).
SYN: SEE: oncogenic virus
A double-stranded DNA virus, the causative agent of cowpox and a member of the Orthopoxvirus genus. Vaccines against smallpox are derived from live cultures of vaccinia virus.
ABBR: VARV A double-stranded DNA member of the Orthopoxvirus family that causes smallpox.
ABBR: WNV A Flavivirus that primarily infects birds but can be transmitted by mosquito bite to humans and other animals. Since West Nile virus was identified in the U.S. in 1999, it has produced a nationwide epidemic of encephalitis.
In 2012 the CDC was notified of 134 deaths and 3142 cases of WNV infection.
SYMPTOMS AND SIGNS
Although infection is usually asymptomatic, signs and symptoms are more likely to be observed in the very young, the very old, or the very sick and include fever, headache, stiff neck, fatigue, loss of appetite, nausea or vomiting, muscle pain, aches, and weakness. Infection is occasionally fatal. Those over age 50 are at greatest risk for serious complications and death.
Enzyme-linked immunosorbent assay is used to detect Immunoglobulin M Antibodies to WNV. But caution must be taken in interpreting findings: Similar viruses, such as dengue and yellow fever virus can sometimes produce false positive results.
There are no direct-acting antiviral agents currently effective against WNV. Treatment is supportive.
IMPACT ON HEALTH
Infected patients sometimes suffer long-term consequences of infection, including fatigue and malaise, difficulty concentrating or thinking, or movement disorders. Poor outcomes primarily occur in patients in whom the disease invades the central nervous system (neuroinvasive disease).
Mosquito-borne transmission can be prevented with mosquito control and mosquito avoidance measures. Health care professionals should advise patients and families to limit time out of doors, esp. at dusk and dawn, to wear protective clothing (long sleeves, long pants, and socks), to place mosquito netting over infant carriers or strollers, and to apply an FDA-approved insect repellant, e.g., DEET, picaridin, or oil of lemon eucalyptus. Mosquito breeding grounds should be eliminated: standing water should be removed from flower pots, bird baths, pool covers, rain gutters, and discarded tires. Window and door screens should be installed and kept in good repair to prevent mosquitoes from entering homes.
The disease is sometimes spread from patient to patient by blood transfusion or organ transplantation. Such transmission can be prevented by screening donated blood or organs.
VECTOR OF WEST NILE VIRUS The Culex mosquito, vector of West Nile virus
ABBR: XMRV virus A human retrovirus that has been isolated from some patients with chronic fatigue syndrome and prostate cancer.
ABBR: ZIKV A mosquito-borne single-stranded RNA Flavivirus was originally identified as a pathogen in Rhesus monkeys; it is transmitted to people primarily by the bite of infected Aedes species. In most individuals, it causes a transient rash, conjunctivitis, and fever. Pregnant women and their developing fetuses may suffer more severe consequences, including microcephaly (underdevelopment of the brain and head of the developing child). ZIKV has also been associated with Guillain-Barré syndrome, a potentially fatal paralytic illness.
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