Chapter 18

How Do Viruses Cause Disease? Viruses multiply within host cells. It is during their escape from those cells—either by cell lysis or budding—that the host cells are destroyed. This cell destruction leads to most of the symptoms of a viral infection, which vary depending on the location of the infection.

Viral Infections of the Skin Chickenpox and shingles Varicella-zoster virus (a DNA virus which is also known as human herpesvirus 3) German measles (rubella) Rubella virus, an RNA virus Measles (hard measles, rubeola) Measles (rubeola) virus, an RNA virus Monkeypox Monkeypox virus, a DNA virus Smallpox Two strains of variola virus (variola minor and variola major), a DNA virus Warts At least 70 different types of human papillomaviruses (HPV), DNA viruses

Chickenpox

Child with Rubella

Child with Measles

Koplik Spots (an Early Sign of Measles)

Viral Infections of the Eyes Adenoviral conjunctivitis and keratoconjunctivitis—caused by various types of adenoviruses Herpes simplex and varicella-zoster viruses can also cause keratoconjunctivitis. Hemorrhagic conjunctivitis—caused by adenoviruses and enteroviruses People with viral infections (e.g., cold sores) should wash their hands thoroughly before inserting or removing contact lenses or otherwise touching their eyes.

Viral Infections of the Upper Respiratory Tract The common cold (acute viral rhinitis, acute coryza) Many different viruses cause colds. Rhinoviruses (more than 100 serotypes) are the major cause in adults. Other cold-causing viruses include coronaviruses, parainfluenza viruses, respiratory syncytial virus (RSV), influenza viruses, adenoviruses, and enteroviruses. Transmission occurs via respiratory secretions by way of hands and fomites or direct contact with or inhalation of airborne droplets.

Viral Infections of the Lower Respiratory Tract Acute, febrile, viral respiratory disease Caused by parainfluenza viruses, respiratory syncytial virus (RSV), adenovirus, rhinoviruses, certain coronaviruses, coxsackieviruses, and echoviruses; transmission occurs via direct oral contact or by droplets. Avian influenza (bird flu) Avian influenza virus type A; three prominent subtypes – H5, H7, and H9; bird-to-human transmission occurs via contact with infected poultry or contaminated surfaces. Hantavirus pulmonary syndrome (HPS) Caused by at least five different hantaviruses (Sin Nombre, Bayou, Black Creek Canal, New York-1, Monongahela); transmission occurs via inhalation of aerosolized rodent feces, urine, and saliva.

Viral Infections of the Lower Respiratory Tract (cont.) Influenza, flu Influenza viruses, types A, B, and C; RNA viruses; transmission is via infected humans; pigs and birds also serve as reservoirs Severe acute respiratory syndrome (SARS) SARS-associated coronavirus (SARS-CoV) (shown here) Transmission occurs via infected individuals by respiratory droplets, or by touching the mouth, nose, or eye after touching a contaminated surface or object.

Viral Infections of the Oral Region Cold sores (fever blisters, herpes labialis) Usually caused by herpes simplex virus type 1 (HSV-1), but can be caused by herpes simplex type 2 (HSV-2) DNA viruses in the family Herpesviridae Either HSV-1 or HSV-2 can also infect the genital tract, although genital herpes infections are most often caused by HSV-2.

Cold Sore Caused by Herpes Simplex

Viral Infections of the GI Tract Viral gastroenteritis (viral enteritis, viral diarrhea) The most common viruses infecting children in their first years of life are enteric adenoviruses, astroviruses, caliciviruses, and rotaviruses. Viruses infecting children and adults include norovirus-like viruses and rotaviruses. Transmission occurs via infected humans, most often by way of the fecal–oral route; possibly from contaminated water and shellfish.

Most Common Types of Viral Hepatitis Type A hepatitis (HAV infection, infectious hepatitis, epidemic Hepatitis) Hepatitis A virus (HAV)—a linear ssRNA virus Fecal–oral transmission Type B hepatitis (HBV infection, serum hepatitis) Hepatitis B virus (HBV)—an enveloped, circular dsDNA virus Sexual transmission or household contact with an infected person; injected drug use; tattooing; needlesticks Type C hepatitis (HCV infection, non-A non-B hepatitis) Hepatitis C virus (HCV)—an enveloped, linear ssRNA virus Primarily parenterally transmitted; rarely sexually Type D hepatitis (HDV infection, delta hepatitis) Hepatitis D virus (HDV, delta virus)—an enveloped, circular ssRNA viral satellite; coinfection with HBV is necessary Exposure to infected blood and body fluids, etc.

Most Common Types of Viral Hepatitis Type E hepatitis Hepatitis E virus (HEV)—a nonenveloped, ssRNA virus Fecal–oral transmission; primarily fecally contaminated drinking water; also person-to-person Type G hepatitis Hepatitis G virus (HGV)—a linear ssRNA virus Parenteral transmission

Viral STDs Anogenital herpes viral infections (genital herpes) Usually caused by HSV-2; occasionally by HSV-1 Transmission occurs via infected humans by direct sexual contact; oral-genital, oral-anal, or anal-genital contact when lesions are present. Genital warts (genital papillomatosis, condyloma acuminatum) Human papillomaviruses (HPV); DNA viruses Transmission occurs via infected humans by direct contact, usually sexual. Genital warts can become malignant.

Viral Infections of the Circulatory System Human immunodeficiency virus (HIV) infection, acquired immunodeficiency syndrome (AIDS) HIV—two types: HIV-1 (most common) and HIV-2; ssRNA viruses Transmission occurs via infected humans, by direct sexual contact; contaminated needles/syringes; transfusion of contaminated blood; transplacental transfer from mother to child; transplantation of HIV- infected tissues or organs; needlestick, scalpel, and broken glass injuries.

Common Modes of Transmission of HIV

Viral Infections of the Circulatory System Infectious mononucleosis ( “mono,” “kissing disease”) Caused by Epstein–Barr virus (EBV), which is also known as human herpesvirus 4; a DNA virus in the family Herpesviridae Transmission occurs via infected humans, person-to-person, direct contact with saliva. Mumps (infectious parotitis) Caused by mumps virus—an RNA virus Transmission occurs via infected humans by droplet spread and direct contact with saliva.

Viral Hemorrhagic Diseases (Viral Hemorrhagic Fevers) Caused by many different viruses, including dengue virus, yellow fever virus, Crimean-Congo hemorrhagic fever virus, Lassa virus, Ebola virus, and Marburg virus Ebola and Marburg viruses are extremely large filamentous viruses. Infected humans serve as reservoirs; infected African green monkeys also serve as reservoirs in Marburg infection. Transmission is person-to-person via direct contact with infected blood, secretions, internal organs, or semen; also needlestick.

Viral Infections of the CNS Poliomyelitis (polio, infantile paralysis) Caused by polioviruses—RNA viruses Transmission is person-to-person, primarily via the fecal–oral route; also throat secretions. Rabies Caused by rabies virus; a bullet-shaped, enveloped RNA virus Many reservoirs, including dogs, foxes, coyotes, wolves, jackals, skunks, raccoons, mongooses, bats Transmission occurs via the bite of a rabid animal which introduces virus- laden saliva; airborne transmission from bats in caves also occurs. Viral meningitis (aseptic meningitis, abacterial meningitis) Caused by many different viruses

Selected Arthropodborne Viral Encephalitides of the United States Disease Reservoirs Vectors Eastern equine encephalitis (EEE) Birds, horses Aedes, Coquilletidia, Culex, and Culiseta mosquitoes California encephalitis Rodents, rabbits Aedes and Culex mosquitoes LaCrosse encephalitis Chipmunks, squirrels Aedes mosquitoes St. Louis encephalitis Birds Culex mosquitoes West Nile virus encephalitis Birds, perhaps horses Western equine encephalitis (WEE)