VIRUSES

Describe some of the characteristics of viruses. Tiny, non-living particles Do not respire, move & grow Cannot reproduce on their own Must reproduce inside of a host cell Typical infection lasts 7-10 days Not given Latin names: named for disease caused, organ infected, or region detected in

Viral structure All viruses consist of 2 basic parts: –nucleic acid –protein coat Capsid: protein coat containing inner core of DNA or RNA May have tail fibers and projections to aid in attachment Some have extra outer layer called envelope Animation:

Viral recognition and attachment Virus must recognize specific receptor site on host cell Lands on host and “locks in” Attachment is specific: virus can enter and reproduce in only a few types of cells Ex: tobacco mosaic virus cannot affect human cells even though we are exposed to it daily

Describe viral reproduction. Viruses must reproduce inside of a host cell They are classified as intracellular parasites The method of entry into the host cell depends on the virus There are two main types of viral reproduction: –1. Lytic cycle –2. Lysogenic cycle

Lytic Cycle Lytic cycle: viral DNA takes over host nucleus, produces new viruses, the cell fills with new viruses, at the end the cell bursts and the cell dies

Steps of the Lytic Cycle 1.ATTACHMENT: virus attaches to the cell surface 2. ENTRY: injects DNA, takes over host nucleus

3. REPLICATION: reproduces virus parts 4. ASSEMBLY: puts virus parts together into mature viruses 5. RELEASE/LYSES: cell splits open releasing viruses that can infect others

Describe the lysogenic cycle. virus attacks cells, but does not kill immediately Viral DNA is mixed with host DNA like lytic cycle Does not immediately begin to reproduce, host may function normally while virus stays dormant Provirus can pop out at any time and begin reproducing Example: Herpes (cold sores, fever blisters)

Lytic Cycle

Viral Multiplication - Basic Steps Virus attaches to host cell Whole virus or genetic material enters host Viral DNA or RNA directs host to make viral genetic material and protein Viral nucleic acids and proteins are assembled New viral particles are released from cell

Lysogenic Pathway Latent period extends the cycle Viral DNA becomes part of host chromosome for a time Viral material integrated Viral material passed on Stimulus may cause cell to enter lytic pathway

Replication of an Enveloped Virus DNA replication Transcription of viral genes Translation Proteins Assembly

Lysogeni c Cycle

Viroids Smaller than viruses Strands or circles of RNA No protein-coding genes No protein coat Cause many plant diseases

Prions Small proteins Linked to human diseases –Kuru –Creutzfeldt-Jakob disease (CJD) Animal diseases –Scrapie in sheep –Bovine spongiform encephalopathy (mad cow disease)

Enveloped, Unsegmented Negative ssRNA Viruses Hemorrhagic Fevers –Marburg virus and Ebola virus are the causative agents –Natural reservoir and mode of transmission to humans unknown –Spread person to person by contaminated body fluids and syringes –Virions attack many cells of the body

Enveloped, Unsegmented Negative ssRNA Viruses Especially macrophages and liver cells –Uncontrolled bleeding under the skin and from every body opening Viral glycoprotein prevents neighboring cells from adhering –Allows blood to leak out of the vessels –The only treatment involves fluid replacement –Up to 90% of human victims die

Filamentous Ebola viruses Figure 25.33

Sites of known human cases of Marburg and Ebola virus Figure 25.34

Laboratory personnel working in level four biocontainment Figure 25.35

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Naked, Positive ssRNA Viruses Common Colds Caused by Rhinoviruses –Cause most cases of the common cold –Infections limited to the upper respiratory tract –A single virus is often sufficient to cause a cold –Virus transmitted by aerosols, by fomites, or via hand-to-hand contact Most commonly transmitted by direct person-to- person contact –Some immunity can be acquired against serotypes infected with in the past The number of infections tends to decrease with age

Rhinoviruses, the most common cause of colds Figure 25.1

Naked, Positive ssRNA Viruses Common Colds Caused by Rhinoviruses –Manifestations of rhinoviruses are usually characteristic –Medications can help relieve the symptoms Do not reduce the duration of the disease –Handwashing is the most important preventative measure

Naked, Positive ssRNA Viruses Diseases of the Enteroviruses –Transmitted via the fecal-oral route Ingestion of contaminated food or water Via fomites Oral contact with infected hands or fomites –Infect the pharynx and intestine Spread via the blood infecting various targets in the body –Three main enteroviruses Polioviruses, coxsackieviruses, and echoviruses

Naked, Positive ssRNA Viruses Diseases of the Enteroviruses –Poliomyelitis Polio is caused by one of three serotypes of poliovirus Last case of wild-type poliomyelitis in the Americas was in 1979 Four conditions caused by polioviruses –Asymptomatic infections –Minor polio –Nonparalytic polio –Paralytic polio »Can result in bulbar poliomyelitis

A hospital ward full of mechanical respirators Figure 25.2

Reports of naturally occurring polio in 2009 Figure 25.3

Violinist Itzhak Perlman Figure 25.4

Naked, Positive ssRNA Viruses Diseases of the Enteroviruses –Poliomyelitis Postpolio syndrome –Crippling deterioration in the function of polio-affected muscles Near elimination of polio due to development of two vaccines –Inactivated polio vaccine (IPV) »Developed by Jonas Salk –Oral polio vaccine (OPV) »Developed by Albert Sabin

Naked, Positive ssRNA Viruses Diseases of the Enteroviruses –Other diseases of enteroviruses Coxsackieviruses and echoviruses also cause human disease Infection occurs via the fecal-oral route Most infections are subclinical or produce mild symptoms

Naked, Positive ssRNA Viruses Diseases of the Enteroviruses –Poliomyelitis Postpolio syndrome –Crippling deterioration in the function of polio-affected muscles Near elimination of polio due to development of two vaccines –Inactivated polio vaccine (IPV) »Developed by Jonas Salk –Oral polio vaccine (OPV) »Developed by Albert Sabin

Naked, Positive ssRNA Viruses Hepatitis A –Hepatitis A virus is the causative agent Survives on various surfaces and is resistant to common household disinfectants –Transmitted through the fecal-oral route –Signs and symptoms are due to the patient’s immune response –Infection does not cause chronic liver disease –Complete recovery occurs most of the time

Comparison of Hepatitis Viruses Figure 25.3

Naked, Positive ssRNA Viruses Acute Gastroenteritis –Caliciviruses and astroviruses can cause acute gastroenteritis –Can cause outbreaks in day care centers, schools, and hospitals –Caliciviruses cause diarrhea, nausea, and vomiting –Astroviruses cause diarrhea but no vomiting –No specific treatment except replacement of fluid and electrolytes

Viruses of the families Caliciviridae and Astroviridae Figure 25.6

Naked, Positive ssRNA Viruses Hepatitis E –Also known as enteric hepatitis –Caused by hepatitis E virus Formerly classified as a calicivirus –Fatal in 20% of pregnant women –No treatment –Prevent by interrupting the fecal-oral route of transmission

Enveloped, Positive ssRNA Viruses Togaviridae and Flaviviridae –Enveloped, icosahedral +ssRNA viruses –Arboviruses Often transmitted by arthropods Coronaviridae –Enveloped, helical +ssRNA viruses

Togaviruses Figure 25.7

Enveloped +ssRNA coronavirus Figure 25.8

Enveloped, Positive ssRNA Viruses Diseases of Positive RNA Arboviruses –Zoonoses Animal diseases that spread to humans –Mosquitoes and ticks transmit arboviruses among animal hosts –Arthropod vectors remain infected Are a continual source of new infections –Most infections result in mild, flu-like symptoms –Arboviruses can occasionally result in second- stage infections Encephalitis, dengue fever, and yellow fever

Enveloped, Positive ssRNA Viruses Other Diseases of Enveloped +ssRNA Viruses –Rubella Also called “German measles” Rubella virus is the causative agent One of the five childhood diseases that produces skin lesions Infection begins in respiratory system but spreads throughout the body Characterized by a rash of flat, pink to red spots Infections in children are usually not serious Adults can develop arthritis or encephalitis Infection of pregnant women can cause congenital defects Vaccination effective at reducing the incidence of rubella

Hosts and transmission of viruses of WEE and EEE Figure 25.9