Chapter 14: Animal Viruses

How do animal viruses differ from bacterial viruses?

• Attachment or entry into the cell

• Replication of viral nucleic acid (remember eukaryotic cells have a nucleus)

• Uncoating step is required by animal viruses

• Exit the host cell by budding or shedding

Entry by membrane fusion

Entry by endocytosis

Replication of nucleic acid and generation of mRNA

Release by budding

Acute viral infections

• Usually short in duration

• Host develops long lasting immunity

• Infection of the virus results in a productive infection…host cells die as a result of infection

General Steps of Acute Viral infection

• Attachment• Entry into host cell• Targeting where it will reproduce• Uncoating of the capsid• Synthesis of proteins, replication of nucleic acid• Maturation• Cell lysis

Can you identify some examples of viruses that produce an acute viral infection?

• Are they naked viruses, or viruses with envelopes?

Persistent infections

• Virus is continually present in the body, released by budding

• Three categories– Latent infections– Chronic infections– Slow infections

Latent Infections

• Persistent infection with symptomless period followed by reactivation of virus and symptoms

• Example of latent viruses are found in the family Herpesviridae – Herpes simplex virus -1– Herpes simplex virus -2

Latent Viral infections

• All of these viruses are in the Herpesviridae family

Herpesviridae Family

• Double stranded DNA (dsDNA), enveloped virus

-herpes simplex 1(cold sores)

-herpes simplex 2 (genital herpes)

-Varicella-zoster virus (chicken pox, shingles)

-Epstein-Barr (infectious mono and Burkitt’s lymphoma)

Herpes Simplex virus-1

HSV-1 reactivation

Herpes simplex-1

• HSV-1 causes fever blisters, HSV-2 genital herpes

• Symptoms: fluid filled skin lesions

• Treatment: Acyclovir

Varicella (chickenpox) and Herpes Zoster (Shingles)

• HSV-3 causes chicken pox and latent activation known as shingles

• Acquired by respiratory route, 2 weeks later see vesicles on skin

• Vaccine established in 1995 for chickenpox

Epstein Barr

• Causes infectious mononucleosis

• Acquire by saliva, incubation period is 4-7 weeks

• Identify by

-lobed lymphocytes

-heterophile antibodies

-fluorescent antibody tests

Chronic infections

• Infectious virus present at all times

• Disease may be present or absent

• Best example is Hepatitis Type B virus– Carriers produce virus detected in blood, saliva,

and semen– Unique replication of dsDNA

Chronic Viral infections

Hepadnaviridae family

• dsDNA virus, enveloped• Hepatitis B

-passes through intermediate stage (RNA)-three particles in blood Danefilamentoussphericle-exposure through blood/body fluids

Hepatitis B

• Incubation period is ~12 weeks

• 10% of cases become chronic, mortality rate is less than 1%

• About 40% of the chronic cases die of liver cirrhosis

Flaviviridae (+) ssRNA, enveloped

• Hepatitis C virus– Obtain from blood/body fluids– Incubation period averages 6 weeks– Hard to screen blood for the virus– 85% of all cases become chronic

Slow Infections

• Infectious agent increases in amount over a long time during which there are no symptoms

• Examples are HIV found in the Retroviridae family

• Retroviruses use reverse transcriptase to replicate ssRNA

Retrovirus

Retroviridae-multiple strands of (-)RNA

• HIV

-infects Helper T cells

-requires the enzyme reverse transcriptase

-integrates as a provirus

-is released by budding, or lyses the cell

Viruses and tumors

• dsDNA viruses are most common to cause viral-induced tumors

• Cancer is result of integration of viral genes into the host chromosome

• Transforming genes are called oncogenes

• Examples: papillomavirus, herpesvirus

Viruses associated with cancers

If multiple forms infect one cell…reassortment can

occur

Orthomyxoviridae-multiple strands of (-)RNA

• Influenza virus– Consists of 8 segments of RNA– Envelope has H spikes (hemagglutinin) and N

spikes (neuraminidase)– Incubation is 1-3 days– Symptoms include: chills, fever, headache,

muscle aches, may lead to cold-like symptoms

Influenza virus

Ways to study viruses

• Since viruses grow in living cells….need a live cell to culture them– Cell culture/tissue culture– Embryonated chicken eggs

Cell Culture

Quantitation of viruses: count plaques

Proteinaceous infectious particles: PRIONS

• 1982 Stanley Prusiner proposed that there were infectious proteins

• Caused the disease “scrapie” in sheep

• Caused the “mad-cow”disease in 1987

• Human forms suggest a genetic component

Prions

• Contain no nucleic acid

• Abnormal protein promotes conformational change to normal protein

• Results in damage to neurons…transmissible spongiform encephalopahthies

Brain with spongiform encephalopathy

Infections caused by prions

Mechanism of prion replication

Picornaviridae (+) ssRNA

• Hepatitis A

-obtain through fecal-oral route, enters GI tract and multiplies

-incubation period is ~4 weeks

-symptoms include: anorexia, malaise, nausea, diarrhea, abdominal discomfort, fever, and chills lasting 2-21 days

Rhabdoviridae (-)ssRNA, enveloped

• Rabies virus

-enters the skin and multiplies in skeletal muscle and connective tissue

-virus travels along nerves to the CNS causing encephalitis

Pathology of rabies