1. Chapter 14: Animal Viruses

2. 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

3. Entry by membrane fusion

4. Entry by endocytosis

5. Replication of nucleic acid and generation of mRNA

6. Release by budding

7. 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

8. 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

9. Can you identify some examples of viruses that produce an acute viral infection?
Are they naked viruses, or viruses with envelopes?

10. Persistent infections
Virus is continually present in the body, released by budding
Three categories
Latent infections
Chronic infections
Slow infections

11. 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

12. Latent Viral infections
All of these viruses are in the Herpesviridae family

13. 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)

14. Herpes Simplex virus-1

15. HSV-1 reactivation

16. Herpes simplex-1 HSV-1 causes fever blisters, HSV-2 genital herpes
Symptoms: fluid filled skin lesions
Treatment: Acyclovir

17. 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

18. Epstein Barr Causes infectious mononucleosis
Acquire by saliva, incubation period is 4-7 weeks
Identify by
-lobed lymphocytes
-heterophile antibodies
-fluorescent antibody tests

19. 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

20. Chronic Viral infections

21. Hepadnaviridae family
dsDNA virus, enveloped
Hepatitis B
-passes through intermediate stage (RNA)
-three particles in blood
Dane
filamentous
sphericle
-exposure through blood/body fluids

22. 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

23. 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

24. 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

25. Retrovirus

26. 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

27. 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

28. Viruses associated with cancers

29. If multiple forms infect one cell…reassortment can occur

30. 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

31. Influenza virus

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

33. Cell Culture

34. Quantitation of viruses: count plaques

35. 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

36. Prions Contain no nucleic acid
Abnormal protein promotes conformational change to normal protein
Results in damage to neurons…transmissible spongiform encephalopahthies

37. Brain with spongiform encephalopathy

38. Infections caused by prions

39. Mechanism of prion replication

40. 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

41. 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

42. Pathology of rabies