1. Eucaryotic Viruses and Other Acellular Infectious Agents
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Chapter 18
Eucaryotic Viruses and Other Acellular Infectious Agents

2. Cytocidal Infections and Cell Damage
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Cytocidal Infections and Cell Damage
An infection that results in cell death is a cytocidal infection
Vertebrate viruses can harm their host cells in many ways; often this lead to cell death
Microscopic or macroscopic degenerative changes or abnormalities in host cells and tissues are referred to as cytopathic effects (CPEs).

3. Mechanisms of host cell damage and cell death
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Mechanisms of host cell damage and cell death
Many viruses can inhibit host DNA, RNA, and protein synthesis eg. Picornaviruses, herpesviruses and adenoviruses.
Cell endosome may be damaged resulting in the cell destruction.
causes release of hydrolytic enzymes into cell
Can drastically alter plasma membrane
can lead to attack of host cell by immune system
HIV destroy helper T-cells (also called T4 or CD4+ cells )

4. Other mechanisms…
High conc. Of viral proteins can have a direct toxic effect on cell and organism.
Intracellular structure called inclusion bodies are formed during virus infection. These may results from the clustering of subunits of virions within the nucleous or cytoplasm. They also may contain cell components such as ribosomes or chromatin.
These inclusion bodies can directly disrupt cell structure
chromosomal disruptions result from herpesvirus and others
transformation of host cell into malignant cell (Latin combination of "mal" meaning "bad" and "nascor" meaning "to be born"; malignant literally means "born to be bad.“)

5. Persistent, Latent, and Slow Virus Infections
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Persistent, Latent, and Slow Virus Infections
acute infections
rapid onset and relatively short duration (e.g. influenza)
persistent infections
can last many years

6. Types of persistent infections
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Types of persistent infections
chronic virus infection
virus almost always detectable
clinical symptoms mild or absent for long periods (e.g. Hepatitis B virus and HIV)
latent virus infection
In this case, virus stops reproducing and remains dormant for a period before becoming active again)
during latency, symptoms, antivirus-antibodies, and viruses are not detectable (e.g. Herpes simplex virus, varicella-zoster virus, cytomegalovirus)

7. Possible causes of persistent infections
The virus genome may be integrated into host genome
decrease in antigenicity of virus
May mutate to less virulent and slower reproducing form

8. Types of Infections and Their Effects on Host Cells
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Types of Infections and Their Effects on Host Cells
Figure 18.13

9. Copyright © The McGraw-Hill Companies, Inc
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Slow virus diseases
A small group of viruses causes extremely slow developing infections, often called slow virus disease or slow infection.
Symptoms may take years to emerge.
Measles virus occasionally produces a slow infection.
HIV also cause slow disease

10. Viruses and Cancer
Cancer is a class of diseases in which a cell, or a group of cells display uncontrolled growth (division beyond the normal limits), invasion (intrusion on and destruction of adjacent tissues).
Tumor (Latin tumere= to swell)
growth or lump of tissue resulting from neoplasia (Greek= new growth)
abnormal new cell growth and reproduction due to loss of regulation
Anaplasia (to form backward). The unregulated proliferation and loss of differentition of tumor cells result in invasive growth that forms unorganized mass of cell
It implies dedifferentiation, or loss of structural and functional differentiation of normal cells

11. Metastasis
There are two major type of tumors with respect to over all form or growth pattern. If the tumor cells remain in place to form a compact mass, the tumor is benign.
In contrast, cells from malignant or cancerous tumors can actively spread through the body in a processes known as Metastasis, often by floating in the blood and establishing secondary tumors.

12. Carcinogenesis complex, multistep process often involves oncogenes
cancer-causing genes
Carcinogenesis can be initiated by a chemical, usually a mutagen, but a cancer does not appear to develop until at least one more triggering event (possibly exposure to another chemical carcinigen or a virus) take place.
At present, viruses have been implicated in the genesis of at least eight human cancers. With the exception of a few retroviruses, these viruses have dsDNA genomes.

13. Viruses implicated in human cancers
Epstein-Barr virus
Burkitt’s lymphoma
nasopharyngeal carcinoma
Hepatitis B virus
liver cancer
Hepatitis C virus
Human herpesvirus 8
HIV
Kaposi’s sarcoma
human papillomavirus
cervical cancer
HTLV-1 (Human T-cell lymphotropic virus)and HTLV-2
leukemia

14. Possible mechanisms by which viruses cause cancer
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Possible mechanisms by which viruses cause cancer
carry oncogene into cell and insert it into host genome
altered cell regulation due to changes in kinase activity or production of regulatory proteins
insertion of promoter or enhancer next to cellular oncogene

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