Viral infections: Epidemiology, Pathogenesis and Pathology

Question 1

Transmission of viral diseases

What is horizontal transmission in virology and what are the common routes?

Answer 1

Horizontal transmission in virology refers to the spread of infectious agents from one individual to another within the same generation.
Common routes of horizontal transmission include: (a) respiratory route, (b) faecal-oral route, (c) venereal route, (d) cutaneous route [animal and insect bites and mechanical abrasions can facilitate penetration of virus], (e) pareneteral inoculation [by transfusion of infected blood or blood products and sharing of the needle among drug abusers]

Question 2

Transmission of viral diseases

What is vertical transmission in virology and what are the common routes?

Answer 2

This refers to the transmission of infection from mother to infant which may occur by:
(a) Transplacental trasmission
(b) Perinatal transmission
(c) Transmission during lactation

Question 3

What is zoonotic transmission in virology?

Answer 3

This refers to the process by which viruses are transmitted from animals to humans.

Question 4

What are two major public health concerns with regards to viral infections in the community, and which accelerate the spread of viruses.

Answer 4

(1) Inapparent infections: Asymptomatic individuals constitute a major source of viral dissemination.
(2) Latent infections: Many viruses, such as herpesviruses cause latent infections and individuals transmit the virus onto contacts.

Question 5

Explain the following effects of viruses on their host cells.
(a) cytolytic/cytocytic growth
(b) noncytocidal productive growth

Answer 5

(a) cytolytic/cytocytic growth: this refers to the process where a virus infects a cell, replicates, and ultimately causes the cell to burst (lyse), releasing new viral particles. The susceptible host cells are termed permissive cells.

(b) noncytocidal productive growth: In noncytocidal productive growth, the virus replicates within the host cell without causing immediate cell death. The infected cell continuously releases new virions over time. This can lead to chronic infections where the virus persists in the host for extended periods.

Question 6

Explain the following effects of viruses on their host cells.
(a) abortive infections
(b) transformation

Answer 6

(a) abortive infections: Abortive infections occur when a virus enters a host cell but fails to complete its replication cycle. Various factors, such as the lack of necessary cellular machinery or an immune response, can prevent the virus from replicating successfully. No new viral particles are produced, and the infection does not spread. The host cell may survive or die depending on the extent of the viral interference with cellular functions.

(b) transformation: Transformation refers to the process by which a virus induces a permanent genetic change in the host cell, often leading to uncontrolled cell growth and cancer. Certain viruses, such as oncogenic viruses, integrate their genetic material into the host cell’s genome. This can disrupt normal cellular regulatory mechanisms and promote uncontrolled cell division. The transformed cells may proliferate abnormally, leading to the formation of tumors. Examples include the human papillomavirus (HPV) causing cervical cancer and Epstein-Barr virus (EBV) associated with certain lymphomas.

Question 7

What are cytopathic effects of viruses on host cells?

Answer 7

Cytopathic effects (CPE) refer to the structural changes in host cells caused by viral infections.

Question 8

List and briefly explain six types of cytopathic effects.

Answer 8

(1) Cell lysis
The virus causes the host cell to burst (lyse), releasing new viral particles.

(2) Cell rounding and detachment
Infected cells round up and detach from the surface they are growing on. [This effect is seen with herpesviruses, adenoviruses, and picornaviruses.]

(3) Syncytium formation
Infected cells fuse with adjacent cells to form large multinucleated cells called syncytia. [Paramyxoviruses and coronaviruses are known to cause syncytium formation.]

(4) Inclusion bodies
Viral components or altered host cell structures accumulate in the nucleus or cytoplasm, forming visible inclusion bodies. [Rabies virus forms Negri bodies in the cytoplasm, while adenoviruses form inclusion bodies in the nucleus.]

(5) Vacuolization
The formation of vacuoles (large vesicles) within the cytoplasm of infected cells. [Polyomaviruses and papillomaviruses can cause vacuolization.]

(6) Chromosomal Margination and Breaking
The virus causes the chromatin in the nucleus to margin and break. [Herpesviruses can cause these changes.]