Course: Virology
Lecturer: Dr. Weam SaadLecture: Epidemiology of viruses
Epidemiology of viruses; Host cells, modes of Transmission, Cytopathogenesis and control
Different viruses have different mechanisms for survival in nature and transmission from one host to the next. The mode of transmission used by the virus depends on the nature of the virus and the host. Viruses infect all types of cellular life including animals, plants, bacteria and fungi. And they are species-specific, for example, small pox infects only human cells while tobacco mosaic virus infects only plants. Also bacteriophages are specific to single strain of bacteria so it is used in detection of clinical infections or bacterial diagnosis and called phage typing.
Outbreaks of a viral infection often result from spreading of a virus (e.g., hepatitis A) into a new location coming from a common source (e.g., food) and can stop after the source identification. Epidemics occur in a larger geographic area and generally result from the spreading of a new strain of a virus into a non-immunized population. Pandemics are worldwide epidemics, usually resulting from the spreading of a new virus (e.g., HIV). Pandemics of influenza A occur approximately every 10 years as the result of new strains of the virus. Endemics occur when a pathogen is native to a place or become a biota in a defined or specific area and continuously present in people living in that area (e.g. tropical endemic Influenza virus).
Viruses can be transmitted by the following ways:
Direct transmission from host to host by contact, transmission ways include droplet or aerosol infection (eg, inflenza, measles, smallpox); by sexual contact (eg, papillomavirus, hepatitis B, herpes simplex type 2, human immunodefiiency virus); by hand– mouth, hand–eye, or mouth mouth contact (eg, herpes simplex, rhinovirus, Epstein-Barr virus); or by exchange of contaminated blood during blood transfusion (eg, hepatitis B, human immunodefiiency virus HIV).
Indirect transmission by the fecal–oral route (eg, enteroviruses, rotaviruses, infectious hepatitis A) or by fomites or non-living objects that able to carry virus (eg, Norwalk virus, rhinovirus).
Transmission from animal to animal, humans can be an accidental host. Spread may be by bite (e.g. rabies) or by droplet or aerosol infection from rodent-contaminated (eg, arenaviruses, Hantaviruses).
Transmission by arthropod vector (eg, arboviruses, now classified as togaviruses, flviviruses, and bunyaviruses). Arthropod act as permanent hosts and reservoirs. E.g. for human is the Human–arthropod cycle of Urban yellow fever and dengue.
Viruses Cytopathogenesis include four outcomes of a viral infection of a host cell are:
Failed infection (abortive infection)
Cell death (lytic infection)
Replication without cell death (persistent infection)
Presence of virus without virus production but with potential for reactivation (latent-recurrent infection). Persistent infections may be:
(1) Chronic (non-lytic, productive).
(2) Latent (limited viral macromolecular but no virus synthesis)
(3) Recurrent (periods of latency then virus production).
(4) Transforming (immortalizing).
Some viral infections have a persistent life cycle or latent infections that last for months to years e.g. (hepatitis B, herpes simplex, cytomegalovirus, retroviruses).
Some viruses are called bioterrorism agents because they are easy in host-to-host transmission and produce high mortality rates (e.g. Variola virus or small pox and hemorrhagic fever Ebola virus).
Genetically designed viruses can be very good delivery systems for genes and called as vectors and used in gene therapy and can be used as new type of vaccines against tumors. The advantage of using viruses is that they can target specific tissues and deliver the DNA or RNA into the cell. Viruses that are used as vectors include retroviruses, adenoviruses, HSV, adeno-associated virus (parvovirus). The viral vectors are attenuated viruses, the virulence genes are replaced by foreign DNA gene, this vector can infect target cell without producing disease. For example; genetically designed Adenovirus and canarypox virus are used to carry and express HIV genes as vaccines.
The control of viruses spreading can be done by Quarantine, good hygiene, changes in lifestyle, elimination of the vector, education and immunization of the population, (e.g. Quarantine was used against smallpox), for example; the proper sanitation of contaminated objectives and materials, also disinfection of the water supply are the ways of limiting the spread of enteric viruses.