Chapter 1 Introduction

Introduction

Many important diseases in animals are caused by viruses. Some are important because they frequently cause death; among such are rabies, hog cholera, Newcastle disease, feline panleukopenia, etc. Others are important because they are very contagious, among such are foot-and-mouth disease, influenza and swine vesicular disease. Still others such as bluetongue virus can cause congenital abnormalities; and finally there are viruses that can cause tumors in animals and humans.

The vast majority of viral infections occur without overt symptoms (subclinical). Thus the destination between infection (viral multiplication in an infected host) and disease (the clinical signs due to viral multiplication and its resultant tissue injury) is a critical one that must be made for all virus infections.

In addition to the fact that most viral infections are inapparent, it is quite clear that many viruses can cause more than one type of clinical infection. Such infections may involve more than one organ system (brain, respiratory tract) and many vary from mild to fatal. Adding further difficulty for the clinician attempting to diagnose a viral infection is the fact that the same clinical picture (e.g. acute respiratory syndrome) may be dependence on the laboratory to identify definitely the etiology of an individual disease, although in the case of certain viral diseases the clinical findings may be quite distinct.

In addition to their medical importance viruses provide the simplest model systems for many basic problems in biology. Viruses are essentially small segments of genetic material encased in protective shells. Since the information encoded in viral genomes differs from that in host cell genome. Viruses afford unrivaled opportunities for the study of mechanisms that control the replication, transcription and translation of genetic information. Knowledge of these mechanisms is fundamental to an understanding of the development and operation of differentiated functions in higher organisms. 

1.1 THE NATURE of VIRUSES

Viruses are small, obligate intracellular parasites which can only replicate in living susceptible cells. They vary in sizes, morphology, chemical composition, host range and in the effect that they have on their hosts. There are certain characters, however, that are shared by all viruses. 

1.2 HISTORIC BACKGROUND

1.2.1 . Animal Host system

• 1. Jenner, 1798. First to record experiments with viral material, established the fact that cow pox could protect human from getting smallpox.
• 2. Pasteur, 1881 introduced the systematic use of laboratory animals for recognition and study of rabies. He proved that rabies virus could be inoculated into a highly susceptible organ or tissue, eg. the brain of a rabbit and caused mortality. When the rabbit died he inoculated brain material from it into another rabbit. With each subsequent passage, the virus caused a more serve disease in rabbit and less severe disease in dogs.
• 3. Iowanowski, a Russian, in 1892 was probably the first to record that tobacco mosaic (a virus disease) can be transmitted by a suspension filtered through a bacterial-proof filter. (proved virus is filterable).
• 4. Loeffler and Frosch in Germany reported in 1898 that foot-and- mouth disease of cattle was caused by an agent that readily passed bacterial proof filters and could not be seen with the microscope. It was the first animal virus discovered.
• 5. Beijerinck in 1898 considered the infectious agents in bacteria- free filtrate to be living but fluid (non particulate), and introduced the term "virus" (means poison in Latin).
• 6. Sanarelli in 1898, proved that a highly contagious rabbit tumor (myxomatosis) was caused by a virus.
• 7. Walter Reed, 1902, reported that yellow fever virus recognized human virus) was present in the blood of patients during the first three days of the febrile illness, that the virus could be transmitted by mosquito bites, and that a number of days had to elapse before the mosquito could further transfer infection, thus defining an extrinsic incubation period.

  1.2.2 Chick embryo culture and cell culture

  Modern Virology has been heavily dependent on the development of chick embryo and cell culture. Cell cultures were initially introduced during the late 1920s.

• 1. Goodpasteur, in the 1930's, introduced the use of chicken embryos to provide another source of susceptible tissue for virus inoculation and growth.
•  2. Enders, 1949, shown that cell cultures were able to support the growth of poliovirus ushered in the modern era of virology and initiated a number of studies that led to the isolation of many of the viruses involved in important human and animal diseases. 

  1.2.3 Bacterial viruses and genetics

• 1. D'Herelle in 1917, discovered some viruses parasitized bacterial cells, the b acteriophage. His discovery paved the road to the study of virus-cell interactions.
• 2. Delbruck and Luria in the late 1930's, using bacteriophage to study quantitative methodology. Carefully developed systems for studying lytic interaction of the bacteriophage, and helped to define the meaning of many of the techniques and technical approaches needed to fully evaluate important genetic concepts.
• 3. Hershey and Chase, 1952, proved that the genetic information of bacterial viruses was encoded in their nucleic acids.

  1.2.4 Tumor indication by viruses

• 1. Rous in 1911 discovered that a virus produced sarcoma in chickens, subsequently other viruses were shown to be capable of producing tumors in animals.

  1.2.5 Biochemically, molecular biology, and cell biology.

• 1. Stanley in 1935, demonstrated that plant viruses consisted of only a nucleic acid and protein, and was able to crystallize tobacco mosaic viruses. Thus, proved that an agent able to reproduce in living cells behaved under certain conditions as typical macromolecules.
• 2. Temin and Baltimore in the 1970's, studies RNA tumor viruses, discovered the presence of a reverse transcriptase enzyme carried by these viruses.