Introduction to viruses by Dr.Ebtisam Eid,M.D.

Introduction Viruses are the smallest infectious agents and contain either RNA or DNA but not both together They are obligatory intracellular parasites, and multiply only in living cells Viruses do not possess all the enzymes necessary for multiplication ,therefore they are dependent on the parasitic cell They are able to pass through bacterial filters

General Properties of Viruses -Viruses are extremely small in size and measured in nanometers[nm] ,therefore, most viruses seen with electron microscope -Viruses are obligate intracellular parasites -Viruses are small and pass from the pores of bacterial filters that retain bacteria -Viruses have few enzymes, they depend on the host cells for protein synthesis and replication -Viruses are not affected by antibiotics -Viruses are inhibited by interferon ,antibodies and antiviral drugs[inhibit viral replication]

Icosahedral=[symmetry] icosai

Structure and morphology of viruses: Viral genome: Nucleic acid which may be RNAor DNA ,never both and called genome . It is the infectious part of virus . It carries the genetic information for virus replication Capsid : It is a protein coat that surrounds the nucleic acid core . It is formed of protein subunits called capsomeres .It protects nucleic acid from the effect of nucleases .It determine the antigenicity of the virus

Nucleocapsid: -Formed of nucleic acid and capsid . In some viruses is the complete virus e.g.[Adenovirus , picorna virus] . Envelope -It is composed of glycoprotein +carbohydrates components -The envelope containing lipid e.g,[Herpes virus,Infleunza virus ] - The envelope can be destroyed by lipid solvents [ ether , chloroform and bile salts ]

Some viruses have one or more envelopes composed of glycoproteins and carbohydrate components The glycoproteins are important antigens because of their position on the outside the virions ,are involved in neutralization reaction Viron: The complete infectious virus , which may be a nucleocapsid or nucleocapsid with envelope

Virus cultivation : -viruses differ from bacteria ,that they have no metabolic enzymes , so can not grow on artificial media --The viral growth can be detected by its pathogenicity as -Cytopathic effects - Inclusion bodies There are three methods used for cultivation of viruses: 1-Laboratory animal e.g. [mice ,rabbit,monkey] 2-Chick embryo [yolk sac, allantoic membrane ] 3-Tissue culture :By inoculating the virus to the living cell from man or animal in a test tube

Effect of physical and chemical agents on viruses : 1-Physical agents -Heat destroys most viruses at 56c for 30 minutes -cooling preserve viruses by freezing at -70 c Preserve by lyophilization [ drying from frozen state under vacum ] -Ultraviolet [UV ] rays : destroys all viruses 2- Chemical agents -Chloroform and ether destroy eveloped viruses -Glycerol 50% : kills bacteria ,and preserves viruses -Oxidizing agents : as K permanganate , iodine and chlorine kill viruses -Phenol : does not affect viruses .

Virus Classification Viruses have several structural properties 1-Size : Viruses from [20nm ]diameter sphere e.g.[Parvoviridae] can be seen with the electrone microscope up to [450nm] e.g,[poxvirus] can be seen by the light microscope 2-Nucleocapsid [helical,icosahedral or complex] e.g.[ herpes virus,poxvirus] 3-Presence of an envelope membrane e.g.[Corona virus, influenza,measles,mumps]

Most viruses can be classified by these criteria into the main groups as: Unenveloped/helical capsid Unenveloped/icosahedral capsid Enveloped/helical capsid Enveloped/icosahidral capsid

4-Type of genome [RNAorDNA ] Viruses: -DNA viruses : e.g. -pox viruses ,Adeno viruses ,Herpes viruses -RNA viruses :e.g. Retroviruses [HIV ] ,Myxo virus [Influenza virus] ,Picorna viruses [polio. Virus ] ,Toga viruses 5-According to disease they produce A-Generallized diseases : Spread via the blood-stream and affect multiple organs . These include measles ,chickenpox,yellow fever,Interoviruses

B- Diseaes affecting specific organs : a -Diseases of the CNS as poliomyelitis ,rabies,aseptic meningitis and herpes simplex b-Diseases of the respiratory system as influenza, adenovirus and respiratory syncytial virus C- Diseases of the liver as hepatitis A,B,C,D,Yellow fever and rubella virus D-Diseases of the skin or mucous membrane as herpes simplex ,warts and herpes zoster.

e-Diseases of the eye : as -adenovirus, herpes and haemorrhagic conjunctivitis f-Diseases of the gasteointestinal tract as Rota virus and enteric adenovirus g-Sexually transmitted as hepatitis B ,retrovirus[HIV] and cytomegallovirus

Virus replication : The growth cycle divided into stages : 1-Attachment [adsorption ]: -The first step in the invasion of a cell by virus is attachment to specific receptors on cell membrane 2-Entry [penetration] : The cell membrane invaginates around the virus then engulfed by the cell

3-Uncoating : -The protein coat[capsid] is removed by host cell enzymes .Virus nucleic acid is then released 4-Eclipse phase : It is the step of transcription using nucleic acid of virus to form copies of the nucleic acid and protein . Expression as double genetic information[mRNA] ,then virus use cell components to translate the[ mRNA] 5-Synthesis : Formation of many separate nucleic acid and capsid

6-Assembly : Each nucleic acid becomes surrounded by a protein coat[capsid] to form virus particle RNA virus ;most of them replicate in cytoplasme DNA virus; most of them replicate in nucleus 7-Release : - non enveloped viruses release by rupture of the infected cell -Enveloped viruses release by budding and the virus takes a part of cell membrane as an envelope

pathogenicity of the viruses At the cellular level : 1-Cytopathic effect [C.P.E] A-Most of viruses kill the cells in which they multiply. The cells recognized by morphological changes in these cells include cell lysis or necrosis B-Inclusion Bodies [I.B] It consist of masses of viral particles in the process of replication e.g.[pox virus] or remnant of viral replication e.g,[herpes] The site of development and multiplication of the virus ,may be intra nuclear as [herpes,polio and yellow fever virus ] or intracytoplasmic as [ pox ,rabies virus ] or in both as in measles P

3-Haemadsorption : Toga virus ,and myxo viruses have the ability to adsorb erythrocytes 4-Interference : The multiplication of one virus in the cell usually inhibits the multiplication of another virus entering subsequently The mechanisms of interference by -The interfering virus may occupy or destroy cellular receptors ,so preventing attachment of the other virus -It prevent mRNA of second virus from being translated

Interferon : - It is formed by any viral infected cell It is a glycoprotein substance produced by the viral infected cell , with the capacity to interfere with the multiplication of other viruses It act on the cell but not on the virus ,it makes adjacent cell resist viral infection and decrease cell division [anti -tumor]

5-Pocks or Plaques : Are localized lesions formed when newly synthesized virus escaping from infected cells to the adjacent cells to form an infected particle or localized lesion 6-Transformation : Where the nucleic acid of the virus gets incorporated with the genetic material of the cell transforming it into cancer cell [oncogenic viruses]

7-Latency: The virus does not kill the cell but remains inside it potentially active e.g.[Herpes virus] which is a latent virus in the trigeminal ganglia ,it causes lesion when the resistance of the body is lowered. B-At the host level : 1-Degenerative necrotic lesion e.g.[polio virus] 2-Hyperplasia :Produce [benign tumour] or [malignant tumour] e.g.Sarcoma virus