“Knowledge is a process of piling up facts; wisdom lies in their

simplification.”

VIRUSES: GENERAL PROPERTIES;

DISEASES, AND HOST RESPONSE.

``Probably the most common infectious agents``

Class of 2011 3rd yearProf. Abbas Hayat

• Definition : 1. Viruses are the smallest

infective agents. Most form of life—not only humans animals and plants, but bacteria also are susceptible to infection with appropriate viruses.

2. Obligate intracellular parasites 3. Have only RNA or DNA. A imp.

distinguishing factor from bacteria and other infectious agents.

Some medically important viruses.DNA Viruses Family Virus DiseasePoxvirus Variola,

molluscamSmallpox, molluscam contagiosum

Herpes virus Herpes simplex, vaicella zoster, cytomegalo virus, EB virus.

Herpes, chicken pox, shingles, infectious mononuleosis.

Adenoviruses Adenoviruses Sore throat, conjunctivitis, G.I.T. infections.

Hepadnavirus Hepatitis B Hepatitis

Papoviruses Pappiloma, polyoma, SV 40

Warts, progressive multifocal leucoencephalopathy

Parvoviruses B19 Erythema infectiosum, hemolytic crisis.

Some medically important RNA viruses Orthomyxovirus

Influenza Influenza

Paramyxoviruses

Para influenza,Respiratory syncytial, measles, mumps

Respiratory inf. Measles, mumps.

Rhabdoviruses Rabies RabiesPicornaviruses Enteroviruses,

rhinoviruses, hepatitis A

Meningitis, paralysis, colds, hepatitis.

Flaviviruses Flavivirus Encephalitis, febrile disease.

Bunyaviruses Buniyaviruses,Hantan virus

Encephalitis, febrile disease

Reoviruses Rotavirus gastroenteritis

Arenaviruses Lynphocytic choriomeningitis, Lassa virus.

Meningitis , febrile disease

Retroviruses HTLV -I, II, HIV -I, II

T cell leukemia,-lymphoma, AIDS

• Virus like particles, or incomplete viruses.

• Bacteriophage: bacteriophage from 'bacteria' and Greek φᾰγεῖν phagein "to eat") is any one of a number of viruses that infect bacteria. Bacteriophages are among the most common biological entities on Earth

• Plasmid: DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. Are double-stranded and, in many cases, circular. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms

• Prion : infectious particles composed of protein and no detectable nucleic acid or envelope. cause of certain slow diseases like Creutzfeldt-Jacob Disease (CJD) in human and scrapie in sheep

Three main properties :1. Small size. 10nm- 300nm. Bacteria are approx. 1000nm and R.B.C 7500nm.

2. Genome . DNA or RNA

3. Metabolically inert . No metabolic activity outside susceptible host cells;

No active Ribosome's or protein synthesizing equipment

Some have enzymes within their particle.

Cannot multiply in inanimate media but only in living cells.

On entry into susceptible cell, viral genome or nucleic acid is transcribed into ----or itself acts as --------virus specific messenger or mRNA which then directs the replication of new virus particles.

STRUCTURE OF VIRUSES • Virus basically consists of a core of

nucleic acid surrounded by a protein coat.

• Protein coat protects the viral genome from inactivation by adverse environment factors e.g. nucleases in the blood stream. It is antigenic and often responsible for stimulating the production of protective antibodies.

The structure which makes up a virus particle are known as :

• VIRION : Intact virus particle.

• Capsid : The protein coat .

• Capsomere : the protein structural units of which the Capsid is composed.

• Nucleic acid : RNA or DNA

• Envelope : particles of many viruses surrounded by a lipoprotein envelope, containing viral antigens, but also partly derived from the plasma or, in some cases the nuclear membrane of the host cell.

THREE TYPES OF SYMMETRY • 1. Cubic. in which particle are

icosohedral protein shells with nucleic acid contained inside.

• 2. Helical. In which the particles contains an elongated nucleocpsid ; the capsomere are arranged round the spiral of nucleic acid. Most helical viruses possess an outer envelope.

• 3. Complex. Does not confirm to any helical or cubic symmetry.

CUBIC SYMMETRY

ICOSOHEDRAL & HELICAL SYMMETRY

THREE METHODS OF CULTIVATION

• Tissue Culture . Cells obtained from man or animal are grown in artificial culture in glass vessels in the Lab, Cells are living and metabolizing and can support viral replication.

• Chick Embryo. Fertile chick eggs are used.

• Lab. Animals. Mice Rabbits Ferrets and Monkeys

THREE EFFECTS OF VIRUSES ON CELLS

1. Cell Death . The infection is lethal : it causes a cytopathic effect (CPE) which kills the cells.

2. Cell Transformation. The cell is transformed from a normal to a malignant or cancerous cell

3. Latent Infection . The virus remains within the cell in a potentially active state but produces no obvious effect on the cells function.

THREE METHODS OF PENETRATION

THREE TYPES OF VIRAL DISEASESViruses are imp. and common

cause of human disease especially in children.

1. Most are mild and patients makes a complete recovery.

2. Many infections are silent and the virus multiplies in the body without causing any symptoms.

3. Few are severe and always have a high mortality rate.

Host responses to viral infection:

SPECIFIC IMMUNE RESPONSES (Adaptive Immunity)

NON SPECIFIC DEFENSES (Innate Immunity)

1. Neutralizing antibody response

1. Interferon - alpha-interferon mainly, stimulated in surrounding cells

2. Cell-mediated responses (i.e., cytotoxic T lymphocytes)

2. Body fluids - stomach acid, tears

3. Complement-mediated lysis

3. Mechanical barriers - respiratory epithelium

Acute infections: • 1.Hit and run infections. Characterized by the common cold viruses (rhinoviruses).

• 2. Short incubation period (36 to 48 hours).

Pathology in the upper respiratory tract due to epithelial cell killing and secondary invasion by normal flora of oral bacteria. Symptoms last for 5-7 days eliminated by the host.

Protection not good since there are >1000 serotypes of rhinovirus. orthomyxoviruses, which include influenza A and B.

Protection against subsequent infection may be poor because these viruses can change their antigenic profile of their surface proteins, e.g. hemagglutinin (HA) and neuraminidase (NA), by either point mutations (drift), or recombination, which results in a completely different gene (shift). (Influenza viruses are capable of antigenic shift because they have segmented single-stranded RNA genomes).

Systemic infections.1. Characterized by the common childhood diseases

caused by measles, chickenpox, mumps, and rubella.

2. Incubation period is long - 14 to 21 days. There are two viremic phases during the incubation period:

Replication at primary site ---- Primary viremia ---- replication in lymphoreticular system and viscera ---- Secondary viremia ---- target organs

Several types of persistent viral infections exist:

• Latent infections: After an acute cytocidal stage, the virus becomes latent.

• The virus when latent does not replicate , produces limited mRNA. Latency may occur in non-dividing cells such as neurons (HSV I, HSV-II and VZV).

• Chronic infections: Persistence of virus after acute phase with active replication of virus. May cause disease in normal individuals, e.g. hepatitis B, or not, e.g. JC and EBV.

• Chronic infection with neoplasia: Transformation of virus infected cells usually after years of persistence, e.g. HTLV-I , HTLV-II, hepatitis B, Hep C, and HPV 16 and 18.

• The outcome can change depending on the state of the individual, i.e. whether they become immunocompromised.

• The site of viral persistence is important as it would be difficult to remain completely latent or with minimal viral transcription and translation in rapidly dividing cells such as those lining the small intestine.

• Viral persistence requires viruses to develop

ways to evade immune responses. Herpes virus.

Thank you & Remember Keep on alert for viruses around you, they can

1. transform you

2. have cyto- pathic effects on you

3. Stay latent inside you…………….

``And you can do nothing much about them…..so relax and keep going as best as you can.``