Medical Virology (MLSC 441) Past Paper PDF

WHAT IS MEDICAL VIROLOGY? Medical Virology is an aspect of Medical Science that deals with the study of viruses that are of medical importance. In other words, it is the study of the biology, pathogenesis, pathogenicity, epidemiology, surveillance, monitoring,...

WHAT IS MEDICAL VIROLOGY? Medical Virology is an aspect of Medical Science that deals with the study of viruses that are of medical importance. In other words, it is the study of the biology, pathogenesis, pathogenicity, epidemiology, surveillance, monitoring, diagnosis, control and prevention of viruses that cause diseases in humans. What is a VIRUS? A virus is a sub-microscopic, obligate, parasitic entity capable of replicating self within a living susceptible host cell. Terms Sub-microscopic- means it can’t be seen with naked eye, but with an electron microscope. Obligate- means it can only survive within a host. Parasite- it survive at the expense of its host. Entity- means it is a living being. Replicating- means can reproduce and generate offspring. Susceptible host- means the host must be vulnerable to virus attack i.e. lack resistance to its infection. DIPHASIC LIFE STYLE OF VIRUSES Viruses have been declared as living and non-living entity based on the following reasons: LIVING FEATURES OF VIRUSES WITHIN HOST i.They can metabolize. ii.They can reproduce/multiply. NON-LIVING FEATURES OF VIRUSES OUTSIDE HOST i.The are inert i.e. do not metabolize outside their host. ii.They exist as crystals. ATTRIBUTES OF A VIRUS Small size relative to other microorganisms Their genome is either composed of DNA or RNA and not both. They are obligate agents i.e. majority survive within living hosts. Being intracellular, few drugs are available to treat viral infection, unlike bacterial infection They are acellular (non cellular structure) They are non-motile. They lack tRNA. They can be visualized with electron microscope. They cannot be Gram stained, but some of them are demonstrable with sellar stain. They are not susceptible to antibiotics but are sensitive to antiviral agents. When they are inert outside the host, they are crystallizable. They are metabolically inert outside the host. They can’t be cultivated in synthetic media but in tissue culture. They are inactivated at alkaline pH. Some of them are sensitive to hypochlorite. DIFFERENCES BETWEEN BACTERIA AND VIRUSES BACTERIA VIRUSES i.They are cellular. i.They are acellular. ii.They may be motile. ii.They are non-motile. iii.They have tRNA. iii.They lack tRNA. iv.Their genome is either iv.Their genome is composed composed of DNA or RNA and of both DNA and RNA. not both. v.They can be visualized with v.They can be visualized with Compound microscope. electron microscope. vi.They can be Gram stained. vi.They cannot be Gram stained, but some of them are vii.They are susceptible to demonstrable with sellar stain. antibiotics. vii.They are not susceptible to viii.Most of them can survive antibiotics. within and outside the host. viii.They are metabolically inert ix.They can be cultivated in outside the host. synthetic media. ix.They can’t be cultivated in synthetic media but tissue culture. TRANSMISSION OF VIRUSES Direct Contact: This occurs when an infected person comes into physical contact with a susceptible individual. Examples include touching, kissing, or sexual contact. Skin-to-skin contact can transmit viruses like herpes simplex and human papillomavirus (HPV). Respiratory Droplets: Many viruses are transmitted through respiratory droplets expelled when an infected person talks, coughs, sneezes, or even breathes. These droplets can be inhaled by people nearby, leading to infections. Influenza, COVID-19, and the common cold are examples of viruses spread through respiratory droplets. Airborne Transmission: Some viruses can remain suspended in the air for extended periods, allowing them to infect people who are farther away from the source. This mode is particularly relevant in closed indoor spaces with poor ventilation. Example include measles. TRANSMISSION OF VIRUSES Fecal-Oral Transmission: Viruses can be present in an infected person's feces, and if proper hygiene measures are not followed, they can contaminate food, water, or surfaces. This can lead to infections when individuals ingest or come into contact with contaminated materials. Hepatitis A and norovirus are examples of viruses transmitted via the fecal-oral route. Vector-Borne Transmission: Some viruses rely on vectors, such as mosquitoes, ticks, or other arthropods, to transmit them from one host to another. These vectors serve as intermediaries, transmitting the virus during blood-feeding. Examples include Zika virus (transmitted by Aedes mosquitoes) and Lyme disease (transmitted by ticks). Vertical Transmission: Viruses can be passed from an infected mother to her child during pregnancy, childbirth, or breastfeeding. This mode is known as vertical transmission. HIV, cytomegalovirus (CMV), and hepatitis B are examples of viruses that can be vertically transmitted. TRANSMISSION OF VIRUSES Indirect Contact: Viruses can survive on surfaces and objects for varying lengths of time. People can become infected if they touch contaminated surfaces and then touch their face, mouth, or eyes. Good hand hygiene and surface disinfection can help prevent this mode of transmission. Bloodborne Transmission: Some viruses are primarily transmitted through contact with infected blood or blood products. This can occur through needlestick injuries, transfusions of contaminated blood, or sharing needles among drug users. HIV and hepatitis C are notable examples of bloodborne viruses. Sexual Transmission: Certain viruses, such as HIV, herpes, and human papillomavirus (HPV), can be transmitted through sexual contact, including vaginal, anal, and oral sex. Zoonotic Transmission: Many viruses originate in animals and can be transmitted to humans through direct contact with infected animals or their products. Examples include Ebola virus (from bats) and SARS-CoV-2 (believed to have originated in bats or other wildlife). TRANSMISSION OF VIRUSES - Man is either the natural or maintenance host for most viruses. - e.g Rotaviruses, polioviruses, hepatitis viruses, HIV, rubella virus, rhinoviruses, measles virus, papillomaviruses, and several herpes viruses. SOME BASIC TERMS IN VIROLOGY Viral nucleic acid core- is the space within the viral nucleus that houses its nucleic acid. Capsid i.It is the proteinous shell that encases/encloses the viral nucleic acid. ii.It is made of polypeptide units or monomers (backbone) called capsomeres. iii.It confers symmetry or shape on the virus. iv.It protects the viral nucleic acid from mechanical injury. v.It bears proteins that serves as the viral epitopes. vi.It is coded for by the viral genome. vii.It can be characterized serologically. Nucleocapsid- Viral nucleic acid + its capsid. Sometimes, a naked virus is also called a “NUCLEOCAPSID”. SOME BASIC TERMS IN VIROLOGY (CONTD.) Envelope or “peplos” i.It is the lipoprotein shell that encloses the viral nucleocapsid. ii.It is built from peptide monomers (building blocks) called peplomeres. iii.It is a stole property from the infected host cell membrane. iv.It is not coded for by the viral genome. v.It bears the viral epitopes i.e. glycoprotein spikes. vi.The glycoprotein spikes it bears can be characterized serologically. SOME BASIC TERMS IN VIROLOGY (CONTD.) Structural proteins- refers to the insoluble proteins (macromolecules) that confer structure/shape on the viral agents e.g. Capsid, envelops, epitopes/glycoprotein spikes. Non-structural proteins-refers to soluble proteins (macromolecules) that do not proteins-confer structure on the virus e.g. Viral enzymes such as transcriptase, integrase, polymerase, protease. Viroids- are an obligate, intracellular, circular RNA particle with no capsid and envelop (contains no gene that encodes them). They cause diseases in both animals and plants. Virusoids- are single stranded, circular RNA (ssRNA) particles, packaged in a stolen capsid from their helper virus, needed for them to complete their replicative cycle. They cause diseases in plants. They do not encode for their structural proteins i.e. capsid. Rous Sarcoma Virus (RSV) is an example of virusoid. It can only acquire its envelop if there is co-infection with Rous Sarcoma-associated Virus, an Avian Leukosis Helper Virus (ALHV) or cultured in cells containing “Chick Helper Factor”. Virino- is an alternative infectious, proteinaceous particle with some nucleic acid (about 10-12nm) together in association. The virus-like particle causes febrile condition in “hamsters”. Prions- are infectious, proteinaceous particles that lacks nucleic acid and capsid, and are capable of causing brain degenerating disorders in humans, animals (particularly sheep and cattle). The followings are examples of Prions: i. Bovine spongiform encephalopathy (BSE) that causes mad cow disease. ii. Sub-acute sclerosing pan-encephalopathy (SSPE) that causes scrapies in sheep. iii. Creuzt-Feldt Jakob Disease (CJD) that causes kuru among human cannibals in Congo. CLASSIFICATION OF VIRUSES The body, vested with the responsibility to classify viruses is called International Committee on Viral Taxonomy.e.i. (ICVT), formerly called International Committee on Viral Nomenclature (ICVN). Taxonomy is a branch of science that deals with the collection, arrangement and classification of organisms while Nomenclature deals with naming of organisms. In viral classification, the family, sub-family, order and genus ends with the following suffixes: -Family-viridae -Sub-family-virinae -Order- virales -Genus- virus Below are 13 criteria for viral classification: 1.Size of the virus- the largest virus is the Poxvirus, the smallest RNA virus is Picornaviridae while the smallest DNA viral family is Parvoviridae. 2.Type of nucleic acids- whether it RNA or DNA. 3.Nature and strandedness of nucleic acids – whether it is single stranded or double stranded, circular, linear, segmented, unsegmented, positively sensed or negatively sensed. 4.Morphology of virus- whether they are enveloped or naked. 5.Physico-chemical properties- whether they are susceptible to ether, chloroform, aldehyde and hypochlorite. Enveloped viruses are susceptible to ether while naked viruses are resistant to it. 6.Symmetry of virus- this is the structure or shape whether they are cubical, helical, binal or complex. 7.Nature of enzyme- HIV virus is classified as “RETROVIRUS” due to the possession of reverse transcriptase enzyme which hence, it 8.Mode of transmission- whether through insect (ARBOVIRUS= Arthropod Borne Virus) or rodent (RODOVIRUS= Rodent Borne Virus). 9.Animal of natural susceptibility 10.Place of isolation- example is Ebola virus from Ebola River in Zaire, Lassa Fever virus from Lassa town, Mokola virus from Mokola etc. 11.Animal pathogenicity 12.Mode of replication- this makes use of Baltimoore’s sheme of classification based on their nature and mode of nucleic acid transcription. 13.Cytopathic effect- makes use of morphological and physiological changes, expressed by virus-infected cells. 14.Symtomatology-is the oldest method of diagnosis (i.e. prognosis), used by physicians. Koplik spot is pathognomonic to measles while kernicterus is associated with Yellow Fever. 15.Sigla- make use of Greek acronymns is assigning name to viruses. DNA VIRAL FAMILIES 1. Adenivridae- 2. Herpesviridae- 3. Hepadnaviridae- 4. Poxviridae- 5. Plasamaviridae- 6. Parvoviridae- 7. Papovaviridae- 8. Irridoviridae-  RNA VIRAL FAMILIES 1. Arenaviridae- 2. Astroviridae- 3. Bunyaviridae- 4. Coronaviridae- 5. Deltaviridae- 6. Filoviridae- 7. Flaviviridae- 8. Orthomyxoviridae- 9. Paramyxoviridae- 10.Rhabdoviridae- 11.Retroviridae 12.Togaviridae-  VIRAL ORDER Mononegavirales- is the virus order, comprising of 3 viral families with single-stranded, negatively sensed and unsegmented nucleic acids. They are Filoviridae (Ebolavirus), Rhabdoviridae (Rabies virus) and Paramyxoviridae (Parainfluenza virus). REPLICATION OF VIRUSES Viruses just like any living creature can reproduce, but this is done within a living and susceptible host cell. Stages involved in viral replication is divided into 7 namely: i. Attachment/adsorption/adhesion. ii. Penetration/entry iii. Uncoating iv. Replication of viral nucleic acid v. Synthesis of viral proteins vi. Assembly and maturation vii. Release or escape of viral off spring Before a host is infected by a virus, there must be a specific interaction between the viral epitope and the target host’s cell receptor site. Without adhesion of the virus to the host receptor, infection cannot take place as it often leads to host resistance to viral infections. Furthermore, loss of infectivity also takes place whenever viral epitopes, envelops or infective enzymes are denatured. i. Adhesion- interaction between the viral epitopes and host’s receptor is optimum at 37oC and also, in the presence of Mg2+ and Ca2+ ions. Examples of viral epitopes or glycoproteins include gp24, gp120 and gp 160 for HIV while the receptor is CD4. ii. Penetration/entry- viruses penetrate their hosts in 3 ways namely: a.Pinocytosis, viropexis and endocytosis- this is a process by which viruses are engulfed or ingested by host macrophages and phagocytes. This process is applicable to naked viruses e.g. Adenovirus. b.Fusion- this process involve condensation and coalition of viral envelop and host cell membrane to facilitate the release of viral nucleo-capsid into the infected host cell. This activity is optimum at 37oC and also in the presence of Mg2+ and Ca2+ ions. It is also enhanced in the presence of fusion protein. This process of entry is common with the enveloped viruses e.g. Hepatitis B virus. c.Direct penetration- is a process by which viral nucleic acid is injected into the host’s cell via post-puncturing by tail-pin. This process is common with bacteriophages. iii. Uncoating- at this stage their is dissolution of the viral capsid in order to release the viral nucleic acid into the host cell. This process is mediated by protease enzyme and it is optimum at 37 oC and in the presence of Mg2+ and Ca2+ ions. After this process the virus integrates itself (i.e. nucleic acid) into the host genus and the entire synthetic activity of the host cell is short down and re-directed towards the synthesis of viral molecules. After the release of viral nucleic acid, there is total disappearance of the viral nucleic acid as it cannot be demonstrated. However, a host of synthetic activity takes place in the host cell. Latency- is a period that lapses between the disappearance of the infectious viral particle within the infected host cell and the reappearance of its off spring within same cell. It is also referred to as “dormancy” because there is no expression of symptoms at this stage. Incubation period- is a period that lapses between the disappearance of the infectious viral particle within the host cell and the re-appearance of its off spring in the supernatant fluid/plasma of the host. This process is also tagged as the “re-activation state” and is characterized by symptoms. The virus with the shortest incubation period of 12hours is the Parvoviridae. iv.Replication of viral nucleic acid- depending on the virus, RNA nucleic acid is mediated by RNA polymerase while the DNA nucleic acid is mediated by DNA polymerase. Replication of all RNA viruses takes place in the cytoplasm except HIV which takes place in the nucleus. Single stranded RNA of HIV is converted to ssDNA by reverse transcriptase and is integrated into the host genome as “Pro-viral DNA” by intrgrase enzyme. Replication of DNA nucleic acid takes place in the nucleus except Herpes virus whose replication takes place in the cytoplasm. v. Synthesis of viral proteins- may be either structural proteins (i.e. epitopes and capsid) or non-structural proteins (i.e. enzymes e.g. Polymerase, transcriptase, protease, integrase, reverse transcriptase). Two processes are involved in viral protein synthesis, they are: a. Transcription- is a process by which the single stranded viral nucleic acid is transcribed into mRNA and this process is mediated by either cellular transcriptase (used Hepatitis B virus) or virion-associated transcriptase (used by Poxvirus). After the synthesis of mRNA, the molecule moves to the ribosome and forms a template for the initiation of “translation”. b. Translation- is a process by which the codon sequences on the mRNA are converted into proteins. This process is mediated by rRNA and tRNA. The rRNA reads the codons while the tRNA arrange the amino acids until the entire reading frame is exhausted. The glycoproteins of enveloped viruses are normally inserted or sandwiched into the membrane of cell compartment where their off springs do escape. Viroplam- is the portion in a virus-infected cell where optimum replication of the viral particles take place. It may be at the nuclear membrane, mitocondria membrane, cytoplasmic membrane or golgi body membrane. Replication of DNA viruses take in the nucleus except for Herpesviridae, Adenoviridae, Poxviridae, Papillomaviridae and Papovaviridae that takes place in the cytoplasm. Replication of all RNA viruses takes place in the cytoplasm except Retroviridae which takes place in the nucleus. vi. Assembly- is the arrangement of the viral particles. The nucleic acid is enclosed in the nucleic acid core, which in turn is enclosed in the capsid. Non-structural proteins such as virion-associated enzymes may be inserted into the capsid. vii. Release or escape- off springs of viruses can escape from the infected host’s cells through 2 major routes namely: a. Lysis- this process involve bursting of the infected cells to release thousands of newly synthesized viral off springs. Naked viruses escape from the host via lysis. b. Budding-off- arrangement and budding-off of viral particles takes place in the nuclear membrane, mitochondrial membrane and golgi apparatus membrane. As the viral off spring buds-off, part of the host cell membrane are stolen by the virus to form their envelops.