DNA and RNA Viruses

Characteriistics of Viruses

Obligate intracellular parasite

DNA viruses replicate and bud off the nucleus

RNA viruses replicate in and are released from the cytoplasm.

Some viruses are strictly human origin and others are zoonotic and transmitted by other means

Some cross the placenta to cause developmental distirbances

Evolutionary origin of viruses

1. Viruses may be derived from DNA or RNA nucleic acid components of host cells that become able to replicate autonomously

2. Viruses may be degerate forms of intracellular parasites

   NB. Pox viruses may exhibit evolutionary product of some cellular ancestors (large and complex)

2 types of DNA viruses

1. Enveloped

   ds genome( poxviruses, herpes

2. No enveloped

Consequences of HPB B19 replicating in immature erythroid cells

Fetal death

Severe anemia in immunocompromised patients

Aplastic crisis in sicklers

Polyomavirus

Induces tumour (JC and BK viruses)

BK- urinary complications

JC- Progressive multifocal leukoencephalopathy (PML)

Latent after primary infection and reactivated in immunocompromised individuals

Adenovirus xtics

Naked icosahedral (70-90nm)

Linear ds DNA

Fibers protrude from each penton vertex of capsomere

Over 100 serotypes . 49 cause infection of the respiratory tract.

Others cause eye and gastrointestinal infections

Used to study viral oncogenesis by tumour induction into new born hamsters

Herpesviridae

Large (150-200)

Enveloped

Icosahedral

Ds circular DNA

Causes latent infections which may last the life span of the person

8 Herpesviridae that infect humans

HSV1

HSV2

HHV6

HHV7

HHV8

Epstein Barr virus

Cytomegalovirus

Varicella-zoster: causes chicken pox. Vesicular rash,spread by direct contact with pus/ aerosol, multiplies in lungs

Xtics of Herpesviridae

Envelope has glycoprotein spikes

Asymmetrical tegument

HSV1 and HSV2

HSV1: orofacial lesions, sore throat , fever and blisters.(gingostomatitis,keratoconjunctivitis sicca,infection in trigeminal nerve)

HSV2: lesions on genitalia (14-29). Reactivated by stress,UV,fever. (Genital herpes,infections in ganglia of sensory nerves, considered STD

Cytomegalovirus

Transmitted via saliva,mucus, milk, urine , semen

Asymptomatic

Latent in bone marrow stem cell, myeloid cells

Epstein Barr virus

Affects lymphoid tissue

Transmission by direct oral contact

Cause mononucleosis (sore throat,high fever ,cervical cancer, lymphadenopathy)30-50 day incubation

Burkitts lymphoma(malaria)

Nasopharyngeal carcinoma

HHV 6

T lymphotropic virus

Transmitted by close contact

Causes roseola, an acute febrile disease

70% of multiple sclerosis patients

Can cause encephalitis,cancer

HHV7&8

HHV 7 - closely related to HHV6

HHV 8 - Kaposi’s sarcoma, common tumours of aids patients, multiple myeloma

Hepadnaviruses

Small (42-48nm)

Enveloped

Icosahedral

Causes acute and chronic hepatitis

4types infect mammals

Poxviruses characteristics

Largest (230-400nm)

Ds DNA

Unique as it replicates entirely in the cytoplasm

Pathogenic to humans( variola, vaccinia, mollusum contagiosum).

Cowpox and monkeypox can infect humans as well

Inhalation/contact

Fever malaise rash

Classification of RNA Viruses

Enveloped : ss segmented (orthomyxoviruses,Bunyaviruses,Arenaviruses) & ss non segmented ( paramyxoviruses, rhabdoviruses, Filoviruses, Togaviruses, Flaviviruses, coronaviruses) & ss encoded reverse transcriptase (retroviruses)

Non enveloped: ss(Picornavirus,Calcivirus) & ds (reovirus)

Picornavirus

Small(28-30nm)

Naked icosahedral

Ss (+) RNA

Enteroviruses and 100 rhinovirus (common cold ) affect humans

6 genera of Picornavirus

Parecho eg. Echo 22 & 23

Hepato eg. Hepatitis A virus

Entero eg. Polio

Aphtho eg. Foot and mouth viruses

Rhino eg. HRV A,B,C

Cardio eg. encephalegaly

Similar viruses to Picornavirus

Astroviruses: Associated with gastroenteritis

Caliciviruses: Norwalk virus and Hepatitis E virus

Reovirus

Ds nonenveloped

Medium (60 to 80)

Naked icosahedral

Segmented

Reclusion strain is used in head and neck cancer treatment

Arboviruses families infecting humans

Togavirus(rubella), flavivirus , bunyavirus

Others : rhabdovirus , arenavirus, reovirus

Corona virus

Longest genome RNA

7 human coronaviruses causing diseases in humans. 4mild causing 30% of common cold

Types of corona viruses

SARS-CoV-2(COVID 19)

SARS-CoV(severe acute respiratory sundrome)

MERS-CoV (Middle East respiratory syndrome

HCoV-NL63

HCoV-229E

HCoV-OC43(mild respiratory disease)

HKU1

Retroviruses

Replication is unique by reverse transcription of DNA copy from RNA genome

Lentivirus: HIV 1&2

Oncovirus:HTLV

Bunyavirus

3 segments : L,M,S

Arthropod vectors

Bunyavirus cause

Rift valley fever

California Serogroup

Sandfly fever virus

It’s a hantavirus

Secondary transmission of Crimean Congo hemorrhagic fever

Cannot be transmitted from person to person except phlebovirus

Orthomyxoviruses (80-120nm)

All influenza viruses are orthomyxoviruses

Undergo a lot of mutation

Antigenic shift and drift

Influenza Type A

Seasonal, epidemic

Acute contagious respiratory illness

Aerosols

Binds to ciliated cells of respiratory mucosa

Influenza Type A causes

Rapid cell shedding

Stripping the respiratory epithelium

Severe inflammation

Fever

Headache

Myalgia

Pharyngeal pain

Shortness of breath

Coughing

Paramyxovirus sub families and six of the seven genera that affect humans

paramyxovirinae

1. Repirovirus : parainfluenza 1,3

2. Rubulavirus: parainfluenza 2,4 and mumps

3. Mobillivirus:measles

4. Henipavirus: Hendra, nipah

   Pneumovirinae

   1. pneumovirus: RSV

   2. Metapneumovirus: Human metapneumovirus

Mumps

Painful swelling of the parotid salivary glands

Measles

Also known as red measles / rubeola

Causes sore throat, conjunctivitis, lymphadenitis, fever , Koplik spots ,oral lesions , rash, dry cough, headache

Complications: Subacute sclerosing panencephalitis,progressive neurological degradation of cerebral cortex

RSV

Infects URT

Produces synctia

Infections in children 6months / younger

Replicates in nasopharynx

Rhinitis, otitis, wheezing, croup

Also called pneumovirus

Rhabdovirus

Resembling a bullet

Envelop has spikes

Genetic arrangement similar to paramyxovirus with 5 genes

Rabies virus

Filovirus

Pleomorphic

Cylindrical

Marburg and Ebola

Bornavirus

Neurotropic and may be associated with neuropsychiatric diseases in humans

Factors enabling viruses to produce diseases

Genetic changes

Growth in privileged sites of the immune system

Possession of enzymes that help in Pathogenesis(neurominidase)

Down regulation of MHC preventing destruction by T-cells.

Host factors determining resistance

Conjunctivae(cleansing action)

Interferon

Cellular and humoral immune response

Stomach acids

Mucus

Bile

Infections may be

Productive

Abortive

Restrictive