virology lec. 6 mechanisms of viral entry and spread of infection in the body

Preferred routes of entry

Viruses enter hosts through skin or mucous membranes by:

• The respiratory tract

• The gastrointestinal tract (alimentary canal)

• The genital tract

• The conjunctiva

• Crossing the placenta (bloodborne viruses)

Explain rabies transmission

From tiny bat bites or scratch

Incubation period usually long enough for post-exposure rabies

vaccination to prevent disease

Explain influenza virus transmission

Thru the respiratory tract and cause respiratory symptoms

Explain measles transmission

Thru the respiratory tract and cause generalized symptoms, usually w/o respiratory syndromes

Respiratory tract entry

• Small particles (viruses and bacteria) inhaled

Ex. Bacillus intractus, rhinovirus, coronavirus, influenza A and B

• Antibodies as defense:

• IgG and IgA

Human viruses that initiate infection thru the respiratory system tract and cause generalized symptoms, usually w/o respiratory symptoms

Mumps, measles, rubella, hantaan, ‘hand, foot, and mouth disease’

GI tract entry

• Viruses enter though oral-fecal route

• mAll naked

• Inflame GI tract

Ex: rotavirus, poliovirus, hepatitis A and E viruses, norovirus

Genital tract entry

• sexually transmitted infections (STIs) that cause sexually transmitted diseases (STDs)

Ex: HIV-1 and HIV-2, hepatitis B and C, Ebola, Zika, herpes

Conjunctiva entry

• thin, transparent tissue that covers outer surface of eye

• Diseases:

• Epidemic keratoconjunctivitis (EKC) caused

by foreign bodies in the eye

• Conjunctivitis (pink eye) sometimes caused

by enterovirus type 70

• Conjunctivitis and Zika virus

• Uveitis and Ebola virus

Other routes of entry

Skin entry

• outer layer of skin is dead cells

• Viruses require metabolically active cells and must enter through breaks in skin

Bone marrow or organ transplants

• in surgery immunosuppressant in taken to avoid organ rejection

Transfusion and blood products

• test for hepatitis C or HIV

Latrogenic induction

• happens in hospital or clinic

• You go the hospital to get something done but return home w/ infection

Mechanisms of viral spread or pathogenesis

replication and infection within the host

• primary viremia

• Systemic infection

Localized viral infections

Primary viremia

where virus enters and is localized.

Ex. Covid-19 nose

Systemic infection

• spread via blood stream and peripheral

Ex; polio goes thru GI and nervous.

Herpes simplex 1, throat → lymphoid cells, CNS

Herpes simplex 2, urogenital tract → lymphoid cells, CNS

Localized viral infections

• concentrated, not spread to other locations of body

Ex: HPV, warts and papillomas

Important target organs

• skin- rashes (exanthems) As macules, papules, vesicles, or pustules

• Lungs and liver

Lungs: respiratory tract viruses

Liver: hepatitis viruses

• kidneys and heart

Infected by only a few viruses

CMV and hantaviruses (kidneys)

Coxsackie B (heart)

Neurotropic viruses

• Infect nervous system (poliovirus, HSV-1 & 2, Varicella zoster and rabies viruses)

• HSV-1 infects epidermal cells and macrophages of the

dermis

• latency- dormant but goes into motion from a stressor

• First neurons infected are located in PNS.

• CNS entry: Crossing the blood–brain barrier

• Blood–brain barrier is semipermeable

Neural entry

1. Through nerves connecting the PNS of the body to the CNS

2. Through the olfactory nerves

3. Through the bloodstream during viremia

Methods of Viral transmission during pregnancy

• transplacental- baby in womb w/ placenta

• Perinatal- hours to days of delivery

• Postnatal- breast feeding. Ex; HIV

Transplacental transmission

• very little known about how viruses cross placenta

• Placental anatomy- created from embryo- tissues foreign to mother

• Immune status changes during pregnancy. In pregnancy, women are immuno compromised to not create antibodies against baby.

What can severe infections cause during pregnancy?

Spontaneous abortion, stillbirth, and neonatal death

Less lethal viruses during pregnancy

Congenital malformation (teratogenic effects)

Zika virus (fetal cerebral malformation (microcephaly)

Four patterns of viral infection

1. Acute infection

2. Acute infection followed by persistent latent infection

3. Chronic infection

4. Slow infection

Acute infection

Rapid onset of viral reproduction w/ short period disease and immune system clearance and memory

• influenza virus, rotavirus, rhinovirus, measles

Persistent infection

is a long-lasting infection where a pathogen is not completely cleared by the host's immune system, allowing it to remain in the body for months, years, or a lifetime.

Ex; DNA virus such as Herpesviruses (HIV)

Chronic infection With continuous shedding of virus

is a type of persistent infection where a virus continually replicates and is actively released (shed) by the host over months, years, or a lifetime.

Ex; HBV and HCV

Slow infection

Have long incubation periods and lead to slow, progressive diseases (HIV)

• integrate into CD4 cells & leave genome to replicate

Shedding of human viruses

• usually shed from their routes of entry

• Viruses infecting brain are shed through other organs, which are the primary sites of replication (Rabies is transmitted by contact w infected saliva)

• Influenza- gets in respiratory, leaves, infects others

Virus shedding and zoonosis potential

• Diseases spread from animals to humans

• Viruses with segmented genomes are emerging pathogens

• Ex: Hantaviruses, SARS, MERS, rabies, Ebola

• Bushmeat a major source

Factors affecting virus survival

• Composition of the virus (enveloped viruses are more sensitive to degradation than naked viruses

• Whether viruses present in human and animal wastes

• Temperature

• Humidity

• pH

Viruses are more ….. in …..than ……

Viruses are more stable in water than bacteria,

Until a person drinks it and activates inside body for host