1. Dr. Mohamed A. Farrag Lecture seven By
450 MIC Course: Medical Virology
Lecture seven
Mechanisms of Virus Infection and Spread
C-Virus Shedding and Transmission By
Dr. Mohamed A. Farrag
Assistant professor of Virology
Botany and Microbiology Dept., KSU

2. √ √ √ Learning outcomes By the end of this lecture students should
Recognize the sequential steps of virus infection and spread. √
Recognize different portals of virus entry into human body. √
Define viremia and its sources and types. √
Recognize different portals of virus shedding.

3. 1 √ 2 √ 3 4 The Sequential Steps in Viral Infection Virus Entry
Virus Spread 2 √
Virus Shedding. 3
Virus Transmission 4

4. Step 3: Shedding
Discharging of viruses into respiratory aerosols, feces or other body fluids or secretions.
Viruses that cause acute infections are usually shed intensively over a short time period, often 1–4 weeks and transmission tends to be relatively efficient.
Viruses, such as HBV and HIV, that cause persistent infections, can be shed at lower titers for months to years, but will eventually be transmitted during the course of a long-lasting infection.

5. Oropharynx and gastrointestinal tract
Step 3: Shedding
Oropharynx and gastrointestinal tract
Enteroviruses may be shed in pharyngeal fluids and feces. The virus replicates in the lymphoid tissue of the tonsil and in Peyer’s patches whence it is discharged into the intestinal lumen.
Reoviruses and rotaviruses can be excreted into feces from the epithelial cells of the intestinal tract
Hepatitis A virus excreted from the liver via the bile duct.
Respiratory tract
Examples: Rhinoviruses and influenza viruses
Aerosols generated by sneezing or coughing or in pharyngeal secretions that are spread from mouth to hand to hand to mouth.
Transmission is via contaminated fomites, such as handkerchiefs, clothing or toys.

6. Step 3: Shedding
Skin
A few viruses, such as variola virus, the cause of smallpox, and varicella virus, the cause of chickenpox, that are present in skin lesions, can be aerosolized and transmitted by the respiratory route.
Papillomaviruses and certain poxviruses that cause warts or superficial tumors may be transmitted by mechanical contact.

7. Mucous membranes, oral and genital fluids
Step 3: Shedding
Mucous membranes, oral and genital fluids
Viruses that replicate in mucous membranes and produce lesions of the oral cavity or genital tract are often shed in pharyngeal or genital fluids.
EX. Herpes simplex virus (type 1 in the oral cavity and type 2 in genital fluids).
Saliva: Epstein-Barr virus, a herpesvirus that causes infectious mononucleosis, sometimes called the ‘kissing disease’, and mumps virus.
Rabies virus replicates in the salivary gland and is transmitted by a bite.
Semen: HBV and HIV, may be present in the semen.
HIV-infected male, an average ejaculate contains 106 mononuclear cells of which 102–104 (0.01–1%) may carry the viral genome.

8. Step 3: Shedding Blood, urine, milk
Blood is an important potential source of virus infection in humans, wherever transfusions, injected blood products and needle exposure are common.
Acute short-term high titer viremias, such as Parvovirus B19.
Persistent viremia such as HIV, HBV, and HCV.
Several arenaviruses are transmitted via aerosols of dried urine (not important mode of transmission).
A few viruses are shed in milk and transmitted to newborns such as HIV.

9. Step 4: Virus Transmission
Following shedding, a virus can be transmitted to a new host in several different ways,
The most common mode of transmission of enteric and many respiratory viruses is probably by oral or fecal contamination of hands, with passage to the hands and thence the oral cavity of the next infected host.
Inhalation of aerosolized virus is also an important mode of transmission for respiratory viruses.
Another significant route is by direct host-to-host interfacing, including oral-oral, genital-genital, oral-genital or skin-skin contacts.
Transmissions from a contaminated common source, such as food, water or biologicals.
Transmissions of arboviruses, insect mediated transmission.

10. Perpetuation of viruses Viruses that cause acute infections
For viruses that can only cause acute infections, transmission must be accomplished during a relatively short time frame, frequently no more than one week of shedding.
Acute viruses are usually ‘fade out’ and disappear.
Prior to measles immunization, measles periodically disappeared in populations of , only to cause an outbreak when it was re-introduced.
Viruses that cause persistent infections
Viruses that cause persistent infections may be transmitted over a long period of time, in some cases for the lifetime of the infected host.
Examples: HIV, HBV and VZV