Micro - 4th Asessment - Epidemiology Of Viral Infections - 29 Jan 2007

EPIDEMIOLOGY OF VIRAL INFECTIONS

Fourth Medical, 2007 Prof. Widad Al-Nakib, FRCPath.

Aspects of Viral Epidemiology • Viruses must enter cells to multiply • Transmission may be horizontal (between different individuals), or vertical (between mother and offspring either before, during, or immediately after birth).

Portals of Entry and Exit Portals of entry and exit in horizontal transmission include all body surfaces, or the blood stream, by arthropod bite Vertical transmission may occur in the ovum, via the placenta, during birth, or in the colostrum or milk The mode of exit is not necessarily the same as the portal of entry.

Viral Zoonoses

• Some human viral infections are acquired from an animal source, which may be via an arthropod in which the virus may multiply.

Epidemiologic Features of Viral Infections • • • • •

Each kind of viral infection is characterized: By a particular mode (s) of transmission A reasonably well-defined incubation period A typical period of communicability A proportion of sub-clinical cases

Many infections have a seasonal incidence.

Portals of Entry and Exit

Skin Penetration Examples: 1. HSV-1 and 2 2. HBV and HCV, through needles and trauma 3. HIV 1 and HIV 2, through needles and trauma 4. Molluscum contagiosum virus (a poxvirus) 5. Human Papilloma viruses (HPV) Communicability Period Long to Very Long

Arthropod Bite Arthropod-Borne viruses or Arboviruses (varies species) • Mosquito-Borne 1. Dengue virus 2. Japanese Encephalitis Virus (JEV) 3. Eastern Equine Encephalitis Virus (EEV) 4. Yellow Fever Virus (YFV) B) Tick-Borne 1. Tick-borne Encephalitis Virus (TEV) 2. Crimean-Congo Hemorrhagic Fever Virus (CCHV) Communicability Period Short

Respiratory Route Local Infections 1) Rhinoviruses, 2) Coronaviruses, 3) Influenza and parainfluenza viruses, 4) Respiratory Syncytial virus, 5) Adenoviruses Communicability Period Short

Generalized Infections 1) Rubella virus, 2) Measles, 3) Mumps, 4) Enteroviruses 5) Smallpox virus, 6) Varicella-Zoster virus,7) Epstein-Barr virus 8) Cytomegalovirus, 9) Hantaan virus, 10) Parvovirus B19 Communicability Period Moderate

Gastrointestinal Route 1. Rotaviruses 2. Caliciviruses 3. Astroviruses 4. Some adenoviruses 5. Enteroviruses, e.g. polio, coxsackie- and echoviruses 6. Hepatitis A virus (HAV) 7. Hepatitis E virus (HEV) Communicability Period Moderate to Long

Genital Route 1. HSV-2 2. Hepatitis B virus (HBV) 3. Hepatitis C virus (HCV) 4. HIV 1 and 2 5. Human Papilloma viruses (HPV) 6. Cytomegalovirus (CMV) 7. HTLV-I and II Communicability Period Very Long

Ocular Route 1. Adenoviruses 2. Enterovirus 70, Coxsackie A 24 3. HSV-1 and 2 4. Vaccinia and Smallpox virus 5. Cytomegalovirus (CMV) 6. Human Papilloma viruses (HPV) Communicability Period Moderate to Long (weeks to years)

Blood-Borne Transmission 1. Hepatitis B virus (HBV) 2. Hepatitis C virus (HCV) 3. Hepatitis D virus (HDV) 4. Other ‘new’ hepatitis viruses (e.g. G virus) 5. HIV 1 and 2 6. HTLV-I and II viruses Communicability Period Very Long ( years & could be for life of the individual)

Vertebrate-Borne Viruses 1. Rabies virus 2. Hantaan virus 3. Poxviruses (monkeypox, cowpox, Orf ) 4. Crimean-Congo hemorrhagic fever virus 5. Arenaviruses (lymphocytic choriomeningitis virus, Lassa fever virus, Junin, Machupo virus) 6. Influenza virus 7. Ebola and Marburg virus (? Monkeys) 8. Rift valley virus Communicability Period Variable

Incubation Periods Short 2-5 days Respiratory and gastrointestinal infections Moderate 2-3 weeks Mumps, rubella, measles, chickenpox Long 4-52 weeks Hepatitis A and E (approx. 4 weeks) Hepatitis B (6-24 weeks), hepatitis C (6-12 weeks), Rabies (4-16 weeks), warts (8-20 weeks) Very long 1-10 years HIV/AIDS

Vertical Transmission • Vertical transmission refers to the transfer of virus from parent to offspring, and may occur via the ovum, across the placenta, during birth, or via the mother's milk • Viruses that cross the placenta include rubella virus and cytomegalovirus, and HIV, which may cause congenital defects or severe neonatal disease

Vertical, Horizontal & Perinatal Transmission • Vertical transmission of cytomegalovirus may occur through the mother's milk, and both cytomegalovirus and herpes simplex virus type 1 can be transmitted from parents to infants by salivary contamination. Then, because of its long latency and the periodic recurrence of lesions, the same virus may be transferred to the next generation • Horizontal transmission. In small, isolated human populations, infections with varicella-zoster virus, chickenpox may be maintained by a similar cycle, zoster in the grandmother causing chickenpox in the grandchild • Perinatal transmission of hepatitis B virus is important in much of Africa and Asia because it is common and often produces a persistent infection that may lead to cirrhosis of the liver or primary hepatocellular carcinoma