Fahareen-Binta-Mosharraf MNS

Disease-causing viruses often grouped by their route of transmission Enteric viruses Generally transmitted via fecal-oral route Often cause gastroenteritis Respiratory viruses Usually inhaled via infected respiratory droplets Generally remain localized in respiratory tract Zoonotic viruses Transmitted from animal to human via animal vector Sexually transmitted viruses Can causes lesions on genitalia or cause systemic infections

Viral Clearance or Persistence  majority of viral infections are cleared but certain viruses may cause persistent infections  Long-term (months / years / lifelong) virus-host interaction may take several forms Viral persistence Transforming infection ChronicLatentSlow

Acute infections Usually short in duration Host may develop long-lasting immunity Result in productive infections Produce large a number of viruses Disease symptoms result from tissue damage and infection of new cells

Acute infections Essential steps include: Attachment Entry Targeting to the site of reproduction Uncoating of virion Removing protein coat exposing nucleic acid Replication of nucleic acid/protein synthesis Maturation Cell lysis Shedding from the host Transmission to other host cells

Persistent infections Viruses are continually present in host Release from infected cells via budding Can be divided into four categories Latent infections Chronic infections Slow infections Transforming infection

Chronic infection  Infectious virus continuously detected (often at low levels)  Mild or no clinical symptoms (for most cases) e.g. HBV  Infection eventually cleared (for most cases) e.g. HBV e.g. chronic hepatitis by hepatitis B/ C virus

Latent infections Viral genome present but infectious virus not produced except during intermittent episodes of reactivations Infection is followed by symptomless period, then reactivation Infectious virus particles are not detected until reactivation Symptoms of reactivation and initial disease may differ Examples: Herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) Chickenpox (herpes varicella) Shingles (herpes zoster)

Epidemiologic strategies of human pathogens Do not persist in humans but must continually infect new human hosts e.g. small pox, polio, measles Maintenance in nonhuman hosts & only incidentally infect humans e.g. Dengue, SARS, rabies

Fig. General patterns of viral infection

Slow virus infection  Prolonged period (usually measured in years) between initial infection & onset of disease e.g. Human immunodeficiency virus; Measles SSPE Transforming infection  Special class of persistent infection  Cell infected by certain DNA viruses or retroviruses may exhibit altered growth properties & begin to proliferate faster than uninfected cells  Viral genetic information may be integrated with host genome  Called transforming infection (because change in cell behaviour) or oncogenic (because transformed cells can cause cancer)

VirusCells & tissues associated with persistent infection Disease syndrome related to persistent infection Herpes simplex virus types 1 & 2 Neurons, particularly in Sensory ganglia Vesicular lesions on skin & mucous membranes, genital herpes, encephalitis VZVSensory gangliaZoster (Shingles) Cytomegalovirus (CMV) Prolonged productive infection of kidney & salivary gland with shedding; site of latency unknown; lymphocytes, monocytes & sinusoidal lining cells Pneumonia, encephalitis, retinitis, colitis Epstein-Barr virus (EBV) B cells, nasopharyngeal epitheliaLymphoma, carcinoma Hepatitis B virusHepatocytes, lymphocytesChronic hepatitis, hepatocellular carcinoma PapillomavirusSkin, epithelial cellsPapillomas, carcinomas Pathologic consequences of major persistent DNA viral infections in humans

Virus Cells & tissues associated with persistent infection Disease syndrome related to persistent infection Measles virus Brain, neurons & supporting cells Subacute sclerosing panencephalitis, inclusion body encephalitis Rubella virusCNS, lymphoreticular cells Progressive rubella panencephalitis Human immunodeficiency virus CD4+ T cells, monocytes/macrophages, microglia AIDS Pathologic consequences of major persistent RNA and retro viral infections in humans

A. Virus must be able to infect cells without being cytopathic B. There must be mechanisms for long-term maintenance of viral genome in host cells C. Virus must avoid detection & elimination by the host's immune system