Priscilla Johanesen; Week 11 MED1011; Microbiology
Viruses are obligate intracellular parasites, infect bacteria, plants, fungi, insects, animals, but most have a limited host range. They are not living cells, and do not grow or undergo division. They replicate and multiply within the host cell and lack complete enzyme systems. They vary in size from 10 to 400nm. Virions are composed of virus genome, protein capsid, and may have an envelope. Viruses replicate in host cells. Pathogenesis is the process in which viral infection leads to disease. Outcome is determined by the nature of host-virus interaction and the host response to infection.
Progression of viral disease is entry, initiation of infection, innate immune response, incubation period, secondary replication in target tissue, host immune response, release and transmission, resolution or persistent/chronic infection. Airborne transmission is common cold, chicken pox; faecal-oral is Hep A, Norwalk, polio. Direct transmission is HIV, herpes; animal bites can be rabies, encephalitis. Foetal and neonatal transmission can be rubella, CMV; vertical with Hep B and C, HIV, herpes can be acquired during birth.
Arboviruses (dengue fever, Ross River) have vectors. Rhinoviruses URT, papillomaviruses epithelia, rotaviruses intestinal epithelium. All of those are local.
Host damage can be from cell lysis, production of toxic substances, cell transformation, suppression of immune mechanisms, induction of non-normal host products and induction of structural alterations in host cell. Spread can be through blood, nerves or lymphatics. Virions in blood from primary replication cause primary viraemia, after spread and further replication cause secondary viraemia. Concentration depends on rate of synthesis. Most viruses preferentially infect some tissues, often depends on cellular receptors, host cellular proteases, temperature of replication; pH lability, cellular transcription factors, anatomic barriers, local intrinsic and innate defences, antibody and cellular response.
Shedding from host may facilitate transmission. Outcomes of infection may be failed infection, acute infection, chronic, latent, recurrent and transforming. Persistent infections may be reactivated, may be associated with virus induced immunopathological disease, may lead to neoplasia, and persist in a population. Herpes is latent in DRG, EBV latent in lymphoid tissue/epithelium/salivary glands, chicken pox is latent with shingles, HIV is latent in lymphocytes, macrophages.
Antivirals are problematic because targets must be virus specific. Suitable targets are virus attachment site, viral penetration and uncoating, and viral replication. Interferons are used for Hep B and C, combination therapy of zidovudine, protease inhibitors and lamivudine used for AIDS.