Pathogenesis of Viral Diseases, lab. diagnosis and treatment

This article discusses the viral pathogenecity, types of viral infections, fate of viral infection, Laboratory Diagnosis and the general outlines of antiviral treatment.
Pathogenesis of Viral Diseases

Entry of viruses: 

  • Viruses enter the body either by:
  • Inhalation (respiratory tract 
  • Ingestion (gastrointestinal tract) 
  • Contact (urogenital system) 
  • Throgh Skin (injections, blood transfusion, insect and animal bites). 

Viral infection may be: 
  • Local infection: where the virus produces disease at the portal of entry .
  • Systemic or deep viral infections: where the virus spreads to distant organs either via the blood (viraemia), or by other means, e.g. along nerves (Table 1).
Table 1: Differences between Local and systemic viral infections:
Characteristic Local infections Systemic infections
Example Common cold (e.g. rhinovirus infection) Measles
Site of pathology Portal of entry At distant sites
Incubation period Relatively short Relatively long
Viraemia Absent Present
Duration of immunity Usually short Usually lifelong
Involved immunoglobulin Secretory IgA IgM and IgG

Fate of viral infections

  1. Inapparent or subclinical viral infections: Viral infection without overt signs and symptoms.
  2. Apparent infections (disease): Local or systemic viral infections with the appearance of clinical signs and symptoms.
  3. Persistent viral infections (chronic): In this form, the virus is continuously detected with mild or no clinical symptoms, e.g. chronic hepatitis B.
  4. Latent viral infections: The virus persists in a dormant form and may flare up intermittently to produce disease, e.g. herpes viruses.
  5. Slow virus infections: Virus infections with long incubation periods (months or years). They are caused by two types of infectious agents:
  • Conventional viruses, e.g. a variant of measles virus which causes subacute sclerosing panencephalitis (SSPE).
  • Unconventional agents (prions).

Laboratory Diagnosis of Viral Infections

The laboratory diagnosis of viral infection involves 2 main diagnostic methods:

a. Direct methods: 
which depend either on the detection of viruses and/or their components in the patient's specimens, or on isolation of viruses.

b. Indirect methods: 
which depend mainly on the detection of antibodies against the suspected virus in the patient's serum, or on skin tests.

The different techniques used in diagnosis of viral infections are discussed in "Practical Microbiology and Immunology".

Treatment of Viral Infections

Viruses can not be treated with antibiotics because they lack the structural targets on which antibiotics can act.
Viruses are obligate intracellular parasites, so antiviral drugs must selectively inhibit viral replication without causing damage to host cells.

The number of antiviral drugs is little compared to antibacterial drugs (Table 2):

Table 2: Selected Antiviral drugs and their mechanism of action and clinical use:
Drug Target Virus
Acyclovir Viral DNA polymerase Herpes simplex virus, varicella-zoster virus
Ganciclovir Viral DNA polymerase Cytomegalovirus
Oseltamivir Neuraminidase Influenza A virus, influenza B virus
Zanamivir Neuraminidase Influenza A virus, influenza B virus
Lamivudine Reverse transcriptase HIV-1, HBV
Efavirenz Non-nucleoside reverse transcriptase inhibitor HIV-1
Indinavir Protease HIV-1
Ritonavir Protease HIV-1
Ribavirin Viral RNA polymerase Hepatitis C virus, respiratory syncytial virus
Interferon-α Interferon Hepatitis B virus, hepatitis C virus, certain types of cancer

Selected Antiviral drugs mechanism

Pathogenesis of Viral Diseases, lab. diagnosis and treatment
Dr.Tamer Mobarak


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