This article discusses the viral pathogenecity, types of viral infections, fate of viral infection, Laboratory Diagnosis and the general outlines of antiviral treatment.
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
- Inapparent or subclinical viral infections: Viral infection without overt signs and symptoms.
- Apparent infections (disease): Local or systemic viral infections with the appearance of clinical signs and symptoms.
- Persistent viral infections (chronic): In this form, the virus is continuously detected with mild or no clinical symptoms, e.g. chronic hepatitis B.
- Latent viral infections: The virus persists in a dormant form and may flare up intermittently to produce disease, e.g. herpes viruses.
- 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 |
