A virus classified in class I possesses double-stranded DNA genomes and do not go through RNA stage.
There are 2 different classes found in class I. Firstly, replication exclusively nuclear; very dependent on the host cell factors.
And secondly, replication in cytoplasm; viral genome contains all factors for genome replication and transcription.
An example of virus in class I is Poxviridae.
Poxviridae
Properties
Poxiviridae, which is the largest family, where viruses can infect both vertebrate and invertebrate animals. It can stay air borne for a long period of time and infect people.
Their virions (viral particles) are generally enveloped though the intracellular mature virions form of the virus, which contains different envelope and is infectious.
They vary in their shape depending upon the species but are generally shaped like a brick or as an oval form similarto a rounded brick.
Four genera of poxviruses may infect humans are:
- Orthopox (Virus examples: Variola Virus, Vaccinia Virus, Cowpox Virus, Smallpox Virus)
- Parapox (Virus examples: Orf Virus, Pseudocowpox, Bovine Frican Stomatitis Virus)
- Yatapox (Virus examples: Tanapox Virus, Yaba Monkey Tumor Virus)
- Molluscipox (Virus example: Molluscum Contagiousum Virus (MCV))
The most common are vaccinia but Monkeypox infections are rising as seen in west and central frican rainforest countries.
Small pox have been eradicated.
Life cycle
1st – virus is bind to a receptor found on the host cell surface; the receptors are thought to be Glycosaminoglycands (GAGs).
2nd – The virus will enter the cell where is will starts to uncoat. Uncoating of virus is a 2 steps process.
First, the outer membrane is removed as the particle enters the cell. Second, the virus particle is uncoated further to release the core into the cytoplasm.
3rd – The pox viral genes are expressed in 2 phases.
These genes known as early genes will expressed first, encoding the non-structural protein, including proteins necessary for replication of the viral genome, and are expressed before the genome is replicated.
Late genes are expressed after the genome has been replicated and encode the structural proteins to make the virus particle.
4th – The assembly of the virus particle occurs in the cytoskeleton of the cell and is a complex process. The replication of this virus is unusual for a virus with double stranded DNA genome because it encodes its own machinery for genome replication and therefore the replication occurs in the cytoplasm.
Most viruses with a double stranded DNA genome replicate in the nucleus and use the host cells genome replication machinery.
5th – This is the last stage of the cycle where the virus is being released to infect other surrounding cells.
Clinical Features 
- Only infect human
- Respiratory Secretions-Saliva
- Always associated with skin lesions
- At least 9 poxviruses cause disease in human
- 7-12 days incubation
- Initially influenza-like symptoms
- Scarring of skins, blindness, DEATH
Lab Diagnosis
- Declared by WHO to be eradicated in 1980
- Not infectious during incubation but during symptoms
- Aerosol or air droplets
- BEST DEFENCE is VACCINATION
Control
- All infected materials must be incinerated
- Last outbreak in Yugoslavia in 1972 due to accidental release
- Vaccination stopped more than 30 years ago
By: Toh Wan Nee