By Bob Dispenza, Director of Education, Allen County Parks
Computer viruses periodically make the news. The only relation these have to biological, “living” viruses is that they are small, insidious, work from the inside and cause damage. Natural viruses do all these things, but they more than this. Little is know about most viruses, though they are around us everywhere. Most of the research into viruses has centered around those which cause human illness, but there are many others that infect bacteria, plants, and both wild and domestic animals.
The term “virus” comes from the Latin for slimy liquid or poison. The term “living” must be used with caution around viruses. There is still argument about whether they are extremely simple life forms or extremely complex chemicals that can produce copies of themselves. Most living things are “on” all the time – that is, enzymes are reacting in their cells constantly. Viruses are only “on” when they are inside a susceptible cell. Outside a cell they are as inert as any other lifeless chemical, and can even assume crystalline form like a mineral. Some can survive for long periods of time outside a living cell, but all are “obligate intracellular parasites”. They can only reproduce by commandeering the metabolic machinery of a living cell, a cell that is killed or drastically altered in the process.
Most viruses consist of two or three parts: a “core” made up of viral DNA or RNA with associated proteins; a “capsid” made of protein which protects the core; and sometimes an “envelope” made of modified material from the host cell. West Nile virus is one of those enveloped viruses, which means it is less hardy and doesn’t usually survive long outside of a host. Capsids often look remarkably like mineral crystals. Many (including West Nile) are icosahedrons, with 20 triangular sides and 12 corners.
Viral RNA or DNA tends to be very simple, sometimes having as few as three genes. A few viruses are so simple that they cannot reproduce without the presence of another, larger virus in the same cell. While the larger virus is parasitizing the cell, the smaller virus parasitizes the larger virus, stealing enzymes and proteins for its own use.
Hosts are often hard-pressed to fight off viruses, since virus genes evolve up to a million times faster than host genes. Viruses are responsible for sicknesses as common as warts and colds. They are also implicated in more deadly diseases – West Nile, dengue fever, AIDS, polio, hepatitis, rabies, and encephalitis. Viruses cause flu, and though it is not now thought of as deadly, it killed over 20 million people in the pandemic of 1918-1919. It’s not all bad news – smallpox may have been eliminated.
Control of viruses can be difficult. Chemical agents are of limited use – viruses are so simple and use so much of the host’s resources that damaging the virus would often involve damaging the host cell. Interferon, a protein secreted by virus-infected cells, can cause resistance to infection in surrounding cells. Immunity is a complex host response that works well, but it requires exposure to the virus itself or specific parts of the virus. Immunity to one virus does not mean immunity to all – there are over 100 different viruses that can cause cold symptoms.
Viruses can sometimes evade host defenses by inserting themselves into the host’s DNA and “disappearing”. Stress, chemicals and other factors may reactivate these viruses, producing disease. These “disappearing” viruses may cause some tumors.
Not usually thought of as wildlife, viruses still play an important part in any ecosystem. By moving DNA around and eliciting immune reactions they play a role in shaping the plants and animals we know today. Next time you get a cold you can take pride in the fact that you are providing habitat for our smallest wildlife.