   What is a Virus? - September 26, 2007 Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County Viruses affect all types of living organisms. We’ve heard of them through the news media because they can affect human health (e.g. avian influenza, West Nile, polio, and AIDS). However, viruses can also infect animals, plants, insects, bacteria, fungi, protozoa, and all other living organisms. Interestingly, a virus is not a free-living organism because it cannot reproduce itself in the absence of a living host. In general, think of a virus as an extremely small particle that contains genetic information (DNA or RNA). It must enter a living cell before the information can be used. The entrance of a virus into a cell is called a viral infection. The virus uses the cell's resources to generate virus parts which are later assembled into thousands of new, mature, infectious viruses which can infect other cells. Structurally, a virus can be envisioned as a tiny capsule that contains the genetic information to reproduce itself. The “capsule” analogy is a generalization and there are many variations on this theme. However, expanding on this generalization, many viruses have an outer membrane (called the envelope) which encases a protein covering (called the capsid). The envelope and capsid protect the genetic material at the core. To visualize this, think of how a baseball is made: the leather cover (envelope) covers a winding of thread (capsid) which covers a rubber core (genetic material). Viruses exist only to replicate themselves. With some exceptions, all viruses appear to be harmful to the host organism because their replication leads to the death of the infected cell. A virus can enter a cell on its own or with the assistance of another organism called a vector. Once inside the cell, the virus material essentially takes over the host cell and uses it to reproduce itself. These viruses are mature and leave the cell either by a process called "budding" (just one or a few viruses at a time leave the cell) or by a process called lysis (the cellular membrane ruptures and releases all of the virus particles at once). The resultant viruses are free to infect other cells and can lead to a variety of symptoms and/or death of the host organism. As gardeners, we are sometimes faced with viruses infecting plants. Some examples include: curly top virus which is vectored by an insect and infects tomatoes, peppers, and other crops; tobacco mosaic virus which infects tomatoes, peppers, petunias, snapdragons, delphiniums, and marigolds; and rose mosaic viruses which cause a variety of symptoms (some of which are relatively harmless to the plant). In all cases, they are not treatable because the active portion of the virus life cycle occurs within the plant cells. Researchers are now exploring how we may effectively use viruses to treat bacterial diseases and cancer. This is done by genetically altering some genes within the virus so that the host cell replicates genes that kill malfunctioning cells (such as cancer). Other viruses are being explored to treat antibiotic resistant bacterial infections. Eventually, these viral therapies may offer cures for plant diseases too. Back in April I wrote about Colony Collapse Disorder (CCD): a mysterious disease that causes honeybees to leave the hive and not return. CCD has been documented across the nation and in foreign countries and has resulted in a loss of 50% to 90% of bee colonies in beekeeping operations across the U.S. Potential causes being investigated are parasitic mites, pesticides, genetically modified crops, and even electromagnetic radiation from cell phones. Scientists now think that a virus is linked to CCD. Work that was published in the journal Science on September 6, 2007 showed a strong correlation between CCD and Israeli acute paralysis virus (IAPV) of bees. Researchers still think that other factors are involved and there is still much work to be done before we can understand the causes of CCD. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |