Viral Growth, Prions, and Sub-Viral Pathogens
Virus Propagation and Culture Methods
Embryonic Growth (Eggs): * Closed shells provide a sterile and protected environment that allows viruses to grow rapidly. * The process involves using a small needle to inject the virus into the embryo. * Embryos contain a variety of cell types, creating perfect conditions for viral replication. * Once the virus has grown, it is harvested and used to produce various vaccines against viral diseases.
Cell Culture (Tissue Culture): * An alternative and often more effective method for certain viruses. * Defined as a simple and effective way to grow populations of isolated animal cells in sterile dishes or bottles. * Specific Cell Targeting: If a virus prefers a specific cell type, such as the COVID virus preferring epithelial cells, a culture of only those cells can be created to maximize growth. * Virology Maintenance: Much of a virologist's daily work involves the labor-intensive task of developing and maintaining these cell cultures. * Evolutionary Study: Cell cultures allow viruses to grow from one culture to the next over many generations. Researchers use this to observe life cycles and study the evolution and mutation of viruses.
Medical Impact and Treatment of Viral Infections
Prevalence and Scale: * It is considered impossible to accurately measure the total number of viral infections worldwide. * Viruses are the most common cause of acute infections that do not require hospitalization. * Examples include colds, chickenpox, influenza, herpes, and warts. * Statistics: Approximately people are estimated to have some form of the herpes virus.
High Mortality and Long-Term Debility: * Rabies: Has a very high mortality rate, estimated at a death rate of approximately without treatment. * Ebola: Cited as another high-mortality viral infection. * Polio and Rubella: Infections that can cause long-term debility. Rubella vaccinations are prioritized for children because infants who contract it can face lifelong physical debilitation.
The Problem of Mutation: * Viruses mutate at a very rapid rate, which makes it extremely difficult for scientists to design effective therapies.
Antibiotics and Viral Infections: * Ineffectiveness: Antibiotics do not work against viruses because they are designed to target living things. * Etymology: "Bio" means life and "Anti" means against; an antibiotic is an agent against life. Since viruses are not considered living organisms, antibiotics have no effect on them. * Misdiagnosis: Patients often self-diagnose viral infections and seek antibiotics, which results in no therapeutic benefit.
Interferon: * A naturally occurring human cell product used to prevent and treat viral infections. * When a cell is infected by a virus, it produces interferon and other chemicals to signal neighboring cells to protect themselves. * Medical applications include giving patients injections of manufactured interferon to boost cellular protection against a viral infection.
Prions and Spongiform Encephalopathies
Definition and Characteristics: * Prions are not viruses and are not living particles; they are purely infectious proteins. * They are classified as noncellular infectious agents. * Incurability: Prion-related diseases are currently incurable, with no known treatments or detection methods in living hosts.
Spongiform Encephalopathy: * This term refers to the brain matter changing to look like a sponge due to the infection. * Autopsies of infected individuals reveal plaques or sponge-like holes in the brain matter caused by these proteins.
Known Prion Diseases: * Creutzfeldt-Jakob Disease (CJD): Affects the central nervous system in humans. It causes gradual degeneration and eventually death. The exact mode of transmission is often unknown. * Bovine Spongiform Encephalopathy (BSE): Commonly known as "Mad Cow Disease." * Scrapie: Found in sheep. * Other Victims: Mink and elk.
Resistance and Control: * Prions are incredibly resilient and are not destroyed by standard cooking temperatures or traditional food sterilization treatments. * In the United States, if a cow is suspected of being infected, the entire herd is typically killed and burned to prevent any possible transmission.
Cultural Context (Oprah Winfrey Lawsuit): * Beef producers in America once sued Oprah Winfrey after she stated on her television show that eating beef could lead to Mad Cow Disease. * Following her statement, beef stock prices dropped significantly, leading to the legal action.
Satellite Viruses and Viroids
Satellite Viruses: * These are viruses that depend on other "helper" viruses to replicate; they essentially piggyback on another infection. * Adeno-associated virus (AAV): A primary example. It was originally thought it could only replicate in cells already infected with an adenovirus, though it is now known to infect cells where DNA has been disrupted by other viruses.
Viroids: * Agents that primarily infect plants and are about the size of an average virus. * They consist only of bare RNA and lack a capsid, which distinguishes them from true viruses. * Economic Impact: They are significant pathogens for tomatoes, potatoes, cucumbers, citrus trees, and chrysanthemums. * Transmission: Frequently spread by insects, such as aphids, that feed on the plants.
Questions & Discussion
Question regarding the naming of prions for an essay: * Student Question: For prions, would "spongiform encephalopathy" be the common or scientific name for the organism? * Instructor Response: Spongiform encephalopathy describes the infection. Because prions are just proteins and not organisms, they do not have a genus or species name like bacteria (e.g., E. coli) or protozoa (e.g., Plasmodium vivax). * Names are often assigned based on the disease or the host: * In humans: Creutzfeldt-Jakob disease. * In cows: Mad Cow Disease (Bovine Spongiform Encephalopathy). * In sheep: Scrapie. * Analogy: The instructor described a prion as a "tank" protein. While there are millions of proteins in the brain, this one large, destructive protein runs into others, crushing and destroying brain matter. It is a sub-viral particle, similar to a single capsomere rather than a full capsid.
Question regarding the importance of spelling: * Student Question: Will points be marked off for spelling on the exam? * Instructor Response (Anecdote): Spelling definitely counts. The instructor shared a story about a doctor friend who was sued. The lawsuit's success hinged on the fact that a nurse had misspelled a term in the medical notes. The lawyer used that misspelling to argue that the medical staff did not know what they were talking about. * In a legal environment, if a patient has a bad outcome, lawyers (sometimes called "ambulance chasers") will scrutinize every note. Misspelling a drug name or a surgical procedure can lead to liability for the doctor, the hospital, and the nurse. Therefore, students are expected to spell medical and scientific terms correctly.
Discussion on Blood Donation and Prions: * Instructor's Story: The instructor, Frederick James, served as the chairman for a sickle cell blood drive in Tennessee on September 11, 2001. It was one of the most successful drives in the state's history. * However, when he attempted to donate blood, he was rejected by a Red Cross worker named Gail. * Reason for Rejection: Because the instructor's father was in the military and they had lived in England during a time of a Mad Cow Disease outbreak, he was permanently deferred. * Because there is no way to detect or treat prion infections in living people, the Red Cross maintains strict exclusion criteria for anyone potentially exposed to the protein, even decades later.