Outline and chapter notes to accompany chapter 17 NEW INFECTIOUS THREATS Dec., 2003 A. ORGANISMS FROM MANY KINGDOMS AND PHYLA CAN CAUSE DISEASE CHARACTERISTICS OF PATHOGENS A pathogen is an organism that can cause disease. A disease is considered ENDEMIC if it persists over a long time in a certain place, usually at low levels. A disease is considered EPIDEMIC if its prevalence and spread are well above endemic levels. A parasite is an organism that lives in or on another and obtains its nutrition by causing harm to its host. Every pathogen must have some means of entering its host, propagating its offspring, avoiding host defenses, and spreading to new host individuals. Pathogens belong to many kingdoms and phyla, and include viruses, bacteria, protists, fungi, and wormlike animals. EVOLUTION OF VIRULENCE A virulent pathogen is one that causes severe harm or death. A pathogen that kills its host no longer has a place to live; it will die out unless it can spread quickly to other hosts. A pathogen that evolves to cause less harm to its host will be able to live longer in one place. Thus, many pathogens, over time, become less virulent. Epidemics are caused by pathogens that have become more virulent or that have spread to new locations or new hosts. FACTORS GOVERNING THE SPREAD OF PATHOGENS Host susceptibility: the pathogen must overcome host defenses. Herd immunity: many disease-resistant hosts will slow the spread of the pathogen. Weather and other transient conditions: these may spread the pathogen and may interrupt host defenses. Routes of transmission include direct contact, airborne transmission, waterborne transmission, foodborne transmission, and transmission by other species (called vectors). INTENTIONAL TRANSMISSION TURNS DISEASES INTO BIOTERRORISM The anthrax scare of 2001 shows the potential for bioterrorism. Many people also fear that smallpox might be spread intentionally. In the past, bubonic plague and other diseases were spread this way. B. SOME DISEASES THAT SPREAD BY DIRECT CONTACT ARE INCREASING IN PREVALENCE THE MAJOR SEXUALLY TRANSMITTED DISEASES include several that are 10 to 30 times more prevalent than AIDS: trichomoniasis (the most prevalent worldwide), chlamydiasis, and gonorrhea. Also much more prevalent than AIDS are syphilis and genital herpes. FACTORS INCREASING PREVALENCE include: Greater levels of sexual activity (and more partners) Greater mobility of affected individuals Greater human population densities in many places Drug-resistant strains of pathogens TUBERCULOSIS Drug-resistant strains are allowing renewed spread of this disease. Tuberculosis and other infections allow greater transmission of AIDS. AIDS allows greater transmission of tuberculosis and other infections. EBOLA VIRUS Ebola virus spreads mostly by direct contact with infected persons or their body fluids, especially by contact with dead bodies. It can also spread through the air by coughing and sneezing and be inhaled. OTHER EMERGING THREATS: Methycillin-Resistant Staphylococcus aureus (MRSA) Severe Acute Respiratory Syndrome (SARS), a coronavirus Middle Eastern Respiratory Syndrome (MERS), also a coronavirus C. FOOD-BORNE DISEASE PATTERNS REFLECT CHANGES IN FOOD DISTRIBUTION More of our food than ever before has traveled many thousands of miles and been handled by many more people before it reaches us. ONE EXAMPLE: VARIANT CREUTZFELD-JAKOB DISEASE This is a variant form of BSE ("mad cow disease"). It is caused by an infective protein particle called a PRION. Several prion diseases are transmitted by diseased brain tissue contained in animal feeds (or consumed by cannibals). SOCIAL AND ECONOMIC FACTORS CONTRIBUTING TO DISEASE OUTBREAKS Food now typically travels very far to reach consumers. More food is now processed in gigantic, centralized facilities, so that a single contamination can spread much more widely and affect many more people than was true in the past. More and more food crosses international borders before it is consumed, and health and safety standards may vary in different countries. IMPROVEMENTS NEEDED More uniform worldwide standards are needed for food handling, food storage, and water for washing foods. D. WATERBORNE DISEASES REFLECT CHANGES IN LIFESTYLE AND CLIMATE CHOLERA is easily transmitted by poor sewage treatment, or by disruption of sewage treatment following hurricanes, earthquakes, floods, and similar disasters. GIARDIASIS is caused by a waterborne parasite that is very hard to kill by standard methods. LEGIONNAIRE'S DISEASE may be spread by the droplets of water that persist and circulate in many air conditioning systems. IRRIGATION PROJECTS and other drainage changes may also help spread certain waterborne diseases. E. ECOLOGICAL FACTORS ESPECIALLY AFFECT PATTERNS OF VECTOR-BORNE DISEASES A VECTOR is a species that transmits a pathogen but does not get sick. WEST NILE VIRUS is transmited by certain bird vectors. Humans and horses are dead-end hosts (from which no further transmission takes place). LEISHMANIASIS is a parasitic disease transmitted by sand flies. It more easily infects HIV positive people. It can also be transmitted by contaminated needles. ---------------------------------- April, 2015 PERMISSION IS HEREBY GRANTED to instructors who have adopted the book BIOLOGY TODAY for classroom use to download, modify, and use these notes as needed to aid them in in their teaching. Students of such instructors may likewise use and modify these notes as study aids.