Infectious Diseases Michelle Garon Biomedical Technology

Infectious Diseases Definition: A disease that is caused by the invasion of a host by agents whose activities harm the host ’ s tissues (that is, they cause disease) and can be transmitted to other individuals (that is, they are infectious)

Pathogens Pathogen: an organism that is capable of causing diseases Most microorganisms DO NOT cause diseases Many microorganisms provide protection against other harmful microorganisms because the effectively compete for the resources that they need to survive, preventing them from growing (example: we have normal flora in our intestines that keep bad bacteria from infecting it)

Pathogens cont … True pathogens: an infectious agent that causes disease in almost any host Opportunistic pathogens: an infectious agent that rarely causes disease in individuals with health immune systems These opportunistic pathogens usually infect elderly, cancer patients on chemo, AIDS patients (ex. Patients with AIDS usually die from pneumonia not the HIV virus)

Infection versus Disease Infection: results when a pathogen invades and begins growing within a host Disease: results only when and if, as a consequence of the invasion and growth of a pathogen, tissue function is impaired

Defense Mechanisms Our body has defense mechanisms to prevent infection and should those mechanisms fail, an infection and/or disease will occur Our body ’ s first line of defense is our skin Other defense mechanisms: hair, eyelashes, eyelids, cilia, fingernails, saliva, ear wax, etc.

Infectious diseases Some infectious agents are easily transmitted (Contagious), but they are not likely to cause disease ( not virulent) Ex: Polio (it infects most people who come in contact with it but only 5-10% of those infected actually develop clinical symptoms)

Infectious diseases In order to cause disease, pathogens must be able to: Enter the host Adhere to host cells Invade & colonize host tissues Inflict damage on tissues

Infectious diseases Entrance into host usually occurs through: mouth, eyes, genital openings, or wounds Some pathogens grow at entry but most invade inside of body They attach to specific host cells Usually the damage to tissues is due to the production of toxins or destructive enzymes by the pathogen (ex. The bacteria that causes diphtheria grows only in nasal & throat surfaces. But the toxin it produces goes through the blood stream to heart, liver, and nerve tissues)

Microbes that cause Infectious Diseases There are 5 major types of infectious agents: Bacteria Viruses Fungi Protozoa Helminthes Prions--- new class that is not recognized

Bacteria Bacteria: unicellular prokaryotic organisms. Some are aerobic (require oxygen) and some are anaerobic (unable to tolerate oxygen) Bacteria are divided into 2 broad classes based on their cell wall structures which influences their gram stain reaction

Bacteria cont … Gram negative bacteria: appear pink after staining (ex. Salmonella typhi which causes typhoid fever & Yersinia pestis which causes plague) Gram positive bacteria: appear purple after staining (ex. Staphylococcus aureus which causes skin, respiratory, & wound infections)

Viruses Viruses: can infect all organisms (plants, animals, fungi, and bacteria) Viruses, however, are not organisms themselves because, apart from a host cell, they have no metabolism and cannot reproduce

Viruses cont … Viruses are made up of a viral genome of nucleic acid that is surrounded by a protein coat called a capsid The viral genome is inserted into the host cell and the host cell replicates its DNA, transcribe the DNA to make messenger RNA, and translate the messenger RNA into viral proteins. The new viruses are released from host cell

Viruses cont … Other RNA viruses, retroviruses, use a unique enzyme call reverse transcripase to copy the RNA genome into DNA. The DNA integrates itself into the host cell genome These retroviruses frequently exhibit long latent periods Example of retrovirus: HIV

Fungi Fungi: most are multicellular organisms (some are unicellular-yeasts) are form spores Fungi: decomposers in environment Example of diseases caused by fungi: ringworm Yeasts of candida genus are opportunistic pathogens that may cause diseases such as vaginal yeast infections and thrush (a throat infection) among people who are immunosuppressed or undergoing antibiotic tx

Protozoa Protozoa: unicellular Do not have cell walls Protozoa can be acquired by contaminated food/water or by the bite of an infected arthropod (mosquito) Malaria, a tropical illness is caused by a protozoan

Helminthes Helminthes: simple invertebrate animals, some of which are infectious parasites They are difficult to treat because physiology is similar to ours. So, the drugs that kill the helminthes are usually toxic to humans as well Diarrhea and abdominal pain are the most common symptoms associated with helminthes (ex. Schistosoma – flatworm). Fatal cases often show congestive heart failure and respiratory paralysis

Prions Prions: proteinaceous infectious particles (ex. Bovine spongiform encephalopathy - Mad cow disease) Causes degenerative disorders of the central nervous system Some prion diseases are inherited Some prion diseases are caused by eating infected tissue or through receiving infected tissue/organ through transplants

Epidemiology Study of the occurrence of disease in populations Epidemiologists are concerned about infectious and non-infectious diseases

Disease Reservoirs Disease reservoir: Site where the infectious agent survives/lives Ex. Humans are the reservoir for the measles virus Animals can also be reservoirs for diseases that infect humans Ex. The reservoir for Yersinia pestis (bacteria that causes plague) is wild rodents

Disease Reservoirs cont ’ d Nonliving reservoirs: Soil is the reservoir for pathogenic fungi and bacteria Ex. Soil is the reservoir for the bacteria, Clostridium Tetanim, that causes tetanus

Modes of Transmission Infectious agents can be transferred through direct and indirect contact Direct contact: when an individual is infected by coming in direct contact with the reservoir Inhalation and sexual contact would also be included here Ex. HIV, influenza, rabies, malaria

Modes of Transmission cont ’ d Indirect contact: Occurs when a pathogen can withstand the environment outside of a host for a long period of time before infecting another person

Host Defenses Against Infectious Diseases Specific defenses: These defenses target particular pathogens Ex. Antibodies, immune system, WBC ’ s Nonspecific defenses: These defenses operate against a wide range of pathogens. Skin, eyelashes, cilia, mucous, skull

Vaccination Vaccine: killed or weakened (attenuated) strain of a particular pathogen, or a solution containing critical antigens from the pathogen The body ’ s immune system will respond to this vaccine as if they contain the actual pathogen, even though the vaccine is not capable of causing the disease So, memory lymphocytes will be present that respond rapidly when the actual pathogen is encountered (this prevents disease)

Objective BT06.02 Analyze the role of public health in the prevention of infectious diseases

Prevention of Infectious Diseases Developed countries have regulations in place that help to protect the general public from infectious diseases This typically involves eliminating the pathogen from its reservoir or from its route of transmission Specific Measures include: Ensuring safe water supply Effectively managing sewage tx & disposal Initiating food safety, animal control, & vaccination programs

Safe Water Many pathogens that cause gastrointestinal diseases (cholera & typhoid fever) are transmitted via water If you visit a developing country you should receive vaccines for these diseases Purification methods include settling, filtration, and chlorination

Safe water cont … When breakdowns in a purification system occur, or when a system is overwhelmed (ex. During unusual flooding), drinking water may not be safe and should be boiled or treated with chlorine before it is ingested Because GI pathogens typically leave the body in the feces, public water must be guarded against contamination from sewage

Sewage Tx and Disposal Sewage includes: Wash water Water from toilets Storm run off All of these carry pathogens To ensure public health safety the U.S. gov ’ t requires that sewage be treated to eliminate pathogens

Sewage Tx and Disposal cont … Minimal acceptable level of tx involves collection and sedimentation of sewage waters, separating solid matter (sludge) from liquid (effluent) portions of sewage The effluent is chlorinated to kill pathogens before it is released to rivers/lakes Sludge is burned or dumped

Food Safety Programs The U.S. has many standards, inspections plans, and regulations about food preparation, handling, and distribution. Meat packing facilities, restaurants, and supermarkets are inspected frequently Milk is pasteurized and dated for sale and is analyzed periodically for contamination

Animal Control Programs Animals are carriers of pathogens Domestic herd animals can pass pathogens to humans through milk and meat These animals are checked for certain pathogens Pets have to have rabies vaccinations Rodents carry pathogens too (ex. Bubonic plaque)

Animal Control Programs cont … Insects carry pathogens like the ones that cause malaria (mosquitoes) The spread of insect-borne disease can be controlled by eliminating breeding areas for insects (ex. Draining areas where stagnant water collects) and using pesticides

Vaccination Programs Most states require that parents show proof that their children have been vaccinated This is important the individaul child ’ s health and the public If a certain proportion of the population is immune to a disease, the pathogen that causes that disease will be unable to reproduce itself at a high enough level to maintain itself in the population

Public Health Organizations All of these agencies report data to the U.S. Public Health Service 1. NIH (National Institute of Health) Does Research 2. CDC (Centers for disease control & prevention Investigates disease breakouts 3. FDA (Food and Drug Administration) Monitors safety of food and drugs 4. WHO (World Health Organization) Provides international surveillance and control of disease

Benefits of public health research Polymerase chain reaction developed Powerful new research technique that allows rapid identification of causative agents HIV proteases inhibitors developed Used with other anti-HIV drugs decrease the death rate in these patients Basic research on ecology of pathogens, reservoirs, modes of transmission leads to preventative measures

Objective BT06.03 Treatment of Infectious Diseases

Antibiotics Antibiotic literally means “ destroyer of life ” or “ against life ” It is a chemical/drug used to treat bacterial infections They have minimal side effects

Antibiotic Groups based on Modes of Action 1. Cell Wall Synthesis Penicillins & Cephalosporins work by interfering with the cell wall 2. Protein Synthesis Erythromycin & tetracycline work by inhibiting protein synthesis These drugs may be toxic to some human tissues when they are used in high doses for too long 3. Nuclei Acid Synthesis Rifampin is used to tx Tuberculosis and it inhibits RNA synthesis

Treatment of Viral Diseases In general, most anti-viral drugs are toxic to the host cell Acyclovir is used to treat outbreaks of genital herpes Amantadine is used to treat moderate influenza AZT is used to inhibit HIV

Treatment of fungal and parasitic diseases Drugs used to treat fungal, protozoan, and helminthic diseases are also highly toxic to mammals

Resistance to antimicrobial agents Alexander Fleming discovered Penicillin He also warned about indiscriminate use of penicillin would lead to the proliferation of resistant pathogenic bacteria Some strains of the TB bacteria are already resistant

Emerging and Re-emerging Infectious Diseases Globally, infectious diseases remain the leading cause of death and they are the 3 rd leading cause of death in the U.S.

Emerging Diseases These diseases: Have not occurred in humans before Have occurred previously but affected only a small number of people in isolated places Have occurred throughout human history but have only recently been recognized as distinct diseases due to an infectious agent Example: Legionnaire disease (the pathogen that causes this disease lives in air conditioning systems

Re-emerging Infectious Diseases These diseases were once major health problems and then declined dramatically, but are again becoming health problems Ex. Malaria and TB (both of these have become resistant to the drugs that used to kill them) Ex. Diphtheria and whooping cough (pertusis) The re-emergence of these diseases is related to inadequate vaccination of population