Emerging & Re-emerging Infectious Disease

Infectious Diseases

• Disease: conditions that impair normal tissue function

• Genetic or Metabolic diseases: ex Cystic Fibrosis

• Disease of aging: ex atherosclerosis

• Infectious Disease: caused by the invasion of a host by agent’s whose activities harm the host’s tissues

• Pathogens: microorganisms that are capable of causing disease

• Opportunistic pathogens are potentially infectious agents that rarely cause disease in individuals with a healthy immune system.

• The terms infection & disease are not synonymous

• An infection results when a pathogen invades and begins growing in a host

• Disease results only if and when, as a consequence of the invasion and growth of a pathogen, tissue function is impaired

Contagious vs. Virulent

• Some infectious agents are easily transmitted (very contagious) but are not likely to cause disease (not very virulent)

• Example: Polio-probably infects most people it contacts, however only 5-10% actually develop the disease.

• Other infectious agents are very virulent, but not terribly contagious.

• Ex: Ebola hemorrhagic fever, virulence of the virus is 50-90% fatality among those infected, however the virus is not transmitted easily by casual contact.

How do infections happen?

• In order to cause disease, pathogens must be able to enter the host body, adhere to specific host cells, invade and colonize host cells, invade and colonize host tissues, and inflict damage on those tissues

• Pathogens usually enter through natural orifices or through breaks in the natural skin barrier such as wounds.

Corynebacterium diphtheria

• Bacteria that causes diphtheria

• Grows only on nasal and throat surfaces

• Damage to tissues is due to the production of enzymes or toxins

• Toxin is distributed to other systems via the circulatory system resulting in damage to the heart, liver and nervous tissues

Streptococcus pyogenes

• Infectious agents associated with “strep throat” and “flesh eating disease”

• Produces enzymes that break down barriers between epithelial cells and remove fibrin clots, helping the bacteria invade tissues

Microbes

• 5 major types of infectious agents– Bacteria– Viruses– Fungi– Protozoa– Helminths

In addition, a new class of infectious agents, the prions, has recently been recognized

Bacteria

• Have no organized internal membranous structure such as nuclei, mitochondria, or lysomes.

• Most bacteria reproduce by growing and dividing into two cells (binary fusion)

• Variety of morphologies. 3 most common– Bacillus (rod shaped)– Coccus (round)– Spirillum (helical rods)

• Some bacteria require oxygen– aerobes

• Some bacteria can not tolerate oxygen– anaerobes

• Some bacteria can grow either with or without oxygen– facultative anaerobes

Gram Stain

• Bacteria are generally divided into 2 broad classes based on their cell wall structure, which influences their Gram stain reaction

• Gram Negative bacteria appear pink after the staining procedure

Gram Negative Bacteria

• Salmonella typhi (causes Typhoid Fever)

• Yersinia pestis (causes the plague)

Gram positive bacteria

• Appear purple after gram stain procedure

• Staphylococcus aureus (causes skin, respiratory & wound infections)

• Clostridium tetani (produces a toxin that can be lethal for humans)

Viruses

• Not organisms themselves, cannot reproduce apart from the host cell

• Cause disease by disrupting normal cell function

• Classified using a variety of criteria including shape, size, and type of genome.

Viruses

• Examples of viruses:• Herpes viruses that cause chicken pox, cold

sores and genital herpes• Poxvirus that causes smallpox• Rhinovirus that causes the common cold• Myxoviruses and paramyxoviruses that cause

influenza, measles and mumps• Rotaviruses that cause gastroenteritis• Retroviruses that cause AIDS and several types

of cancer

Fungi

• Reproduce primarily by forming spores

• Together with bacteria, fungi work to be the “decomposers” in our environment

• Examples of diseases caused by fungi– Ringworm– Histoplasmosis (lung infection caused by bat

or bird droppings)

Fungi

• Yeasts of the Candida genus are opportunistic yeast infections and cause diseases such as vaginal yeast infections and thrush (a throat infection) among people who are immunocompromised or undergoing antibiotic therapy

Protozoa

• Do have cell walls so are able to have rapid and flexible movements

• Can be acquired through contaminated food or water or by a bite of an infected anthropod such as a mosquito

Protozoa Diseases

• Giardia lamblia & cryptosporidium parvum are two protozoa parasites that cause diarrheal disease in the US

• Malaria, a tropical illness, is caused by several species of the protozoan Plasmodium

Helminths

• Are simple, invertebrate animals, some which are infectious parasites

• Because they are animals, their physiology is similar to humans, making this parasite difficult to treat because drugs that kill helminths are frequently very toxic to humans

Helminths

• Have complex reproductive cycles and many require a host

• Examples: – Schistosoma, Caused by a flat worm

• Results in diarrhea and abdominal pain

– Trichinella spiralis, caused by a roundworm• Results in vomiting, diarrhea and fever, later

symptoms include muscle pain, congestive heart failure and respiratory paralysis

Prions

• Infections particles that consist only of proteins

• Pronounced (pree-ons)

• Linked to some CNS degenerative disorders in humans and animals

Prions

• Examples of diseases:

• Creutzfeldt-Jakob disease in humans

• Scrapie in sheep

• Bovine spongiform encephalopathy (“mad cow disease”) in cattle

• Some prion diseases are inherited, others are caused from eating infected tissue

Epidemiology

• Defined as the study of the occurrence of diseases in populations

• Epidemiologist are concerned not only with infectious disease but also with non-infectious disease such as cancer and environmental diseases such as lead poisoning

Disease reservoirs

• The reservoir for a disease is the site where the infectious agent survives

• Example: Humans are the reservoir for the measles virus because it does not infect other organisms

• Wild rodents are the reservoir for Yersinia pestis, the bacteria that causes the plague

• Soil is the reservoir for the bacteria Clostridium tetani which causes tetanus

Modes of transmission

• Transmitted by either direct or indirect contact

• Direct contact is when an individual is infected by contact with the reservoir

• Example: touching an infected person, eating infected meat, being bitten by an infected animal, inhaling infectious droplets from a sneeze or cough, intimate sexual contact

Direct contact diseases

• Some diseases spread by direct contact include:

• Ringworm• AIDS• Trichinosis• Influenza• Rabies• Malaria

Modes of Transmission

• Indirect contact occurs when a pathogen can withstand the environment outside its host for a long period of time before infecting another individual

• Examples: tissues or toys handles by sick people, ingesting food or beverages contaminated by contact with a disease reservoir

Indirect contact

• Fecal-oral route is a significant form of indirect transmission for gastrointestinal diseases such cholera, rotavirus infection, cryptosporidiosis, and giardiasis.

• Also included in indirect transmission are diseases transmitted from parent to child such as AIDS and herpes encephalitis

Role of research in prevention

• Understanding the infectious cycle is critical in order to identify accessible targets for control strategies

• Examples: Vector-borne diseases may be prevented by control methods that either kill the vector or prevent it’s contact with humans. Vaccines may prevent the development of a pathogen within a host, drugs may be used to prevent infections or suppress the disease process

Host Defenses against Infectious Disease

• The humans body has several mechanisms for preventing diseases

• These include anatomical barriers to invading pathogens (skin, bone, nasal passage), physiological deterrents to pathogens (tears, sweat, blood, vaginal secretions), and the presence of normal flora (microorganisms present on and in the body)

Immunity

• When a host encounters an antigen that triggers a specific immune response for the second or later time, the memory lymphocytes recognize it and quickly begin growing and dividing. The immune response occurs so quickly the second time that the pathogen does not have time to reproduce before the body has destroyed it.

Vaccination

• A vaccine is either a killed or weakened strain of a particular pathogen

• The body’s immune system will respond to these vaccines as if they contain the actual pathogen.

• As a result, memory lymphocytes will rapidly respond when the actual pathogen is encountered, destroying the pathogen before it can reproduce

Vaccination

• Many diseases are not easily prevented by vaccination

• Some pathogens have a way to evade the immune response either by not allowing antibodies to pass through their cells, by disguising themselves as a host cell or by mutating.

• Cold and influenza viruses are examples of rapidly mutating pathogens

Public Health measures to prevent Infectious Disease

• Safe water: Many pathogens that cause gastrointestinal diseases (cholera & typhoid fever) are transmitted in water.

• Travelers to developing countries are asked to be immunized

• Unnecessary in the US and developed countries because the water used for drinking, washing and preparing food is purified before it goes into homes.

Water purification

• Purification methods include settling, filtration and chlorination.

• Well water is usually safe if guidelines about distance from sewage disposal facilities is followed.

• When purification systems breakdown, such as with unusual flooding, drinking water may not be safe and should be boiled or treated with chlorine prior to ingesting.

• Public water must be protected from sewage

Sewage treatment and disposal

• Sewage includes wash water, water from toilets and storm run-off

• These fluids may carry pathogens for many waterborne diseases, including giardiasis and hepatitis A.

• To ensure safety, the US government requires that sewage be treated to eliminate pathogens

Food Safety programs

• The US has many standards, inspection plans and regulations about food preparation, handling and distribution.

• Meat packing plants, restaurants and supermarkets are all inspected

• Milk is pasteurized and dated for sale and is analyzed periodically for contamination

• Canning and preserving foods is maintained through quality control checks.

Animal control programs

• Aniamls are carriers of many diseases that also affect humans

• Inspecting herds animals for tuberculosis (due to the bacterium Mycobacterium bovis) and brucellosis (a disease that causes spontaneous abortion in herd animals and abscesses of the liver, spleen, bone marrow and lymph nodes in humans) has helped prevent passage of those organisms in milk and infected meat

Animal Control programs

• Before dogs can be licensed, owners must show proof of rabies vaccination

• Most cases of rabies in the US is due to bites from wild and stray animals, these animals are usually impounded and destroyed

• Bubonic plague is spread by rats. Rat control in urban areas is a major component of public health efforts

Animal control measures

• Insect-borne diseases such as malaria can be controlled by eliminating breeding areas for insects and using pesticides.

• Examples: draining areas where stgnant water collects

Vaccination Programs

• Most states now require parents show proof of vaccination before enrolling their children in daycare or public schools.

• If a certain proportion of the population is immune to the disease, the pathogen that causes the disease will be unable to reproduce itself at a high enough level to maintain itself in the population. Eventually the pathogen cannot spread any further and may be eliminated from the population.

Public Health Organizations

• City and state agencies are responsible to enforce regulations, provide public health services such as vaccinations, and monitor and report the incidence of a particular disease to state and federal agencies.

• All these agencies report data to the US Public Health Service

Public Health Organizations

• NIH: National Institute of Health. One of the 8 health agencies of the US Public Health Service– Supports health related research aimed at

understanding, preventing, treating and controlling infectious and other diseases of humankind

Public Health Organizations

• Centers of Disease Control (CDC)– In Atlanta, Ga. Investigates disease outbreaks,

publish epidemiological reports, sponsor a variety of educational programs, research projects and reference laboratories

• Food & Drug Administration (FDA)– Monitors the safety of our food, medicines, and many

other products we use daily

• World Health Organization (WHO)– Provides international surveillance and control of

disease

Treatment of infectious diseases

• Antibiotic literally means “destoyer of life”

• Antibiotics refer to a chemical substance used to treat bacterial infections

• Antimicrobial refers to anything that inhibits the growth of microbes which does not include antihelminthics because worms are not microscopically small

Antibiotics

• Drugs used to treat bacterial diseases• Grouped into categories based on their modes

of action• In general, these drugs inhibit cell wall synthesis,

nucleic acid synthesis, or other enzyme-catalyzed reactions

• Examples: penicillins, cephalosporins, tetracycline and erythromycin

• Some antibiotics can be toxic to human tissues when used in high doses or prolonged periods of time

Antibiotics

• Rifampicin-used to treat TB (tuberculosis)

• Sulfonamides interfere with the synthesis of folic acid, a vitamin necessary for nucleic acid synthesis. Most bacteria must synthesize their own folic acid because their membranes are impermeable to external folic acid.

Treatment of Viral Diseases

• In general, drugs that effectively inhibit viral infections are highly toxic to host cells because viruses use the host’s metabolic enzymes in their reproduction.

• For this reason, most viruses are treated symptomatically until the host’s immune system controls and eliminates the virus or the host dies.

Antiviral drugs

• Drugs that are typically target virus specific

• Acyclovir-used to treat outbreaks of genital herpes

• Amantadine-used to prevent or moderate influenza among those at high risk of severe illness from the disease

Antiviral Drugs

• AZT-inhibits that replication of the HIV genome

• AIDS patients today are prescribed proteases that interfere with the packaging of the HIV genome into virus particles

Treatment of fungal and parasitic disease

• The development of drugs to treat fungal, protozoan, and helminthic diseases is challenging because the agents that kill or inhibit growth in the organisms are also highly toxic to human cells.

• Amphotericin B is an antifungal that disrupts the cell membrane

• Antihelminthic drugs target the adult worm

Protozoa Treatment

• Malaria was successfully treated with chloroquine

• In recent decades, Plasmodium species that are resistant to this drug have appeared and spread to areas where malaria is a common threat. In those areas, a combination of the drugs sulfonamide and pyrimethamine is frequently used to treat the disease

Resistance to antimicrobial agents