Unit 7 HOST PARASITE RELATIONSHIP

• A Delicate relationship exist between pathogenic microorganismand the body defenses.

• When the defenses resist the pathogens, the body remains healthy.

• But when the pathogens overcome the defenses, the result is disease.

• So inspite of our defenses, we are still susceptible to pathogenicmicroorganisms.

• Pathology; the scientific study of diseases.

PATHOLOGY:

1. First concerned with cause (etiology) of disease.

2. Deals with pathogenesis ( manner in which diseases develops)

3. Concerned with pathology (structure) and functional changes brought about by disease and with its final effect on body.

INFECTION AND DISEASE

• Infection: Invasion or colonization of body by pathogenic microorganism.

• Also defined as the presence's of a particular type of microorganism in a part of the body where it is not normally found.

• Ex E. coli normally found in intestines but considered an infection if found in the urinary tract.

• Disease; Occurs when infection results in any change from a state of health.

• Normal flora; the population of microorganisms found where the body tissues interface with the environment.

• Much of the normal flora is permanent, but some portions are transient.

• Transient flora: present for a time and then disappears.

Types of relationships between normal flora and body:

1. symbiosis; means living together

a. commensalism: when one organism benefits and the other remainsunaffected.

b. mutualism; when both organisms benefit one another.

c. parasitism; when one organism damages the other.

d. phoresy; to carry something Ex flies carry parasites on legs

2. Opportunistic organisms; are potentially pathogenic organisms that normally do not cause disease.

• However in a compromised host, the organism may see"opportunity" to invade the tissues.

• Ex AIDS individuals often are susceptible to

• Pneumocystic carinii which invade the lung tissues and causes a lethal pneumonia

ETIOLOGY OR DEVELOPMENT OF INFECTIOUS DISEASE;

• In order to relate a particular organisms to a particular disease, KOCH'S POSTULATE must be fulfilled.

• Remember; Koch's postulates are a series of procedures for identifying the cause of a particular disease.

EXCEPTIONS to Koch's postulates;

• There are a few exceptions to Koch's postulate.

• Ex Treponema pallidum is the causative agent of syphilis, but has never been cultured in artificial media.

• Some pathogens cause several pathologies ex. Mycobacterium tuberculosis disease of lungs , skin, bones and intestinal organs

SYMPTOMS AND SIGNS;

• Infections diseases are usually characterized by changes in body function known as symptoms.

•Symptoms are subjective changes not always apparent to the observer.

• The patient may also exhibit signs, which are objective changes that can be measured. Ex fever, skin rash.

• When a specific group of symptoms or signs accompanies a disease, the group is called a syndrome.

• Predilection site: where you expect to find the parasite or disease in the host.

TRANSMISSION AND INCIDENCE

• Diseases are often classified in terms of how they behave within the host or within a given population.

• Infections diseases may be classified according to theirtransmissibility.

a. Communicable disease: a disease that spreads from one host to another. Ex chicken pox, measles, herpes

b. Contagious; Those communicable diseases transmitted with ease.

c. Noncommunicable; diseases that are not spread between host. Ex Clostridium tetani

• Incidence; refers to the percent of the population that contracts a disease over a particular period of time.

• Prevalence of a disease; the percentage of a population having the disease at a particular time.

Classification of Disease Occurrence;

1. Sporadic disease: when a disease occurs only occasionally. Ex Typhoid fever

2. Endemic; When a disease is present in a population at all times. Ex common cold

3. Epidemic; A disease that breaks out in a population in a short period of time. Ex influenza, AIDS

4. Pandemic: an epidemic disease occurring throughout the world. Ex AIDS

Categories of human population during an epidemic;

1. suffering class: show all symptoms

2. suffering asymptomatic class: source of infection, don't know they have it.

3. immune or vaccinated class

4. susceptible but not yet infected: not exposed.

• The more there are of classes 1,2,4 the greater the epidemic spread.

• The more there are of class 3, the less probability of a major epidemic.

To limit or end an epidemic;

1. isolate victims

2. isolate potential victims

3. treatment

4. quarantine: typical time period about 40 days.

TYPES OF DISEASES; (based on severity and duration)

1. acute disease: occurs rapidly and last a short time. Ex influenza

2. chronic disease: develops slowly and lasts a long time. Ex tuberculosis

3. subacute disease: disease with vague symptoms and lasts relatively long time. Ex sclerosing panencephalitis (rare brain disease decreases intelligence and nervous function)

4. latent disease: remains inactive in host for a time then becomes active. Ex shingles

Scope of diseases;

1. local infection: those restricted to a small area of the body.

2. systemic infection: spread throughout body system. Also calledgeneralized infection.

3. septicemia: presence's of multiplying microorganisms in the blood.(Toxemia— toxins in blood, Viremia=\\rus in blood,Bacteremia=bacteria in blood)

4. focal infection', agent of local infection enters blood/lymph and spread to other body parts.

CAUSE OF DISEASES:

1. bacterial

2. fungal

3. viral

4. rickettseal

5. protozoan

MODES OF DISEASE TRANSMISSION

1. Reservoir of infection: when a disease remains in a population, this is the source of the pathogen ( soil, animal, human, plant, inanimate objects...ect)

2. Carrier: human/animal reservoir who has had the disease and recovered but continues to shed the infectious organisms (zoonose: animal disease spread to humans)

3. Contact: may be direct or indirect

a. direct transmission: form person to person ex touching, kissing ect.

b. indirect transmission: when a nonliving object is intermediary between 2 humans.

• Fomite: an inanimate object that serves as a means to transmit a disease. Ex nail fomite for Clostridium tetani

• droplet nuclei: bits of mucus, and saliva that spread between individuals

4. Vectors: an arthropod that transmits an infectious agent from an infected host to a susceptible host. May be mechanical (carried on body part such as legs) or biological (in saliva, or feces)

5. Vehicles; lifeless objects such as food, water, and air. Which may be contaminated by human feces, pathogens from soil or droplet nuclei.

Ways Arthropods transmit diseases

1. bite2. regurgitation3. abrasion4. contamination5. combination of above• HOST: an organism, simple or complex including

human, capable of being infected by a specific agent.• PORTALS OF EXIT: In order for infection to be

transmitted, microorganism must leave the body. Ex intestinal, mouth, skin surface, are generally the same as the infected body part.

DEVELOPMENT OF AN INFECTIOUS DISEASE:

• Infectious diseases result from a competition for supremacy between the parasite and the host.

• If the parasite overcomes the host, there is a change in the general state of good health and disease develops

• Several contributing factors are involved in the establishment of infectious diseases.

CONTRIBUTING FACTORS:

1. PORTAL OF ENTRY• The way an infectious agent enters a host.

• The most common portals of entry are: respiratory tract, and gastrointestinal tract, but may also include skin wounds and urogenital tract.

• Many pathogens have a preferred portal of entry and using it is a prerequisite to their causing a disease. Ex Salmonella typhi: must be swallowed to cause typhoid fever, hookworms(Necator americanus) through the skin

• Nosocomial infections: Infections acquired during a hospital stay. 5 to15% of all hospital patients acquire noscomial infections.

• In the past it was gram positive Staphylococcus aureus.

2. DOSE:

• The dose of an organism refers to the number ofmicroorganism required to establish an infection.

• LD50 The does of microorganisms that will kill 50% of the hosts it enters.

• ID50: (infectious dose) dose required to produce a demonstratable infection in 50% test animals

3. INVASIVENESS:

• The ability of the pathogens to penetrate into thetissue. Ex Entamoeba histolytica (causes intestinal ulcers) penetrates tissue effectively.

• Tissue invasion often begins with adherence, the ability of the pathogens to attach to the tissue by using structures such aspili.

• Capsules: structures possessed by some pneumonia• Ex Streptococcus pneumonia

• COMPROMISED HOST:• • One whose resistance to infection is impaired by

disease, therapy or burns.• —3 conditions that can compromise a host

a. broken skin/mucous membranes

b. suppressed immune system

c. impaired cell activity

DISEASE PATTERNS:1. Period of incubation: time between the entry of the parasite into toehost

and the appearance of symptoms. Varies.

2. Prodrome period: mild symptoms such as aches, fever and early signsof the disease.

3. Period of illness: disease is most acute. Signs and symptoms are most apparent and each disease has its own characteristic appearance.

4. Period of decline: signs and symptoms subside

5. Period of convalescence: person returns to normal.

• EPIDEMIOLOGY: The study of the distribution and determinant of a diseases in the human population. (1854 John Snow—questioned victims of cholera and traced the path of the disease)—giving rise to modern epidemiology.

ENZYMES AND TOXINS: • Many pathogens produce a series of enzymes to help overcome

body defenses and establish themselves in host.

• Enzyme Examples:1. leukoicidens (enzymes that destroy WBC)2. hemolysins (bacterial enzyme destroys RBC)3. coasulases: enzyme that clots blood...helps staphylococci avoid

body'sphagocytes and contributes to its pathgenicity.

4. streptokinase: streptococcal enzyme that dissolved blood colts, helpsorganism escape body's attempt to wall off infection

5. hyaluronidase: destroys hyaluronic acid, a polysaccharide that"cements" cells together in tissue.

6. collasenase: breaks down collagen in the connective tissue of musclesthus spreading the disease.

• Toxins: poisonous substances produced by bacteria, they act on the body's cells, tissues, and organs and interfere with important body processes, thereby interrupting normal body functions.

Types of toxins(2):

• 1. Exotoxins: proteins produced by bacteria during their growth and released into their surroundings.

• Produced mainly by gram positive bacteria. Ex neurotoxins: interfere with the nervous system, enterotoxins: interfere with gastrointestinal tract

• The body produces special antibodies called antitoxins: which neutralize toxins and provide defense against the disease.

• 2.Endotoxins: portions of the cell wall of Gram negative bacteria.

• They consist primarily of lipopolysaccharides and are released when bacteria break apart during the process of lysis.

• Since lysis occurs during antibiotic therapy, the effects ofendotoxins can bring about worsening of symptoms during the recovery period, (endotoxin shock: fever, chills, aches and cardiovascular collapse)

PATHOGENIC VIRUSES:

• Because viruses lack metabolic capabilities, they rely on other means for overcoming body defenses and causing disease.

• Viruses avoid body defenses by multiplying within host cells, where antibodies and other components of the immune system cannot reach them.

• The effect occurring in host cells during viral invasion is referred to as the cytopathic effect (in some cases the virus causes the cell's lysosomes to release enzymes which then destroy the cell)