Download Principles of Parasitology

Chapter 14: Host-Microbe Relationships and Disease Processes
Host-Microbe Relationships
•Pathogen: A parasite capable of causing disease in a host
•Host: Any organism that harbors another organism
•Symbiosis: An association between two (or more) species
•Symbiosis includes: mutualism, commensalism, and parasitism
Mutualism: Both members of the association living together benefit from the relationship
Parasitism: One organism, the parasite, benefits from the relationship, whereas the other
organism, the host, is harmed by it (e.g. bacteria, viruses, protozoa, fungi, helminths)
Commensalism: Two species live together in a relationship such that one benefits and the other
one neither benefits nor is harmed
Contamination, Infection, and Disease
•Can be viewed as a sequence of conditions in which the severity of the effects microbes have on their
hosts increases
Contamination: means that the microorganisms are present
Infection: refers to the multiplication of any parasitic organism within or on the host’s body
Disease: A disturbance in the state of health wherein the body cannot carry out all its normal
functions
Pathogens, Pathogenicity, and Virulence
•Pathogenicity: the capacity to produce disease
•Virulence: refers to the intensity of the disease produced by pathogens, and it varies among different
microbial species
The virulence of a pathogen can increase by animal passage, the rapid transfer of the pathogen
through animals of a species susceptible to infection by that pathogen
Attenuation: the weakening of the disease-producing ability of the pathogen
Normal (Indigenous) Microflora
•Organisms that live on or in the body but do not cause disease
•Have well-established associations with humans
•Two categories of organisms can be distinguished:
Resident microflora: comprise microbes that are always present on or in the human body
Transient microflora: microbes that can be present under certain conditions in any of the locations
where resident microflora are found
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Opportunists
•Organisms that take advantage of particular opportunities to cause disease. Conditions that create
opportunities for such organisms include:
1. Failure of the host’s normal defenses (immunocompromised)
2. Introduction of the organisms into unusual body sites
3. Disturbances in the normal microflora (microbial antagonism)
Koch’s Postulates
•Must be satisfied in order to prove that a specific organism is the causative agent of a particular disease
1. Specific causative agent must be observed in every case of a disease
2. Agent must be isolated from a diseased host and grown in pure culture
3. Agent from pure culture is inoculated into a healthy host, the agent must cause same
disease
4. Agent must be reisolated from the inoculated, diseased experimental host and ID as being
identical to the original causative agent
Kinds of Diseases
•Human diseases are caused by infectious agents, structural or functional genetic defects, environmental
factors, or any combination of these causes
•Infectious Diseases: caused by infectious agents such as bacteria, viruses, fungi, protozoa, and helminths
•Noninfectious Diseases: caused by any factor other than infectious organisms
Classification of Diseases
1. Inherited Diseases are caused by errors in genetic information
2. Congenital Diseases are structural and functional defects present at birth
3. Degenerative Diseases are disorders that develop in one or more body systems as aging occurs
4. Nutritional Deficiency Diseases lower resistance to infectious diseases and contribute to the
severity of infections
5. Endocrine Diseases are due to excesses or deficiencies of hormones
6. Mental Diseases can be caused by a variety of factors (emotional, psychogenic or infection)
7. Immunological Diseases such as allergies, autoimmune diseases, and immunodeficiencies
8. Neoplastic diseases involve abnormal cell growth that leads to harmless or cancerous tumors
9. Iatrogenic diseases are caused by medical procedures and/or treatments
10. Idiopathic diseases are diseases whose cause is unknown
The Disease Process
•Virulence factors are structural or physiological characteristics that help organisms cause infection and
disease
•Factors include:
1. Structures such as pili for adhesion to cells and tissues
2. Enzymes that help in evading host defenses
3. Protect the organism from host defenses
4. Toxins that can directly cause disease
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Direct Actions of Bacteria
•Adherence or attachment: A critical point in the production of bacterial disease
•Adhesins are proteins or glycoproteins found on attachment pili (fimbriae) and capsules
•Colonization refers to the growth of microorganisms on epithelial surfaces, such as skin or mucous
membranes or other host tissues
•Invasiveness is the ability to invade and grow in host tissues (hyaluronidase enzyme is the spreading
factor)
Hyaluronidase: enzyme digests hyaluronic acid, a gluelike substance that helps hold the cells of
certain tissues together
Coagulase triggers blood plasma clotting, allowing bacteria protection from immune
defenses
Streptokinase dissolves blood clots
Bacterial Toxins
•Any substance that is poisonous to other organisms
•Exotoxins are soluble substances secreted into host tissues
Some exotoxins are enzymes (e.g. hemolysin)
Leukocidins are exotoxins that damage white blood cells
•Endotoxins are part of the cell wall and are released into host tissues from gram negative bacteria
Types of hemolysis:
A: Alpha or partial hemolysis of red blood cells results in a greenish zone around
colonies of Streptococcus pneumoniae
B: Streptococcus pyogenes colonies release B-hemolysins, which produce complete
breakdown of hemoglobin, causing clear zones to form around colonies on blood agar
Clinical Use of Botulinum Toxin
Help victims of dystonia which refers to a group of neurological disorders characterized by
abnormal, sustained, involuntary movements
Blepharospasm: patient’s eyes remain tightly closed at all times. Toxin blocks nerve impulses to
muscles thereby relieving spasms of eyelids
Oromandibular dystonia in which the patient’s jaws are clenched so tightly that the jaw bones may
break are being helped by botulinum toxin injections
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Intoxications
•Diseases that result from the ingestion of a toxin rather than infections
•Many exotoxins have a special attraction for particular tissues:
1. neurotoxins: act on tissues of the nervous system to prevent muscle contraction (botulism) or
muscle relaxation (tetanus)
2. Enterotoxins: act on tissues of the gut
•Toxoid: an altered toxin that has lost its ability to cause harm but that retains antigenicity
How Viruses Cause Disease
•Viruses can replicate only after they have attached to cells and then penetrated specific host cells
•Cytopathic effect (CPE): In tissue culture systems, once inside a cell, viruses cause these observable
changes
•Productive viral infection: occurs when viruses enter a cell and produce infectious offspring
•Abortive viral infection: occurs when viruses enter a cell but are unable to express all their genes to
make infectious offspring
•Latent Viral Infections are characteristic of herpesviruses. A weakened immune system allows the virus
to multiply
•Persistent Viral Infections involve a continued production of viruses over many months or years. The
hepatitis B virus (HBV) infects the liver in such a chronic fashion that there may be no outward signs of
an infection
How Fungi, Protozoa, and Helminths Cause Disease
•Most fungal diseases result from fungal spores that are inhaled or enter cells through a cut or wound
•Certain fungi produce mycotoxins
•Some protozoans invade and reproduce in red blood cells, and Giardia intestinalis attaches to tissues and
ingests cells and tissue fluids. Virulence factor: adhesive disk
•Helminths are extracellular parasites that inhabit intestines or other body tissues and many release toxic
waste products and antigens in their excretions
Giardia intestinalis: The suction forces of the adhesive disk are so strong that they leave
markings behind on the intestinal surface
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Signs, Symptoms, and Syndromes
•Most diseases are recognized by signs and symptoms
Sign: a characteristic of a disease that can be observed by examining the patient (e.g. swelling,
redness, rashes, coughing, pus, runny nose, vomiting)
Symptom: a characteristic of a disease that can be observed or felt only by the patient (e.g. pain,
shortness of breath, nausea, sore throat, headache)
Syndrome: a combination of signs and symptoms that occur together and are indicative of a
particular disease or abnormal condition
Sequelae: even after recovery, some diseases leave after-effects (e.g. valve damage)
Types of Infectious Disease
Acute disease develops rapidly and runs its course quickly (e.g. measles and colds)
Chronic disease develops more slowly than an acute disease, is usually less severe, and persists for
a long, indeterminate period (e.g. Tuberculosis)
Subacute disease is intermediate between an acute and a chronic disease (e.g. gingivitis)
Latent disease is characterized by periods of inactivity either before signs and symptoms appear
(e.g. herpes virus)
Stages of an Infectious Disease
Incubation period is the time between infection and appearance of signs and symptoms
Prodromal phase is a short period during which nonspecific, often mild, symptoms such as malaise
and headache
Prodrome is a symptom indicating the onset of a disease
Invasive phase is period during which the individual experiences the typical signs and symptoms
of the disease
Decline phase is the period of illness during which host defenses and effects of treatment
overcome the pathogen
Convalescent period tissues are repaired, healing takes place, and body regains strength and
recovers
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