Ph 162A Lec 6: Host-Parasite Relationship

Front 1

nosocomial infections

Back 1

hospital acquired, esp. urinary tract, surgical wounds, resp tract (via catheter)

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symbiosis

Back 2

association btwn 2+ species:
1.) mutualism
2.) parasitism
3.) commensalism
4.) antagonism

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indigenous microflora (normal flora or human microbiome)

Back 3

-colonization at birth
-humans require MO that permanently or temporarily become associated with it, usually commensal
-transiensy- persistance of MO for hours to months depending on environment

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central concern of epidemiologists

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the host-parasite relationship

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parasite and host

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-parasite- an organism dependent on another organism for its existence
-host- the organism the parasite depends on

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infection

Back 6

est. of a host-parasite relationship by growth and multiplication of the parasite in or on the host

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disease (infectious and non)

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-disease- an injury or abnormal finding in a living organism which results in recognizable effects
-infectious- one that results from an infection with an organism
-non- results from something else

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morbidity and mortality

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morbidity-illness
mortality- death

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epidemiology and reservoir

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-ep- the study of the distrib/transmission and causes (etiology) of disease in a pop (animals, plants)
-reservoir- source of an infectious agent i.e. host or place in which the organism normally lives and multiplies

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common reservoirs

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1.) soil- esp. saprophytes living there eg. Coccidioides immitis, anthrax
2.) animals; zoonoses-diseases caused by organisms whose reservoir is a nonhuman animal eg. anthrax, plague, salmonella, rabies, ringworm, histoplasmosis, tapeworms
3.) humans eg. typhoid mary (NY)- salmonella

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portals of entry and exit

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how microorganism enters and exits the host

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mode of transmission from reservoir to host

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vector- an arthropod which carries the organism from reservoir to host eg. flies, mosquitoes
1.) biological vector
2.) mechanical vector
3.) fomites
4.) direct contact
5.) food
6.) air and water

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biological vector

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different phase of the MO multiplies i.e. partof the life cycle takes place in the vector eg. mosquitoes and malaria, plague

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mechanical vector

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transfers the MO passively with no multiplication of he MO eg. flies, E coli, diarrhea, salmonella

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fomites

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indirect contact; inanimate objects that spread MOs from reservoir to host, eg. nail for tetanus

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direct contact, water

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direct contact- rabies, rat bite fever
water- droplets

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routes of transmission

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1.) respiratory route
2.) fecall-oral route
3.) blood-borne route
4.) contact route

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respiratory route

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-reservoir-humans or animals
-exit- resp tract of infected indiv.
-entry- resp tract of uninfected indiv
-usually via MOs in saliva or mucous by breathing, coughing, sneezing, fomites

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fecal-oral route

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-reservoir- humans or animals
-exit=GIT (feces), entry=mouth
-usually involves fomites esp human hangs, contaminated food and water (eg. typhoid mary)

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blood-borne route

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-reservoir-humans or animals
-exit= wound, entry=wound
-vectors and fomites involved
-special case of mother to fetus transplacental transmission

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contact route

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-reservoir- human, ainmals, soil
-exit= lesion or sore, entry=any
-fomites commonly are involved

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communicable and noncommunicable disease

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-comm- can be transmitted from one organism to another; contagious
-non- cannot be or is usually not transmitted

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infectious disease patterns

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1.) endemic
2.) sporadic
3.) epidemic
4.) pandemic
5.) emerging

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endemic

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disease constantly present in an area but involving relatively few persons at any given time, eg. plague, rabbit fever

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sporadic

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disease that occurs only occasionally and involves only a few individuals eg. encephalitis viruses

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epidemic

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disease involving a large number of persons for a limited period of time eg. whooping cough

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pandemic

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describes a series of epidemics that involve a large part of the world eg. 2009 H1N1 flu

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emerging infectious disease

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disease of infectious origin whose incidence in humans has increased within the past two decades and threatens to increase in the near future
1.) new disease, new MO, eg. HIV
2.) old disease, new MO
3.) new disease, old MO
4.) reemerging infectious disease eg. diphtheria in russia

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covert and overt infection

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-covert- subclinical/asymptomatic; elicits hidden host responses (eg. Ab production)
-overt- clinical/symptomatic; infection that causes conspicuous injury to host

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incubation period (latency)

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-time between exposure to an MO and appearances of signs and symptoms of disease
-signs- objective evidence of disease observed by another
-symptoms- expressed by patient

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endotoxin vs. exotoxin

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-endo- released by dead cells as a result of autolysis, esp gram -, polysacch and lipid, weak, heat resistant, poor immune stim.
-exo- released by living cells, esp gram +, protein, very potent, heat labile, good immune stimulus

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host factors determining infection

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1.) species
2.) certain genetic traits
3.) age
4.) hormonal balance
5.) nutritional status
6.) integrity of an organ system eg. physical barriers
7.) natural secretions eg. chemical barriers (lysozyme-gram +)
8.) host's immune system
9.) endogenous orgs in host
10.) host's behavior

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host's immune system

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-immunodeficient- cells of the immune system are impaired
-immunosuppressed- immune system is suppressed, usually by drugs or a severe disease
-host's immune response to the specific parasite

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endogenous orgs already present on the host

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-interactions among aprasitic organisms:
1.)interference-microbial organisms
2.) synergism- working together to cause disease

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host's behavior

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hygiene, lifestyle- a set of daily life habits, in public health, used to mean habits that affect health, particularly substance abuse, sexual practices, diet, exercise

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adaptation to epidemics

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natural selection of those genetically resistant to the disease until the majority of the pop is able to survive

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co-evolution of host and parasite

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-more advantageous for parasite to not kill host, host can undergo changes to become more resistant
-parasitism --> commensalism --> mutualism
-our indigenous organisms once caused disease but now are in a state of commensalism or mutualism with us

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evolution to increase survival between hosts

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if # of hosts has declined or trickier to get from host to host
eg. cysts- Giardia intestinalis
eg. spores- Bacillus anthracis, fungi

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increase survival by using a vector for transmission so there are two host species to survive in

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1.) using another species as an alternative host eg. helminths
2.) temporary survival until hosts are around eg. Plasmodium sp. (malaria)

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increase survival by developing a long latency period

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increases spread by asymptomatic carriers eg. HIV

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increase survival by an increase in virulence

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to cause a disease that enhances transmission eg. cough --> T, skin lesion, diarrhea

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increase survival by a decrease in virulence

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be so well tolerated by host there is no immune rxn eg. ringworm fungus, endogenous organisms (E coli.)

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host adaptations to increase resistance to the parasite

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euro agriculture:
1.) MO's from domesticated animals
2.) influenza virus-pigs
3.) measles virus-cattle
4.) mycobacterium tb-cattle
5.) smallpox-cow pox
6.) diphtheria bacillus- cattle

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epidemics due to first exposure

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1942- columbus spread smallpox and tb to Na. Am.
1763- differential disease resistance in Britain exploited for germ warfare
1850s- euro --> EAf. >95% TB dead
today- amazon indians- little genetic diversity; af --> us ebola!

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susceptible introduction

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1.) never exposed before
2.) less genetic diversity in pops
3.) 90% death rate upon exposure