Transcript Document

Lecture 4 : Agents / Transmission
Overview
CAUSAL AGENTS
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Metazoa
Fungi
Protozoa
Bacteria
Viruses
MEANS OF TRANSMISSON
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Airborne
Food- And Water-Borne
Vector-Borne
Direct Physical Contact
Indirect Physical Contact
Metazoa
• Diseases caused by worms include:
o Onchocerciasis (or river blindness) caused by
Onchocerca volvulus
o Schistosomiasis (or bilharzia) caused by different
species of blood fluke belonging to the genus
Schistosoma
• May also get infestations by arthropods (e.g. lice, fleas,
mites), but they are not very serious.
Fungi
• Fungi are plant-like multi-celled organisms.
• They form a separate kingdom with about 50,000 species.
• Fungi cause many plant diseases. A fungus was the cause
of the Irish Potato Famine (Phytophthora infestans).
• Fungi only cause diseases (known as mycoses) in humans
but most are fairly mild, e.g. ringworm, thrush, and
athletes foot.
Protozoa
• Protozoa are single celled microorganisms belonging to the
Protista kingdom.
• Diseases caused by protozoa include:
o Malaria (Plasmodium)
o Nagana (African Sleeping Sickness) (Trypanosoma)
o Chaga’s Diseases (Trypanosoma)
Bacteria(1)
• Bacteria are single-celled organisms belonging to the
Monerans kingdom.
• Bacteria contain DNA but they do not have a nucleus.
• There are 1,600 known species, but there may be 10 to 100
times more waiting to be discovered.
• Major sub-types include:
o cocci - round or oval
o bacilli - rod-like
o spirochetes - spiral or corkscrew shaped
o vibrios - curved rods shaped like commas.
Bacteria(2)
• Bacteria are about 1000 times smaller than animal cells
(i.e. 1-10 micrometres).
• They can replicate every 15-20 minutes. A single cell can
produce 5,000,000,000,000,000 cells in 24 hours.
• Bacteria perform many useful functions in nature, and have
been put to use by humans (e.g. sewerage farms, food
industry).
• Only a minority of around 200 species cause health
problems.
Bacteria(3)
Diseases caused by bacteria include:
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anthrax (Bacillus anthracis)
bubonic plague (Yersinia pestis)
cholera (Vibrio cholerae)
diphtheria (Corynebacterium diphtheriae)
gonorrhoea (Neisseria gonorrhoeae)
leprosy (Mycobacteria leprae)
syphilis (Treponema pallidum)
tetanus (Clostridium tetani)
tuberculosis (Mycobacteria tuberculosis)
typhoid (Salmonella typhi)
typhus (Rickettsia prowazeki)
whooping cough (Bordetella pertussis)
Bacteria(4)
• Many bacteria can be killed by heat (e.g. boiling, cooking,
pasteurisation).
• Many bacteria are susceptible to antibiotics.
• However, there is a growing problem of antibiotic
resistance due to natural selection.
Viruses(1)
• Viruses are little more than a strand of DNA or RNA
enclosed in a protein shell (capsid).
• Viruses vary in size and shape, but they are about 1,000
times smaller than bacteria. Most are too small to be seen
by a light microscope.
• Viruses are completely parasitic. It is questionable whether
they are a lifeform as they are incapable of an independent
existence.
• They repoduce by hijacking a cell’s ribosomes, often
causing the cell to burst apart.
Viruses(2)
Diseases caused by viruses include:
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AIDS
chicken pox
common cold
encephalitis
herpes
hepatitis
influenza
measles
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mumps
poliomyelitis
rabies
shingles
smallpox
rubella (German measles)
yellow fever
Viruses(3)
• Antibiotics have no effect on viruses.
• Most drugs that destroy viruses also destroy the host cell.
• The best protection against viruses is provided by vaccines
(i.e. weakened strains of the virus that trigger the immune
system).
• Many viruses mutate continuously rendering vaccines
ineffective.
Other Causal Agents
• Viroids cause diseases in plants, but not have been
identified in human diseases;
• Prions cause Creuzfield Jacob disease, mad cow disease,
and similar diseases in other animals.
Means of Transmission
• Most infectious diseases are associated with a particular
means of transmission.
• There are numerous means of transmission, but a few
broad categories may be identifed.
Airborne
• Agents present in nasopharynx or respiratory tract are
disseminated in salivary droplets during coughing,
sneezing or breathing.
• The droplets are very small and soon evaporate leaving the
agent floating in the air.
• They are breathed in by the next victim.
• Diseases spread in this manner include mumps, measles,
chicken pox, colds, and influenza (viral); and whooping
cough, diphtheria and tuberculosis (bacterial).
Water- And Food-Borne
• Food may become contaminated (e.g. Salmonella,
Clostridium botulinum).
• Agents passed in faeces may find their way into the water
supply (the faecal-oral route). Cholera and typhoid are
transmitted in this manner.
• Sanitary reforms in the late-19th century reduced the risk of
these diseases by ensuring clean water supplies.
• Risks of infection may be reduced by good personal
hygiene, but infection is still easier than might be realised.
Vector-Borne
• Transmission is by an organism (the vector), in most cases
an arthropod (and generally an insect).
• Three main mechanisms:
a) Mechanical – e.g. flies, cockroaches, mice, rats.
b) Bites – e.g. mosquitoes (malaria, yellow fever, dengue),
fleas (bubonic plague), ticks (Lyme disease) and mites
(typhus).
c) Urine / faeces. May subsequently be ingested in
fodd/water, breath in dust, or enter cuts.
Direct Physical Contact
• Venereal diseases (e.g. syphilis, gonorrhoea, genital
herpes) are transmitted by direct contact during sexual
intercourse.
• A few diseases (e.g. leprosy, yaws) may be transmitted by
direct contact of a non-sexual nature.
Indirect Physical Contact
• The causal agent may be transmitted via an inanimate
object (fomite).
• Sharp instruments which penetrate the skin may carry
infection (e.g. surgical intruments, syringes, razor blades,
needles).
• Dust and soil carrying an infectious agent may gain acces
through a cut to cause diseases such as tetanus
(Clostridium tetani) or gas gangrene (Clostridium
perfringens).