1. Lecture 4 : Agents / Transmission Overview
CAUSAL AGENTS
Metazoa
Fungi
Protozoa
Bacteria
Viruses
MEANS OF TRANSMISSON
Airborne
Food- And Water-Borne
Vector-Borne
Direct Physical Contact
Indirect Physical Contact

2. Metazoa Diseases caused by worms include:
Onchocerciasis (or river blindness) caused by Onchocerca volvulus
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.

4. 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.

5. Protozoa
Protozoa are single celled microorganisms belonging to the Protista kingdom.
Diseases caused by protozoa include:
Malaria (Plasmodium)
Nagana (African Sleeping Sickness) (Trypanosoma)
Chaga’s Diseases (Trypanosoma)

6. 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:
cocci - round or oval
bacilli - rod-like
spirochetes - spiral or corkscrew shaped
vibrios - curved rods shaped like commas.

7. Bacteria(2)
Bacteria are about 1000 times smaller than animal cells (i.e micrometres).
They can replicate every 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.

8. Bacteria(3) Diseases caused by bacteria include:
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)

9. 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.

10. 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.

11. Viruses(2) Diseases caused by viruses include: mumps AIDS
poliomyelitis
rabies
shingles
smallpox
rubella (German measles)
yellow fever
AIDS
chicken pox
common cold
encephalitis
herpes
hepatitis
influenza
measles

12. 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.

13. 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.

14. 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.

15. 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).

16. 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.

17. 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.

18. 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.

19. 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).