Microorganisms of medical and veterinary importance

Question 1

In humans what are the 6 infectious diseases that account for 90% of deaths?

Answer 1

Acute respiratory infection, TB, diarrhoeal disease, malaria, measles, HIV

Question 2

In the UK, cancer accounts for about 24% of all deaths each year, while in Africa around 4% of people die of cancer
Why do you think that might be?

Answer 2

More likely to die of infectious disease in africa, partly lifestyle, longevity, diseases you are exposed to and diagnosis

Question 3

Infectious agents contribute to cancer

Answer 3

E.g. Hepatitis B virus (HBV)
E.g. Human Papilloma Virus (HPV ) types 16, 18
E.g. Helicobacter pylori
Estimated that around 2 million cases of cancer each year attributable to infectious cause

Question 4

Burden of disease in animals important for three key reasons:

Answer 4

Economic (If cows caught TB - loose money)
Conservation (zoos)
Epidemiological (spread between humans and animals)

Question 5

Co-evolution of hosts and parasites

Answer 5

Requires capacity for genetic variation in both host and parasite populations
Resistance / susceptibility factors among host population
Virulence/ reduced virulence factors in parasite population

Question 6

Zoonoses often unexpected (to both host and parasite!) and can be disastrous to one or both:

Answer 6

e. g. Haemorrhagic fever viruses (such as Lassa fever, Ebola)
e. g. Trichinellosis ( caused by Trichinella spiralis)

Question 7

Important to remember that the symptoms of infectious diseases are often due to the body’s immune response

Answer 7

e.g. rhinitis – sign of allergy or a cold!
Infection = disease
Asymptomatic infections because host and parasite in ‘familiar’ relationship

Question 8

Adaptation to the environment
Attaching to cells
Evading chemical defences

Answer 8

Attaching to cells – e.g. Influenza virus cell surface glycoprotein Haemagglutin attaches to sialic acid receptors on surface of respiratory epithelial cells
Evading chemical defences- e.g. Helicobacter pylori secretes copious amounts of urease, which breaks down stomach urea to form ammonia, thus avoiding effects of gastric acid

Question 9

Adaptation to the environment
Evading immune system
Changing life cycle stage

Answer 9

Evading immune system – e.g. Trypanosoma brucei keeps changing the antigenic glycoprotein that it expresses on its cell surface
Changing life cycle stage – e.g. Entamoeba histolytica changes from ameobic to cyst stage to prepare for excretion from gut.

Question 10

3 routes of transmission

Answer 10

Direct contact
Droplet /respiratory infection
Faecal –oral route

Question 11

6 mechanisms of infection

Answer 11

Blood borne 
Sexually transmitted 
Vector-borne 
Interspecies (animal to human/human to animal)
Congenital infection
Opportunistic infection

Question 12

Respiratory viruses

Answer 12

Spread by droplet infection ( sneezing, coughing)
Secretions contain a lot of viral material, but not all fully formed /functional virus particles
Most of the secretory material will end up on inanimate surfaces etc
Virus induces the host to produce copious amounts of mucous etc; virus remains viable in this for a few hours
Virus produces many progeny viruses
Only need one to reach the respiratory epithelial cell of a new host for life cycle to continue

Question 13

Faecal –oral parasites

Answer 13

E.g. Giardia, Entamoeba histolytica
Spread via cysts contaminating water and food
Faeces contains a lot of cysts
If untreated, parasite can live for several years so possibility to excrete large numbers of cysts over long period of time
Cysts are resistant to environment, 4-60C and Cl
Cysts only last for days/weeks unless at low temperature
Sewage often treated
Hand hygiene not always adequate
Large concentrations of cysts in faecal material ( 105/106 per gram!)
Only need one cyst to be ingested by new host and to reach duodenum

Question 14

STIs

Answer 14

E.g. Chlamydia trachomatis, Neisseria gonorrhoeae, Herpes simplex virus
Spread through intimate contact
Relies on human behaviour
Most individuals will not make many such contacts in life time
Infected people often asymptomatic
Symptomatic people often reluctant to seek help
Contact usually successful in transmitting organism

Question 15

Tuberculosis

Answer 15

Tuberculosis in humans and animals
Tuberculosis ( TB) caused by bacteria in Mycobacterium spp . genus
‘Acid fast bacillus’
At least 9 million people newly diagnosed with TB each year and at least 2 million deaths
Reactivation a problem in immunocompromised people

Question 16

TB symptoms, transmission, latent infection

Answer 16

Pulmonary TB ( infection of the lungs)
Symptoms – persistent cough, thick mucous and often blood in sputum, pyrexia , breathlessness, weight loss, anorexia, lethargy
Transmitted from person to person via respiratory route
Latent infection – reactivation especially when immunocompromised (e.g. HIV infection)
Can also affect other sites and organs (extra pulmonary) and be disseminated to many sites ( ‘miliary TB’)

Question 17

Mycobacterium spp.

Answer 17

Clinical disease tuberculosis in human caused by:
Mycobacterium tuberculosis
and also three other species: M.bovis, M.africanum and M.microti
Called Mycobacterium tuberculosis complex (MTBC)
M.bovis, M.africanum and M.microti not normal human pathogens, but can be passed from animals to humans (‘zoonosis’)

Question 18

What is the same and what is different between Mycobacterium spp.?

Answer 18

Hard to distinguish between them in the laboratory, clinical disease and treatment the same
But transmission route different
In UK, most human TB cases associated with M. tuberculosis infection, but also some M. bovis

Question 19

Transmission route of Mycobacterium spp. to humans

Answer 19

M. tuberculosis - respiratory route: Bacteria present in sputum of infected person and aerosol created by their coughing
M.bovis – usually ingestion route: Bacteria present in milk and meat of infected cow
TB caused by M.bovis used to be very common in UK, but few cases now

Question 20

Prevention of M.bovis transmission in humans

Answer 20

M.bovis is denatured by pasteurisation of milk
Meat generally cooked adequately
Also if present in cattle, farmer has to notify DEFRA and herd destroyed , so milk and meat cannot be human food source

Question 21

Reasons for transmission of M.bovis in cattle

Answer 21

M.bovis is spread between cows in herd by respiratory infection
Also found in other animals
In spite of careful vigilance among farmers and vets , still cases of bovine TB
Suggestion that spread between herds via badgers
Infected badgers urinating in cattle feeding troughs during the night ; when cattle feed, ingest or inhale bacteria
Badgers become concentrated in certain areas so opportunities for infection are greater
Controversial scheme to cull in certain areas in response to bovine TB in cattle herds
Randomised Badger Culling Trial (RBCT) tested the effectiveness of this approach

Question 22

Problem with Culling (TB)?

Answer 22

Data analysis reported in 2012 by Vial and Donnelly showed that this ‘reactive culling’ tends to exacerbate the problem
Badgers are very territorial and culling disrupts their groupings and interactions
After culling, surviving badgers move into different areas increasing contact with other badgers and other cattle herds - ‘Perturbation effect’
Tend to move back into original territory area once culling over – now even more likely to be infected with bovine TB!

Question 23

Main form of TB transmission in cattle?

Answer 23

Further analysis of the RBCT data indicated that direct badger to cattle transmission in 6% of cases
94% cattle to cattle – due to undetected infections in cows

Question 24

Prevention against TB in cattle and badgers?

Answer 24

Vaccine available - no perturbation
Does not necessarily prevent disease, but reduces serious illness ( both cattle and badgers)
Suggested that vaccine most effective when given to young animals – but may require boosters
Current vaccine is given intravenously

Question 25

How we know when a person has a particular disease?

Answer 25

Recognise the symptoms

Do specific diagnostic tests if necessary

Question 26

Symptoms of a cold

Answer 26

Sore throat
 Slight pyrexia ( raised body temperature)
 Sneezing
 Runny nose ( rhinitis)
 Coughing
 Headache

Question 27

If you have a cold and you isolate group A streptococcus from a throat swab, does this mean cold is caused Streptococcus pyogenes?

Answer 27

Virology result
Electron micrograph showing coronavirus particles
Bacteriology
Gram stained slide showing Streptococcus pyogenes
Colonies of Streptococcus pyogenes on blood agar

Question 28

Koch’s postulates

Answer 28

Robert Koch
devised the following steps to prove that a particular organism causing a particular disease
1- The organism must be found in every case of the disease
2- It should be possible to grow the organism (from appropriate samples) in pure culture in the laboratory
3-The purified organism must be introduced to a suitable animal model and produce the expected symptoms
4- The organism must be isolated from this animal used as a host in the experiment

Question 29

Problem with Koch’s postulates

Answer 29

Not always possible to grow organism in laboratory culture
Suitable animal model not always available
Some illnesses caused by more than one organism all of which might be present in host
Some conditions are caused by combinations of organisms