infectious disease pathology

Question 1

Staph Aureus
Describe the virulence factors of s.aureus

Answer 1

Surface protein - involved in adherence and binding to host cells as well as evading the immune
response
Secreted enzymes- to degrade proteins and enhance capacity for invasion
Secreted toxins - to damage host cells i.e. alpha toxin, beta toxin, superantigens

Question 2

What diseases can be caused by staph aureus?

Answer 2

● Skin and soft tissue infections - cellulitis, impetigo, abscesses, folliculitis, paronychia.
Necrotising skin infections and scalded skin syndrome
● Pneumonia
● Endocarditis
● Osteomyelitis

Question 3

What are the risk factors for toxic shock syndrome?

Answer 3

● Use of tampons
● Post operative wound infections
● Postpartum period
● Use of nasal packs
● Staph or strep infections

Question 4

What are the clinical features of toxic shock syndrome

Answer 4

● Hypotension
● Acute renal failure
● Coagulopathy
● Respiratory failure
● Soft tissue necrosis at the sight of infection
● Generalised rash

Question 5

Streptococci
What is the microscopic appearance of streptococci?

Answer 5

Gram positive cocci in pairs or chains

Question 6

What are some post-infectious syndromes caused by streptococcal infections?

Answer 6

● Rheumatic fever
● Immune complex glomerulonephritis

Question 7

What are some infections that can be caused by streptococci?

Answer 7

● Mouth – dental caries caused by S. mutans
● Skin – erysipelas (group A strep) or scarlet fever also s.pyogenes
● Pharyngitis – s.pyogenes
● Pneumonia – s.pneumoniae (pneumococcus)
● CNS infections – meningitis S. agalactiae (group B strep) – also causes Neonatal sepsis
● Endocarditis via S. viridans

Question 8

Which bacterial class does e. Coli belong to?

Answer 8

E.coli is a gram negative rod which is a facultative anaerobe. It is a normal GIT commensal

Question 9

List some types of infections that are commonly caused by e.coli?

Answer 9

● Urinary tract infections
● Prostatitis
● Epididymo-orchitis
● Infectious enterocolitis
● Cholecystitis
● Bacterial peritonitis

Question 10

List the types of e.coli enteritis and their features (

Answer 10

Enterotoxic e.coli (ETEC)
Food and water borne, travellers diarrhoea
LT Heat labile toxin, stimulates adenylyl cyclase increases cAMP increases Cl- secretion
(cholera-like toxin)
ST Heat stable toxin, guanylyl cyclase increase in cGMP

Enterohaemorrhagic e.coli (EHEC)
Found in meat
O157:H7 toxin
Shiga like toxin
Responsible for large outbreaks, clinically causes bloody diarrhoea,
Can result in haemolytic uraemic syndrome or thrombotic thrombocytopaenic purpura in 2% of
cases

Enteroinvasive e.coli (EIEC)
Present in food and water, can be spread from person to person
NO toxins, invades the mucosa and causes colitis

Enteroaggregative E.coli (EAEC)
Adheres via adherence fimbriae
Shigella like toxin and ETEC ST toxin
Non-bloody diarrhoea, seen in patients with acquired immunodeficiency syndrome

Question 11

What is the difference between endotoxin and exotoxin?

Answer 11

● Endotoxins are lipopolysaccharides in the outer membrane of the cell wall of gram
negative bacteria which cause injury via the host immune response
E.g. E coli, shigella, pseudomonas, neisseria

● Exotoxins are proteins that are secreted by the bacteria and cause direct injury
○ tetanospasmin produced by Clostridium tetani.
○ botulinum toxin produced by Clostridium botulinum

Question 12

Neisseria Meningitidis
How does Neisseria meningitidis cause infection

Answer 12

● Common coloniser of the oropharynx
● Spread by respiratory droplets
● Most people develop an immune response and clear it
● Invasive disease happens when there is exposure to a new serotype
● Invades via the respiratory epithelium then to the bloodstream
● Capsule helps the pathogen evade immune response
● Mortality still approximately 10% even with abx cover

Question 13

What are the consequences of Neisseria Meningitidis infection?

Answer 13

Sepsis, meningitis, seizures, SIADH, stroke, hearing loss, cognitive impairment, death

Question 14

Apart from neisseria, what else can cause meningitis?

Answer 14

● Other bacteria - e.coli, group B strep (infants), strep pneumoniae, listeria, haemophilus,
listeria
● Viral: enterovirus, measles
● Other: TB, rickettsial, carcinoma, autoimmune, chemical

Question 15

Gastroenteritis
What are the common causes of infective gastroenteritis?

Answer 15

VIRAL: rotavirus, enteric adenovirus

BACTERIAL:
- Ingestion of preformed toxin i.e. s.aureus, vibrio and chlostridium perfringens
- Toxogenic organism i.e. those that proliferate in the gut and release toxins: e.coli,
vibrio cholera
- Enteroinvasive organisms: e.coli, shigella, salmonella

PARASITES: giardia lamblia, entamoeba histolytica

Question 16

Salmonella
What type of bacteria is salmonella?

Answer 16

Gram negative bacillus, flagellated

Question 17

Describe the pathogenesis of typhoid fever

Answer 17

● Caused by salmonella typhi and paratyphi
● Invades the epithelium
● Taken up by macrophages in the lymphoid tissue in the gut.
● Invades M cells causing reactive hyperplasia in lymph tissue
● Disseminates via the blood

Question 18

What are the clinical features?

Answer 18

● Fever, anorexia, vomiting, bloody diarrhoea via gut wall inflammation
● BC positive in 90% of those with fevers
● Subsequent bacteraemia which can present with flu like symptoms

Question 19

CHOLERA
What is the causative organism of cholera?

Answer 19

Vibrio cholera – a gram negative bacteria that is comma shaped

Question 20

Describe the pathogenesis of cholera (prompt – describe how the toxin causes diarrhoea)

Answer 20

● A non invasive pathogen
● Flagella proteins for attachment and colonisation
● Release of a pre-formed enterotoxin (the cholera enterotoxin, which has 5 beta subunits
and 1 alpha subunit)
● Alpha subunit goes into the cytoplasm and binds a G protein which stimulates adenylyl
cyclase increase intracellular cAMP
● This opens the CFTR channel and results in release of Cl into the lumen
● This results in secretion of large amounts of HCO3, Na and water
● Clinically, it is seen as secretory diarrhoea (ricewater stools) which overwhelms the
colonic resorption capacity

Question 21

Glandular fever
Describe the pathogenesis of glandular fever

Answer 21

● EBV transmitted by close contact (saliva)
● B cells infected
● Viral infection begins in the oropharyngeal lymphoid tissue
● There can be lysis of infected cells with virion release or latent infections
● Symptoms appear with the activation of the host immune system
● Proliferation of T cells in spleen and liver causes organomegaly
● Can result in recovery or in development of B cell lymphomas

Question 22

What are the classical features of glandular fever?

Answer 22

Fever, sore throat, lymphadenitis, splenomegaly, fatigue, hepatitis, rash

Question 23

What are the possible outcomes of glandular fever?

Answer 23

● Recovery - usually takes 4-6 weeks, fatigue can be longer
● Hepatic dysfunction - jaundice, abnormal LFTs, appetite
● Splenomegaly and splenic rupture
● Transformation to lymphomas

Question 24

Please give some examples of clinical herpes simplex infection

Answer 24

Cold sores, gingivostomatitis, encephalitis, genital herpes, keratitis, oesophagitis, pneumonia,
hepatitis

Question 25

After a primary HSV infection, how does reactivation occur?

Answer 25

● Viral nucleocapsids travel from the skin to the nucleus of the sensory neuron
● During the latent period, only viral mRNA is produced, no viral proteins
● Reactivation occurs via avoiding immune recognition and moving along the sensory
nerve again

Question 26

Varicella
What are the 2 clinical conditions caused by this virus

Answer 26

Chicken pox and shingles

Question 27

Describe the pathogenesis and clinical course of infection with this virus including
chicken pox and shingles

Answer 27

● Starts with aerosol or direct contact spread
● Heamatogenous dissemination
● Vesicular skin lesions
● Vesicles rupture, crust over and then heal;
● Some virus lies dormant in the dorsal root ganglia and are reactivated later with
immunosuppression
● Characterised by vesicular eruption along the dermatome of one or more sensory nerves
● May also cause nerve dysfunction i.e. ramsay hunt syndrome

Question 28

What are the complications of chicken pox?

Answer 28

● Lung – interstitial pneumonia
● Nervous system – transverse myelitis, encephalitis
● Skin – shingles, bacteria superinfection
● Gut – necrotising visceral lesions

Question 29

Describe the pathogenesis of varicella zoster

Answer 29

● Patient has an exposure to varicella zoster virus either chickenpox or subclinical
● Virus evades immune defences and infects the sensory neurons in and around the dorsal
root ganglia
● Able to remain latent here for many years
● Uswually a single episode of recurrence in the form of shingles
● Reactivation often occurs in the elderly or immunocompromosed

Question 30

. Influenza
Describe the structure of the influenza virus

Answer 30

Single stranded RNA

Question 31

What are the types and subtypes in influenza?

Answer 31

● Types A, B and C - determined by a nucleoprotein
● Subtypes depend on differences in the Hemagglutinin and neuraminidase proteins on the
envelope

Question 32

What is the pathological basis of pandemics and epidemics?

Answer 32

● This only occurs in influenza A
● Antigenic shift - for pandemics, H and N antigens are replaced by recombination of RNA
with those from animal viruses
● Antigenic drift - for epidemics, mutation in H and N proteins over time allowing for escape
from host antibodies

Question 33

Measles
What type of virus is measles?

How is it spread

Answer 33

● Single stranded RNA virus
● Member of the paramyxovirus family

Respiratory droplet spread

Question 34

Describe some of the clinical manifestations of measles infection

Answer 34

● Viral pneumonia
● Conjunctivitis and keratitis
● Acute measles encephalitis
● Longer term - subacute sclerosing panencephalitis
● Diarrhoea
● Immunosuppression
● Croup

Question 35

What immune responses occur as a result of measles infection?

Answer 35

T cell mediated immunity controls the infection and produces the rash
Antibody mediated immunity protects against reinfection

Question 36

Malaria
What organisms cause malaria?

Answer 36

Malaria is a protozoal infection and an intracellular parasite - plasmodium
The types are P. falciparum, P ovale, P vivax and P malariae

Question 37

Describe the pathogenesis of malaria

Answer 37

Infectious stage (sporozoite) is found in saliva of female anopheles mosquito
Sporozoites are released into the blood and attach & invade hepatocytes
Multiply rapidly
Hepatocyte ruptures, releasing merozoites
These merozoites bind to the surface of RBCs and then grow in a vacuole
The RBCs lyse and spread further

Question 38

How does P falciparum present clinically?

Answer 38

Fever, severe anaemia, cerebral symptoms, pulmonary oedema, DIC
Splenomegaly
Acute renal failure