Lecture notes Year 1 MBChB: Introduction to Medical Sciences (IMS)

INFECTION


INTRODUCTION TO INFECTION



Causes of infection Size Parasites in Medicine
 Helminths - Tapeworm  Parasite: organisms that depends on another for its survival to the
 Insects - Fleas detriment to the host
 Protozoa – Malaria  All infections are parasites
 Fungi - Candida  Medical parasite: helminths, insects and protozoa
 Bacteria - Staphylococci  Endoparasites: live inside body; major cause of illness
 Viruses - Influenza  Ectoparasites: live outside body; minor symptoms but can transmit other
 Prions - Kuru infections e.g. fleas, lice, bed bugs, ticks


Human endoparasites

Protozoa Helminths
 Unicellular organisms  Complex organisms
 Some have complex lifecycles involving more than  Some have complex lifecycles involving more than one
one host host
 Other species have their own Helminths which can
accidently cause human disease

Protozoa
Amoebae  Example: Entamoeba histolytica
 Invades large bowel lining Worms – Helminths
 Causes dysentery  Cestodes (tapeworms) –
o Abdominal cramps segmented, flat
o Bloody diarrhoea  Trematodes (flukes) –
 Excreted with faeces unsegmented, flat
 Spread via contaminated food & water  Nematodes (round worms) –
 Risks: poor hygiene/sanitation cylindrical, have digestive tract with lips,
teeth and anus
Sporozoa  Example: Plasmodium falciparum (malaria)
 Lifecycle in both humans and mosquitoes
Ringworm – colloquial term
 Infects red blood cells and liver
 Symptoms: fever, headache, joint pains
Cestodes
Trematodes
– tapeworms
- flukes
 Complications: kidney failure, coma, death
 Example:
 Example: taeniaSchistosoma
saginata (beef
haematobium
tapeworm)(bilharzia)
 Risk: geographical
 Intestinal
 Human parasite
host: infection
of humanof(obligatory
veins around
host)
bladder
 Largely
 Causes asymptomatic
bladder inflammation, bleeding into urine
o(haematuria)
abdominal pain
 oIntermediate host freshwater snail
malnutrition
 Diagnosis: urine microscopy for eggs
 Diagnosis
o patient
o stool microscopy for eggs
 Cattle are intermediate hosts

Ectoparasite – bedbug
 Example: Cimex lectularius
 Wingless insect
 Worldwide infestation of human dwellings
 Hide in cracks in furniture & walls
 Emerge at night to feed – 5-10 minutes for a blood meal
 Itchy rash after bite
 Can transmit other infections - protozoa in South America (trypanosomiasis)

Fungal infections
 Yeasts – single cells which bud
 Moulds – filaments strands
 Some can switch between yeast & mould (dimorphic fungi)

, Severe invasive fungal infections
Superficial fungal infections  Example: cryptococcus neoformans (yeast)
 Tinea pedis – athletes foot  Infects patients with low resistance due to failing immune
 Tinea corporis – ringworm system e.g. HIV
 Usually due to 3 common species of mould  Causes meningitis (inflammation of membranes lining brain)
 Headache, neck stiffness, confusion, coma, death

Medical importance of bacteria
Bacteria - prokaryotes  Responsible for:
 Unicellular organisms o Pneumonia
 Cell membrane, cell wall o Urinary tract infection
 No nucleus – non-linear DNA o Cellulitis - skin
 Reproduce asexually o Meningitis
 Move using flagellae and pili o Cholecystitis
o Diverticulitis
o Appendicitis


Gram stain
 Some retain crystal violet stain, others do not
 Gram-positive (purple)
 Gram-negative (pink)
 Allows you to predict which antibiotics will be effective quickly
 Tells you what antibiotic to prescribe




Bacterial infection
 Example: streptococcus pneumoniae aka pneumococcus
 Gram-positive cocci in pairs (diplococci)
 Colonise nose & throat (40-50% adults)
 Invade other sites e.g. lungs causing pneumonia
 Cough, dirty sputum, chest pain breathlessness, fever
 Complications: blood stream infection, meningitis, death, sepsis

Viruses
 Dependent on infection of host cell for metabolism & replication
 Contain protein core surrounding genetic material (DNA or RNA), protein coat +/- outer
membrane
 Very small: 1/100th size of bacteria
 Can only be seen with powerful electron microscopes
Virus life cycle – HIV
 Turns RNA into cDNA integrates it into our own cell, protease
packages it and send it out of the host cell

Viruses cause a wide spectrum of disease
 Some cause trivial infections e.g. rhinovirus – common cold
 Some cause severe chronic disease e.g. HIV
 Some cause acute life threatening disease e.g. Ebola – viral
haemorrhagic fever
Viruses infect host cells for differing
lengths of time
 Acute infection: norovirus infects hosts for days, causing diarrhoea and vomiting (gastroenteritis)
 Chronic infection: hepatitis C virus causes liver inflammation for years
 Latent infection: herpesviruses can be dormant for decades before reactivating to cause disease