Lecture 28 - What causes infection Flashcards
Causes of infection
helminths - tapeworm insects - fleas protozoa - malaria fungi - candida bacteria - staphylococci viruses - influenza prions - kuru
What is a parasite
an organism which depends on another for its survival tot e detriment of its host
all infections are
parasites
traditionally which are considered medical parasites
helminths, insects and protozoa
What are the 2 types of medical parasites
edoparasites - live inside the body - major cause of illness
- helminthes and protozoa
Ectoparasites
- live outside body - minor symptoms, transmit other infections
- fleas, lice, bed bugs, ticks
Endoparasites
protozoa
- unicellular organisms
- have some complex lifecycles involving more than 1 host
helminths
protozoa (amoebae)
example: entamoeba histolytica invades large bowel lining causes dystentery -abdominal cramps -blood diarrhoea -excreted with faeces -spread with contaminated water risks: poor hygiene/sanitation
Protozoa - sporozoa
Eg. Plasmodium falciparum (malaria)
- lifescycle in both humans and mosquitoes
- infects RBC’s and liver
- symptoms: fever, headache, joint pains
- complications include: kidney failure, coma and death
- risk: geographical
Life cycle of malaria
gametocyte - sporozite - liver stages - merozite - trophozite and schizont
Helminths (worms)
complex organisms
- some have complex life cycles with more than one host
- other species have their own helminths which can accidentally cause human disease
Examples of helminths -worms
cestodes - tapeworms -segmented, flat trematodes - flukes -unsegmented, flat nematodes - round worms -cylindrical, have digestive tract with lips, teeth and anus
cestodes
eg. taenia sanginata - beef tapeworm
- intestinal parasite of human obligatory host
-largely asymptomatic
-abdominal pain and malnutrition
diagnosis
-patient
-stool microscopy for eggs
-cattle are intermediate hosts
What is the lifecycle of a cestode
-eggs passed into faeces
cattle become infected by -ingesting vegetation
-oncospheres hatch penetrate intestinal wall and circulate to musculature
oncopheres develop into cysticerci in muscle
- humans infected by ingesting raw or undercooked infected meat
- scolex attaches to intestine
-adults in small intestine
Trematodes - flukes
example: schistosoma haematobium
Human host: infection of veins around bladder
causes bladder inflammation, bleeding into urine
-intermediate host freshwater snail
diagnosis: urine microscopy for eggs
Life cycle of schistosoma spp
eggs passed in urine of faeces into fresh water
miracadium hatches from egg, infects, first intermediate host (snail)
cercariae leave snail penetrate skin wall
immature worm enters the blood stream and eventually ends up in veins near the intestine or bladder
worms reach sexual maturity in veins of abdominal cavity.
females produce eggs, eggs enter intestinal tract or bladder
Ectoparasite - bedbug
example: cimex lectularius wingless insect worldwide infestation of human dwellings -hide in cracks in furniture and walls - emerge at night to feed - 5-10 minutes for a blood meal -ithcy rash after bite can transmit other infections -protozoa in south america
forms of Fungal infections
2 main forms - yeasts - single cells which bud
moulds - filamentous strands
- some can switch between yeast and mould
Superficial fungal infections
skin and related structures
- tinea pedis (athletes foot)
- tinea corpis (ringworm)
- usually due to 3 common species of mould
Sever invasive fungal infections
example: cryptococcus neoformans (yeast)
-infectious patients with low resistance due to failing immune system - HIV
Causes meningitis -
headache, neck stiffness, confusion, coma and death
features of Bacteria
unicellular (prokaryotes) cell membrane and wall no nucleus genetic material in form of DNA reproduce sexually move using flagellae and pili
grouping of bacteria
shape - round (coccus), rod (bacillus)
grouping - clusters, chains or pairs
Medical classification of bacteria
gram stain
- cell walls vary in composition
- some retain crystal violet stain, others do no
- gram -+ve - purple
- -ve pink
What is the clinical importance of classification by appearance on shape grouping and gram stain
allows you to predict which antibiotics will be effective quickly
-which ones to prescribe
Bacterial infection - eg. streptococcus pneumococcus
Gram - positive cocci, (diplococci)
- colonise nose and throat
- invade other sites eg. lungs causing pneumonia
- cough, dirty sputum, chest pain breathless, fever
- complications: blood stream, infection, meningitis, death
Viruses
depend on infection and host cell for metabolism and replication
-contain protein core surrounding genetic material, protein coat outer membrane
very small : 1/100th size of bacteria
-can only be seen with powerful electron microscopes
Lifecycle of a virus
HIV ( GP120) binds to cell through CD4 receptors
- reverse transcription occurs, integrase allows it to integrate with cell - protease leaves cell
eg of Common viral infections
rhinovirus
sever viral infections
HIV
Acute life threatening viral disease
ebola - viral haemorrhagic disease
Examples of short terms and long term viral infections
acute: norovirus - causing diarrhoea and vomitting (gastroenteritis)
chronic: hep C virus causes liver inflammation for years
Latent infection - herpes virus can be dormant for decades before reactivating to cause disease
Viruses that cause latent infection
varicella zoster virus
- primary infection causes chickenpox
- characteristic rash and fever
- virus becomes dormant in sensory nerve roots
- reactivates years later as shingles
- same rasg
- confined to dermatome
What happens in glandular fever and what is it caused by
what can it contribute to
Epstein-Barr virus
- usually causes mild illness
-infects immune system cells (B cells) and epithelial cells of nose and mouth (nasopharynx)
- very common - 90%
-causes latent lifelong infection
contributes to certain cancers in southern china
lymphoma in HIV infection
Prions
smallest infective agents known proteinaceous infectious particles proteins are abnormal and lack nucleic acid, not a living organism accumulate main in neural tissue -difficult to destroy - standard sterilisation doesnt work
CJD = Creutzfeldt–Jakob disease
prion diease
rare fatal, degenerative neurological disease
transmitted via human growth hormone, surgical instruments and corneal grafts
Variant CJD
typically occurs in young adults
Thought to be derived form bovine spongiform encephalopathy (BSE or mad cow disease)
Kuru
similar to vCJD
occurred in Papua New Guinea in 1950s
spread by cannibalism
especially brains of deceased relatives
