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Introduction to Medical Sciences > Infection > Flashcards

Infection Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the different types of organisms that cause disease? Give examples

A 
Helminths - Tapeworm
Insects - Fleas 
Protazoa - Amoeba, Plasmodium 
Fungi - Candidia and Trinea 
Bacteria - Staphylococcus 
Virus - Influenza 
Prions - Kuru
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2
Q

What is a parasite?

A

An organism which depends on another for its survival to the detriment of its host,. Strictly all infections are parasites however only helminths, insects and protozoa are classed as medical parasites.

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3
Q

What is the mode of transmission of Malaria? What are the symptoms?

A

Malaria is transmitted by the protozoa Plasmodium falciparum. It affects the red blood cells and the liver. Its symptoms are headache, fever and joint pains but can lead to kidney failure, coma and death. It is common in Southern Africa, South America and south-east Asia. Plasmodium travels via the vector the Anopheles mosquito. This mosquito bites the human, passing Plasmodium sporozites into the blood. This then travels to the liver where it forms merzoites. these merzoites replicate and travel throughout the blood infecting red blood cells forming trophozoites.

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4
Q

What is the effect of Amoeba?

A

The Entamboeba histolytic invades large bowel lining and causes dysentery. It is excreted with faeces. It sis read via contaminated food and water.

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5
Q

What is the mode of transmission of tapeworms?

How do you test for it?

A

Tapeworms are part of the groups Cestodes in the Helminth family. These worms are an intestinal parasite of humans. The effect os largely asymptomatic. You diagnose this using a stool sample. Cattle are its intermediate host and so is spread through infected meat.

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6
Q

What is the mode of transmission of Schistosoma spp?

How do we test for it?

A

These are helminths - part of the trematodes group - that spread through contaminated water. There eggs are found in infected bodies of water; these then use the fresh water snails as hosts. Humans that swim in this water, or drink for it can be infected. This causes bladder inflammation and haematuria. The test for this infection is using urine.

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7
Q

What are the two forms of fungi?

A

Yeast - unicellular

Moulds - Multicellular (filamentous strands - mycelium)

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8
Q

Give examples of common fungal infections and their cause.

A

Ring worm: Tina corporis. Usually due to 3 common species of mould.

Athlete’s foot: Tinea pedis.

Crytocococcus neoformans (yeast) is a severe invasive fungal infection. It affects those with a high immunodeficiency.

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9
Q

How do we name bacteria?

A
  • Coccus = round
  • bacillus = rod-like shape

Can be grouped in clusters, chains or pairs

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10
Q

How do we classify bacteria using gram stain?

A

Gram positive = Purple due to which peptidoglycan cell wall. Retain crystal violet stain.
Gram negative = Pink. Due to thin wall of peptidoglycan. But also made up of lipoproteins.

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11
Q

Give an account of the bacteria that causes pneumonia:

  1. Name of organism
  2. Method of transmission
  3. Symptoms
  4. How is it tested for?
A
  1. Streptococcus pneumoniae. Gram positive -dicocci bacteria.
  2. Colonise the nose and throat. Invade other sites such as the lungs. Complications can lead to a blood stream infection, meningitis and death. Droplet transmission.
  3. Symptoms cough, dirty green sputum, chest pain, breathlessness and fever.
  4. Nose swab.
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12
Q

How do viruses effect us? Use HIV as an example

A
  1. The virus binds to the cell surface membrane receptor. In the case of HIV it is the CD4 receptor.
  2. It then gains access to to the cell.
  3. It then uses reverse transcriptase to convert its mRNA into DNA.
  4. It uses integrate to incorporate its DNA into the DNA of the host cell.
  5. The host cell then is able to produce the protein, the virus needs to survive.
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13
Q

What is the structure of viruses?

A
  • Protein coat surrounding genetic material

- No nucleus - but a loop of genetic material

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14
Q

Which virus causes the common cold?

A

Rhinovirus

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15
Q

Which virus causes the winter vomiting disease?

A

Norovirus

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16
Q

Which virus causes chicken pox? What is shingles? What is the difference in symptoms?

A

Varicellar Zoster virus. It’s primary infection causes chickenpox. This produces the characteristic rash and fever. Virus can become dormant in sensory nerve roots and reactivate in later years as Shingles. This also produces a rash but the rash is confined to the dermatome (the area supplied by the single sensory neurone)

17
Q

Which virus causes glandular fever?

A

The Epstein-Barr Virus. This virus infects immune cells (B cells) and epithelial cells of the nose and mouth. It is very common. Can cause latent lifelong infection and contribute to cancer - nasopharyngeal carcinoma or lymphoma in HIV positive patients.

18
Q

What can chronic Hepatitis B lead to in the future?

A

Individuals chronically infected with hepatitis B have a 25% to 40% lifetime risk of developing liver cancer.

19
Q

What are prions? What disease do they cause?

A

Prions are infectious proteins. They are not living organisms as they do not contain DNA. Prions can cause normal proteins to transform into infectious proteins.
Examples of the diseases they cause:
- Creutzfeldt–Jakob disease: A rare fatal, neurodegenerative disease transmitted via hCH, surgical instruments and corneal grafts.
- Kuru. Occurred in Papua New Guinea in 1950s.

20
Q

What are normal microbial flora? How can they be a source of infection?

A

Most mucous membranes in humans harbour multiple types of bacteria. These vary according to site e.g. in the gut, reproductive system and upper airways.

21
Q

What is an endogenous infection? Give examples.

A

An infection that arises due to normal flora e.g. those found in the genital tract, gut, skin and upper airways. An infection results when the flora gets in the wrong place.
e.g. UTI such as cystitis. This is usually due to flora from the gut (E.coli) leaking into the bladder. This results in inflammation, haematuria, change in frequency, urgency and lower abdominal pain.
Another example is MRSA. Can be found on the skin but when it enters the bloodstream, it can cause life0threatening infections.

22
Q

What are exogenous infections?

A

Infections that arise from parasites that are usually found outside the body. These are communicable disease. Can be spread from person-to-person, from non-human sources such as birds and insects and from the environment (such as eggs in the soil and water).

23
Q

What are the routes of transmission of endogenous infections? Give examples

A
  • Migration: For example bowel flora (such as E.coli) gain access to the urethra and cause a local infection. This can spread to the bladder and form a UTI.
  • Perforation: e.g. Diverticulitis. Diseases of the bowel - especially the colon. Perforation of the bowel leads to contamination of the abdominal cavity with floral flora. Can lead to severe, fatal infection - faecal peritonitis.
  • Blood: e.g. endocarditis. Dental work may allow mouth flora to enter the blood stream. Circulation of organisms allows them to reach the distant sites e.g. heart valves. Causes inflammation and structural damage.
24
Q

What is the method of transmission of exogenous infections? Give examples

A
  • Direct contact e.g. Impetigo. superficial skin infection due to staphylococcus/streptococcus bacteria.
  • Indirect contact e.g. MRSA, C. difficult and Norovirus via hands, equipment, furniture etc.
  • Injuries e.g. Tetanus - Clostridium tetani. present in soil. Contaminates wounds. Releases toxin causing muscle spasm.
  • Bites e.g. Malaria
  • Airbourne e.g. Influenza. Droplet transmission
  • Oral route e.g. Cholera which is spread via drinking contaminated water.
  • Blood Bourne e.g. Hepatitis B. spread by using infected needles and transmission.
  • Sexual transmission e.g. Chlamydia
  • Vertical (Mother-to-baby) e.g. Herpes

Some however can be transmitted in more than one way e.g. HIV - blood Bourne, vertical (pre-natal and breast feeding) and sexual.

25
Q

How does Staphylococcus aureus infect?

A

It requires more than one step. It is an opportunistic bacteria despite being a part of the natural microbiome - found naturally on the skin and in the upper airways.

  1. Colonisation of sin - joins skin flora
  2. Penetration of skin - spreads and damages
26
Q

What are the pathogenic factors that make an infection serious?

A
  • Virulence factors and toxins
  • Pathogen dose
    There is a minimum number of organisms required to produce disease.
  • Antibiotic resistance
  • Direct infection of cells/tissue
27
Q

What are virulence factors?

A

Virulence factors are molecules produced by bacteria, viruses, fungi, and protozoa that add to their effectiveness and enable them to achieve the following:

  • colonization of a niche in the host (this includes attachment to cells)
  • immunoevasion, evasion of the host’s immune response
  • immunosuppression, inhibition of the host’s immune response
  • entry into and exit out of cells (if the pathogen is an intracellular one)
  • obtain nutrition from the host
28
Q

How does Cholera infect the host?

A 
Cholera produces toxins. These bind to the mucosa. This leads to a massive loss of fluid and electrolytes. It does this increasing the activity of Adenyl class and so increasing cAMP production. As a result it decreases Na+ absorption at the kidneys and increases Cl- secretion. This leads to severe dehydration and diarrhoea. 
The diarrhoea is often described as rice water.
29
Q

What is antibiotic resistance?

A

Micro-organisms readily mutate e.g. HIV and staphylococci. Frequent mutations increases the chance of resistance. increasing antibiotic use increases mutation rate.

30
Q

What are the host factors that affect the severity of an infection?

A
  • Environment
    Geography; climate; poverty - and so availability of health care; Public heath infrastructure and distribution of other infection hosts.
  • Barriers to infection
  • genetics
31
Q

What is dengue fever?

A

A virus infection, transmitted by Aedes mosquito. results in a fever, rash and muscle pain. Complications can result in bleeding, shock and multi-organ failure.

32
Q

What are the barriers to infection?

A
  • Mucus membranes
  • Genetic resistance
  • Antibodies
  • Stomach acid
  • Inflammation
  • Behaviour
  • Naive bacteria
33
Q

Why are patients on some drugs to treat stomach ulcers at a higher risk of food poisoning?

A

The medication, increases the pH of the stomach acid. This means that more parasites can survive the environment.

34
Q

What is the effect of antibodies destroying normal flora?

A

Normal flora prevent colonisation of pathogenic bacteria. If destroyed by antibiotics, pathogenic bacteria can colonise and so reproduce and cause disease. For example C. difficult infection can arise after the use of broad spectrum antibiotics, leading to diarrhoea.

35
Q

What basic tests can be used to diagnose infection?

A
  • Neutrophil count - if high can signal bacterial infection.
  • Lymphocyte count - If high can signal viral infection.
  • Eosinophil count - If high can signal parasitic infection.
  • C-reactive protein (CRP) - marker of inflammation
  • Liver function tests (LFT)
  • U&E - Urea and electrolytes levels (Kidney function)
  • Lumbar puncture - test CSF for meningitis
36
Q

What tests can be done on the sample?

A
  • Bacterial culture
  • Antibiotic sensitivity testing
  • Microscopy
  • Identify DNA/RNA using PCR
37
Q

What are antimicrobial agents? What are the factors of a perfect antimicrobial agent?

A

Agents that kill microorganisms.

Factors:

  • Selective toxicity (effective against microbe without harming host)
  • Bacteriocidal (= kills the organism) rather than bacteriostatic (= inhibits its growth)
  • No resistance
  • Good pharmacokinetics (i.e reach high level in body and stay there, allowing fewer doses/day)
  • No side effects
  • Not inactivated by enzymes secreted by microbes, or by the host
38
Q

What are the problems with antimicrobials?

A

Variable spectrum of activity:

- Broad spectrum (kills lots of different organisms)
- Narrow spectrum (kills few organisms)

Some cannot be given orally

Many cause side effects which can be serious, e.g. anaphylaxis (severe allergy)

Bacterial resistance
some antibiotics are now effective against very few organisms, e.g. penicillin
some organisms are now resistant to many antibiotics, e.g. MRSA

39
Q

Give examples of prevention of infection?

A 
Food/water borne:
Sanitation, clean water supplies, food preparation hygiene
Contact
e.g MRSA:
Handwashing
Wearing gloves, gowns, masks
Airborne
e.g. TB:
Isolate infected patient in a negative pressure room (that filters out-going air)
Vector borne
e.g. malaria:
Avoid mosquito bites: insect repellent, bed nets, lonf sleeves

Introduction to Medical Sciences (43 decks)

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