Diagnostics in infectious disease Flashcards
(25 cards)
What are the two main approaches to diagnosing infectious disease in animals?
Detection of the organism itself or evidence of infection via antibody response.
List five questions to consider when diagnosing an infectious disease.
- Does signalment/history match the suspected agent?
- Do clinical signs support the suspicion?
- Has the agent or antibody response been proven?
- Has prior treatment affected diagnosis?
- Have other causes been excluded?
Why is sampling technique critical in diagnosing infections?
To avoid contamination and improve diagnostic yield (e.g., cystocentesis for urine vs. free catch).
Give an example where timing and organism lifecycle affect detection.
Giardia is intermittently excreted; pooled samples over 3 days increase detection chances.
What factors affect bacterial culture success?
Aerobic bacteria are easier to culture than anaerobic; some (e.g., mycobacteria) are difficult; others (e.g., Haemoplasma) can’t be cultured.
Define Minimum Inhibitory Concentration (MIC).
The lowest antimicrobial concentration that inhibits macroscopic bacterial growth in a standard lab culture.
How is MIC useful in treatment decisions?
It helps classify bacteria as susceptible or resistant, guiding drug choice and dosing strategy.
What zoonotic risk does fungal culture pose?
Uncommon fungi (e.g., Paecilomyces) may infect humans, requiring strict lab precautions.
Why is viral culture less common now?
PCR is faster and more sensitive, though culture remains useful when PCR targets are undefined (e.g., Feline calicivirus).
What is the value of direct organism observation?
Provides rapid evidence to guide initial therapy before culture results return.
Name four special stains and their uses.
Gram stain: Gram bacteria; Ziehl-Neelsen: acid-fast bacteria (mycobacteria); PAS/KOH: fungi; Immunohistochemistry: microorganism antigens.
How does an ELISA test detect antigen?
Antigen binds to a plate; labelled antibody binds the antigen; color change indicates presence when substrate is added.
Name three other immunoassays besides ELISA.
Direct immunofluorescence (IFA), indirect immunofluorescence, microscopic agglutination assay.
What is PCR and why is it useful?
Polymerase Chain Reaction amplifies tiny amounts of DNA or RNA, providing sensitive and rapid pathogen detection.
List two drawbacks of PCR.
- Detects DNA of dead organisms (false positives).
- Primer variability between labs affects reliability.
What does serology detect?
Host antibodies (IgM early, IgG later) indicating exposure or infection.
When is paired serology useful?
When single results are unclear; a 4-fold titre rise over 10–14 days suggests active infection.
How should antibody samples be stored?
Stable at 7–10 days refrigerated; freeze if stored longer.
What does test sensitivity measure?
The ability to correctly identify animals with the disease (true positives).
What is Sn-N-Out?
High Sensitivity, Negative result rules Out disease.
What does test specificity measure?
The ability to correctly identify animals without the disease (true negatives).
What is Sp-P-In?
High Specificity, Positive result rules In disease.
Define positive predictive value (PPV) and negative predictive value (NPV).
PPV: Probability that a positive test indicates disease.
NPV: Probability that a negative test indicates no disease.
How does disease prevalence affect predictive values?
High prevalence increases PPV; low prevalence increases NPV.
