Infectious Disease Diagnostics

Question 1

what are the two classes of diagnostic tests

Answer 1

1. pathogen detection
2. host response to pathogen

Question 2

pathogen detection diagnostics

Answer 2

detect the pathogen itself

• whole pathogen/culture
• nucleic acid based
• pathogen antigens

Question 3

host response to pathogen diagnostics

Answer 3

detects aspects of the patient’s response to the pathogen

• antibody to the pathogen
• cell mediated response to the pathogen

Question 4

culture

Answer 4

use of specialized media and culture conditions to isolate live and whole organisms from a specimen collected from the patient

Question 5

what are the types of cultures

Answer 5

1. agar media
2. cell culture
3. whole animal

Question 6

agar media culture

Answer 6

bacteria or fungi
- not all organisms grow on agar

does NOT ID organism - need additional testing

Question 7

cell culture

Answer 7

obligate intracellular bacteria and viruses

have to grow a cell line from same species as the patient

does NOT ID organism - need additional testing

Question 8

whole animal culture

Answer 8

rarely done

used when pathogen cannot grow outside a living creature

Question 9

when should culture be used

Answer 9

• when you need the organism to do further testing
• organisms that grow quickly in standard conditions
• discovering novel organisms

Question 10

what does a + and - culture mean

Answer 10

+: pathogen present but does not always mean it is causing disease

-: does not rule out disease but makes it less likely

Question 11

nucleic acid testing

Answer 11

PCR testing - detects DNA (or RNA converted to DNA)

Question 12

when to use PCR

Answer 12

• slow growing/unculturable pathogens
• need rapid results (hours)
• acute infections
• biohazardous agents that you do not want to culture
• in conduction with culture to ID organisms

Question 13

what are the downfalls of PCR

Answer 13

• have to know the DNA sequence
• cause only use certain sample types
• DNA and RNA can degrade - have to test quickly w/ adequate sample size
• may detect subclinical disease

Question 14

what are PCR panels

Answer 14

tests for multiple organisms at once
- less expensive
- more difficult to interpret - not all pathogens detected may be causing disease

Question 15

what does a + and - PCR test indicate

Answer 15

+: above threshold; piece of DNA of interest was detected in the patient sample

-: below threshold; pieces of DNA of interest was not detected in the patient sample

Question 16

what causes false positive and negatives on PCR

Answer 16

false positive: previous infection, contamination

false negative: inhibitory substances in sample, lower amount of DNA than test sensitivity

Question 17

antigen detection tests

Answer 17

detects one or more antigens on/from the pathogen

• IHC
• DFA
• lateral flow assays
• ELISA

Question 18

what is required for an antigen detection test to work

Answer 18

• a known antibody that can detect antigen
• a way to visualize the antigen/antibody complex once formed (color, fluorescence)

Question 19

steps of antigen ELISA tests

Answer 19

1. antibody to the antigen is immobilized on substrate
2. sample is processed and put on/in substrate
3. second antibody is added to also recognizes bound antigen
4. wash
5. detection of second bound antibody

Question 20

steps of IHC and DFA

Answer 20

1. take a tissue section or smear containing suspected pathogen
2. antibody is added to the sample with an enzyme or fluorescent tag
3. color change or fluorescence occurs if antibody binds the antigen

IHC: color change; use with tissue biopsies
DFA: fluorescence; use with smears/stains

Question 21

what does a + and - antigen detection test indicate

Answer 21

+: pathogen present but not always causing disease
-: does not rule out disease but makes it less likely

Question 22

antibody detection tests

Answer 22

detects the presence of antibodies against a pathogen in patient’s sample

• ELISA
• IFA
• agglutination
• immunoblot
• AGID

Question 23

when to use antibody detection tests

Answer 23

• chronic disease
• paired measurements/titers (acute sample + chronic sample)
• pathogens that are difficult to culture

Question 24

benefits of antibody tests

Answer 24

• good for hard to culture pathogens
• paired tests can be used for acute disease
• detection of IgM can indicate acute exposure

Question 25

downfalls of antibody detection tests

Answer 25

- not good for acute disease - takes time for patient to mount an antibody response - may have low sensitivity - false positives can occur with vaccination/subclinical disease - positive results doesn't always mean infection - not good for immunocompromised animals - may need multiple samples

Question 26

what does a + and - antibody test indicate

Answer 26

+: indicates exposure or vaccination; not necessarily live infection -: indicates no exposure/infection OR possible acute infection OR patient is immunocompromised

Question 27

cell-mediated immune response tests

Answer 27

detects cell mediated cytokine responses - skin tests (Tb) not commonly done

Question 28

what tests should generally be done for acute disease

Answer 28

pathogen detection - culture - antigen

Question 29

what tests should generally be done for chronic disease

Answer 29

host response detection - antibody - cell mediated

Question 30

what tests should generally be done for immunocompromised patients

Answer 30

pathogen detection - culture - antigen

Question 31

what tests should generally be done for undetectable organisms

Answer 31

host response detection - antibody - cell mediated

Question 32

what diagnostic test should be used for a cat with a large growth on its nose w/ suspected Cryptococcus

Answer 32

collect an aspirate of the mass for direct smear staining and microscopy

Question 33

why might only 1 species of bacteria be on an aerobic culture if 2 species were visualized on direct gram stain

Answer 33

the second species may be a strict anaerobe

Question 34

what is the best way to transport samples from a horse transtracheal wash and pleural fluid for culture

Answer 34

ship each in a sterile white top or red top with a cold pack overnight to the lab

Question 35

what is the best diagnostic test for a horse with pleuropneumonia? samples are from transtracheal wash and pleural fluid

Answer 35

- gram stain - aerobic and anaerobic culture - susceptibility testing

Question 36

what stain would be most beneficial for an FNA from a 7 yo indoor/outdoor cat with ulcerative, proliferative lesions on the paw that do not respond to antimicrobial therapy

Answer 36

acid fast stain (can ID mycobacterium)

Question 37

what is the best way to test for Francisella tularensis

Answer 37

PCR from a small piece of spleen (do NOT culture due to Tularemia risk)

Question 38

why might a 3 month old goat doe receive a positive CAE ELISA antibody assay

Answer 38

a 3 month old goat is young enough to still have circulating maternal antibodies, causing a false positive

Question 39

what diagnostic test would you select for an 18 month old quarter horse with stumbling/incoordination, fever, muscle fasciculations, and ataxia

Answer 39

West Nile virus ELISA on serum that detects IgM because the assay detects an ACUTE response to the virus

Question 40

what diagnostic test would you select for a 2 yo ram showing signs of infertility and unilateral epididymal enlargement

Answer 40

brucella ovis ELISA on serum

Question 41

what diagnostic test would you select for a pleural fluid sample from a 3 yo indoor/outdoor cat with lethargy, inappetence, dyspnea, tachypnea, and lung retraction on rads

Answer 41

aerobic and anaerobic culture on pleural fluid (expecting nocardia)