Testing for infectious diseases

Question 1

Which organisms can be visualised by microscopy?

Answer 1

Bacteria, fungi, rickettsial, protozoal

More difficult to see mycobacteria, viruses, and spirochaetes

Question 2

What kind of bacteria are always pathological?

Answer 2

Intracellular bacteria

Question 3

Culturing bacteria

Answer 3

Relatively easy to culture

Culture of anaerobic bacteria requires more careful handling of the samples

Question 4

Culturing viruses

Answer 4

Requires specialist facilities, only performed at some laboratories

Question 5

Culturing fungi

Answer 5

Can be performed, can require special media

Fungi grow quite slowly so can take a while to get results

Question 6

Culturing mycobacteria

Answer 6

Quite difficult to culture, requires prolonged culture times

Generally not very useful

Question 7

Considerations when taking samples for culture

Answer 7

Aseptic technique

Suitability of sample (lots of normal flora etc.)

Avoid use of dry swabs (use charcoal transport medium)

Culture from fluid or tissue where possible

State on form where it was taken from

Ideally do cytology at the same time

Transport to laboratory, should be stored at 4 degrees, unless blood and then should be incubated at 37 degress. Samples for anaerobic culture should not be refrigerated.

Question 8

Anaerobic culture

Answer 8

More difficult

Take from tissues or fluid, or swabs in charcoal

Don’t refrigerate (kills some and increases rate od oxygenation)

Question 9

Urine culture

Answer 9

Commonly performed

Best performed on cystocentesis samples

Put in plain tube

Question 10

Blood culture

Answer 10

If you suspect they are septic

Inject correct volume into bottle of blood culture medium, using strict aseptic technique

Do not use indwelling catheters

Take 2-3 samples over 24hrs to maximise sensitivity

Incubate tubes at 37 degrees

Question 11

Joint fluid culture

Answer 11

Can be difficult to culture, false negatives possible

Use of meat broth increases sensitivity

Question 12

Faecal culture

Answer 12

Generally of minimal diagnostic value

Culture for Salmonella and Campylobacter may be useful

Question 13

Virus culture

Answer 13

Relatively slow, requires special transport medium

Will detect viable/replicating virus so can be more specific than PCR

Question 14

Fungal culture

Answer 14

Slow - 10-14 days

Can then be further speciated by PCR

Commonly carried out for Dermatophyte culture (ringworm)

Question 15

Antigen detection by PCR

Answer 15

More sensitive than visualising
More rapid than culture
Detects organisms alive or dead
RNA viruses can also be detected
Can use quantitative methods to identify the abundance (burden)

Only detects the organism you are testing for

False positives possible (contamination), can pick up from modified live vaccines

False negatives possible if low level, or if mutations have occurred

Question 16

Antigen detection by antibody capture

Answer 16

Lateral flow tests

Antibodies tagged with coloured latex beads and incorporated onto a membrane - antigens pick them up and are immobilised by a line of stuck antibodies

E.g. FeLV ‘SNAP’ tests, Angiodetect ‘SNAP’ test (angiostrongylus)

Question 17

Serology

Answer 17

Testing for endogenous antibodies to specific agents

Can be run from blood sample alone

Antibodies not generated immediately after infection
Only indicates exposure not current infection

IgM produced in response to recent infection, persists for about 3mo

IgG produced in response to infection but persists for longer, may reflect exposure or infection

Can look for rising titres on paired testing (4 weeks apart)

Question 18

Testing for bacterial infections

Answer 18

Visualisation of bacteria is often possible and can be used to guide empirical therapy.

Culture is the gold standard for diagnosis, and will allow full speciation and antimicrobial susceptibility testing to be performed.

Question 19

Testing for FeLV and FIV

Answer 19

First line test is an in-house ‘SNAP’ test which detects the presence of antibodies to FIV (serology) and the presence of the p27 protein (antigen) from the FeLV virus in blood - as vaccinate for FeLV

Question 20

Testing for FIP

Answer 20

Mutated form of feline coronavirus

Serology - poorly specific as infection with non-mutated cornoaviruses common, high titres can be suggestive

PCR of effusions/tissue - useful if coronavirus detected in unusual location

Immunocytochemistry of effusions - allows antigen detection and identification of coronavirus in macrophages, specificity can be variable

Immunohistochemistry on histopathology - gold standard

Question 21

Detection of feline herpesvirus and calicivirus and Chlamydophila felis

Answer 21

Can cause nasal, respiratory, and ocular infections

Feline herpes and caliciviruses can be detected on oral and conjunctival swabs by PCR or virus isolation
False positives if recent vaccine

Chlamydophila felis identified by PCR from conjunctival swabs

Question 22

Testing for canine parvovirus

Answer 22

Causes diarrhoea and GI signs

First line test is ParvoFASTtest (antigen capture), sensitivity may be as low as 30%

PCR of faeces likely to be more sensitive but takes longer

Question 23

Testing for canine leptospirosis

Answer 23

Can present with signs of marked liver damage/dysfunction and renal dysfunction

Usually diagnosed with PCR of blood and urine

Serology can also be tested, can get lower titres after vaccine

Question 24

Testing for canine distemper virus

Answer 24

Can present with GI, resp, or neuro signs

Can be diagnosed by PCR on CSF, or blood, or serology on CSF or blood

Question 25

Testing for fungal infections

Answer 25

VIsualisation possible due to large size Culture can be quite slow PCR can detect some contaminant fungal agents rather than clinically important organisms Serology for Aspergillus (causing canine rhinitis) available but not that sensitive

Question 26

SNAP 4Dx test

Answer 26

Serological testing for: - Anaplasma phagocytophilum and platys - Ehrlichia canis and ewingii - Borrelia burgdorferi Positive results interpreted in light of clinical signs, travel history, and liklihood PCR may be needed to confirm positive result

Question 27

Testing for mycobacterial disease

Answer 27

Visualisation challenging as do not stain with normal stains Staining with Ziehl-Neelsen can be helpful PCR of tissue most commonly used test as culture requires 2-3mo Blood test for interferon gamma indicated exposure to mycobacteria

Question 28

Testing for Angiostrongylus vasorum

Answer 28

Lungworm Can soetimes be visualised within BAL fluid, or larvae can be seen on faecal smear examination Antigens can be detected using Angiodetect SNAP test Both antigen detection and faecal examination are <50% sensitive

Question 29

Testing for Giardia

Answer 29

Parasitic infection associated with diarrhoea Antigens can be detected in faeces using SNAP test Highly sensitive, specific for infection Giardia trophozoites can also be seen on faecal smear, low sensitivity