Page 902 - Clinical Small Animal Internal Medicine

840 Section 9 Infectious Disease

Table 79.1 Advantages and disadvantages of common diagnostic methods for the detection of infectious diseases in dogs and cats
VetBooks.ir Diagnostic techniques Advantages Disadvantages

Microscopic ● Permits direct detection of the pathogen and nature ● Relatively low sensitivity for the detection of
examination (cytology of the host response. pathogens in tissues or body fluids
/histopathology) ● Findings could be suspicious of infection or allow ● Requires the performance of other diagnostic
exclusion of other differential diagnoses tests such as immunohistochemistry and/or
● Cytology is rapid and less invasive than obtaining PCR when pathogens are not visualized
tissues for histopathology by biopsy ● Does not distinguish between
● Cytology usually permits easier visualization of morphologically similar organisms
pathogens when compared with histopathology ● Requires expertise
Serologic testing ● Detection of specific antibodies against the ● Does not detect the pathogen itself
(antibody detection) pathogen causing infection ● Typically does not discriminate between
● Permits evaluation of seroconversion to confirm vaccinated and naturally infected dogs or cats
recent infection ● Serologic cross‐reactivity between related
organisms is possible
– Qualitative ● Rapid in‐clinic test ● Provides only positive or negative results
● Variable sensitivities and performance with
risk of false‐negative results due to
conservative cut‐off level and use of
recombinant proteins
● A positive result will benefit from additional
validation by quantitative serology
– Quantitative ● Determines the antibody level which is of major ● Performance and accuracy of cut‐off vary
(IFA, ELISA) importance in some diseases such as leishmaniasis between laboratories
where high antibody levels in the presence of ● Frequent lack of sufficient standardization of
compatible clinical signs and/or clinicopathologic techniques between laboratories
abnormalities are conclusive of clinical leishmaniasis
● Low antibody levels frequently require further
testing
Serologic testing ● Allows the detection of pathogen ● May not be sensitive when low numbers of
(antigen detection) pathogens are present
Molecular testing ● Allows the detection of pathogen DNA and RNA ● False‐positive results possible due to DNA
● High sensitivity and specificity for target loci contamination or to amplification of
erroneous targets including host DNA
● Allows determination of pathogen load (by real‐time
PCR) ● Different techniques used by diagnostic
laboratories and lack of standardization
Culture ● Permits the isolation of pathogens and their ● Time‐consuming and laborious
maintenance for further comparisons and analysis ● Requires special equipment and biohazard
● Facilitates in‐depth identification of pathogens conditions and therefore restricted to
specialized laboratories
● May require up to one month to provide a
result
Source: Modified from Solano‐Gallego and Baneth (2016). Reproduced with permission of John Wiley & Sons.
ELISA, enzyme‐linked immunosorbent assay; IFA, immunofluorescence assay; PCR, polymerase chain reaction.

based solely on morphology is often not possible and or low tissue pathogen load, which frequently make
molecular analysis is required for speciation. Sensitivity pathogen observation by microscopic evaluation diffi-
will depend on the time spent searching for microorgan- cult. Therefore, the use of molecular diagnostic assays is
isms, type of pathogen, tissue sampled, amount of organ- strongly recommended in those cases. In addition,
ism in the tissue (parasite load), and clinical status of the identification of protozoa in formalin‐fixed, paraffin‐
patient. Generally speaking, higher diagnostic sensitivity embedded sections of different tissues may be facilitated
is found in sick animals compared with subclinically by immunohistochemical methods such as immunoper-
infected dogs or cats. The diagnosis of chronically oxidase staining or in situ hybridization techniques.
infected and carrier animals remains a diagnostic The optimal tissue or body fluid for sampling will
challenge due to low and often intermittent parasitemia, depend on types of pathogen involved and lesions found