Page 1462 - Small Animal Internal Medicine, 6th Edition

1434 PART XIV Infectious Diseases

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FIG 91.8
Bartonella spp. antigen recognition pattern by feline serum antibodies determined by
Western blot immunoassay. MW, Molecular mass standards; Post, weeks after infection.

to the puppy or kitten in the colostrum. Most infectious by vaccination. Examples include feline coronaviruses, some
agents can induce disease within 3 to 10 days of initial expo- B. burgdorferi assays, some Leptospira spp. assays as well as
sure; with many assays serum IgG antibodies are usually not those for FHV-1, parvoviruses, FIV calicivirus, and canine
detected until 1 to 2 weeks after initial exposure. On the basis distemper virus.
of these facts, falsely negative serum antibody tests during The clinician should interpret positive results in serum
acute disease can be common in small animal practice. If antibody tests only as evidence of present or prior infection
specific serum antibody testing is initially negative in an by the agent in question. Recent or active infection is sug-
animal with acute disease, repeat antibody testing should be gested by the presence of IgM, an increasing antibody titer
performed in 2 to 3 weeks to assess for seroconversion. Doc- over 2 to 3 weeks, or seroconversion (negative antibody
umentation of increasing antibody titers is consistent with result on the first test and positive antibody result on conva-
recent or active infection. Assessment of both the acute and lescent testing). However, detection of recent infection based
convalescent sera in the same assay on the same day is prefer- on antibody testing does not always prove disease. Con-
able to avoid interassay variation. versely, failure to document recent or active infection based
Sensitivity is the ability of an assay to detect a positive on serologic testing does not exclude a diagnosis of clinical
sample; specificity is the ability of an assay to detect a nega- disease. For example, many cats with toxoplasmosis develop
tive sample. Sensitivity and specificity vary with each assay. clinical signs of disease after serum antibody titers have
Positive predictive value is the ability of a test result to predict reached their plateau. The magnitude of antibody titer does
presence of disease; negative predictive value is the ability of not always correlate with active or clinical disease. For
a test result to predict absence of disease. Many of the infec- example, many cats with clinical toxoplasmosis have IgM
tious agents encountered in small animal practice infect a and IgG titers that are at the low end of the titer scale; con-
large percentage of the population, resulting in serum anti- versely, many healthy cats have IgG titers greater than
body production. However, they only induce disease in a 1:16,384 years after infection with T. gondii. Similarly, Bar-
small number of animals in the infected group. Examples tonella spp. antibody magnitude does not correlate to clinical
include coronaviruses, canine distemper virus, T. gondii, illness in cats.
Bartonella spp., and Borrelia burgdorferi. For these examples,
even though assays with good sensitivity and specificity for BODY FLUIDS
the detection of serum antibodies are available, the predic- Some infectious agents induce disease of the eyes or central
tive value of a positive test for presence of disease is extremely nervous system (CNS). Documentation of agent-specific
low. This is because antibodies are commonly detected in antibodies in aqueous humor, vitreous humor, or CSF can be
healthy carriers. Diagnostic utility of some serologic tests is used to support the diagnosis of infection of these tissues.
also limited because of the presence of antibodies induced Quantification of ocular and CSF antibodies is difficult to