Page 904 - Clinical Small Animal Internal Medicine

842 Section 9 Infectious Disease

in peracute or acute diseases. Evidence of seroconversion
VetBooks.ir is fundamental in the diagnosis of some acute diseases. In
these cases, the measurement of acute and convalescent
antibody levels (paired samples) is confirmatory for acute
infection. Antibody level determination may vary between
different laboratories, and therefore, it is recommended to
use the same laboratory and assay for comparison of anti-
body levels. Furthermore, variations are also likely
between assays carried out on different days. Therefore,
ideally, paired samples should be tested at the same time.
Seroconversion is considered when a fourfold or
greater increase in antibody level is demonstrated in
paired samples over a 2–4‐week period, and in some
particular infections longer than that, for instance in
L. infantum infection where a serologic response develops
months after initial infection. In addition, antibodies
Figure 79.1 Lymph node aspirate from a dog. Note the increased
numbers of plasma cells and macrophages with abundant induced by vaccination may be detected by serologic
intracellular Leishmania amastigotes (modified Wright stain, assays, making it impossible to discriminate between vac-
original magnification ×400). cinated and naturally infected animals. Examples of this
include vaccination against B. canis and L. infantum.
sensitivity for the detection of infection, depending on High antibody levels are associated with high tissue
the antigen employed and the level of serum antibodies. parasite loads and disease in some protozoal and arthro-
Serologic cross‐reactivity is common between patho- pod‐borne chronic diseases. In contrast, cases of animals
gens of the same genus or otherwise closely related with suspected clinical signs and compatible clinico-
organisms. Therefore, cross‐reactivity between antibod- pathologic abnormalities and low antibody levels require
ies directed at different pathogens is possible with some the use of additional detection methods to exclude or
serologic tests, especially those based on whole‐parasite confirm the disease, because low antibody levels may
antigen, and is less likely to occur when using recombi- also be detected in subclinical carriers suffering from a
nant protein antigens. It is important to note that the use different clinical disease. Canine leishmaniasis is a good
of antigens of similar species will frequently result in example of a disease in which moderate to severe clinical
cross‐reaction at a high level. For example, antibodies disease is usually manifested by medium to high anti-
against Babesia canis will cross‐react with Babesia vogeli body levels, whereas subclinical infection is more often
antigen. Pathogens of closely related different genera are associated with lower antibody levels or seronegativity.
more likely to cross‐react at low levels, such as the cross‐
reactivity between antibodies to Trypanosoma cruzi and Antigen Detection
Leishmania infantum. The detection of circulating plasma or blood antigen
Antibody detection can indicate past exposure or specific for a pathogen is an additional and usually very
present acute or persistent infection. Acute diseases specific serologic technique. This type of assay detects
may be difficult to diagnose due to the lack of detectable the presence of a circulating plasma or blood antigen.
antibody production and low pathogen load in blood or For example, the most common detection technique
other tissues. Therefore, false‐negative results are possible of canine heartworm infection caused by Dirofilaria

Figure 79.2 Blood smears from two
different dogs. (Right) A large form of
Babesia. (Left) Small forms of Babesia
(modified Wright stain, original
magnification ×1000).