Page 906 - Clinical Small Animal Internal Medicine

844 Section 9 Infectious Disease

presence of parvoviral antigen in the feces of dogs shed- Bacterial Infections
VetBooks.ir ding this virus. The rapid assays that detect antibodies Microscopic Examination
against a virus are often not quantitative and only pro-
duce a positive or negative result, in contrast to tests
done in a specialized laboratory which frequently Most bacteria are detectable by high‐power light micros-
provide an antibody titer or quantitative measure related copy of cytology specimens (Figure 79.3) but this is usu-
to the level of antibodies, such as the optical density in ally not sufficient for species identification and may
the ELISA. serve as a relatively insensitive measure for the presence
of bacteria in the sample. Furthermore, some anatomic
sites such as the skin, oral cavity, gastrointestinal tract,
Molecular Diagnostics and upper airway normally contain bacteria and it is usu-
Molecular diagnosis of viruses includes the detection of ally difficult to distinguish between the normal flora and
specific nucleic acid sequences present in the targeted infection with pathogenic bacteria just by microscopic
virus. Viruses may contain DNA or RNA, depending on examination of the organisms. Gram stain, acid‐fast
the viral family to which they belong. PCR assays are staining, and other staining methods are ancillary
commonly associated with the amplification of the techniques that can assist in the initial assessment of
nucleic acid sequence selected as the target for the assay bacterial infection. Dark‐field microscopy is helpful in
and visualization of the product on a gel in conventional the detection of spirochetes such as leptospires in urine
PCR, or its amplification and release of measurable fluo- samples.
rescence by real‐time PCR which can also be quantita-
tive. Amplification of nucleic acids from RNA viruses Serologic Testing
may require an additional reverse transcription step in
which RNA is translated into cDNA and then amplified. Serology can be useful in the detection of some bacterial
RNA is less stable than DNA and is susceptible to rapid infections of dogs and cats. Detection of specific anti-
degradation unless stored in an RNA stabilization solu- bodies is helpful in the diagnosis of leptospirosis and a
tion and frozen at –70 °C or below. A multiplex reverse battery of serogroup antigens is used to find which sero-
transcription‐nested PCR assay has been developed to var demonstrates the highest antibody titer and is either
differentiate between wild‐type and vaccine strains of responsible for infection or closest to the Leptospira
canine distemper virus. serovar present in the animal. Serology is also useful for
An additional molecular diagnostic technique is the the detection of exposure to intracellular bacteria such as
use of nucleic acid probes which attach specially to Ehrlichia canis or Anaplasma phagocytophilum.
nucleic acid sequences in viruses and do not involve
amplification of the virus’s DNA or RNA. Isolation and Identification (Including
Molecular Testing)
Isolation
Culture has been the major technique for detection and
Isolation of virus from blood, secretions, or organ tissues characterization of bacterial pathogens for the past cen-
is a common method of diagnosis for viral infections of tury. There are a number of different media for growth of
dogs and cats. Isolation requires the use of appropriate bacteria from various groups with specific metabolic
media to transport the tested sample, a specialized labo- requirements. Often, one type of medium is used for the
ratory, and rapid transportation to the laboratory in transport or initial growth of bacteria and other media
refrigeration. Viruses are usually taken from the trans- are subsequently used for subculture with enrichment
port media or the original fluid in which they were sub- and selection of the bacteria suspected as cause of infec-
mitted (e.g., blood or CSF) and grown in various cell tion. The MacConkey agar medium selects for gram‐
culture lines. Often virus isolation is attempted in multi- negative bacteria. Cultures from an infection site are
ple cell lines simultaneously as the host cell preference of often placed initially into more than one medium when a
the particular virus strain is unknown. Virus isolation broad spectrum of infective agents is suspected, for
from animals is usually performed with the addition of instance in aerobic and anaerobic specific media which
antibiotics and antifungal drugs to cell lines to prevent also require specific growth conditions such as an
contamination with bacteria and fungi. While feline ret- anaerobic environment enriched with carbon dioxide for
roviruses such as FIV and FeLV are usually isolated from anaerobes.
blood, isolation of canine distemper virus is usually Identification and characterization of isolated bacteria
attempted from the CSF, bronchoalvelor lavage, and are usually done based on biochemical qualities and tests
transtracheal lavage specimens. such as the catalase test and coagulase production.