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Toxoplasmosis - Cat and Dog (view source)
Revision as of 08:37, 13 August 2010
, 08:37, 13 August 2010→Laboratory Tests
===Laboratory Tests===
===Laboratory Tests===
HAEMATOLOGY, BIOCHEMISTRY AND
URINALYSIS FINDINGS
There are no laboratory findings pathognomonic for toxoplasmosis
(Lappin 1996). However, if the clinical history
is consistent with toxoplasmosis, the following laboratory
abnormalities raise the index of suspicion: non-regenerative
anaemia, neutrophilic leucocytosis, lymphocytosis,
monocytosis, neutropenia, eosinophilia, increased activities
of creatine kinase, alanine aminotransferase, alkaline
phosphatase and lipase, hyperbilirubinaemia, hyperproteinaemia,
proteinuria and bilirubinuria.
RADIOGRAPHIC FINDINGS
Interstitial and alevolar patterns are common radiographic
findings in cats with pulmonic toxoplasmosis, while
pleural effusion is rarely detected. Abdominal radiographic
findings are non-specific but can include a
homogeneous increase in density due to peritoneal effusion,
hepatomegaly, lymphadenopathy, intestinal masses,
or loss of contrast in the cranial right quadrant of the
abdomen due to pancreatitis. In cats with CNS involvement,
mass lesions may be detected by myelography,
computed tomography or magnetic resonance imaging.
CYTOLOGY AND CEREBROSPINAL FLUID
ANALYSIS
In a series of cats with suspected CNS toxoplasmosis,
cerebrospinal fluid (CSF) protein levels ranged from
normal to 149 mg/% and nucleated cell counts from less
than 5 to 28 cells/,l (M. R. Lappin, unpublished data).
Small mononuclear cells were the predominant white
blood cells. Cytological examination may reveal tachyzoites
in blood, CSF, transtracheal wash fluids, peritoneal
effusions and pleural effusions from clinically ill
animals.
FAECAL EXAMINATION
T gondii oocysts are 10 x 12 ,um in size and can be
demonstrated microscopically in feline faeces following
0 2 4 6 8 12 16 20 26 34
Weeks after inoculation
Temporal appearance of
T gondii-specific IgM, IgG
and IgA antibodies in the
serum of experimentally
flotation using solutions with a specific gravity of 1-18. inoculated cats
Oocysts of the non-pathogenic coccidians Hammondia
hammondi and Besnoitia darlingi cannot be distinguished
microscopically from those of T gondii; definitive diagnosis
relies on laboratory animal inoculation. Most cats
with clinical toxoplasmosis have completed the oocyst
shedding period and so the diagnostic utility of faecal
examination is limited. However, due to the potential
zoonotic risk, a faecal examination should be performed
for any cat with clinical signs referable to toxoplasmosis.
SEROLOGY
T gondii-specific antibodies, antigens and immune
complexes have been detected in the serum of cats
(Lappin 1996). Tests for antigens and immune complexes
are currently only available in research laboratories
and so will not be discussed further.
Antibodies can be detected using a range of techniques
that are commercially available, including ELISA,
immunofluorescent antibody, western blot immunoassay,
Sabin-Feldmann dye test, and a variety of agglutination
tests. ELISA, immunofluorescent antibody assay
and western blot immunoassay have been adapted to
detect IgM, IgG and IgA antibody responses (see graph
above).
A latex agglutination assay and an indirect haemagglutination
assay are also available commercially. These
assays, which can be used with serum from multiple
species, theoretically detect all classes of immunoglobulin
directed against T gondii. However, these assays rarely
detect antibody in feline serum samples positive only for
IgM (Lappin 1996, Dubey and Lappin 1998). A combination
of modified agglutination tests using formalin-fixed
and acetone-fixed tachyzoites can be used to accurately
predict recent infection, but the assays are not commercially
available.
Using ELISA, approximately 80 per cent of healthy,
experimentally infected cats have detectable T gondiispecific
IgM in serum within two to four weeks of inoculation
with T gondii; the titres are generally negative
again by 16 weeks post-infection. Persistent IgM titres
(>16 weeks) have been documented commonly in cats
coinfected with FIV and in cats with ocular toxoplasmosis.
In some chronically infected cats, IgM can be detected
again after repeat inoculation with T gondii, primary
inoculation with the Petaluma isolate of FIV, and administration
of glucocorticoids. Because of these findings,
IgM titres cannot accurately predict when a cat is
InPractice i NOVEMBER/DECEMBER 1999 583
shedding oocysts or if a cat has clinical toxoplasmosis.
However, detectable IgM titres were present in the
serum of 93-3 per cent of cats in one study of clinical
toxoplasmosis, while IgG titres were only detected in
60 per cent (Lappin and others 1989). Hence, the IgM
antibody class appears to correlate more closely to clinical
disease than IgG.
T gondii-specific IgG can be detected by ELISA in
serum in the majority of healthy, experimentally inoculated
cats within three to four weeks of infection (Lappin
1996, Dubey and Lappin 1998). IgG antibody titres can
be detected for at least six years after infection (Dubey
1995); since the organism probably persists for life, IgG
antibodies probably do as well. Single, high IgG titres do
not necessarily suggest recent or active infection -
healthy cats commonly have titres of above 10,000 six
years after experimental induction of toxoplasmosis. The
demonstration of an increasing IgG titre can document
recent or active disease but, in experimentally infected
cats, the time span from the first detectable positive IgG
titre to the maximal IgG titre is approximately two to
three weeks. Many cats with sublethal clinical toxoplasmosis
have chronic, mild clinical signs and may not be
evaluated serologically until their IgG antibody titres
have reached their maximal values. In humans and cats
with reactivation of chronic toxoplasmosis, IgG titres
rarely increase.
Western blot immunoassay can be used to determine
the T gondii antigens recognised by humoral immune
responses. Transplacentally infected kittens could be distinguished
from T gondii-naive kittens with antibodies in
serum from colostrum ingestion by comparing antigen
recognition patterns between the queen and kittens.
AQUEOUS HUMOUR AND CSF ANTIBODY
AND DNA MEASUREMENT
Detection of T gondii-specific antibodies produced by the
eyes or CNS can be used to document clinical toxoplasmosis.
While IgG and IgA class antibodies are produced
transiently by the eyes and CNS of healthy cats after
experimental inoculation, IgM has only been detected in
the aqueous humour or CSF of cats with clinical disease
(Lappin 1996, Dubey and Lappin 1998). Most cats with
local production of T gondii-specific antibodies in aqueous
humour have responded to the administration of anti-
Toxoplasma drugs, suggesting that aqueous humour antibody
testing can aid in the diagnosis of clinical ocular
toxoplasmosis in cats (Lappin and others 1992b).
T gondii DNA has been detected in the aqueous
humour of cats with uveitis by polymerase chain reaction
(PCR) (Lappin and others 1996). However, the
organism is also detected occasionally in the aqueous
humour of cats without uveitis and so positive results do
not provide a definitive diagnosis of clinical toxoplasmosis
(Burney and others 1998).
DEMONSTRATION OF T GONDII IN TISSUES
In addition to using cytology to demonstrate T gondii
tachyzoites, PCR, tissue culture and animal inoculation
techniques can be used to detect the organism in whole
blood, aqueous humour or CSF. Tissue biopsy sections
can be assessed for the presence of T gondii by haematoxylin
and eosin (H&E) staining, immunohistochemical
staining, PCR, cell culture or animal inoculation.
Immunohistochemical staining procedures are superior
to H&E staining because they are specific for T gondii
(see figure on page 580). It may be difficult to document
the organism in the tissues of some clinically ill cats
because of the small sections of tissue evaluated
histopathologically and because the pathogenesis of disease
in some may be immune-mediated. This appears to
be particularly true for the ophthalmic form of the disease
in cats.
DIAGNOSIS OF CLINICAL FELINE
TOXOPLASMOSIS
Definitive diagnosis of clinical feline toxoplasmosis
requires demonstration of the organism in the tissues in
association with inflammation. This is usually achieved
at necropsy in cats with overwhelming tachyzoite replication.
Occasionally, a definitive diagnosis of clinical
feline toxoplasmosis is made antemortem by demonstrating
bradyzoites or tachyzoites in tissues or effusions.
Since T gondii-specific antibodies can be detected in
the serum, CSF and aqueous humour of normal animals,
as well as those with clinical signs of disease, it is not
possible to make an antemortem diagnosis of clinical
toxoplasmosis based on these tests alone. However, a
presumptive antemortem diagnosis of clinical feline
toxoplasmosis may be based on the following combination
of findings:
* Demonstration of antibodies in serum, aqueous
humour or CSF (documents exposure to T gondii);
* Demonstration of an IgM titre of above 1:64 or a
fourfold or greater increase in IgG titre, or the documentation
of local antibody production or DNA in aqueous
humour or CSF (suggests recent or active infection);
* Clinical signs of disease referable to toxoplasmosis;
* Exclusion of other common aetiologies;
* Positive response to appropriate treatment (see
below).
Diagnosis is made by biologic, serologic, or histologic methods, or by some combination of the above. Clinical signs of toxoplasmosis are nonspecific and are not sufficiently characteristic for a definite diagnosis. Antemortem diagnosis may be accomplished by indirect hemagglutination assay, indirect fluorescent antibody assay, latex agglutination test, or ELISA. IgM antibodies appear sooner after infection than IgG antibodies but generally do not persist past 3 mo after infection. Increased IgM titers (>1:256) are consistent with recent infection. In contrast, IgG antibodies appear by the fourth week after infection and may remain increased for years during subclinical infection. To be useful, IgG titers must be measured in paired sera from the acute and convalescent stages (3-4 wk apart) and must show at least a 4-fold increase in titer. Additionally, CSF and aqueous humor may be analyzed for the presence of tachyzoites or anti- T gondii antibodies. Postmortem, tachyzoites may be seen in tissue impression smears. Additionally, microscopic examination of tissue sections may reveal the presence of tachyzoites or bradyzoites. T gondii is morphologically similar to other protozoan parasites and must be differentiated from Sarcocystis spp (in cattle), S neurona (in horses), and Neospora caninum (in dogs).
Diagnosis is made by biologic, serologic, or histologic methods, or by some combination of the above. Clinical signs of toxoplasmosis are nonspecific and are not sufficiently characteristic for a definite diagnosis. Antemortem diagnosis may be accomplished by indirect hemagglutination assay, indirect fluorescent antibody assay, latex agglutination test, or ELISA. IgM antibodies appear sooner after infection than IgG antibodies but generally do not persist past 3 mo after infection. Increased IgM titers (>1:256) are consistent with recent infection. In contrast, IgG antibodies appear by the fourth week after infection and may remain increased for years during subclinical infection. To be useful, IgG titers must be measured in paired sera from the acute and convalescent stages (3-4 wk apart) and must show at least a 4-fold increase in titer. Additionally, CSF and aqueous humor may be analyzed for the presence of tachyzoites or anti- T gondii antibodies. Postmortem, tachyzoites may be seen in tissue impression smears. Additionally, microscopic examination of tissue sections may reveal the presence of tachyzoites or bradyzoites. T gondii is morphologically similar to other protozoan parasites and must be differentiated from Sarcocystis spp (in cattle), S neurona (in horses), and Neospora caninum (in dogs).