Intestinal Disease
Clinicopathological examinations are an important adjunct to a thorough clinical examination,
including diagnostic imaging, for the investigation of intestinal disease. Important examinations
include:
Rectal examination which may allow palpation of neoplasms, abscesses or thickened
intestine.
Ultrasonographic examination may provide evidence of peritoneal effusion, intestinal
thickening, diffuse pathology of parenchymatous organs or the presence of neoplasms or
abscesses.
Gastroscopic examination may provide evidence of equine gastric ulcer syndrome or (rarely)
gastric neoplasia and provides a means of examining and biopsying the duodenum.
ORAL GLUCOSE ABSORPTION TEST
The OGAT is a valuable test for the detection of small intestinal malabsorptive syndromes including
chronic infiltrative inflammatory diseases. This test has no relevance to large intestinal diseases such
as parasitism. It is important to not confuse the sampling protocols for the OGAT and the oral
glucose test for assessment of insulin resistance (see endocrine chapter).
12 hour fast prior to testing (allow water)
Take ‘baseline’ oxalate-fluoride blood sample
Give 1g/kg glucose as warm 20% solution by stomach tube. Take oxalate-fluoride bloods
hourly for 5-6 hours or until there is a return to baseline
Oxalate-Fluoride blood can be taken once at 2 hours for a shortened version of the test
that is still quite accurate.
Analyse samples for glucose and calculate percentage increases above baseline
Interpretation
A ‘normal’ response is at least a 65% increase of glucose from baseline at 90-150 minutes post-
dosing. However, severely hypoalbuminaemic (<15 g/L) cases may have depressed peaks in the
absence of small intestinal pathology possibly as a result of bowel oedema.
A ‘partial malabsorption’ (+15-65% increase) is often significant and merits a retest for monitoring
purposes following treatment.
A ‘total malabsorption’ is regarded as a no greater than 15% rise in blood glucose at 2 hours post-
dosing. This is often a highly significant finding leading to a poor prognosis although occasional cases
of total malabsorption have been known to improve.
PERITONEAL FLUID ANALYSIS
Although usually successful there are a few common reasons for failure to collect a peritoneal tap.
These include:
Deep retroperitoneal fat layer (which can be up to 10 cm thick) requiring a 9 cm spinal
needle or canula to obtain a sample. This is easily determined using abdominal
ultrasonography.
Splenic tap due to enlargement of the spleen (physiologic or pathologic) causing it to cross
the midline. Fit Thoroughbredds commonly have a spleen across the midline.
Ultrasonography will easily identify the position of the spleen and allow repositioning of the
peritoneal tap site.
Enterocentesis usually occurs due to striking colon with a needle as it is too heavy to move
out of the way, or due to distended small bowel (not distended small bowel usually moves
out of the way of a needle). Ultrasonography can be used to target non-distended small
intestine or a gap between colons. Teat cannulas are less likely to cause enterocentesis into
distended small bowel but can still penetrate colon.
Dehydration may decrease the amount of free fluid. A further attempt following rehydration
with fluid therapy is warranted.
Interpretation
Normal peritoneal fluid has a total nucleated cell count of <2.0 x 109/L (with approximately two-
thirds of the cells being neutrophils) and a total protein concentration of <15-20 g/L. Peritoneal fluid
glucose concentration is normally slightly greater than blood glucose in the range of 4-7mmol/L.
With septic peritonitis cell counts are generally >50 x109/L and total protein concentration >50g/L.
Glucose is metabolised by bacteria and concentration decreases to <2 mmol/L. Lactate will normally
be increased in septic samples.
Mild increases in total nucleated cell counts around 5-10 x 109/L represent a modified transudate
and are more difficult to interpret but infer intra-abdominal disease. Intra-abdominal neoplasia in
horses is rarely identified by exfoliated neoplastic cells in a peritoneal tap. However, mild to
moderate increases in total nucleated cell counts and protein concentrations are often found though
they may be confused with low-grade septic peritonitis or equine grass sickness. Horses with
inflammatory bowel disease and other causes of hypoalbuminaemia with peritoneal effusions may
have dilute peritoneal fluid with low cell counts and protein concentrations.
INTESTINAL BIOPSY
The site for biopsy (rectal vs. duodenal) will depend on the clinical presentation and whether disease
of large or small intestine is suspected to be predominant.
Rectal biopsy
This is undoubtedly the easiest and most accessible part of the gastrointestinal tract but is only likely
to be pathologically affected in a proportion of cases showing signs of distal intestinal tract disease
(i.e. diarrhoea). In the absence of diarrhoea it is highly questionable whether this test is justified. It is
easy and relatively safe to perform with light sedation and stocks.
Although pathologic change is fairly commonly found in rectal biopsies taken from cases of chronic
weight loss and diarrhoea, the changes are often highly non-specific and sometimes quite
misleading. Overall there is a fairly poor correlation between rectal biopsy findings and intestinal
pathology confirmed later at post-mortem. Nevertheless, it is a simple and straightforward test to
use and is certainly justifiable in many cases.
Biopsy forceps are the best and most appropriate tool with which to collect the rectal biopsy. The
site to choose is in the dorsal midline. Some prefer to take a true rectal biopsy with the hand inside
the rectum no more than “wrist deep”, others prefer to take a colonic biopsy by performing the
biopsy at full arm’s length. Whilst the latter technique may give results that are more representative
of the large intestine, any inadvertent penetration through the rectum is likely to result in
intraperitoneal rather than retroperitoneal infection. The biopsies should be placed in 10% formalin
for histopathologic analysis and/or cultured for enteropathogenic bacteria such as Salmonellae.
Small bowel biopsies
There are three methods for taking small intestinal biopsies:
- Duodenal pinch biopsies can be taken via duodenoscopy by passing a gastroscope through
the pylorus. The resultant samples are superficial mucosal biopsies that are often damaged
by crush artefact in the collection procedure. However, they are relatively simple to take and
can sometimes be diagnostic in suspected small intestinal disease especially if grossly
abnormal mucosa is identified endoscopically. - Laparoscopic full thickness biopsies may be collected from the small intestine in the standing
sedated horse. - Full thickness biopsies from all levels of the intestinal tract may be collected via laparotomy
under general anaesthetic. Exploratory laporotomy also enables thorough examination of
the intestinal tract and whilst it is the most invasive technique in most cases it offers the
best chance of achieving a definitive diagnosis.
DIAGNOSIS OF PARASITISM FROM BLOOD SAMPLES??
Nematode infections in the adult horse were once typified by intra-luminal adult worms and intra-
arterial larval migration associated with Strongylus vulgaris. These were often associated with an
eosinophilia detectable in blood samples in response to intra-arterial larvae and also, in some
instances, a detectable increase in γ-globulins (especially IgG(T)). S. vulgaris has declined and
cyathostomins are now assumed to account for the vast majority of nematode eggs detected in
equine faecal samples in this country. Cyathostomin infection results in encystment of larvae locally
in the caecal and colonic wall but is not associated with larval parasitic migration outside the
intestinal tract. An eosinophilia is not associated with cyathostomin infections and a raised ß1-
globulin fraction is a very occasional and non-specific finding.
Several research studies have failed to confirm any clinically useful relationship between serum
protein electrophoresis and parasitism in horses. Normal concentrations of IgG(T) and ß1 -globulins
are usually found in parasitised adult horses and ponies although changes may be more likely in
young horses. In an investigation of horses with chronic diarrhoea, less than half of horses with
parasitic diarrhoea had raised ß1-globulins and this finding was also common in horses with non-
parasitic disease. ‘Cyathostominosis’, the acute diarrhoea and weight loss syndrome associated with
en masse larval emergence, is consistently associated with a neutrophilia, hypoalbuminaemia and
hyperfibrinogenaemia (all non-specific findings). Blood samples taken from parasitised horses show
no consistent abnormalities in haematology or protein analysis therefore are only one piece of the
diagnostic jigsaw.
The recently marketed serologic assay for cyathostomes will be inescapably compromised by the
temporal limitations of serologic tests. The main concerning issue with this test is that following
successful elimination of cyathostomin infection, a horse is likely to remain seropositive for a
variable, indeterminate but possibly very long period of time. This may then lead to unnecessary
anthelmintic treatment be applied to such cases, which is exactly the opposite from what we are
currently trying to achieve with responsible anthelminitic usage. For example, information produced
by the manufacturer indicates that almost half of horses with worm egg counts under 200 epg will
test positive with the serologic test. Additionally, protein losses in clinically parasitised horses may
serve to decrease the magnitude of the test result.