PATHOGENIC E. COLI OF PIGS AND CALVES
Some strains of E. coli are intestinal
pathogens, but other strains are part of the normal flora,
making the diagnosis and interpretation of intestinal colibacillosis
difficult. Consequently, a number of laboratories test E.
coli isolates for the presence of virulence factors. Pathogenic
E. coli can cause one or more of the following disease
syndromes, depending upon which virulence factors they possess.
I. Enterotoxigenic (secretory) diarrhea.
II. Enteropathogenic diarrhea (necrotic or
hemorrhagic).
III. Systemic toxemia (i.e. Edema Disease
of pigs).
IV. Systemic (enteroinvasive) septicemia.
To determine whether an isolate of E. coli is
pathogenic, it is most useful to test for functional factors
that either permit the E. coli to colonize the gut
(attachment factors such as pili), or permit the E. coli
to cause damage (cytotoxic factors). Due to the number of
virulence factors and the confusing terminology, interpretation
of test results can be difficult. For your convenience, a
table of commonly tested virulence factors and associated
diseases are listed on the following page.
The table is intended to help you interpret results from
various labs and is, therefore, redundant. For example, edema
disease principle (EDP), verotoxin (VT), and shiga-like toxin
II (SLT-IIv) are names used by various labs for the same toxin.
In addition, bacteria don't always read the books. For example,
F18ab or F18ac pili are found on E. coli that colonize
post-weaned pigs. They may either be associated with SLT-IIv
(Edema disease), STB (enterotoxin-diarrhea) or both. When
a single F18+ strain of E. coli secretes both SLT-IIv
and STB, edema disease and post-weaning diarrhea can occur
simultaneously. Also, there are undoubtedly other virulence
factors that have not yet been discovered. For example, we
sometimes isolate disease causing toxigenic strains that don't
possess any known colonization or attachment factors.
-By: Duane Murphy, ADDL-SIPAC
Tom Hooper, ADDL-SIPAC
-Edited By: Greg Stevenson, DVM, PhD
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