Pythiosis in Dogs
Pythiosis is an infectious disease caused by
a fungus-like organism, Pythiuminsidiosum, that
naturally inhabits wetlands, ponds, and swamps. In dogs,
the disease usually is manifested by gastroenteritis or dermatitis.
Presumably dogs become infected by ingesting, or swimming
in contaminated water. Pythiosis is endemic in states that
border the Gulf of Mexico, but has
been diagnosed in dogs from southern Indiana
(with no history of travel outside the state). Dogs with
gastrointestinal pythiosis often have a history of retrieving
objects, such as sticks, from water and then chewing on them;
young male retriever-type dogs are particularly at risk.
Dogs with open skin wounds are probably predisposed to acquiring
cutaneouspythiosis.
Gastrointestinal pythiosis is usually a fatal
disease. Pathologically, the organism is highly invasive
and angiotropic, so the infection can be complicated by disruption
of vascular supply. The infection typically involves the
full-thickness of the stomach or intestine and sometimes extends
to adjacent organs. For example, pythiosis of the pylorus
or duodenum can extend to the pancreas, omentum, lymph nodes,
or contiguous viscera. The tissue response is typically a
combination of macrophages, multinucleated giant cells, lymphocytes,
plasma cells, and eosinophils. Pythiumhyphae are usually
found in necrotic foci, surrounded by inflammatory cells and
sometimes coated by proteinaceous material that is probably
derived from eosinophil granules. In H&E-stained sections,
this coating (termed a Splendore-Hoeppli reaction) allows
for microscopic visualization of the outline of the otherwise
non-staining Pythiumhyphae. Necrotic foci and granulomas
associated with these organisms may be grossly visible as
yellow granules. Extensive fibrosis often results in stenosis
of the affected segment of gut producing clinical signs referable
to the site of infection. For example, pythiosis of the gastric
pylorus often results in projectile vomiting, and pythiosis
of the duodenum in obstruction.
Lesions of cutaneouspythiosis usually develop
on the limbs, tail, or face. They are typically firm or spongy
(depending on the degree of fibrosis) and ulcerated, often
with a draining fistulous tract. Clinically, these lesions
may resemble acral lick granulomas.
Because both the organism morphology and the
inflammatory reaction are so characteristic, the diagnosis
of pythiosis is usually made by microscopic examination of
affected tissue(s). A diagnostic biopsy sample of gastrointestinal
tissue usually requires the submucosa (i.e. a biopsy of only
the mucosa may not provide a diagnostic sample). Cytology
and histopathology of the sample is recommended. The organism
produces non-septatehyphae, 4 to 8 microns in diameter. They
are best visualized in histologic sections stained with Gomori'smethenamine
silver (GMS). Culture and immunohistochemistry are ancillary
techniques that are usually reserved for research.
Successful treatment of dogs with pythiosis is
difficult. Although morpho-logically resembing fungi, the
cell wall - plasma membrane of Pythium - differs bio-chemically
from that of fungi. So it is essentially resistant to antifungal
drugs. The only treatment option available for a potential
cure is surgery and the goal is complete resection of the
affected tissue. In the gastrointestinal tract, pythiosis
clinically mimics an invasive carcinoma so aggressive surgical
extirpation must be attempted. But the prognosis is poor.
For cutaneouspythiosis involving extremity, amputation may
be necessary.
- byYumi Yuasa, ECFVG
- edited by Evan Janovitz, DVM, PhD
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