Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 27 Clinical Biochemistry in Toxicology
primarily carnivores. The coccidiostat, amprolium, is a
thiamine antagonist that produces polioencephalomalacia
in ruminants. Calves early in the course of polioencephalomalacia
may have reduced blood transketolase
(which requires thiamine pyrophosphate as a cofactor) and
increased pyruvate ( Jubb and Huxtable, 1993 ).
XI . TOXINS AFFECTING THE INTEGUMENT
Most toxins affecting the skin ( Table 27-9 ) will induce
no or nonspecific alterations in clinical biochemistry.
Fortunately lesions are usually readily visible by physical
examination. Topical exposure to strong acids or alkalis
may induce immediate and severe damage of the stratum
corneum and epidermis.
Ergot and ergot-like syndromes are produced by the
fungi Claviceps purpurea in infected rye and other cereal
grains, and Acremonium coenophilaum in fescue and other
pasture grasses. These fungi produce vasoconstrictive
alkaloids, which are derivatives of lysergic acid including
ergotamine, ergometrine, ergotoxine ( C. purpurea ), and
ergovaline ( A. coenophilaum ). Skin lesions are the result
of ischemic necrosis that is usually most impressive in the
distal extremities.
Dermatotoxic heavy metals include thallium and arsenic
( Yager and Scott, 1993 ). Thallium is still used as a
rodenticide in some countries, but it is mainly of historical
interest in many developed countries in which it has
been banned. Thallium induces parakeratosis and alopecia.
The mechanism is unknown, but it is speculated to center
around alteration of sulfhydryl groups in keratin resulting
in parakeratosis and alopecia. Arsenic toxicity exerts similar
influences.
Mimosine is a toxic amino acid occurring in Mimosa
pudica and Leucaena leucocephala . This toxin produces
alopecia by mechanisms that are incompletely
understood but may involve metal chelation that inhibits
metalloenzymes.
Organochlorines and organobromines include chlorinated
naphthalenes, polychlorinated biphenyls (PCBs),
polybrominated biphenyls (PBBs), and dibenzofurans.
These industrial toxins are cumulative and result in alopecia,
hyperkeratosis, and squamous metaplasia of columnar
epithelium of the respiratory tract. Molybdenum toxicosis
results in a relative copper deficiency in cattle and sheep
that produces depigmentation as a consequence of the
decreased activity of tyrosinase.
Toxic photosensitization, enhanced susceptibility of
the skin to actinic radiation, occurs primarily in lightly
pigmented skin of herbivores and may be primary or secondary
to chronic hepatotoxicity ( Yager and Scott, 1993 ).
Primary disease is due to exogenous photodynamic agents
that include treatment with the anthelmintic phenothiazine
and grazing of toxic plants such as Ammi majus (Bishop’s
weed, furocoumarin), Cymopterus watsoni (spring parsley,
furocoumarin) Fagopyrum spp. (buckwheat, fagopyrin),
Hypericum perforatum (St. John’s wort, hypericin), and
Thamnosma texana (Dutchmen’s britches, furocoumarin).
Secondary, or hepatogenous, photosensitization occurs
in herbivores with diffuse liver damage that reduces the
ability to excrete phylloerythrin. This photodynamic agent
is formed from chlorophyll by gastrointestinal flora and
is transported by the portal system to the liver where it is
normally conjugated and excreted in the bile. When phylloerythrin
escapes into the systemic circulation, it is poorly
excreted by the kidneys and accumulates in tissues including
the skin.
TABLE 27-9 Toxins Affecting the Integumentary System
Toxins Disease Onset Geography Species Affected
Acids/alkalis Acute Worldwide All
Ergotism Acute to Chronic Worldwide All
Kerosene Chronic Worldwide B
Metals (As, Tl) Chronic Worldwide All
Mimosine Chronic Worldwide B, E, O, P
Organobromines/organochlorines Chronic Worldwide All
Photosensitization, primary Acute to chronic Worldwide All
Photosensitization, secondary Acute to chronic Worldwide B, O
Selenium Chronic Worldwide All
Tricothecenes Acute Worldwide All
Vicia villosa (hairy vetch) Chronic Worldwide B, E
Abbreviations: B, bovine; Cn, canine; Cp, caprine; E, equine; F, feline; O, ovine; P, porcine.