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VetBooks.ir Chapter 12

Nervous System Toxicity

Donna Mensching and Camille DeClementi

INTRODUCTION functional integrity of the nervous system can be affected
by insults to its structural integrity depending on the
The nervous system is a susceptible target for toxicity
severity and location of the insult.
because of its complex anatomy, specialized functions,
high metabolic requirements, limited ability to repair
itself, and the potential for life-threatening complications STRUCTURAL TOXICOSES
when disequilibrium occurs. A plethora of neurotoxicants
Histopathological abnormalities associated with structural
exists including man-made pesticides, agents of chemical
toxicoses can be subdivided into neuronopathy, axonopa-
warfare, medications, illicit drugs, and all-natural com-
thy, or myelinopathy. The following discussion addresses
pounds produced by a variety of organisms, such as spi-
each of these pathologies, providing examples relevant to
ders and snakes, which confer advantages to their makers
veterinary toxicology.
such as increased predatory efficiency, avoidance of pre-
dation, or increased survivability in environmental
extremes. Neuronopathy
As is the case with any toxicant, exposure to a signif-
icant dosage of a neurotoxicant warrants appropriate A neurotoxicant that causes neuronopathy directly targets
decontamination in an otherwise healthy and asymptom- the neuronal cell body, resulting in cell death and second-
atic patient. In the symptomatic patient, alleviation of ary axonal degeneration. Gliosis, proliferation of astro-
clinical signs may be nonspecific or symptomatic (e.g., cytes and/or microglial cells, is a common response to
diazepam for seizures) or specifically antidotal based on loss of neurons (Anthony et al., 2001). With few excep-
the mechanism of toxicity (e.g., pralidoxime for organo- tions, this type of injury is irreversible. Examples of such
phosphorus (OPs) insecticides). Table 12.1 lists exam- toxicants include methyl mercury, which preferentially
ples of antidotes used for select neurotoxicants. Clinical targets the cell bodies of the occipital cortex and the cere-
signs of nervous system toxicity (Table 12.2) can be bellum via an unproven mechanism. Blindness and motor
divided roughly into stimulatory and depressant catego- incoordination are common manifestations of lesions in
ries, although an overlap of these categories can occur these areas. In veterinary medicine, methyl mercury intox-
with varying dosages of a given toxicant and/or class of ication is most likely seen in animals that subsist on a diet
toxicant. When severe signs are not minimized, potential of contaminated fish. A classic example from the 1950s
complications can result including extremes of body involved the cats of mercury-contaminated Minamata
temperature and blood pressure, hypoxia/anoxia, dissem- Bay, Japan. Because of their advanced cerebellar ataxia,
inated intravascular coagulation, rhabdomyolysis, organ these cats often fell into the water of the bay and were
failure, and trauma resulting from an inability to assess described as “dancing” or “suicidal” (Smith and Smith,
the environment and/or inability to avoid environmental 1975; Francis, 1994). In recent years, awareness of envi-
hazards. ronmental contamination with mercury has created bio-
The general nature of this chapter avoids discussion of monitoring programs that attempt to assess the impact of
every known neurotoxicant. Instead, neurotoxicants are dietary mercury on piscivorous wildlife such as bald
divided into those that affect the structural integrity of the eagles (Hinck et al., 2009).
nervous system and those that affect only its functional Domoic acid, the neurotoxicant responsible for amne-
integrity. It is important to note, though, that the sic shellfish poisoning in people and wildlife, is produced

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