Page 895 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Poisonous Plants of the United States Chapter | 61 851

VetBooks.ir additional 4 6 weeks of grazing may be obtained by will reduce losses. In addition, it is recommended that ani-
mals not be watered or provided mineral supplementation
grazing these ranges early, before larkspur elongates flow-
in areas that have high densities of larkspurs.
ering racemes. The risk of losing cattle is low when graz-
ing before flowering, even though larkspur is very toxic,
because larkspur consumption is typically very low. Once
pods are mature and begin to shatter, larkspur ranges can Graze Sheep Before Cattle
usually be grazed with impunity because pod toxicity Sheep can be herded into or bedded on the patches to
declines rapidly, and leaf toxicity is low. Based on limited reduce larkspur availability or acceptability to cattle on
study, cattle increase consumption of low larkspur after tall larkspur-infested ranges where larkspur grows as dis-
flowering, and increases in grazing pressure increase crete patches. In those areas in which larkspur is uni-
amounts of low larkspur eaten by cattle. No consistent formly spaced over a pasture, sheep must eat immature
consumption patterns of plains larkspur (D. geyeri)by larkspur and leave sufficient feed for subsequent grazing
cattle were found; thus, management recommendations by cattle. This can be problematic because early growth
need to be tailored to each specific year and location tall larkspur may not be palatable to sheep. Our observa-
(Pfister et al., 2002). tions (Pfister, unpublished data) indicate that sheep eat lit-
Studies have been performed to characterize the tle low larkspur (D. nuttallianum) unless a high stock
kinetic profile of larkspur alkaloids in cattle (Green et al., density is used.
2009b, 2011). In these studies, the elimination half-life of
MLA from cattle dosed once with dried and finely ground
larkspur via oral gavage was approximately 20 h. In addi- Drug Intervention
tion, the T max , or time to maximal toxin concentration, A variety of remedies have been applied in the field when
was approximately 10 h. These data suggest that cattle ranchers find intoxicated animals (e.g., bleeding by cut-
that have consumed larkspur will be most susceptible to ting the tail), but most are without a solid scientific ratio-
poisoning 10 h after consumption, and that after the cattle nale. Any imagined success with these treatments was
have stopped consuming larkspur, greater than 99% of the probably related to the dose. If less than a lethal dose
toxins will be eliminated after 6 days. were ingested, the animal would likely recover despite
any treatment, unless bloat or aspiration pneumonia
occurred during recumbency. Treatment for overt poison-
Prevention and Management of Poisoning ing is usually symptomatic, and recovery is often sponta-
neous if animals are not stressed further by driving. Once
Grazing Management the animal is observed showing muscular tremors, it
A simple and low-risk grazing management scheme can should be allowed to drop back and proceed at its own
often be used based simply on tall larkspur growth and pace. Poisoned animals should never be forced to con-
phenology: (1) graze during early summer when sufficient tinue moving because this will exacerbate the clinical
forage is available until larkspur elongates flowering effects and can result in death. Drugs that increase acetyl-
racemes (4 6 weeks depending on elevation and choline effectiveness at the neuromuscular junction have
weather); (2) remove livestock, or contend with poten- potential for reversing larkspur toxicosis or reducing sus-
tially high risk from flowering to early pod stages of ceptibility. The cholinergic drug physostigmine (0.08 mg/
growth (4 or 5 weeks); and (3) graze with low risk during kg i.v.) has been successfully used under field and pen
the late season when larkspur pods begin to shatter (4 6 conditions to reverse clinical larkspur intoxication (Nation
weeks). This scheme can be refined substantially if live- et al., 1982; Pfister et al., 1994). Similarly, i.v. adminis-
stock producers periodically obtain an estimate of the tox- tration of neostigmine (0.04 mg/kg) significantly reduced
icity of tall larkspur, and if ranchers spend time clinical signs in cattle (Green et al., 2009a), and neostig-
periodically observing and documenting larkspur con- mine administered intramuscularly at 0.02 mg/kg can be
sumption by grazing cattle. used as a rescue treatment for cattle in recumbency. This
Management to reduce losses to low larkspur begins reversal lasts approximately 2 h, and repeated injections
with recognition of the plant during spring. Vegetative low of physostigmine are sometimes required. Under field
larkspur plants will typically begin growth before the major conditions, physostigmine temporarily abates clinical
forage grasses. Low larkspur populations fluctuate with signs and animals quickly (B15 min) become ambulatory.
environmental conditions (Pfister, unpublished data). Risk Depending on the larkspur dose, the intoxication may
of losing cattle is much higher during years with dense recur. The use of physostigmine-based treatments may
populations. During those years, recognizing the plant, and aggravate losses in the absence of further treatment if sud-
finding alternative pasture or waiting to graze infested pas- denly ambulatory animals later develop increased muscu-
tures for 4 6 weeks until the low larkspur has dried up, lar fatigue, dyspnea, and death.