Page 598 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Toxicity of Herbicides Chapter | 44 563

VetBooks.ir normal use has not been reported in domestic animals. uncouple and/or inhibit oxidative phosphorylation. Ioxynil,
The nitrile herbicides, ioxynil and bromoxynil, may
Dicamba either alone or combined with another herbicide
presumably due to its iodine content, causes enlargement of
induced significant levels of apoptosis in mouse preim-
plantation embryo assay (Greenlee et al., 2004). It is a the thyroid gland in the rat (Marrs, 2004). Members of poly-
skin and eye irritant, and high doses may cause neurobe- cyclic alkanoic acids (diclofop, fenoxaprop, fenthiaprop,
havioral symptoms in rats and rabbits. The compound did fluazifop and haloxyfop) have moderately low toxicity,
not show any adverse effects in a three-generation study whereas haloxyfop-methyl is an exception, and has high
in rats (Harp, 2010). toxicity. They tend to be more toxic if exposure is dermal.
Dicamba induces peroxisomal enzymes in rat liver The dermal LD 50 of diclofop in rabbits is only 180 mg/kg
and causes transcription up-regulation of the peroxisome (Susan, 2003). Some members of the amide group, such as
proliferator-activated receptor. Long-term exposure to bensulide and propanil, are used as plant growth regulators,
dicamba may induce tumors in rats due to its action as a and some of them are more toxic than others. A lethal dose
peroxisome proliferator; however, the implications of of bensulide for dogs is 200 mg/kg.
these findings are not clear and may require further study The prominent clinical sign is anorexia; other signs and
(Espandiari et al., 1998). Dicamba-induced oxidative lesions are not definitive and are similar to those of chloro-
stress-mediated cytogenotoxicity has been demonstrated phenoxy acid poisoning. Hemolysis, methemoglobinemia
in an in vitro cell model (Gonzalez et al., 2009; Harp, and immunotoxicity have occurred after experimental
2010). exposure to propanil (Lorgue et al., 1996). The toxicity of
sulfonylureas (chlorsulfuron, sulfometuron, metsulfuron,
chloremuron, and kensulfuron) appears to be quite low
Carbamates, Thiocarbamates (Susan, 2003).
and Dithiocarbamate Compounds A number of substances are used as defoliants in agri-
culture, including sulfuric acid to destroy potato haulms
The compounds in this category include derivatives of
and two closely related trialkylphosphorothioates (DEF
carbamic acid (asulam, barban, chlorpropham, chlorbu-
and merphos) to defoliate cotton. A notable feature of the
fam, karbutilate and phenmedipham), derivatives of thio-
latter is that they produce organophosphate-induced
carbamic acid (butylate, cycloate, diallate, EPTC,
delayed neuropathy in hens (Baron and Johnson, 1964).
molinate and triallate) and derivatives of dithiocarbamic
Chlomequat is used as a growth regulator on fruit trees.
acid (metham sodium). These herbicides have low to
The signs of toxicity in experimental animals indicate that
moderate toxicity in rats and do not pose acute hazards.
it is a partial cholinergic agonist (JMPR, 2000).
They are used at low concentrations, and poisoning pro-
blems have not been reported. In general, these herbicides
do not produce skin or eye irritation. With repeated expo- ENDOCRINE DISRUPTION
sure, there is a possibility of alopecia for some time after
In both males and females, some herbicides affect
ingestion (Lorgue et al., 1996; Hurt et al., 2010).
reproduction through different mechanisms of action of
In ruminants, diallate results in anorexia, ataxia, mus-
endocrine disruption; exogenous agents interfere with
cular contractions, exhaustion, prostration, and alopecia
reproduction and the development process. In males,
in sheep, which is an indication of chronic poisoning.
normal reproductive function involves interaction of the
Thiobencarb has induced toxic neuropathies in neonatal
hypothalamic-pituitary-testis axis and the thyroid gland.
and adult laboratory rats. It appears to increase permeabil-
In females, increased concentrations of xenoestrogens
ity of the blood brain barrier. The nonspecific lesions
may affect ovarian function through the disruption of
include hepatic, renal and pulmonary congestion, enteritis,
feedback mechanisms in the hypothalamus-pituitary-
ascites, and hydrothorax (Susan, 2003).
gonadal axis (Flaws and Hirshfield, 1997; Bretveld et al.,
2006). Herbicides, like other chemicals, may disrupt all
stages of hormonal function of the reproductive system.
Others
In females, during pregnancy and, to a greater extent, dur-
Bromacil and terbacil are commonly used methyluracil ing lactation, a portion of the maternal body burden of
compounds. These compounds can cause mild toxic signs these chemicals is transferred to the offspring. For exam-
at levels of 50 mg/kg BW in sheep, 250 mg/kg BW in cat- ple, herbicides such as linuron produce hypothyroidism
tle and 500 mg/kg BW in poultry when given daily for (Gupta, in press). The herbicide glyphosate in low non-
8 10 days. Signs of toxicity include bloat, incoordina- toxic concentrations caused disruption of the aromatase
tion, depression and anorexia. Toxic doses of bromacil enzyme in human placental cells in vitro. It reduced the
can be hazardous, especially for sheep, but no field cases aromatase enzyme activity responsible for the synthesis of
of toxicity have been reported. estrogens (Richard et al., 2005). A study indicated that