Phenylarsonic organic arsenicals are relatively less toxic
than inorganic compounds or aliphatic and other aromatic organic compounds.
Aliphatic organic arsenicals include cacodylic acid and acetarsonic
acid. These were historically used as stimulants in large animals, but their use is
now uncommon. Some aliphatic arsenicals such as monosodium methanearsonate (MSMA)
and disodium methanearsonate (DSMA) are occasionally used as herbicides or grassburr
and crabgrass killers. Ruminants, especially cattle, are very sensitive to MSMA and
DSMA. Clinical signs, lesions, and treatment of aliphatic organic arsenicals are
similar to those of inorganic arsenicals, except that ruminants may have necrosis of
the mucosa of the rumen and omasum and gelatinous serosal edema of the omasum and
Aromatic organic arsenicals include trivalent phenylorganics, such as
thiacetarsamide and arsphenamine for the treatment of adult heartworms in dogs, and
pentavalent compounds such as phenylarsonic acids and their salts. Thiacetarsamide
and arsphenamine are no longer used commonly, especially since the introduction of
melarsomine dihydrochloride (also see Heartworm Disease).
Phenylarsonic compounds were widely used as feed additives to improve
production in swine and poultry rations and to treat dysentery in pigs. The three
major compounds in this class are arsanilic acid, roxarsone
(4-hydroxy-3-nitrophenylarsonic acid), and nitarsone (4-nitro-phenylarsonic
Toxicosis results from an excess of arsenic-containing
additives in pig or poultry diets. Severity and rapidity of onset are
dose-dependent. Signs may be delayed for weeks after incorporation of 2–3 times
the recommended (100 ppm) levels or may occur within days when the excess is
>10 times the recommended levels. Chickens are tolerant of arsanilic acid;
however, roxarsone can produce toxicosis in turkeys at only twice the
recommended dose (50 ppm). In pigs, roxarsone also has a higher toxicity than
Clinical Findings and Diagnosis
The earliest sign in pigs may be reduced weight gain,
followed by incoordination, posterior paralysis, and eventually quadriplegia.
Animals remain alert and maintain good appetite. Blindness is characteristic of
arsanilic acid intoxication but not of other organic arsenicals. In ruminants,
phenylarsonic toxicosis is similar to inorganic arsenic poisoning. There are
usually no specific lesions present in phenylarsonic poisoning. Demyelination
and gliosis of peripheral nerves, the optic tract, and optic nerves are usually
seen on histopathology. Analyses of feed for the presence of high levels of
phenylarsonics confirm the diagnosis.
Phenylarsonic poisoning in pigs should be differentiated
from salt poisoning, insecticide poisoning, and pseudorabies. In cattle, arsenic
poisoning should be differentiated from other heavy metal poisoning, insecticide
poisoning, and infectious diseases such as bovine viral diarrhea.
Treatment and Prognosis
There is no specific treatment, but the neurotoxic effects
are usually reversible if the offending feed is withdrawn within 2–3 days of
onset of ataxia. Once paralysis occurs, the nerve damage is irreversible.
Blindness is usually irreversible, but animals retain their appetite, and weight
gain is good if competition for food is eliminated. Recovery may be doubtful
when the exposure is long and the onset of intoxication slow.
Last full review/revision April 2015 by Tam Garland, DVM, PhD, DABVT