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; however, their use now is 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 arsenical toxicosis are similar to those for inorganic arsenical toxicosis, except ruminants may have necrosis of the mucosa of the rumen and omasum and gelatinous serosal edema of the omasum and abomasum.
Aromatic organic arsenicals include trivalent phenylorganics, such as thiacetarsamide and arsphenamine, available for the treatment of adult heartworm disease Heartworm Disease in Dogs, Cats, and Ferrets Heartworm disease (dirofilariasis), produced by Dirofilaria immitis, primarily affects the pulmonary arteries, producing inflammation, vascular dysfunction, and pulmonary hypertension... read more 
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.
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-nitrophenylarsonic acid). However, ash from treated lumber ingested by cattle is metabolized to a trivalent arsenical. As of 2013, the compounds for use in swine and poultry were voluntarily withdrawn by the sponsors of the compounds. In 2015, the US FDA withdrew all approvals for the use of phenylarsonic compounds and arsanilic acid compounds. Some arsanilic acid may be used in laboratories in association with nanoparticles. Other countries outside the US may have these products from other sources.
Etiology of Organic Arsenical Toxicosis
Organic arsenical toxicosis results from an excess of arsenic-containing additives in pig or poultry diets. Severity and rapidity of onset are dose-dependent. Clinical signs may be delayed for weeks after incorporation of 2–3 times the recommended (100 ppm) concentrations or may occur within days when the excess is >10 times the recommended concentrations. Chickens are tolerant of arsanilic acid; however, roxarsone can produce toxicosis in turkeys at only twice the recommended concentrations (50 ppm). In pigs, roxarsone also has a higher toxicity than other phenylarsonics. Although the US has ended the use of the phenylarsonic compounds in animal feeds, it may still be used in other countries.
Clinical Findings and Diagnosis of Organic Arsenical Toxicosis
Often, clinical findings of acute incoordination and hind limb paralysis are almost pathognomonic for organic arsenical toxicosis in swine. These animals should be suspected of having been exposed to an arsanilic acid or a phenylarsonic compound. Affected animals may also be acutely blind. No specific serologic test is available for such a presentation. However, finding and removing the source of the arsanilic acid or phenylarsonic compound may resolve the clinical signs in a few days. Differential diagnoses in swine include salt poisoning, insecticide poisoning, and pseudorabies Pseudorabies in Pigs Pseudorabies is an acute, often fatal, viral disease with a worldwide distribution. Swine are the primary host, but other species are also occasionally infected. Clinical signs include reproductive... read more . Differential diagnoses in cattle include inorganic arsenic intoxication, other heavy metals, insecticide poisoning, and infectious diseases, especially bovine viral diarrhea Bovine Viral Diarrhea and Mucosal Disease Complex Bovine viral diarrhea/mucosal disease is a pestivirus infection of cattle and other ruminants. Infection leads to immunosuppression and can cause signs in multiple body systems in addition to... read more .
Gross pathology photograph of a bovine rumen displaying sloughing (arrow) in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of a bovine rumen displaying necrotic (large arrow) and hemorrhagic (small arrow) lesions in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of a bovine rumen displaying ulcerative (small arrow) and hemorrhagic (large arrow) lesions in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of bovine intestines filled with blood (arrows) in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of a bovine urinary bladder, showing hemorrhagic lesions (arrow, plus all other hyperemic areas) in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of a bovine intestine showing ulcerative lesions (arrows) in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of a bovine intestine with mucosal perforations (arrows) in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
Gross pathology photograph of a horse stomach with mucosal ulcers (arrow) in a case of arsenic poisoning.
Courtesy of Dr. Tam Garland.
The earliest clinical sign of organic arsenical toxicosis in pigs may be decreased weight gain, followed by incoordination, hind limb paralysis, and eventually quadriplegia. Animals remain alert and maintain good appetite. Blindness is characteristic of arsanilic acid intoxication but not of toxicoses due to other organic arsenicals. In ruminants, phenylarsonic toxicosis is similar to inorganic arsenic poisoning. Usually, no specific lesions are present in phenylarsonic poisoning. Demyelination and gliosis of peripheral nerves and optic nerves are usual histopathologic findings. Analyses of feed samples for the presence of high levels of phenylarsonics confirm the diagnosis.
Treatment and Prognosis of Organic Arsenical Toxicosis
There is no specific treatment for organic arsenical toxicosis other than supportive care. Animals must have access to feed and water but must be prevented from falling into or drowning in the water. There is no treatment for blindness in affected animals; however, the animal will still grow and mature normally. Arsenic poisoning must be differentiated from other heavy metal poisonings and insecticide poisonings in cattle.
The neurotoxic effects of organic arsenical toxicosis 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; however, 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.
Key Points
Phenylarsonic compounds were withdrawn from the market in 2013 and have been illegal to use in the US since 2015. This may not be the case in other countries.
Incoordination and hind limb paralysis may be the first clinical signs and must be differentiated from pseudorabies, salt poisoning, insecticide poisonings, and other heavy metal poisonings.
There is no treatment other than supportive care and time.
