Pigs allowed access to pastures or lots containing pigweed (Amaranthus retroflexus) may be poisoned.
Most poisonings occur in the late summer or fall. Signs appear within five to ten days after exposure and include trembling, weakness, incoordination, knuckling, and almost complete rear leg paralysis. Morbidity is variable; mortality can be high (75-80%) in pigs showing signs.
Lesions are those associated with acute nephrosis and heart failure. There is a marked perirenal edema. The kidneys are normal in size but may appear blanched. Other lesions include ascites, hydrothorax and edema of the ventral body wall; long-standing cases may have chronic fibrosing nephritis. Microscopic lesions in the kidneys of acutely affected pigs include necrosis of both proximal and distal convoluted tubules with numerous casts in tubules. Many glomeruli will be atrophic and have a distended Bowman’s capsule containing filtrate. Diagnosis can usually be made after identification of the plants, obtaining a history of sudden access to them, and observing the clinical signs and rather unique kidney lesions. The toxic principal is not known.
There is no widely accepted treatment. Pigs should be denied access to the plants immediately, but new cases may develop for as long as ten additional days.