Pyrrolizidine alkaloid (PA) toxicity is a common cause of chronic hepatic disease in cattle. It is caused by repeated consumption of PA-containing plants. There are over 6,000 different types of plant species that contain PAs.
Many of these plants are considered invasive noxious weeds in certain regions, meaning that they may abundantly grow and be difficult to control. The concentration of toxins within PA-containing plants fluctuate depending on many factors, such as stage of growth of the plant, soil medium, environmental conditions (drought, frost, flooding), application of herbicides, season, and plant part. Toxin concentrations tend to be highest during budding and flowering stages.
Due to their bitter taste, PA-containing plants are generally disliked and avoided by cattle when exposed to them in pastures. However pastures which contain an overabundance of these plants with little grass, increases the chance of cattle eating them. Also, if PA-containing plants are mixed in with hay, cattle often don't recognize their presence. Repeated ingestion of contaminated hay is one of the most frequent ways cattle are poisoned.
Toxicity occurs cumulatively over a course of a lifetime. Cattle' must consume 2% to 5% of their body weight in PA-containing plants prior to developing liver disease. This often takes weeks or months after initial consumption before clinical signs develop. Unfortunately, by the time the clinical signs become apparent, the horse has already lost more than 80% of its liver function.