Biochemistry of hemlock (Conium maculatum L.) alkaloids and their acute and chronic toxicity in livestock. A review.

ABSTRACT

The literature on Conium maculatum biochemistry and toxicology, dispersed in a large number of scientific publications, has been put together in this review. C. maculatum is a weed known almost worldwide by its toxicity to many domestic animals and to human beings. It is an Umbelliferae, characterized by long, hollow stems, reaching up to 2 m height at maturity, producing a large amount of lush foliage during its vegetative growth. Its flowers are white, grouped in umbels formed by numerous umbellules. It produces a large number of seeds that allow the plant to form thick stands in modified soils, sometimes encroaching on cultivated fields, to the extent of impeding the growth of any other vegetation inside the C. maculatum area of growth. Eight piperidinic alkaloids have been identified in this species. Two of them, gamma-coniceine and coniine are generally the most abundant and they account for most of the plant acute and chronic toxicity. These alkaloids are synthesized by the plant from eight acetate units from the metabolic pool, forming a polyketoacid which cyclises through an aminotransferase and forms gamma-coniceine as the parent alkaloid via reduction by a NADPH-dependent reductase. The acute toxicity is observed when animals ingest C. maculatum vegetative and flowering plants and seeds. In a short time the alkaloids produce a neuromuscular blockage conducive to death when the respiratory muscles are affected. The chronic toxicity affects only pregnant animals. When they are poisoned by C. maculatum during the fetuses organ formation period, the offspring is born with malformations, mainly palatoschisis and multiple congenital contractures (MCC; frequently described as arthrogryposis). Acute toxicity, if not lethal, may resolve in the spontaneous recovery of the affected animals provided further exposure to C. maculatum is avoided. It has been observed that poisoned animals tend to return to feed on this plant. Chronic toxicity is irreversible and although MCC can be surgically corrected in some cases, most of the malformed animals are lost. Since no specific antidote is available, prevention is the only way to deal with the production loses caused by this weed. Control with herbicides and grazing with less susceptible animals (such as sheep) have been suggested. C. maculatum alkaloids can be transferred to milk and to fowl muscle tissue through which the former can reach the human food chain. The losses produced by C. maculatum chronic toxicity may be largely underestimated, at least in some regions, because of the difficulty in associate malformations in offspring with the much earlier maternal poisoning.