Rodenticides and Herbicides: Superwarfarins and Paraquat

Superwarfarins: warfarin's chemistry, stretched over months

If you understand warfarin, you already understand the mechanism — you only have to stretch it. Long-acting anticoagulant rodenticides — brodifacoum, difenacoum, bromadiolone and their relatives — are known as "superwarfarins," and the name is honest. They inhibit exactly the enzyme warfarin does: vitamin K epoxide reductase (VKOR). That enzyme's job is to recycle vitamin K, regenerating the reduced (active) form after it is spent. Reduced vitamin K is the essential cofactor that lets the liver carboxylate — and thereby switch on — clotting factors II, VII, IX and X. Block VKOR and reduced vitamin K runs out; the liver then churns out clotting factors that are functionally dead. The result is a coagulopathy and bleeding, exactly as in warfarin toxicity, monitored by the same rising INR. This is the identical pharmacology taught in the Haematology chapter on antithrombotics — read the two side by side.

So why do these rat poisons last for months when a warfarin dose wears off in days? Two reasons compound. First, potency: the superwarfarins bind VKOR far more tightly and were engineered to be lethal to rodents that had grown resistant to plain warfarin. Second, storage: they are highly lipophilic and accumulate in the liver and fat, from which they leach back into the blood for weeks. A single significant ingestion can therefore keep the INR deranged for one to several months. The clinical tempo is deceptive — the patient may feel perfectly well at first, then present with delayed bleeding days later, or turn up (as many do) with an unexplained coagulopathy and no history of exposure at all.

The antidote: vitamin K1 — high dose, long haul

The good news is that superwarfarin poisoning has a clean, specific antidote: vitamin K1 (phytomenadione). Give enough reduced vitamin K downstream of the blocked enzyme and the liver can carboxylate its clotting factors again — the coagulopathy corrects. The catch is dose and duration. Because the poison is potent and keeps leaching from fat stores, patients often need high oral doses (tens of milligrams daily, sometimes far more) continued for weeks to many months, with the INR guiding when it is finally safe to stop and watched carefully for rebound after withdrawal. This is the opposite of therapeutic warfarin reversal, where a small dose of vitamin K for a day or two is plenty. Vitamin K is not, however, an emergency treatment for active bleeding: it takes hours to work because the liver must synthesise new factors. When the patient is bleeding seriously here and now, you must replace the missing factors directly — with prothrombin complex concentrate (PCC), which delivers factors II, VII, IX and X in a small volume, or fresh frozen plasma (FFP) where PCC is unavailable. Vitamin K is then given alongside for the sustained correction. These enhanced-elimination and antidote principles are drawn together in the Antidotes chapter.

Paraquat: the herbicide that burns the lungs from within

Everything about superwarfarin is hopeful. Almost nothing about paraquat is. Paraquat is a widely used herbicide and one of the most lethal poisons a person can swallow. Its toxicity comes from redox cycling: inside the cell paraquat is repeatedly reduced and re-oxidised, and each turn of the cycle spits out reactive oxygen species (ROS) — superoxide and its downstream radicals — while draining the cell's NADPH reserves. This oxidative storm damages every organ it reaches, causing acute kidney injury and hepatic failure, but its cruelty is anatomical: paraquat is actively taken up and concentrated by the lung's alveolar (type I and II) pneumocytes, using the cell's own polyamine transporters. The lung therefore accumulates the highest doses and bears the worst of the ROS. Over days to weeks this drives a relentless, progressive pulmonary fibrosis — the lungs scar shut. A patient who looks stable on day one can slide into irreversible respiratory failure a week later. The lung injury and the reason it is so specific are explored further in the Respiratory chapter.

There is no effective antidote. Deliberate ingestion of a concentrated formulation is frequently fatal, and prognosis tracks the dose and the plasma concentration measured against time. Management is therefore a race to limit absorption and support the organs, not to reverse the poison: give activated charcoal or Fuller's earth as early as possible to bind paraquat in the gut — the decontamination principles covered in the Decontamination chapter — with aggressive supportive care for the kidneys and circulation. Haemodialysis and haemoperfusion help clear renal failure but do little for lung outcome once the poison is in the pneumocytes. Experimental strategies aimed at the oxidative injury — high-dose corticosteroids with cyclophosphamide (immunosuppression), and antioxidants — have been tried with, at best, marginal benefit. Above all, do not send these patients to a transplant only after the fibrosis is fixed; recognise the trajectory early.

Other rodenticides worth a mention

Not every rodenticide is an anticoagulant. Zinc and aluminium phosphide — common in grain fumigants and "rat pellets" in many countries — react with stomach acid (and moisture) to release phosphine gas, a mitochondrial poison that blocks cellular respiration and causes profound shock, metabolic acidosis and cardiac failure; there is no antidote, and it is a leading cause of fatal self-poisoning in parts of the world. Strychnine, an older rodenticide, antagonises inhibitory glycine receptors in the spinal cord, producing agonising muscle spasms and opisthotonus with a fully conscious patient — management is supportive, with control of the spasms. Thallium, historically a rodenticide (and infamous poison), causes a classic triad of gastrointestinal upset, painful peripheral neuropathy and later alopecia; here there is a specific binder — Prussian blue — that traps thallium in the gut for elimination, a nice contrast to paraquat's therapeutic emptiness.