Vetnews | Maart 2026 18 « BACK TO CONTENTS supportive treatment, including intravenous fluids to restore blood volume and correct dehydration. Kidney and liver function should be monitored during treatment. Bracken Fern Poisoning Bracken fern is widely distributed in open woodlands with sandy soils and disturbed (clear-cut or burned) areas throughout North and South America, Europe, Australia, and Asia. Ingestion of a large quantity of bracken fern results in signs of poisoning related to thiamine deficiency. The toxic effects appear to be cumulative and may require 1 to 3 months to develop, depending on the species of animal, quantity consumed, time of year, and other factors. Both leaves and roots may be toxic. Most severe poisonings are seen after periods of drought when food is scarce. However, the plant is toxic even when present as a contaminant in hay, and cases have occurred in stabled animals. Horses seem to be particularly susceptible. In horses, signs of bracken-induced thiamine deficiency (bracken staggers) include loss of appetite, weight loss, lack of coordination, and a crouching stance with the back and neck arched and the feet placed wide apart. When the horse is forced to move, its muscles may tremble. In severe cases, the heartbeat is very rapid and irregular. Death (usually 2 to 10 days after onset) is preceded by convulsions, muscle spasms, and spasms of the back muscles that cause the head and lower limbs to bend backwards and the trunk to arch forward. The rectal temperature is usually normal but may reach 104°F (40°C). Plants other than bracken fern, such as horsetail and turnip, can also cause thiamine deficiency. In horses, the condition must be distinguished from other nervous system disorders, including rabies or poisoning from Crotalaria species or ragwort. Blood tests can confirm the diagnosis. Treatment is highly effective if thiamine deficiency is diagnosed early. Injection of a thiamine solution followed by oral supplementation is suggested. Animals similarly exposed but not yet showing signs should also be treated with thiamine, because signs can develop days or weeks after the source of bracken has been removed. Bracken is usually grazed when more suitable food is not available, although individual animals may develop a taste for the plant, particularly the young, tender shoots and leaves. The problem most often shows up in early spring (tender bracken shoots) or late summer (poor pasture conditions). The disease has been prevented in horses by improving pasture management and fertilisation or by alternating bracken-contaminated and noncontaminated pasture at 3-week intervals. Bracken fern growth can be hindered by close grazing or trampling in alternate grazing pasture systems. In time, bracken can be eliminated from a pasture using this approach or by regular cutting of the mature plant or, if the land is suitable, by deep ploughing. Herbicide treatment using asulam or glyphosate can be an effective method of control, especially if combined with cutting before treatment. Cantharidin Poisoning (Blister Beetle Poisoning) Cantharidin is found in more than 200 species of beetles throughout the continental USA. Beetles of the genus Epicauta are most often associated with toxicosis in horses. The striped blister beetles are particularly troublesome in the southwestern US. Blister beetles usually feed on various weeds and occasionally move into alfalfa fields in large swarms. These insects live in groups and may be found in large numbers in hay when it is baled. One flake of alfalfa may contain several hundred beetles, but a flake from the other end of the same bale may have none. Animals are usually exposed to eating alfalfa hay or alfalfa products that contain blister beetles. Cantharidin is odourless, colourless, and highly irritating. It causes blisters when in contact with skin or mucous membranes. As little as 0.1 to 0.2 ounces (4 to 6 grams) of dried beetles may be deadly to a horse. The toxicity of cantharidin does not decrease in stored hay, and cantharidin is also toxic to people, dogs, cats, rabbits, rats, cattle, sheep, and goats. The severity of signs associated with cantharidin poisoning varies according to dose. Signs can range from mild depression or discomfort to severe pain, shock, and death. Common signs include abdominal pain, depression, loss of appetite, frequent attempts to drink small amounts of water or submerge the muzzle in water, and frequent attempts to urinate. Signs can last from hours to days. Affected horses always have dark, congested mucous membranes, even if other signs are barely noticeable. Horses that ingest a large amount of toxin may show signs of severe shock and die within hours. Laboratory tests can detect cantharidin in stomach contents or urine. The amount of cantharidin in urine becomes too small to be detected in 3 to 4 days, so urine should be collected early in the course of the disease for analysis. Microscopic evaluation of stomach contents of fatally poisoned horses may reveal fragments of blister beetles. There is no specific antidote for cantharidin, and supportive treatment must be prompt to be successful. Administration of mineral oil helps flush the gastrointestinal tract, and repeated treatments may be recommended. Activated charcoal may be helpful if given early. Calcium and magnesium supplementation for prolonged periods is almost always recommended. Other supportive treatment includes the administration of fluids, pain relievers, and medication that increases urine output. The outlook for affected horses improves daily if no complications occur. Prevention is aimed at feeding beetle-free hay. The hay field must be scouted before it is cut and during baling, because the insects can be crushed during cutting, crimping, or baling of hay. Areas of the field that contain swarms of beetles must be avoided for a few days because most of the insects will leave. Once the beetles have left, these areas can be harvested. First-cutting hay is almost always free of blister beetles, because the insects overwinter as subadults and usually do not emerge until late May or June in the southwestern US. Likewise, the last cutting of hay is often safe, because it is usually harvested after the adult insects are no longer active. v Article Poisoning..... <<< 17 Visit: https://www.msdvetmanual.com/special-pet-topics/poisoning/
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