Poisoning in horses refers to the ingestion, inhalation, or absorption of toxic substances that can adversely affect their health. These substances may include plants, chemicals, medications, or environmental toxins. Poisoning can lead to a range of clinical signs, from mild gastrointestinal upset to severe neurological or systemic effects, depending on the toxin involved. Common toxic substances for horses include certain plants like oleander and yew, heavy metals such as lead and arsenic, and specific pesticides or herbicides. This page compiles peer-reviewed research studies and scholarly articles that investigate the causes, effects, diagnosis, and management of poisoning in horses.
Baird JD, Hutchins DR, Lepherd EE.A 12-month-old thoroughbred horse, after being treated with 30 g. of micronized phenothiazine, developed an acute haemolytic anaemia characterized by Heinz-body formation. Elevation of serum glutamic oxaloacetic transaminase, isocitric dehydrogenase, lactate dehydrogenase and blood urea nitrogen also occurred. The colt's clinical condition improved after receiving blood transfusions, fluids, antibiotics and corticosteroids. After 10 weeks, the haematological and biochemical picture returned to normal. [A.S.] D.A.Cz.
James LF, Van Kampen KR, Staker GR.Of 70 Aberdeen-Angus cows which ate locoweed (Astragalus lentiginosus) during rotational grazing in Utah, because it was the only green feed available at the time, a large number aborted. All the cattle showed signs of poisoning, but recovered when removed from the pasture. There were signs of poisoning also in 12 horses, and though they seemed to recover signs of permanent damage to the central nervous system remained. Hereford cattle grazing plots with a similar proportion of locoweed were not affected, but for them other green herbage was available
Barr AC, Reagor JC.Horses poisoned by a plant may show a myriad of clinical signs. Awareness of the poisonous plants growing in a given area and those that are likely to appear in hay and their associated clinical signs can be instrumental in making diagnoses. More importantly, the information can be shared with clients to help prevent plant poisonings in horses.
Gish A, Robveille C, Gicquel T, Allorge D, Gault G, Gaulier JM.Analytical detection of Oenanthe crocata toxins in biological samples is challenging because of their instability, the lack of commercially available standards and the exceptionally low detection of these molecules using mass spectrometry. This work aims to report the used analytical methods that allowed identification of the main plant toxins in biological samples from an equid (an Arabian horse) fatality related to hemlock water dropwort (Oenanthe crocata Linnaeus) intake. Using both LC-DAD and LC-HRMS methods allowed identification (i) of oenanthotoxin in roots found on the site, root fragm...
Dewes HF, Lowe MD.Events leading to the deaths of two fillies at pasture are described. Pasture hay containing the flowering stages of Senecio jacobea (ragwort) had been fed three to four months earlier. Paddocks were subdivided with posts and rails treated with copper chrome arsenate. Six horses on the property chewed rails spasmodically. Both fillies presented with haemoglobinurea. Values in liver of 83 mg Cu kg and kidney 35 mg Cu kg wet weight and serum 1.4 mg Cu/l together with histophathology of seneciosis support a sequence of ragwort poisoning followed by copper accumulation in liver and kidney terminat...
Bischoff K, Moiseff J.Feed as a cause of poisoning in horses can occur on small or large scales. It is challenging to work up cases of suspected feed contamination, but there are resources available to veterinarians and owners. Feed contamination can be chemical or biological. This article focuses on and provides examples of chemical feed contamination including misformulation, adulteration, and natural contaminants. Additionally, recommendations for feed sampling and diagnostic submission, including legal documentation, are included.
McConnico RS, Brownie CF.Two horses with red maple (Acer rubrum) toxicity responded to treatment with high doses of vitamin C (ascorbic acid), in addition to blood transfusions, and intravenous fluid therapy. The clinical course included Heinz body anemia, marked methemoglobinemia, depression, and evidence of severe tissue anoxia. Clinical recovery was dramatic with stabilization achieved 36 hours following the initiation of ascorbic acid therapy.
Mirck MH.The use of trichlorfon as an anthelmintic in horses has increased since it was combined with mebendazole and febantel, and became commercially available as Telmin-trichlorfon and Rintal-plus respectively. Field studies showed that these combined preparations frequently gave rise to cases of poisoning. Mild to severe colic was observed up to six hours after treatment, diarrhoea and/or anorexia continuing to be present for one to six days after administration. The 'horse weight measurer' introduced by Janssen Pharmaceutica is not sufficiently accurate to prevent overdosage. It is advisable only ...
Kowalczyk DF, Naylor JM, Gunson D.A diagnosis of lead poisoning in a foal was established based on clinical signs and the presence of an elevation in the concentration of blood lead (40 microgram/dl). Additionally, ZPP was determined in whole blood and found to be significantly elevated compared to 10 normal horses not exposed to lead (634 vs 73 microgram/dl). An elevation in ZPP in man has been found to be a sensitive biochemical indicator of lead toxicity even before overt clinical signs are present. This report suggests that ZPP may be a useful screening test for evaluating biological toxicity due to lead in horses and poss...
van den Dikkenberg MI, Holtkamp BM.Seven cases of poisoning in horses are reported. This was caused by ingestion of branches of the alder buckthorn (Frangula alnus (mill.) syn. Rhamnus frangula L.), which had been thrown on the pasture. The biological characteristics and the toxic action of the plant are discussed more fully.
Miyazaki S, Fukumura M, Yoshioka M, Yamanaka N.From 1997 to 1999, 29 cases of disorders were detected in cattle and horses that had been fed ryegrass straw imported from the U.S.A. These animals showed symptoms resembling ryegrass staggers and the clinical signs disappeared after removal of the straw. Endophytic hyphae were detected in the seeds of all straw samples that were responsible for the clinical cases. Lolitrem B concentrations in the straw ranged between 972 and 3740 ppb. Ergovaline concentrations were between 355 and 1300 ppb. Even though the concentrations of lolitrem B were lower than the toxic threshold proposed by Oregon Sta...
Knight HD, Costner GC.During the summer and fall of 1973, a few horses, goats, and cattle in the Napa Valley of California became intoxicated by bromide via the ingestion of volunteer oat hay that had been cut from a field treated with methyl bromide, a soil fumigant. The bromide content of the hay ranged from 6,00 to 8,400 ppm. Signs of intoxication were lethargy, weakness, and ataxia. Animals experimentally fed the contaminated hay developed signs of intoxication between the 7th and 9th days.
Spoo W.Poisoning resulting from exposure to a wide variety of industrial chemicals is not a common occurrence in horses, but it does happen on occasion. A wide range of toxicosis can occur from a wide range of industrial pollutants, such as dioxin, carbon tetrachloride, and tetrachloroethylene, to heavy metals, such as cadmium and zinc. The equine practitioner must consider industrial chemical toxicosis in differential diagnoses and work with a reputable veterinary diagnostic laboratory to confirm or rule out industrial chemical poisoning.
Blanchard T, Fantinati M, Domange C, Priymenko N.The present report describes two novel cases of suspected intoxication with Galega officinalis in 6- and 21-year-old Arabian mares displaying acute respiratory signs. Both animals showed signs of pulmonary edema at physical examination, with the oldest of the two also manifesting severe dyspnea and foamy nasal discharge. The mares were grazing on the same meadow with hay available ad libitum. Botanical analysis of the latter showed traces of the toxic plant Galega officinalis (L.), which has been daily ingested at a dose of around 14 g of dry matter for three days. Based on the respiratory sig...
van Miert AS.A study of the literature was done because of questions asked in a court of justice concerning possible poisoning in a jumper, resulting from administration of both phenylbutazone and a coumarin derivative within a particular period. In view of the mechanisms of action and the pharmacokinetic characteristics of the agents, these forms of combined treatment are also highly inadvisable in horses.
Puschner B, Galey FD.Poisoning in the horse can present a highly complex case. The practitioner, owner, toxicologist, and pathologist play important roles, and all contribute information that may be important to the case. Once all the information is available, all the evidence is collected (historical, clinical, pathologic, and analytic), and proper sampling of specimens has occurred, a complete summary of the findings can be provided to the client. Based on identification of a potential toxic source and, ultimately, the diagnosis, specific treatment of affected animals and prevention of additional cases can be in...
Westermann CM, Boerma S, van Nieuwstadt RA.Since the only registrated anti-ectoparasiticum for horses (foxim) is no longer available in The Netherlands, some cases of amitraz intoxication of the horse have occurred. In this article the literature concerning amitraz intoxications and experiments will be discussed and some cases will be described.
Kroneman J, Goedegebuure SA.The clinical and pathomorfological symptoms of a two month's zinc-overload in a foal are described. After an exposure of about two weeks symptoms of unthriftness and increasing stiffness develope. The stiffness is caused by severe intra-articular damage.
Olson CT, Keller WC, Gerken DF, Reed SM.Of 10 horses in a heavily overgrazed pasture, 4 died within 1 week. Clinical signs included muscle tremors, ataxia, reluctance to walk, heavy sweating, and myoglobinuria. Serum creatine kinase, aspartate transaminase, and alkaline phosphatase activities were high. Histopathologic findings were nonspecific. On the basis of clinical signs, clinicopathologic findings, nonspecific histopathologic findings, the condition of the pasture, the identification of numerous white snakeroot plants from which trematone was extracted, and evidence that these plants had been heavily browsed, it was believed t...
Sloet van Oldruitenborgh-Oosterbaan MM, Schipper FC, Goehring LS, Gremmels JF.In the course of several days most of the 40 riding-school horses turned out in paddocks developed ataxia of variable severity. Five of these horses showed severe ataxia and tremors, became paralyzed and were euthanized. Eleven privately-owned horses which were stabled on the same premises showed no clinical signs. The most likely diagnosis seemed to be the 'neurological form of EHV1', although the signs were not entirely typical. A few weeks later a second outbreak occurred among the riding-school horses and one of the privately-owned horses also showed signs of ataxia. In the meantime it had...
Fox JH, Porter BF, Easterwood L, Hildenbrand JRV, Hélie P, Smylie J, O'Toole D.Metal phosphides, particularly zinc and aluminum phosphide, occasionally poison horses and other equids following their use as rodenticides and insecticides. Grain-based aluminum phosphide baits are used to control rodents such as prairie dogs. The clinical course in intoxicated horses is short (<24-48 h), and animals may be found dead. Hepatic lesions caused by phosphine poisoning are not well described. Laboratory confirmation depends on detecting phosphine gas in gastric contents. Eight horses and a mule were exposed to zinc phosphide used to control prairie dogs on a Wyoming ranch. Three o...
Hooser SB.Incidences of feed contamination are rare even though enormous amounts of animal feed are manufactured. However, there are still some cases of feed-related illness in horses. Veterinarians play a crucial role in recognizing and mitigating these events and in assessing the severity of risks. Due to these risks, proper reporting and consultation with government and state agencies are crucial. Accurate diagnosis and identification of the source of poisoning are promising when a thorough case workup is performed and agencies such as veterinary diagnostic laboratories and the US Food and Drug Admin...
Stegelmeier BL, Davis TZ.Range and pasture toxic plants can poison horses. Many of these plants are noxious weeds that can dominate plant populations and replace healthy forages. Poisoning is often difficult to diagnose as the resulting plant-induced disease is similar to other infectious, toxic, and nutritional diseases. Identifying potentially problem plants, and observing what plants horses are eating, is essential in determining the risk of poisoning. If the risk is significant, it can drive management to invest in strategies to avoid exposure, animal disease, and suffering.
Machado M, Queiroz-Machado CRR, Gardner DR, Castro MB, Câmara ACL, Pimentel LA, Galiza GJN, Riet-Correa F.Leucaena leucocephala poisoning is reported in horses in different Brazilian regions. The poisoning occurred one month after the horses were introduced into paddocks invaded by the plant or after 10 days of consuming cut Leucaena administered as the only food. Affected horses showed moderate to severe hair loss on the mane and tail, orthokeratotic hyperkeratosis with marked follicular telogenization, and hyperplasia of thyroid follicular cells. Mimosin concentration in leaves (5.5 mg/g) was determined by a new HLPC-UV method which is also reported.
Sander J, Terhardt M, Janzen N, Renaud B, Kruse CJ, François AC, Wouters CP, Boemer F, Votion DM.Equine atypical myopathy is caused by hypoglycin A (HGA) and methylenecyclopropylglycine (MCPrG), the known protoxins of sycamore maple (Acer pseudoplatanus). Various tissues from five atypical myopathy cases were analyzed but only HGA was found. Whether deamination of MCPrG has already occurred in the intestine as the first stage of metabolization has not been investigated. Activation of the protoxins to methylenecyclopropylacetyl (MCPA)-CoA and methylenecyclopropylformyl (MCPF)-CoA, respectively, occurred mainly in the skeletal muscles, as evidenced by very high concentrations of MCPA-carnit...
Mello GW, Riet-Correa F, Batista MC, Carvalho CJ, Dias AC, Franklin FL, Silva SM, Dias A.Farmers in the State of Piauí in northeastern Brazil reported nervous signs in ruminants and donkeys after ingestion of Brunfelsia uniflora at the start of the rainy season when the plant is flowering. Leaves of the plant, collected at the start or at the end of the rainy season, were administered in single doses of 5-20 g/kg body weight to 8 sheep and 3 donkeys. Two sheep and 1 donkey that ingested 10 g/kg of the plant in November at the start of the rainy season, when the plant was flowering, developed severe convulsions and diarrhea. One sheep was euthanized and autopsied, and no significa...
González Medina S, Hyde C, Lovera I, Piercy RJ.Hypoglycin A (HGA) toxicity, following ingestion of material from certain plants, is linked to an acquired multiple acyl-CoA dehydrogenase deficiency known as atypical myopathy, a commonly fatal form of equine rhabdomyolysis seen worldwide. Whilst some plants are known to contain this toxin, little is known about its function or the mechanisms that lead to varied HGA concentrations between plants. Consequently, reliable tools to detect this amino acid in plant samples are needed. Analytical methods for HGA detection have previously been validated for the food industry, however, these technique...
