Chapter 13 Toxicological disorders
Illustrations of toxicological disorders in cattle present problems. The clinical signs may be transient, with death occurring within a few minutes, such as in yew (Taxus baccata) poisoning (p. 249). In many cases the signs may be nonspecific. Where the effects are confined largely to one system, the description has been given in the appropriate section, e.g., ergot and fescue foot are considered under locomotor disorders (7.158–7.160). In this chapter, toxicoses have been broadly grouped into plant, organic, and inorganic chemical sections, and a few examples are given of each group. They are taken from Brazil, China, Germany, New Zealand, South Africa, the UK, the USA, and Zimbabwe.
Food safety issues arising from residues of chemicals in meat, offal, and milk are extremely important. Farmers are obliged, under food safety regulations, to avoid contaminating the food chain and must withdraw produce contaminated with chemical residues from the food chain. Food businesses such as dairies and supermarkets will reject produce from farms with unresolved poisoning incidents.
acute or chronic worldwide toxicity following ingestion of large quantities of bracken (bracken fern), which contains several bone marrow toxins (aplastic anemia factor) which kill precursor cells in the bone marrow.
bracken toxicosis is widespread in several continents. Bracken (Pteridium aquilinum) contains ptaquilosides and other compounds which act as a cumulative poison in two syndromes. Firstly, after ingesting large quantities for a few weeks, cattle may show an acute syndrome resulting from aplastic anemia and thrombocytopenia. Sudden death is occasionally seen. In 13.1 the vulva of the crossbred Angus cow is pale from severe anemia. The pinpoint hemorrhages result from thrombocytopenia. Hemorrhages elsewhere can cause epistaxis, hyphema (13.2) (bleeding into the anterior chamber), or hematuria from bladder mucosal hemorrhage (13.3).
Secondly, long-term ingestion of considerable quantities of bracken for several months can lead to a chronic syndrome of enzootic hematuria. The ptaquilosides are carcinogens and cause bladder neoplasia, resulting in enzootic hematuria and malignancies such as hemangiosarcoma (13.4). Numerous discrete masses, seen protruding from the mucosal surface, bleed readily as the bladder distends and contracts. Some mucosal areas (top right, lower left) appear normal. The hemangiomata can develop into ulcerating tumors of various types. Alimentary tract neoplasms include squamous cell carcinomas and papillomas affecting the pharynx and esophagus, respectively. 13.5 shows pharyngeal squamous cell carcinomas (A) and esophageal papillomas (B) identified in Brazil. Bovine papillomavirus (types 2 and 4) may also be involved in upper alimentary neoplasms.
a history of exposure to bracken, clinical signs of severe anemia and pancytopenia, and low platelet counts. Most outbreaks in grazing cattle have many subclinically affected animals with markedly abnormal blood parameters.
treatment of individual cases is usually hopeless, possibly blood transfusion (5–10 liters); pasture and grazing management (bracken eradication). Food safety implications, resulting from the excretion of bone marrow toxins into milk, have led to the recommendation of a 4-day withdrawal period (UK) before milk can be consumed directly from bracken-grazed dairy cows.
the toxic principle causes renal and gastrointestinal changes. The signs include abdominal pain, often with a hemorrhagic diarrhea, thirst, polyuria, and ventral edema as a result of subacute and chronic toxicity. The esophageal mucosa can be hemorrhagic (13.6). The enlarged, swollen kidneys (13.7) show scattered hemorrhages and a nephrosis, which accounts for the ventral edema, ascites, and hydrothorax seen in cases with renal failure.
signs usually follow acute exposure and include nervousness, ataxia, dyspnea, and collapse. The opened rumen in 13.8 shows normal ingesta mixed with needle-like yew leaves. Cattle usually die minutes after ingesting a few mouthfuls of yew twigs or berries, typically encountered as fresh or dried clippings thrown over a graveyard hedge into a bare winter pasture. The lethal dose in adult cattle may be as little as 1 kg of leaves. Some cattle may survive up to 3 days before they die.
Senecio jacobea is one of many species of ragwort worldwide which contain the pyrrolizidine alkaloid jacobine, that causes acute and more often chronic liver disease. Several other plants e.g. Crotalaria, contain similar pyrrolizidine alkaloids which also can result in chronic hepatotoxicity.
early signs include dark-colored diarrhea, photosensitization, jaundice, abdominal pain, and central nervous system abnormalities. Prolonged ingestion (chronic exposure) results in progressive weakness, weight loss, and liver failure due to cirrhosis, and severe lung disease. In the mature Hereford cow in 13.9, the resulting right heart failure led to the edema affecting the ventral body wall, brisket, and head. Clinical signs may not be seen until several months after digestion, by which time the animal is no longer consuming the toxin, and diagnosis can be difficult. Liver biopsy may be useful for diagnosis. The fresh plant is bitter and usually avoided. Intoxication frequently occurs following its incorporation into conserved forage as this renders the plant more palatable. See also photosensitization (p. 30).
some forms of forage of the Brassica family, such as kale and rape, contain glucosinolates which with plant myrosinase can be hydrolyzed to S-methylcysteine sulfoxide (SMCO) which can cause a hemolytic (Heinz-Ehrlich body) anemia following production of dimethyl disulfide by ruminal bacteria. The flowers and seeds of mature plants are particularly poisonous, whether ingested by grazing cattle or in cake or meal. Glucosinolates cause goiter, encephalomalacia (“rape blindness”), pulmonary edema, and interstitial pneumonia.
postparturient hemoglobinuria (possibly related to SMCO via a cumulative effect in red cells), bacillary hemoglobinuria, nitrate/nitrite poisoning (13.27, 13.28), hypomagnesemia (9.5), babesiosis (12.39–12.43), anaplasmosis (12.44–12.47), acute bracken poisoning (13.1–13.3), chronic copper poisoning (13.33).