Poisoning in Cattle – II. Post-mortem and Laboratory Examination
Post-mortem examination of the animals died of suspected poisoning is extremely important as it gives valuable clues and specimens for laboratory examination for making a final diagnosis. Pre-mortem signs such as struggling, frothy discharges from the mouth or nostrils, dilated eyes and fang marks may be found in cases of poisoning. Cherry red color mucus membranes in cyanide poisoning; brown discoloration of mucus membranes in nitrate, nitrite or chlorate poisoning; Presence of grayish-white flakes of arsenic trioxide with rose-red color of the alimentary tract in arsenic poisoning are some of the characteristic necropsy findings that are useful for making a tentative diagnosis in field conditions.
The animals died of poisoning usually have lesions in the gastrointestinal tract, liver and kidneys necessitating a close examination of these organs. Tissue samples from liver and kidney, stomach contents and samples of suspected sources of the poison (feed, fodder, grass, soil, water, etc.) should be collected invariably for further laboratory investigations. Some very simple, yet effective qualitative screening tests are available that can be successfully performed by veterinarians in the field without much laboratory requirements for detection of toxic metals (Reinsch test, Spot Test), hydrocyanic acid (Ferrous sulphate test, Steyn test) and nitrate (Diphenylamine test, Salicylic acid test).
Reinsch Test is useful for screening of toxic heavy metals including arsenic, mercury and antimony in urine or stomach contents. The procedure entails boiling of a small copper coil in an acidified solution of test material. A spiral copper wire is first cleaned with 35% nitric acid and after rinsing with distilled water, it is immersed in 15 ml of sample acidified with hydrochloric acid (4ml). After heating for an hour the wire is examined for the deposition of metal ions. The test is positive when there is deposition, the color of which may be matte black for arsenic and silver gray for mercury.
Steyn test is used on rumen ingesta or plant material for qualitative detection of hydrocyanic (prussic) acid (HCN). The test requires a strip of filter paper, a wide mouth jar or test tube, chloroform, sodium carbonate, picric acid and distilled water. The alkaline picrate solution is prepared by dissolving 5 g sodium carbonate and 0.5 g picric acid in 100 ml water. Freshly prepared paper strips are wet with this reagent. Suspected material is placed in a wide mouth jar containing a little water. Few drops of chloroform are added and the filter paper strip impregnated with sodium (alkaline) picrate solution is suspended on mouth of the jar or test tube. Close the mouth of the jar or test tube and observe the color of picrate paper. Change in color of picrate paper from yellow to red within 5-10 min indicates presence of large amounts of HCN in the test sample. Change in color in 24 hours indicates trace quantity. There is no change in color in the absence of HCN.
Diphenylamine test (DPT) can be used in the field to test the presence of nitrate/ nitrite in body fluids (ocular fluid, serum, and urine) as well as in plants. The test reagent is prepared by dissolving 0.5 mg diphenylamine salt in 20 ml distilled water and then bringing volume to 100 ml by adding concentrated sulfuric acid. A drop of reagent is added to test the sample. An intense blue coloring indicates a positive test. The test is considerably effective for detection of nitrate content in common fodder such as corn stalks, sorghum and weeds used to feed cattle. A drop of DP reagent is placed on the freshly cut inner tissue of the plant stem. The development of blue color within 5-10 seconds suggest the presence of nitrate. The content of nitrate can be assessed by the time of development and intensity of color. Forage samples developing blue black color within 5 seconds may contain at least 1% (10,000 ppm) nitrate, a highly dangerous level capable of causing fatal poisoning in cattle. A commercial test strip is available for detecting nitrate in water samples.
It is important to note that the above tests are qualitative screening tests and are not confirmatory and the test results need to be supported by laboratory investigations to reach a final and valid diagnosis of poisoning.
Laboratory examinations are necessary for confirming the diagnosis of poisoning as well as patient management. These include biochemical tests on blood and other body fluids, chemical analysis for detecting causes of poisoning, histopathological and radiological examinations. Once an animal has been exposed to a toxicant, the clinical outcome depends on its absorption, distribution, metabolism and excretion (ADME). Interspecies and intraspecies (individual susceptibility) variations in response to toxicants are also found. Laboratory tests are therefore an important tool to monitor the extent of exposure and determine pathophysiological consequences of the poisoning.
Biochemical tests, including biomarker assays are often useful in making differential diagnosis and also in monitoring the success of treatment. For example, delta aminolevulinic acid (δ-ALA) level in plasma is a reliable biomarker of lead poisoning in cattle, and its higher values can differentiate acute lead poisoning, with other diseases of cattle such as rabies, listeriosis, trypanosomiasis, polioencephalomalacia, hypovitaminosis A, hypomagnesemic tetany, nervous acetonemia and insecticide poisoning, which are also marked by signs referable to brain dysfunction. Enzyme delta aminolevulinic acid dehydratase (δ-ALAD), another biomarker for lead poisoning, is so sensitive to lead that its activities remain inhibited even after lead exposure has ceased. The biomarker is useful to monitor past exposure to lead as well as to monitor the success of antidotal therapy in lead poisoning. Increase choline esterase activity in blood is an important biomarker of organophosphate poisoning, and high values of alkaline phosphate and the gamma glutamyle transpeptidase in urine are accurate and early biomarkers of mercury poisoning. It is however important to note that biochemical tests constitute only one aspect of diagnostic approaches and should be supported by other evidences of poisoning. Further laboratory investigations, including histopathological examination and chemical analysis of a rightly selected, preserved and transported specimens collected from live or dead animals and the suspected source of poisoning are undertaken for the final diagnosis of poisoning.
Dr. Devendra Swarup
BVSc & AH, MVSC (Vet Med), PhD
Former Director, ICAR-CIRG and Head Division of Medicine, ICAR- IVRI