Close attention to detail in evaluation of history and clinical findings, accurate and appropriate sample collection, maintenance of chain of custody of evidence and judicious use of analytical testing are imperative when investigating suspected intentional animal poisonings. Veterinarians should be willing and able to work with the necessary public authorities (e.g. animal control and/or law enforcement) in order to ensure that sufficient evidence is collected to provide for successful prosecution of suspected poisoners.

Necropsy considerations

Ideally, forensic necropsy of suspected malicious poisoning victims should be performed by a board-certified pathologist at a veterinary diagnostic laboratory. While most veterinary practitioners have been trained to perform a basic necropsy, cases of malicious poisoning may have few or subtle lesions that may be overlooked. Additionally, forensic necropsies require detailed record keeping in order that accurate testimony can be given, oftentimes years after the necropsy had been performed, and the lack of pathology credentials may be used by the defense to impeach the veterinarian's testimony.if referral of a body to a diagnostic laboratory is not possible, the veterinary practitioner should perform a thorough and complete necropsy, maintaining copious records. A tape recorder may be used to dictate findings, which can later be transcribed into a written report. Initial examination should include photographs of the body and any external abnormalities. Foreign material on the haircoat or in the oral cavity should be swabbed and saved for possible future analysis. Once the external examination is complete, a thorough examination of internal organs should be performed. It is important that the prosector not jump to conclusions or make assumptions during the necropsy. Each organ system should be thoroughly examined, photographs taken, and samples collected methodically. Epidermis and subcutaneous tissues around any identified injection sites should be excised and saved. For exclusionary purposes, samples may be submitted for non-toxicology related tests (e.g. Microbiology) based on lesions found at necropsy. Stomach contents should be examined closely for evidence of foreign objects such as granules and pellets, plant material, foods, pill casings, tablet fragments, or illicit drugs. Samples of all major organs should be taken and preserved in fixative for histopathologic examination; for large organs such as liver, sections should be taken from multiple areas rather than a single site.

When investigating potential malicious poisonings, sample collection is essential in order for confirmatory tests to be performed. When collecting samples, it is always best to err on the side of taking too many samples, since one can always throw away unneeded samples but it is not possible to resurrect material that has been discarded. Live animal samples include stomach contents from lavage and/or vomitus, urine, feces, whole blood, serum or plasma, and hair. Inspection of the patient's oral cavity for agents lodged in the teeth or trapped in mucosal folds; these items should be swabbed away and saved for future analysis. Because some analytes may be damaged by contact with red blood cells, it is best to collect both whole blood and serum or plasma. In general, glass containers are preferred over plastic because plastic can leach contaminants into samples over time; if plastic containers must be used, harder plastics pose less risk of sample contamination. Never store samples in syringes, as leakage may occur and syringes with needles are hazardous to receiving personnel. Serum and plasma may be frozen prior to shipping, while whole blood should be refrigerated, never frozen. Urine may be either refrigerated or frozen. Vomitus, gastric contents and feces should be stored in glass or hard plastic and may be frozen prior to shipping. Hair samples are most useful for topical exposures and they may be stored in hard plastic or glass vials. For necropsy specimens, liver, kidney, urine, stomach/intestinal contents, and feces should be saved for toxicologic analysis. If exposure to anticholinesterase agents (organophosphates or carbamates) is suspected, samples of brain and retina (submit entire eyeball) may be evaluated for cholinesterase activity.

It is important to remember that there is no one "toxicology screen" that will detect all known toxic agents, and testing at random can prove to be expensive and futile. Determination of which toxicant to look for in a chemical analysis is based on the clinical, historical and environmental findings in the case which hopefully will provide the clinician with a list of potential rule outs to consider. Some laboratories will offer specific screening tests based on clinical findings. For instance, Michigan State University's diagnostic laboratory offers a convulsant screen which will analyze for toxicants frequently associated with seizures or convulsions including bromethalin, metaldehyde, organophosphate insecticides, carbamates, strychnine and tremorgenic mycotoxins. In general, however, one needs to have an idea of what toxicant is suspected in order for the appropriate analysis to be requested.

An important aspect in the interpretation of toxicology results is to realize that exposure to a potential toxicant does not necessarily indicate that a toxicosis has occurred. With all toxic agents, a threshold below which signs will not develop exists (i.e. "The dose makes the poison"). This is especially important to remember as our ability to analyze and detect agents in samples improves, and our ability to measure the presence of agents at minute levels means we will at times be detecting the presence of agents at levels consistent with casual exposure but not toxicosis. Close attention should be paid to the normal background levels indicated by the testing laboratory in order that accurate interpretation of analytical results occurs.

Finally, once a diagnosis of toxicosis has been confirmed through evaluation of clinical findings and laboratory results, the determination of malicious intent can still be difficult to establish. This is especially true in cases of malicious animal poisonings, as the victims cannot testify that they saw the perpetrator expose them to the poison. Ancillary trace evidence should be collected and retained, including any potentially poisoned foods materials (e.g. cans of tuna or pet food mixed with rodenticides or ethylene glycol) along with their containers. The food materials can be analyzed for the presence of the toxic agent, and fingerprints or trace evidence on the containers may provide sufficient evidence of that the suspected perpetrator was in possession of the tainted material at some time.