Comprehensive Study Guide for Diagnostic Veterinary Medicine and Veterinary Diagnostic Medicine

Fundamentals of Diagnostic Veterinary Medicine

• Diagnostic medicine is primarily composed of laboratory work, though it can also incorporate imaging and other diagnostic modalities.

• The central purpose of diagnostic medicine is to provide a framework so a clinician can confirm or diagnose a case with greater accuracy.

• It serves as a tool to help the veterinarian make the most accurate diagnosis possible, ultimately facilitating the treatment and health of the patient.

• Core concerns within this field include:

  • Sampling for toxins versus infectious agents.

  • Overcoming sampling barriers depending on the diagnostic goal.

  • Considering public health concerns.

  • Navigating medical-legal issues encountered in clinical practice.

The Diagnostic Process and Clinical Assessment

• When assessing a patient, clinicians must look for the most obvious signs first and work backward to determine the etiology.

• Clinical signs and findings include pale mucus membranes, which are a hallmark of anemia.

• A specific etiology for anemia includes parasites.

• Physical exams are conducted on both sick and healthy animals. Even when not actively treating a patient, a veterinarian must make assessments regarding body condition, clinical signs, and general health (e.g., weight loss, bad teeth, or debilitating disease).

• Types of Diagnosis:

  • Clinical Diagnosis: Based on the evaluation of the animal's physical presentation and vital signs.

  • Differential Diagnosis: The process of listing various potential causes ($A$, $B$, $C$, or $D$) and using diagnostic medicine to narrow down the list.

  • Etiological Diagnosis: Identifying the specific disease or condition causing the illness.

  • Confirmatory Diagnosis: The final conclusion or overall diagnosis reached after all evidence is gathered.

  • Morphological Diagnosis: A diagnosis based on biopsy or necropsy, typically performed by pathologists.

Diagnostic Laboratory Infrastructure

• Veterinary diagnostic labs are often state-run and provide critical support to clinicians.

• Location Examples:

  • Alabama: The state diagnostic lab belongs to the state of Alabama and is situated on the Auburn University campus.

  • Texas: There are approximately three diagnostic labs in the state.

  • Specialized Labs: Some facilities are located near major industries, such as the poultry diagnostic lab in Hanceville, Alabama, which serves the local poultry houses and slaughter plants.

• Accreditation and Regulation:

  • The American Association of Veterinary Laboratory Diagnosticians ($AAVLD$) is the organization that accredits state labs.

  • They run programs where tests are compared for accuracy.

  • The $AAVLD$ holds annual meetings to discuss cases common to specific parts of the country or certain species, as well as new testing methodologies.

• Laboratory Experts: These facilities are staffed by toxicologists, pathologists, and other specialists. Owners and clinical veterinarians collaborate with these experts by submitting animals or samples for analysis.

Toxicology and Specialized Analysis

• Diagnostic medicine is practiced daily, even if samples are sent to outside labs.

• Types of Toxicological Analysis:

  • Organic Analysis: Uses state-of-the-art procedures such as High-Performance Liquid Chromatography ($HPLC$) and Gas Chromatography-Mass Spectrometry ($GC-MS$). These tests look for drugs, pesticides, rodenticides, and mycotoxins.

  • Inorganic Analysis: Focuses on heavy metals and minerals. For example, feedlots may use these tests to determine the $selenium$ or $copper$ levels in a herd to improve healthcare rather than just diagnosing death.

• Microbiology and Other Labs: Diagnostic labs also handle bacteriology, parasitology, and pathology (biopsies and necropsies). While many places lack a dedicated virology lab, federal labs are often used for viral diagnostic tests.

Sample Integrity and Submission Guidelines

• Veterinarians are responsible for ensuring sample integrity because they are trusted to take and handle samples properly.

• General Sampling Rules:

  • Samples from live animals (such as blood) should be taken by a veterinarian or technician, centrifuged if necessary, and stored cool ($4^{\circ}C$) until they reach the lab.

  • The concentration of toxic chemicals is the strongest evidence of poisoning, but detection depends on equipment being able to identify residues above specific thresholds.

  • Samples must be taken immediately before treatment because chemicals can be metabolized, inactivated, or excreted quickly.

• Specific Sampling Matrices:

  • Vomitus: Best taken from the stomach or at the time of vomiting.

  • Ocular Fluid: Used for diagnosing $nitrate$ poisoning.

  • Skin or Hair: Used for $selenium$ or $arsenic$ poisoning.

  • Fat: Used for lipid-soluble pesticides where toxics accumulate.

  • GI Tract/Feces: Secondary sources if the toxin has already passed the stomach.

Case Studies in Toxicosis

• Mycotoxins: The mere presence of a mycotoxin does not confirm a disease. Diagnosis requires confirming exposure, documenting clinical manifestations, and performing tissue analysis to determine if levels are high enough to cause adverse effects.

• Blue-Green Algae ($Cyanobacteria$):

  • Associated with stagnant water that appears "funky" and colored blue, green, or red.

  • Safety Warning: Practitioners must wear protection as they can be exposed to toxins during sampling.

  • Preservation: Samples should be capped and kept cool. Uniquely, blue-green algae is the only time it is recommended to put tissues in $formalin$ for toxic analysis to preserve the chemical structure and freeze it. In all other toxicological cases, $formalin$ should be avoided as it disintegrates chemical structures.

• Blister Beetles ($Cantharidin$):

  • Caused by the toxin $cantharidin$, found especially in male blister beetles.

  • These beetles often contaminate alfalfa hay. As few as $15$ beetles can kill a $1,500\,lb$ horse.

  • Clinical signs include lesions in the oral cavity and the gastrointestinal tract.

  • The best location for analysis is a urine sample.

  • Historical Note: $Cantharidin$ was historically used as an aphrodisiac because it causes clitoral irritation.

• Strychnine:

  • Often used maliciously in baits (meat) to kill roaming dogs.

  • Strychnine is frequently identified as small green pellets or materials laced into food.

• Lightning Strike: A differential diagnosis for animals found dead in good health near water or specific landmarks.

Necropsy and Pathological Procedures

• Entire carcasses (e.g., horses or dogs) can be submitted for necropsy.

• Clinicians should look for lesions and even "far out" findings that might not be related to the cause of death but should be noted.

• Toxicology Preservation: Samples for toxicology should be frozen. Do not fix them in $formalin$.

• Pathology Preservation: For infectious disease or biopsy, tissues should be cut into thin, small sizes and fixed in $10\%$ buffered $formalin$. The volume of $formalin$ must be $10$ times the volume of the tissue.

Medical-Legal Issues and Forensics

• Veterinary Forensics: An emerging field investigating cases of animal poisoning and malicious intent. Experts like Ernie Rogers provide scenarios and evidence on how to prove a condition occurred due to poisoning.

• Expert Witnesses: Veterinarians and toxicologists often serve as expert witnesses in lawsuits. Maintaining sample integrity and documentation is essential for keeping the lab and the veterinarian on the right side of the law.

• Documentation Requirements:

  • Chronology of illness and death.

  • Total number of animals dead versus sick.

  • Environmental conditions and time of year.

  • Precise location of the animal and vaccination/deworming records.

• Estimating Time of Death:

  • Death typically occurs at least $4$ to $6$ hours after the last meal if food is still in the gastrointestinal tract.

  • If the small intestine is empty, death likely occurred $12\text{ hours}$ or more after the last meal.