- refers to the evaluation of cells and tissues using a microscope.

-As a follow-up to the post-mortem exam, histology can be a valuable tool in assessing flock health. Some poultry diseases can only be diagnosed by histopathology.

- For example, the clinical presentation of infectious laryngotracheitis virus or wet pox within a flock can be virtually identical, but the diseases cause distinctly different and characteristic histopathologic changes that allow a definitive diagnosis.

 -Successful use of histopathology practice requires the availability of appropriately selected and preserved samples.

-Sample Collection Collect specimens for histopathology as soon as possible after death to avoid deterioration of tissues.

-Fresh tissue samples from birds humanely euthanized immediately prior to postmortem examination provide the best quality slides.

-If mortality must be used for tissue collection, they should be determined to be fresh as possible, and not decomposed.

NB: Do not collect samples from birds that have been previously frozen. The freeze and thaw processes can disrupt cellular features, leading to poor quality slides. An individual sample should be no larger than 1 cm3 (1x1x1 cm) to allow for adequate penetration of the tissue with fixative.

- Larger pieces of tissue will decompose in the center before adequate penetration by the fixative (formalin).

-Sampling for Specific Diseases When there is concern for a particular disease based on regional risk, a suspicious result on surveillance testing, or clinical signs in the flock, specific tissues should be collected.

-Samples preservation for histopathology:

- Submerged in a solution of 10% neutral buffered formalin for preservation.

-The volume of formalin solution in a single container should be at least 10 times the volume of all tissues.

-Samples must be fully immersed in the solution to be adequately saturated by fixative to prevent deterioration.

- Lung tissue and other air-containing tissues may be wrapped gently in absorbent cotton to aide immersion.

-Gently open the lumen of trachea and intestine samples to release trapped air.

-Sample submission, when submitting samples to a diagnostic laboratory, it is important to provide thorough and relevant flock information on the laboratory submission form.

-Critical information that should accompany all diagnostic sample submissions beside, Special shipping regulations may apply for formalin filled containers, locally appropriate biohazard labelling on all transport containers.  This information is vital to the flock veterinarian and diagnostician to make a meaningful interpretation of diagnostic results and provide recommendations to improve flock health and/or production.

-Sample processing, after arrival at the diagnostic laboratory, the formalin preserved tissues are embedded into a paraffin block, and then sectioned with a microtome into thin slices. Tissue slices of this thickness (4 micron) are thin enough to be examined by the pathologist under a light microscope. These slices are fixed on glass slides and stained. Various stains can be used to highlight different cell types, or other aspects of the tissue. The most frequently used stain for disease diagnosis is hematoxylin and eosin (H&E) stain.