From Janet Tobiassen Crosby, DVM,
Your Guide to Veterinary Medicine.
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Definition: Pronunciation Key: Patho-no-monick

A bullseye rash (erythema migrans) lesion is considered to be pathognomonic for Lyme disease. What does this difficult word mean? Find out in this glossary entry.

Pathognomonic is defined as a clinical sign or lesion is so characteristic of a specific disease that one can make a diagnosis (definitive or highly suggestive diagnosis) based on simply seeing the sign or lesion.

This word is greek in origin: pathos (disease) and gnome (judgment).¹

There are few true single pathognomonic findings that will allow a doctor to definitively diagnose a disease, but sometimes a group of clinical findings and lab test results will be termed as pathognomonic for a particular disease.

Here are some examples of pathognomonic signs seen in veterinary medicine:

• Increased intraocular pressure (IOP) of the eye is often pathognomonic for glacoma.
• Negri Bodies, cellular changes seen on microscopic exam, are considered pathognomonic for Rabies, seen in approximately 70% of Rabies cases.²
• Symmetrical hair loss and enlargement of the vulva in female ferrets is considered to be pathognomonic for ferret adrenal disease.
• Some skin lesions, such as the diamond to rectangular ones seen in swine erysipelas are pathognomonic lesions.
• A "moth-eaten" microscopic appearance of bovine brain tissue is pathognomonic for Mad Cow Disease, or Bovine Spongiform Encephalopathy.

References
Word Origin¹ Wikipedia
Rabies - Hydrophobia ref.² eMedicine

Veterinary Glossary Index

Pronunciation: Patho-no-monick

Examples: When he saw the large red bullseye rash around the original tick bite area, he knew that Lyme disease was the primary concern.