Differential Diagnosis for the Patient's Condition

The patient's symptoms, which started after petting a stray cat and include a small red itchy patch near her right eye that has continued to spread, suggest several potential diagnoses. These can be categorized as follows:

• Single Most Likely Diagnosis

  • Cat scratch disease (Bartonellosis): This is the most likely diagnosis given the history of contact with a stray cat and the development of a red, itchy patch near the site of the scratch. Cat scratch disease is caused by the bacterium Bartonella henselae, which is commonly found in cats.

• Other Likely Diagnoses

  • Allergic contact dermatitis: This could be a reaction to substances on the cat's fur or other allergens the patient came into contact with. The itchy, red patch is consistent with an allergic reaction.
  • Flea bites or other insect bites: If the cat had fleas, the patient could have been bitten, leading to itchy, red lesions.
  • Ringworm (Dermatophytosis): Although less common, ringworm can be transmitted from cats to humans, causing circular, itchy, red patches.

• Do Not Miss Diagnoses

  • Rabies: Although extremely rare and unlikely if the cat seemed healthy, rabies is almost always fatal if not treated promptly after exposure. Any bite or scratch from a potentially rabid animal must be taken seriously.
  • Tularemia: This bacterial infection can be transmitted through contact with infected animals, including cats. It causes skin ulcers, fever, and can be severe if not treated.
  • Plague: Another rare but potentially deadly disease that can be transmitted through the bites of infected fleas, which could be present on cats.

• Rare Diagnoses

  • Erysipeloid: A bacterial infection that can be transmitted from animals to humans, typically through scratches or bites, causing red, itchy lesions.
  • Leptospirosis: Although more commonly associated with water exposure, this bacterial infection can be transmitted through contact with infected animal urine, potentially including cats.
  • Sporotrichosis: A fungal infection that can enter the body through cuts or scratches in the skin, potentially from contact with contaminated soil or organic matter on the cat's fur.