Primary photosensitization in horses associated with ingestion of Malachra fasciata Jacq. (Malvaceae) in the Northeast of Brazil.

Overview

• This study reports the first confirmed case of primary photosensitization in horses caused by the plant Malachra fasciata in Northeast Brazil.
• The condition was observed in horses grazing on pastures heavily infested with M. fasciata, leading to skin lesions on sun-exposed areas.

Background

• Malachra fasciata Jacq. (Malvaceae): An invasive plant species common in Northeastern Brazil.
• Previously documented to cause severe dermatitis and photosensitization in sheep during experimental studies.
• No prior documented cases of toxicity or photosensitization in horses attributed to this plant.

Case Description

• Subject: Four Quarter Horses grazing in a pasture heavily infested with M. fasciata in Paraíba, Brazil.
• Clinical Signs:
  • Alopecia (hair loss) on sun-exposed skin areas.
  • Erythema (redness), crusting, and ulceration of the skin.
  • Severe pruritus (itching) and behavioral distress likely due to discomfort.

Diagnostic Findings

• Histopathology of skin lesions revealed:
  • Epidermal ulceration covered with serocellular crusts (consisting of serum and cellular debris).
  • Marked hyperkeratosis (thickening of the stratum corneum of the skin).
  • Necrosis of keratinocytes (skin cells), indicating cell death due to damage.
  • Spongiosis (intercellular edema in the epidermis).
  • Dermal infiltration predominantly composed of eosinophils, cells commonly associated with allergic or toxic reactions.

Treatment and Outcome

• Removal of horses from the M. fasciata-infested pasture resulted in clinical improvement.
• Animals housed in shaded environments to minimize further sun exposure, facilitating healing.
• Relapse of lesions upon re-exposure to the infested pastures, confirming causation.

Significance

• This is the first documented and confirmed case of primary photosensitization in horses caused by Malachra fasciata.
• Findings alert equine owners and veterinarians to the risks posed by this invasive plant, especially in endemic regions.
• Highlights the need for careful pasture management to avoid outbreaks of photosensitization, which can lead to animal distress and economic loss.
• Encourages further research on plant toxicology affecting horses in tropical and subtropical environments.