Platelet activating factor mimics antigen-induced cutaneous inflammatory responses in sweet itch horses.

The study examines the role of platelet activating factor (PAF) in mimicking the symptoms of sweet itch in horses, a skin condition caused by hypersensitivity to biting flies. The researchers examined how the PAF and a Culicoides antigen affect the immune response and blood vessel permeability in affected and healthy horses, and suggest a role for PAF and histamine in the progression of the disease.

Methodology and Findings

• The researchers injected PAF and a Culicoides antigen directly into the skin of affected horses during both the active and inactive stages of the disease.
• They tracked cellular responses and changes to vascular permeability, using histamine as a positive control.
• The results showed a dose-dependent increase in blood vessel permeability in response to PAF in both affected and normal horses.
• Interestingly, they also noted that the antigen caused wheal formation (swelling of the skin) in affected horses only during the active phase of the disease. This effect was biphasic, with maximum response at 1 hour and 8 hours after administration.
• In contrast, only the highest dose of the antigen induced changes to vascular permeability in normal horses.
• Furthermore, histamine-induced wheals were significantly smaller in affected horses, both during the active and inactive phases.

Cellular Responses

• The introduction of PAF and the antigen led to the accumulation of neutrophils (a type of white blood cell) in the skin of both the affected and normal horses, across both the active and inactive phases of the disease.
• However, recruitment of eosinophils (another type of white cell) was noted only in affected horses and only during the active phase in response to PAF.
• The antigen also resulted in a higher concentration of mononuclear cells (a third type of white blood cell) in the skin of affected horses during the active phase.
• These findings show that PAF has similar effects to the Culicoides antigen during the active phase of the disease.

Conclusion and Implication

• Based on the results of the research, it is suggested that PAF and histamine may play significant roles in the progression of sweet itch in horses.
• Understanding these mechanisms could contribute to future advances in therapeutic options for sweet itch, an intensely pruritic skin condition in horses.