Headshaking associated with Trombicula autumnalis larval infestation in two horses.

This research discusses the problem of headshaking in horses, which can lead to difficulty or danger in riding them. It explores numerous causes of this behavior, specifically highlighting a connection between headshaking and the infestation of the horses’ skin by Trombicula autumnalis larvae in two cases studied.

Overview of Headshaking in Horses

• The paper describes headshaking as a condition in horses where they shake their heads so frequently and violently that it becomes challenging or unsafe to ride them.
• Headshaking is not a disease itself, but is a clinical sign indicating several potential health issues.
• Although many ailments may cause headshaking, it is usually difficult to diagnose the exact cause.

Potential Causes of Headshaking

• Various potential causes for headshaking have been listed such as diseases of the nervous system, skin, ear, eye, upper respiratory tract, upper alimentary tract, and musculoskeletal system.
• One theory suggested that most headshakers are partially asphyxiated and are expressing unwillingness to comply with the riders’ requests for head flexion and precise leg movements.
• A review of 100 headshaking cases found possible causes in only 11 horses, including ear mite infestation, otitis externa, cranial nerve dysfunction, cervical injury, and other conditions.
• The remaining cases presented a range of clinical signs associated with nasal irritation, sneezing, lacrimation, and nasal discharge, indicating a potential connection to allergic reactions.

Seasonal Aspects of Headshaking

• Seasonal patterns were observed in the onset of the disease and the recurrence of symptoms in subsequent years, similar to the seasonal patterns seen in humans suffering from hay fever or seasonal allergic rhinitis.
• Another form of rhinitis, known as vasomotor rhinitis, has also been observed in headshaking horses.

The Role of Trombicula autumnalis Larvae

• A very specific type of infestation by Trombicula autumnalis, also known as the ‘harvest mite’, was found on the skin around the nares of two horses.
• These horses presented with sudden-onset headshaking, suggesting a connection between this type of infestation and the headshaking behaviour.

Light-Triggered Headshaking

• There were also instances where horses were found to exhibit headshaking triggered by light, paralleling the human condition known as photic sneeze syndrome.