Did you know ticks can transmit bacteria to horses that cause Anaplasmosis (formerly known as Ehrlichiosis) and Lyme disease?
Anaplasma bacteria damage blood vessels and cause vasculitis (vascular inflammation), which causes fluid to leak out of blood vessels under the skin and cause the horse's legs to "stock up" (become swollen, like in the photo below) so badly that the horse might not want to move. The horse will also usually have a fever (temperature over 101.5), and may have a poor appetite or act lethargic as a result.
In most cases the tick causing the disease has already fallen off, but if you find a tick you should remove it from your horse. Horses with a healthy immune system will usually improve with supportive care and anti-inflammatory medication in a few days, but more severe cases with swelling of the chest/abdomen, purple/yellow gums, or neurologic signs may require specific antibiotic treatment to cure the disease. Anaplasmosis is definitively diagnosed with a PCR blood test (which can take a few days for results) or by looking at a white blood cell smear under the microscope (this is less likely to give a positive result than PCR), but can be suspected if the horse's white and red cell counts and/or platelets are low. Horses that recover can be immune to re-infection up to 20 months.
Tick prevention is key: Check your horse daily when bringing in from pasture (especially under the tail, underside of the neck/chin/throatlatch, and in the armpits and groin regions) and remove ticks promptly. Apply fly spray containing permethrin regularly, and/or use a spot-on product that is more sweat-resistant for longer-term tick control. Keep the grass/weeds in the pasture short, remove dead leaves, and don't ride through tall grass on trails.