It’s called the Herxheimer effect. (Named after Karl Herxheimer, a German doctor who discovered it while developing a treatment for Syphillis before being murdered by the Nazis.)
Herx-ing happens when the antibiotics do their job and kill off lots of the targeted bacteria (In Lyme cases, this would be Borrelia burdorferi.)
But then the bug die-off has a toxic impact. The waste-eliminating organs (kidney, liver) have to work overtime to rid the body of the die-off and detoxify.
In humans, patients feel crappy, with bad headaches and flu-like symptoms. They feel even worse than they felt before being treated.
In horses, a Herxheimer reaction will be similar to humans but it can have a potentially devastating effect on the laminae.
Laminae are tiny features that secure the coffin bone to the hoof wall. When they swell or become irritated, all hell breaks loose. If these comb-like structures get really inflamed, the coffin bone will separate from the hoof wall.
No one wants a foundered horse.
Monitoring your horse as it responds to treatment is vital, Bushmich said.