The 5-year-old leisure horse developed meningitis—an infection of the meninges, the membranes that surround the brain and spinal cord—within a day of undergoing surgery to remove an upper molar with a tooth root abscess and sinus surgery to debride and flush her infected sinuses. With rapid diagnosis and treatment, the mare defied the odds and survived, said Lindsey Boone, DVM, PhD, Dipl. ACVS, an associate professor of equine surgery and sports medicine at Auburn University’s College of Veterinary Medicine, in Alabama.

First an Abscess, Then a Sinus Infection …

Left untreated, tooth root abscesses can lead to sinusitis, Boone said. The mare in this study had been diagnosed with a tooth root abscess in an upper cheek tooth (first molar), and the treating veterinarian recommended extraction. However, due to several mitigating circumstances, the extraction was delayed for a year.

“It is not uncommon for treatment of dental disease to be delayed for various reasons,” she explained.

During the yearlong wait, the mare developed an intermittent foul-odor discharge from the right nostril and was reported to shake her head occasionally. Otherwise, she appeared healthy.

… Then Meningitis

Because the mare’s case had become complicated with the chronic sinus infection, she was sent to the referral hospital at Auburn University for surgery. There, Boone and her colleagues performed diagnostics that confirmed an infected upper cheek tooth and right sinus infection, but also identified a sinus infection with suspected infected upper cheek tooth on the left side. They removed the affected upper right molar using a peripheral (maxillary). Following extraction, they inserted catheters into the left and right sinuses for lavage (flushing).

Approximately 24 hours later, the mare developed an increased heart rate and lost her appetite, Boone said. Veterinarians postponed removing the affected left sided upper cheek tooth until her clinical status improved.

Two days later, the mare developed fever, and over the course of the next few days she became stiff in the neck. Her fever increased to nearly 105 degrees F despite treatment with anti-inflammatories and antibiotics, she said. Because the fever and neck stiffness made Boone suspect meningitis, she and colleagues performed a spinal tap on the horse; the results of the cerebrospinal fluid analysis confirmed her suspicions.

They treated the mare “aggressively,” she said, with broad-spectrum antibiotics that can pass the blood-brain barrier—a protective membrane around the brain that blocks many kinds of medications. They prevented further inflammation of the meninges by treating the mare with corticosteroids and anti-inflammatory medication.

Beating the Odds Against a Fatal Disease

The mare’s veterinarians knew her chances of survival were not favorable, Boone said. Previous studies of meningitis in horses following dental or sinus surgery have painted a grim picture: All the reported cases died or were euthanized.

Yet, in this case, the mare pulled through, she said. Her fever dropped within 12 hours of targeted therapy. Within a week, she had fully recovered to the point the veterinarians could extract the affected upper molar as planned.

“To our knowledge, this is the only report in the literature of survival of a horse with an acquired bacterial meningitis after cheek tooth extraction and sinus surgery,” Boone said. “(We) feel that rapid recognition of clinical signs, diagnosis, and treatment aided survival.”

Other factors related to the kind and amount of bacteria that invaded the meninges as well as the mare’s overall health status could have resulted in a milder case of meningitis than previously reported cases.

“This horse did not have any neurologic deficits on neurologic examination, other than a dull mentation, and it may be inferred that this was a mild case of bacterial meningitis,” she said. “But without appropriate treatment, her clinical signs would have rapidly progressed.”

Meningitis After Dental/Sinus Surgery

It’s not clear why some horses develop bacterial meningitis after dental and/or sinus surgery, Boone said. In complicated cases like this one, bacteria from the mouth and sinuses find a path into the meninges either through disruption to the bony plate within the sinus that shields the meninges or via the bloodstream or the cranial nerves, she explained.

“It is important to recognize that complications can and do occur with treatment of complicated dental and sinus disease,” Boone told The Horse. “While meningitis is rare, veterinarians should be aware of presenting clinical features associated with meningitis such as post-extraction fever, tachycardia (rapid heart beat), neck pain, and changes in neurologic status. The risk of meningitis is extremely low. But if it does occur, it is often fatal for the horse.”

The Bottom Line: Dental Care Shouldn’t Wait

A good way to reduce risks is to manage dental disease quickly, without waiting for it to become more advanced and complex, said Boone.

“What is most important is for dental disease to be recognized early and be treated appropriately for resolution,” she said. “This starts with the owner.

“It is imperative that owners recognize that veterinarians should be the professionals that are evaluating their horse’s health and providing preventive health services,” she continued. “Maintenance of dental health is vital to your horse’s overall health. This is why annual to semiannual dental examination is recommended and should be performed by your veterinarian—and only your veterinarian.”

Owners should understand delaying the cost of dental care could end up making the situation not only riskier but also much more expensive, she added.

“Resolution of chronic dental and sinus disease can be challenging, and sometimes it is best to make the initial investment, if possible, for a rapid and aggressive resolution,” said Boone. “In my opinion, often times the ‘cheaper’ treatment just prolongs the disease process because it ‘covers it up’ rather than addressing the issue at hand, and it makes things more complicated and expensive in the long run.”

Meningitis After Tooth Extraction and Sinus Lavage in a Horse was published in the Journal of Equine Veterinary Science in 2021.