NAVLE Nuggets: Equine Mosquito-Borne Neurologic Diseases

Let's jump into our first case.

Case number one.

It's late August.

A six-year-old quarter horse is presented after developing a fever yesterday.

Overnight, the horse became depressed and began stumbling.

This morning, the owner reports that the horse is circling, pressing its head into the stall wall, and is now having difficulty standing.

The horse lives in the southeastern United States.

What's at the top of your differential list? The answer in this case is Eastern Equine Encephalitis or EEE.

Eastern Equine Encephalitis is caused by an alpha virus transmitted by mosquitoes.

It is most common along the Atlantic and Gulf Coast.

Wild birds serve as the natural reservoir for the virus.

And mosquitoes become infected after feeding on birds and then can transmit the virus to horses and people.

So note that this is a zoonotic disease, but it's not directly transmitted from horses to humans.

It's instead transmitted from the mosquitoes that bit the birds and then bit the humans.

One important Natalie Pearl, horses and humans are considered dead end hosts, which means they can become infected, but generally do not develop high enough levels of virus in the bloodstream to continue the transmission cycle.

So why is EEE important? Among the mosquito-borne encephalitis viruses affecting horses, EEE is considered the most severe.

It progresses rapidly and carries a very high mortality rate.

So this is one of the biggest board pearls.

EEE equals highest mortality.

Let's talk about the clinical signs.

Many horses initially develop fever, depression, and anorexia.

And as the disease progresses, neurologic signs become more obvious, including ataxia, head pressing, circling, blindness, muscle weakness, difficulty swallowing, seizures, recumbency with paddling, and overall a rapid progression.

Some horses deteriorate over just 24 to 48 hours.

In order to diagnose eastern equine encephalitis, we want to pull together clinical history, including the season and high possibility of mosquito exposure, with diagnostic testing, including, most commonly, an IgM capture ELISA.

Let's talk about IgM for a second.