Disease Du Jour: Lab Tests for Horses

This podcast can help veterinarians in the field better understand which samples to send to the lab to help diagnose a sick horse.
vet taking blood sample from horse
Knowing what samples to take, how to store them properly and how quickly they need to get to a lab is critical in learning more about a horse’s issue. Amy Dragoo

As a veterinarian in the field or in the hospital, what tests should you run on a specific equine case? The list can be short or long, depending on what you are trying to find out.

In Episode 89 of the Disease Du Jour podcast, we talk to Toby Pinn-Woodcock, DVM, DACVIM (Large Animal), who is an Assistant Clinical Professor in the Department of Population Medical and Diagnostic Sciences at Cornell University’s College of Veterinary Medicine. She received her DVM from the University of Wisconsin and did her internal medicine residency at Cornell.

Pinn-Woodcock will help us understand from the lab side how they can help veterinarians in the field better use a lab vet’s expertise and know which tests might be helpful in individual cases.

She said that it helps the lab vet when the submitter provides pertinent history, including what was found on the physical exam, what other tests have been done, if any, and the attending vet’s list of differential diagnoses.

“Lots of labs have consulting veterinarians,” said Pinn-Woodcock. “At Cornell we have three on call.”

She said with better input of what the veterinarian is facing in the field, the lab vet can offer guidance on what tests and samples are recommended.

“Sample handling is critical,” said Pinn-Woodcock. “Some samples, such as anaerobic culture, need to be kept at room temperature, other samples must arrive frozen, while others  should be chilled but never frozen. Most labs have a test and fee list that gives specimen handling recommendations.”

One issue that can make a difference in a conclusive test is a delay from collection until the sample gets to the lab, she noted.

“With some tests, it’s okay for the specimen to sit in the fridge,” she said. “With others, how the specimen is handled after collection and the time it takes to get the lab will make a difference.”

Lab Test Consulting

Pinn-Woodcock said some of the most difficult diagnoses for equine veterinarians in the field are fever of unknown origin, “ain’t doin’ right”, off feed, seems dull or lethargic.

“There isn’t a leading clinical sign—it’s not neurologic, it’s not colic, it’s not obviously respiratory,” said Pinn-Woodcock. “That means it could be any of a number of things, including GI or respiratory. And you need to considera variety of differentials, from bacterial to parasitic or viral.”

She said astute owners notice when a horse is “off” before there is a leading clinical sign, so it’s hard for the attending veterinarian to “key in” to a body system.

In non-specific cases like these, Pinn-Woodcock recommends taking a fresh manure sample and keeping it refrigerated, a nasal swab or nasopharyngeal wash, a purple-top tube blood sample and a red-top tube serum sample.

For swabs, she recommends taking two and placing one in a red top tube with a small amount of sterile saline (0.5ml, just enough to keep the swab moist) for PCR and the other in aerobic transport media for bacterial culture.

Pinn-Woodcock noted to veterinarians that she is hearing of more equine respiratory cases involving EHV-2 in horses, often in conjunction with other viral causes of equine respiratory disease.

Diarrhea

Pinn-Woodcock said signalment will lead you down different paths when trying to diagnose the cause of diarrhea in horses.

Equine diarrhea can be secondary to infectious and non-infectious causes. She recommends doing a float test to look for sand, checking for parasites and appropriate diagnostics to rule out infectious diseases such as Salmonella, Clostridium difficile and perfringens, Coronavirus and Potomac horse fever.

“With yearlings and weanlings, add in Lawsonia,” noted Pinn-Woodcock.

Rotavirus A in foals is also a potential cause of diarrhea, while recently cases of rotavirus type B have been reported, particularly in Kentucky.

“There might be toxic plants to consider in different parts of the country as well,” she added.

She stressed the need for a fresh fecal sample that has been refrigerated. She also recommended taking a swab of the feces and sending it in aerobic and anaerobic bacterial transport media.

If you are considering a clostridial issue, “take a fresh fecal sample and freeze it quickly if you are doing clostridial toxin testing,” said Pinn-Woodcock.

She said different labs have different tests available, so check with your lab to get recommendations.

Other Considerations

Pinn-Woodcock said there can be regional and seasonal variances in what a veterinarian should test for.

The Cornell Animal Health Diagnostic Center (AHDC) has a website that can help attending veterinarians determine what test is needed and the costs. It also contains contact information and forms and submission guidelines. The AHDC also offers equine diagnostic plans, which can aid as a brainstorming tool when developing a differential list for various disease presentations (https://www.vet.cornell.edu/animal-health-diagnostic-center/testing/diagnostic-plans-and-panels). 

Pinn-Woodcock also recommended that veterinarians make unstained, air-dried slides of samples (blood or fluids) to preserve cell morphology in case of shipping delays. “That way if a package gets lost on the way to the lab, there is a path to still look at that sample,” she said. She recommended using slide protectors to avoid damage if you have to ship the slide.

“I would recommend vets consult their submitting lab specifically for guidance,” said Pinn-Woodcock.

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