A Dangerous Age: Weanlings and Yearlings at Risk

Laura Kennedy, DVM, Dipl ACVP and associate professor at the University of Kentucky Veterinary Diagnostic Laboratory, spoke at the third session of the virtual 10th annual UK Equine Research Showcase, an event focused on weanling and yearling horses. Presenting sponsors included BET, Kentucky Performance Products, McCauley’s, Merck, Rood & Riddle Equine Hospital and Tribute Equine Nutrition.

Reviewing three years of cases submitted to the UKVDL, Kennedy shared that 141 weanlings submitted between Sept. 1, 2018 and Dec. 31, 2020 and 182 yearlings submitted between Jan. 1 and Dec. 31, 2020, were evaluated. Submissions were 95% Thoroughbreds, with the vast majority from within 25 miles of the central Kentucky laboratory.

“We have a group of horses that is often very intensely managed, intensively housed – for better or for worse – and the owners have access to a lot of specialized medicine and surgery. So, our horse population is a little bit different than most,” Kennedy said.

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A look at weanlings

Beginning with weanlings, Kennedy shared the most common causes of death based on category. Of the weanlings submitted, 43% were euthanized and sent to UKVDL due to gastrointestinal disorders and 31% for musculoskeletal disorders. The remaining 26% were categorized in other, smaller groupings.

For the weanling mortalities due to gastrointestinal issues, Kennedy shared the main causes of death within that category.

“This is where we see quite a bit of differentiation from our other age groups,” she said. “In this gastrointestinal group, 24% of submissions are due to what we would consider colic for lack of a better term. Colic cases are going to be intestinal strangulations, small or large intestine volvulus, any of those other things we would generally think about as colic surgery types.”

According to Kennedy, the remaining mortalities in this category come from a variety of gastrointestinal problems, from miscellaneous bacterial diseases, congenital malformations or sequala to infections or diseases.

“These guys get turned out in the fall, the wind picks up, they’re out for the first time with all of their buddies, and they tend to either run into each other or a solid object.”

“In weanlings we also look at two infectious diseases; 25% due to disease from a bacterium called Lawsonia intracellularis and 15% from Rhodococcus equi,” she said.

Lawsonia intracellularis is an intracellular bacterium that resides in the rapidly dividing cells of the intestinal crypts, causing a disease called Equine Proliferative Enteropathy (EPE). It was first and most commonly recognized in swine but was identified within horses in 2000. Since then, there has been an increase in identification of this disease, yet the question remains if that’s due to simply being aware of the disease or if there really is an increase in cases, she said.

A disease of weanling foals, it typically occurs within 3-8 months of age, according to Kennedy, who said one hypothesis of why this disease occurs in this age group is that the stress of weaning predisposes them to EPE. Lawsonia does sporadically affect and cause disease in yearlings and adults as well.

“The typical clinical presentation for a foal with Lawsonia is very characteristic. These are going to be foals that have ill thrift, be poor doers. They’re going to be thin without a lot of muscle, be hairy and have a potbelly. They’re just not going to look like they are doing well,” she said.

Kennedy also said that these foals tend to be depressed and she hears from owners that the clinical history often includes a rapid weight decline. They can also suffer from dependent edema, which means an accumulation of excess fluid along their ventrum (belly line) and the lower parts of their limbs.

“They may or may not have diarrhea, they generally don’t have a fever, they have an increased white blood cell count and high fibrinogen. This just signals that there is inflammation. The primary feature of the Lawsonia foals is that they have a very low (serum) protein level,” Kennedy said.

According to Kennedy, the bacteria is interesting because it infects the cells down in the crypts of the lining of the small intestine. Crypt epithelial cells are rapidly dividing cells and the precursor of mature enterocytes, which are the functional cells. Infection with the bacteria stimulates crypt epithelial division and inhibits maturation, and without these mature enterocytes, the nutrients are lost.

With regard to Rhodococcus equi, Kennedy said this disease is a primary cause of pneumonia in young foals (those between 1 and 4 months old). In constrast, with weanlings, you are more likely to see an abdominal manifestation of Rhodoccoccus.

“This can cause inflammation of the colon, plus or minus the caecum, and sometimes the distal end of the small intestine, which is the ilium and also affects lymph nodes. These horses present for signs of abdominal disease, which is fever, depression, anorexia, weight loss, colic and diarrhea,” Kennedy said.

According to research, the most common clinical signs of Rhodococcus equi are massively enlarged lymph nodes with extensive adhesions between the nodes, mesentery and intestines. Once these foals develop abdominal abscesses, they become difficult to manage and treat.

“Rhodococcus is soil-borne, nearly ubiquitous and a normal component of feces. It favors very dry, dusty conditions. It’s gram-positive, and a facultative-intracellular, which means it does best while it’s inside a cell, but that’s not required,” Kennedy said.

For musculoskeletal causes of death in weanlings, Kennedy also broke down the epidemiological data into percentages. Trauma made up 65% of deaths in weanlings, followed by 19% from inflammatory responses, 12% from degenerative affects and, finally, congenital defects/gunshot wounds each made up 2% of the total number.

“These guys get turned out in the fall, the wind picks up, they’re out for the first time with all of their buddies, and they tend to either run into each other or a solid object,” Kennedy said.

Inflammatory responses included another problem linked Rhodococcus equi, sites of infection within a bone, Kennedy said. This may occur in conjunction with, following or independent of disease in other systems.

“Some of our equine cases can present in a very traumatic fashion with a vertebral abscess due to Rhodococcus equi and acute fractures,” Kennedy said.

Neurologic disease in weanlings makes up only 5-6% of the overall population. Yet of the weanlings that are presented with neurologic disease, 86% of them are due to Wobbler syndrome and the remaining 14% are due to EPM.

A look at yearlings

Causes of mortality look a little different for yearlings compared to weanlings. Yearlings are approaching maturity and causes of death start to become more similar to adult horses.

“When we look at diagnosis for yearlings, we have 34% musculoskeletal, 22% gastrointestinal and 27% present with neurologic disease, followed by a scattering of other things,” Kennedy said.

Musculoskeletal issues in yearlings present almost exactly like the weanlings. In this age group, 66% of deaths were related to trauma, 20% due to conditions that cause a major proinflammatory problem, 11% due to degenerative health effects and 3% due to congenital defects.

“The preponderance of our yearlings that are euthanized for neurologic disease have Wobbler syndrome – 82% of them. Then just a handful, 10% of them, had EPM,” said Kennedy.

Wobbler syndrome, (Cervical Stenotic Myelopathy) is a neurological disease that results from spinal cord compression due to vertebral malformation. Even though Wobblers presents with neurologic signs, it is actually a skeletal disease process. It is not a primary disease process of the nervous system. Males are more commonly affected, and it is a multifactorial disease, which makes it difficult to treat. It primarily occurs in young growing horses. Advanced imaging is helping to better characterize the disease.

Looking at yearlings and gastrointestinal disease, this is where they are more like adult horses, said Kennedy. Gastrointestinal disease in yearlings is due to inflammatory causes 68% of the time while colic causes 32%.

“With these horses, their gastrointestinal system is going to be mature. Everything is functioning the same as it would within an adult horse. They’re eating the same way an adult horse would, and their management is also often similar, so you have horses/yearlings that are kept with extensive access to pasture with limited concentrates. This tends to happen from winter through late spring/early summer for these horses. Horses that are going to a yearling sale are going to be brought in at that point and have limited access to pasture but increased concentrates, and they are going to start becoming athletic,” Kennedy said.

Takeaway message

“Young horses are silly and tend to run into one another and solid objects. It’s one of the most common things we see. Young Thoroughbreds are predisposed to the development of Wobbler syndrome, and because of the breeds highly represented in central Kentucky, we see a lot of them. In general, specific disease profiles decrease with age. Neonatal foals have a very specific set of diseases, weanlings have a specific set of diseases, more or less, but then as they mature further to become yearlings, they can have Wobblers, but that tracks out for a couple more years. As horses get older than a year or two, the specificity of disease decreases,” Kennedy said.

~ reprinted with permission