Ascarid parasites (Parascaris species) can become so numerous in the small intestines of young horses that they lead to serious or even fatal outcomes. These ascarids usually infect foals 0 to 8 months of age but also can affect yearlings.
The most significant concern with heavy infestations of these roundworms is impaction or rupture of the small intestine. In these cases there is a guarded to poor prognosis for foal survival. Sixty percent of hospitalized cases survive until dismissal, while only 21 percent survive beyond a year after the incident. Signs of pain always accompany impaction, and lack of response to medical treatment frequently requires surgical intervention.
Though egg counts are useful for a qualitative diagnosis of infestation, no good diagnostic techniques are currently available to estimate actual ascarid burdens in foals. This leads to the need for an alternative technique. One strategy that does not work? Ongoing, broad-spectrum deworming in an attempt to reach zero-parasite status.
“A lot of ascarids are resistant to ivermectin, the most widely used anthelmintic for the past three decades to combat parasitic infection,” says Martin K. Nielsen, DVM, PhD, DEVPC, DACVM, of the University of Gluck Equine Research Center in Lexington, Kentucky. “The other two anthelmintic classes are also starting to lose efficacy against ascarids. Because we don’t have any new anthelmintics on the horizon, we don’t expect another product to reach the market soon to ‘save us.’ We are up against a wall.”
Nielsen says veterinarians need to be more selective when it comes to the dewormers used to treat infestations. “We can’t just rotate drugs and eliminate the resistance problem as once thought,” he says. “We need to know what we are deworming, what to deworm with and at what point in time to do it.”
An important part of the process is educating clients that some level of parasite burden is normal, Nielsen says. “We’ve tried to pursue a goal of keeping our horses and foals parasite-free, but we have to realize this is not an attainable goal,” Nielsen states. “If we continue to try to reach that goal, we are going to get drug resistance instead.”
However, he continues, veterinarians and horse owners do need to continue to treat ascarids in most foals. “For the first six to eight months of their lives foals are accumulating a lot of ascarid parasites in their system, until their own immune system starts to kick them out,” Nielsen says. “This early period is very critical—it’s when we see impactions, stunted growth and ill thrift as a consequence of heavy ascarid burdens.”
The challenge is determining whether an individual foal has accumulated a high enough burden that it’s at risk, Nielsen says. “Impaction can also be considered an adverse reaction to anthelmintic treatment, since foals accumulate numerous dead parasites that block their intestine and ‘clog the system,’ especially at the junction of the ileum and cecum,” he says. “Normally the worms live in the proximal half of the small intestine, but when they’re treated with a dewormer that works, they tend to accumulate in the ileum, right before they would make it into the cecum. That seems to be the ‘bottleneck’ in the system … where we see a lot of the impactions—even intussusceptions of the ileum—and ruptures.”
Basically, all foals 4 to 5 months of age can be expected to have ascarid parasites, Nielsen says. “There is little need to run tests to determine whether the parasites are there,” he says. “We know they are in the foals’ environment.” The real question is to determine which foals have a larger burden. Egg counts do not provide this information. That’s where transabdominal ultrasound comes in.
How it all started
The quest for an alternative diagnostic technique began with a question from an equine practitioner in Lexington, Kentucky. “I understand that egg counts don’t really tell us how many worms a horse has,” she said to Nielsen.
“Correct,” Nielsen agreed. “We don’t have a direct correlation between egg count [i.e., the number of eggs per gram of feces] and how many worms a foal or horse might have. It would be an advantage to have a way to detect a higher burden of ascarid parasites in the small intestine of the foal before we deworm it.”
“Well, we’re currently using ultrasound to diagnose Rhodococcus equi lung infections,” the practitioner continued. “Would it be useful to take the ultrasound probe and move it a little bit further back on the animal to look at intestinal parasite infection within the abdominal cavity?”
“I don’t know,” Nielsen replied, “though I do know from clinical workup on individual clinical cases that worms are pretty easy to see since they’re so echogenic.”
“I’m wondering if transabdominal ultrasound would be a useful surveillance tool for detecting ascarid parasites,” the veterinarian concluded. She wanted to use the technique as a screening tool for 50 foals on a farm in her practice.
“I initially thought it sounded unrealistic to ultrasound 50 foals for worms,” Nielsen says. But the practitioner reminded him that major breeding operations with valuable foals are already having veterinarians do ultrasounds for Rhodococcus species. It wouldn’t be a stretch for them to go ahead and perform ultrasound for ascarids at the same time, she said.
That was the premise of Nielsen’s study.1 “We got a grant from Zoetis,” he says. “They were looking to fund something that would have an economic aspect to it, be beneficial to the industry, be useful to veterinarians in practice, and lead to a technique of significant value. That’s why we included cost-benefit calculations in the study. With the grant, we looked at transabdominal ultrasonography to detect ascarid worms in young foals 5 to 8 months of age.”
The first portion of the the study was to develop a technique and a scoring system, validating true worm counts in foals the group already had in a terminal study anyway. In the second portion of the study, researchers evaluated the technique with groups of foals treated with anthelmintics and an untreated control group. During that portion they used the ultrasound for five days in succession. “Clara Fenger, DVM, PhD, DACVIM, a clinician on the study, was an excellent collaborator,” Nielsen says. “She had a PhD in parasitology and was board-certified in internal medicine, but most importantly, she had excellent rapport and internal communication skills.”
The technique did not require sedation or clipping of the foal’s coat. Therefore the researchers realized they could assess foals in minutes to determine the prevalence and concentration of ascarids in relation to concerns surrounding GI impaction.
Nielsen and his colleagues used a portable ultrasound device (Honda HS 2100V) with a convex probe. “Frequency was typically set at 2.8 MHz, depth at 12 cm and focal point at 10 cm, although some individual examinations varied in order to achieve optimal visualization of the small intestine,” Nielsen says.
The scan, which takes place along the foal’s ventral midline, is situated “immediately caudal to the xiphoid and continued caudally, ending immediately cranial to the umbilicus.”1 In order to get optimal images, the clinician situates the probe transversely and longitudinally along the ventral midline or paramedian until a proper image is obtained.1
In the study, three examination regions were scored: immediately caudal to the xiphoid, midway between the xiphoid and umbilicus and immediately cranial to the umbilicus. If loops of small intestine could not be obtained from these regions, a fourth region, the left parainguinal area, was examined. Once researchers established a clear ultrasonographic window to visualize the small intestine, they evaluated the bowel loops for the presence of mature ascarids.1
Ascarids were identified in the intestines by parallel hyperechoic lines or circular structures present intraluminally. Nielsen and his colleagues found that in transverse views, the cuticle of the ascarid showed a “doughnut” appearance: a white circle with a darker center. In longitudinal views, the worms had a “train-track” appearance with double parallel lines. These were much easier to identify than the doughnuts.
The work conducted by Nielsen and his colleagues showed that transabdominal ultrasonography is a beneficial diagnostic tool to determine the burden of ascarid infection in foals, with the large majority (81 percent) of examinations generating useful images. The study also confirmed that:
- The technique can reliably identify ascarid infections of more than 10 worms.
- Ultrasound scores increase and decrease with age in untreated foals.
- Anthelmintic treatment reduced ultrasound scores post-treatment.
- The ultrasound technique becomes cost-effective in environments with high incidence rates of ascarid impactions in foals (more than 5 percent of all foals), with variations depending on the value of the foals, the cost of the ultrasonographic procedure, and the costs for medical and surgical treatment of a verminous impaction.
The technique was also shown to be valuable in comparison with fecal egg counts. At the age of 5 to 8 months, when ascarid egg counts typically decline in foals, significant numbers of ascarid worms may still be present in the small intestine (as they were in necropsied foals of this age). Researchers speculate that this may be due to immunity building up against the parasites—the first effect is a decrease in egg shedding before the worms eventually get expelled. Therefore employing ultrasound to detect the presence of ascarids was shown to be especially useful at 5 to 8 months.
The key takeaways for the study were as follows: Ascarid parasites were sufficiently large enough to recognize on the ultrasound screen. The technology is beneficial to potentially reduce the number of foals succumbing to impaction due to heavy ascarid infection. And based on the use and study of this transabdominal ultrasonographic technique, individual equine practitioners can consider using this tool to help diagnose equine parasites.
Training the technique
“After the study we trained several local veterinarians in the use of the technique,” Nielsen notes. “First we demonstrated the technique using our ultrasound system. Then the visiting practitioners, who brought their own individual ultrasound equipment, learned on that equipment to ensure their comfort and the reliability of the technique. … None of them had ever looked for worms with ultrasound and they were surprised how easily they could be localized. They were all using ultrasonography on a regular basis, not realizing its value to assess parasite infection.
“The process began as one practitioner’s question about the possibility of ultrasound, and we were able to ‘complete the circle’ by developing the technique and then offering these training sessions as a learning tool to practitioners,” Nielsen concludes. “I hope we’ve added a useful technique for equine practitioners’ assessment of ascarids in foals.”
1. Nielsen MK, Donoghue EM, Stephens ML, et al. 2016. An ultrasonographic scoring method for transabdominal monitoring of ascarid burdens in foals. Eq Vet Jour 2016:48:380-386.
Nielsen MK. Evidence-based considerations for control of Parascaris spp. infections in horses. Eq Vet Ed 2016:28:224-231.