Parasite Burdens in Horses with Severe Asthma

Mirroring a phenomenon observed in humans, equine parasitologists recently reported that horses with severe equine asthma (SEA) shed fewer internal parasite eggs compared to their healthy herdmates. The mechanisms underlying this link remain unclear, leaving researchers uncertain as to whether internal parasites hinder asthma from developing or if asthmatic animals are more resistant to intestinal parasites.

The most common intestinal parasite of adult horses is small strongyles, also known as cyathostomins. Horses become infected by ingesting infective larvae that hatched from eggs passed in feces from affected horses. Those larvae then burrow into the lining of the intestinal wall, forming cysts. Within these cysts, larvae are protected from the horse’s immune system as well as deworming products (anthelmintics). After emerging from the cyst, they develop into adult worms. Female worms produce eggs that pass in the horse’s feces, perpetuating the cycle.

“The inverse relationship between internal parasites and severe asthma in humans has been known for the past two decades, and some small studies in horses reported a similar relationship,” said Ashley Fowler, Ph.D., a nutritionist for Kentucky Equine Research.

Using a larger number of horses than in previous studies, Simões and colleagues from Portugal collected fecal samples from 20 healthy horses and 20 horses diagnosed with SEA.^* All horses were unrelated, client-owned, and managed under similar conditions. Owners of horses included in the SEA group reported coughing, nasal discharge, and respiratory effort while at rest. All horses underwent testing to either diagnose SEA or confirm their inclusion in the healthy control group. Specifically, all horses had X-rays of the chest performed, followed by an endoscopic examination to evaluate tracheal mucous. A bronchoalveolar lavage was also performed to analyze the fluid for inflammatory cells consistent with SEA.

All horses were dewormed two to three months before enrolling in this study.

Fecal samples were collected from all horses on the same day and analyzed using standard fecal testing protocols. The number of small strongyle eggs per gram of feces (EPG) was determined for all horses. Further, the number of infective larvae were calculated per gram of feces (LPG).

“This study found that 24 of the 40 horses (60%) had small strongyle eggs in their feces. The average EPG was significantly higher in the healthy horses than the SEA group. Similarly, 60% of the horses had infective larvae in their feces. Again, the LPG was significantly higher in the healthy horses than SEA horses,” Fowler said.

The two most dominant explanations for the observed decreased eggs and infective larvae in the feces of SEA horses included the following:

1. Hygiene hypothesis. “This theory suggests that due to improved population hygiene, the prevalence of chronic diseases such as parasitism has decreased. With chronic parasite infections, the parasites have the ability to suppress the immune system, decreasing the occurrence of allergies,” explained Fowler.
2. Genetic hypothesis.  “Scientists in favor of this theory suggest that genes of patients with allergies grant them an advantage to being resistant to parasitic infections,” Fowler said.

Regardless of the exact reasons for the decreased EPG and LPG in the SEA group, those horses still had parasites, so deworming based on fecal egg count testing under the guidance of a veterinarian is recommended. This contrasts with outdated methods of deworming, such as rotational deworming programs based on the calendar or season.

“Your veterinarian can also provide information on important management changes to best help your SEA horse breathe easier. Key management strategies focus on decreasing respirable dust particles, including housing outdoors and offering only steamed or soaked hay. Further, experts recommend offering the long-chain polyunsaturated omega-3 fatty acid DHA, which is found in EO-3,” Fowler said.

^*Simões, J., J Paulo Sales Luís, L. Madeira de Carvalho, and P. Tilley. 2023. Severely asthmatic horses residing in a mediterranean climate shed a significantly lower number of parasite eggs compared to healthy farm mates. Animals (Basel) Sep 15;13(18):2928.