Infectious Diseases

Deadly Worm Crosses Placenta, Aborting Pregnancies

By Adam Marcus

In what is believed to be a first, researchers in Germany have documented 2 cases of spontaneous abortion in a mare that was infected with a parasitic worm that crossed the placenta and terminated the pregnancies.

The causal organism was a soil-dwelling species of nematode called Halicephalobus spp., which causes rare but life-threatening infections in equids and other mammals, including humans. According to the researchers, the afflicted mare experienced spontaneous abortions in 2 consecutive pregnancies after the worms managed to cross the placenta and disrupt gestation.

The mare—a Connemara pony brought to a stud farm in Germany from Ireland in 2021—was dewormed routinely 2 to 4 times per year using a regimen that included ivermectin, fenbendazole and praziquantel, in various combinations.

In 2022 and 2023, the mare birthed healthy foals. But in April 2024, a stillbirth of a mature foal occurred. The foal’s internal organs, including the kidneys and the central nervous system, showed signs of widespread nematode infection.

The mare, which appeared clinically healthy, was treated with a course of fenbendazole (50 mg/kg body weight daily for 3 days) and was mated successfully later. However, that pregnancy aborted roughly 6 months later, and a subsequent necropsy revealed similarly invasive infection with the parasitic worms.

No Other Explaination Likely
Although transplacental infection was not directly observed, Andrea Springer, DVM, a parasitologist at the University of Veterinary Medicine Hannover, in Germany, said no other route of transmission appears plausible.

“Given the fact that 2 foals of the same mare were affected in a row, other routes seem highly unlikely. The only other route that could be imagined would be venereal transmission. However, according to the owner, no other mares were affected, although the stallion was mated with many other mares,” said Dr. Springer.

Could similar cases explain other mysterious, spontaneous terminations of pregnancy?

Dr. Springer said that the question “cannot be answered based on the current state of knowledge. So far, Halicephalobus spp. infections are considered very rare,” so much so that the literature does not include a prevalence.

Still, she added, the new report could prompt equine veterinarians to perform postmortem examinations of placentas and fetuses to help illuminate the potential role of the nematode in equine reproduction. MeV

For more information:
Springer A, Krüger C, Strube C, et al. Worms and reproductive failure: First evidence of transplacental Halicephalobus transmission leading to repeated equine abortion. Curr Res Parasitol Vector Borne Dis. 2025 Aug 20:8:100309.
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