The findings have helped researchers determine that free-roaming Przewalski’s horses experience notable changes in their gut microbiota after annual deworming, said Dini Hu, PhD, in the School of Ecology and Nature Conservation at Beijing Forestry University, in Beijing, China.

These observed shifts following anthelmintic treatment—which occur in all categories of the microorganisms living in the digestive system, including bacteria, viruses, eukaryotes, and archaea (single-celled organisms without a defined nucleus)—warrant further study, Hu said.

“Anthelmintic drugs may have some negative effects on horses while deworming, so they should be used with caution,” she said.

Sampling Przewalski’s Horses’ Feces Before and After Anthelmintics

Scientists reintroduced Przewalski’s horses from European zoos to the Mongolian steppes starting in 1992, long after they went extinct in their native region. As part of the health care program for these wild horses, management teams round the animals up every winter to assess their parasite loads and treat them for infestations of botfly eggs and larvae.

Hu said she and her fellow researchers wondered how the anthelmintic treatments affected the microbiotal community makeup in the animals’ digestive tracts. In particular, she wanted to know how it affected all the microorganisms—not just the bacteria. Although researchers have already investigated how anthelmintics affect the gut bacteria, scientists have yet to explore what happens to other kinds of gut microorganisms—viruses, eukaryotes, and archaea—after oral dewormer administration.

Her team ran shotgun metagenomic sequencing on DNA extracts taken from the fecal samples of six adult Przewalski’s horses in Xinjiang before and after their annual ivermectin treatment. The researchers detected nearly 500 million organisms total, with bacteria representing slightly fewer than 98% of them. Viruses made up 0.3%, whereas eukaryotes and archaea comprised 1.85% and 0.87% of the fecal microbiota populations, respectively.

They found that, just after deworming, many of the balances among genre within each kind of microorganism had adjusted, Hu said.

Shift Trends in Przewalski’s Horse Microbiota After Deworming

In particular, the research team noted that:

• Among bacteria, Firmicutes and Bacteroidetes continued to be the dominant phyla, but Clostridium and Eubacterium genera increased, while Bacteroides and Prevotella
• Among viruses, the genus of unclassified_d__Viruses increased, while those of unclassified_f__Siphoviridae and unclassified_f__Myoviridae
• Among eukaryotes, genera related to digestion increased, but the relative abundances of identified species in general decreased “substantially.”
• Among archaea, Euryarchaeota remained the dominant phylum. However, the genus Methanobrevibacter increased, and the genus Methanocorpusculum decreased, along with two of its usually dominant species: Methanocorpusculum labreanum and Methanocorpusculum bavaricum.

The anthelmintic treatment also increased the abundance of 128 pathogens and reduced that of 59 others, Hu said. That’s worth banking against the changes noted in the bacteria, as their altered genera are associated mainly with immunity and digestion.

“In this study, we have observed that anthelmintic drugs can influence the microbial composition and its functions,” said Hu. “The truth is that the intestinal microbiota also can affect the physiology, metabolism, and immunity of their host. Thus, the change in microbiota following anthelmintic treatment merits our attention.”

Changes Noted, But Deworming Still a Necessity

Despite the observed changes in various kinds of microbiota, the findings do not support discontinuing deworming programs for Przewalski’s horses, said Hu.

“Przewalski’s horses are a reintroduced and endangered species, and the number of them is very low,” she said. “The released horses show that they have been suffering from severe infestations of botflies, up to two to three times more so than other (equids) in the same region.” The infection rate in these herds is 100%, she added.

While the changes they saw appeared relatively consistent across the Przewalski’s horses they observed, that does not mean the same modifications would occur in domestic horses after anthelmintic treatments, said Hu. “Multiple parameters can affect the microbial composition, including the differences in (host) species,” she said.

Metagenomic Analysis of Fecal Archaea, Bacteria, Eukaryota, and Virus in Przewalski’s Horses Following Anthelmintic Treatment was published in the Frontiers in Veterinary Science in 2021.