Nematode-Trapping Fungi Provide a New Approach to Control Equine Nematodes

Thomas R. Klei, PhD
Boyd Professor, Department of Veterinary Microbiology & Parasitology
and Department of Veterinary Science

Alexandra Baudena
PhD Candidate, Department of Veterinary Microbiology & Parasitology

Current Methods of Equine Parasite Control

Most horse owners rely solely on the regular use of modern anthelmintics to control internal parasites in their horses. Some of these endectocides, such as ivermectin and moxidectin, are effective against not only a wide range of nematodes but also stomach bots, increasing the frequency of their application. Although these newer drugs remain highly effective, resistance to other anthelmintics, including the benzimidazoles and pyrantel salts, has been demonstrated in small strongyle populations throughout the world. Many parasitologists and pharmacologists feel that it is only a matter of time before ivermectin and moxidectin resistant cyathostomes emerge. This is particularly important in that the small strongyles, or cyathostomes, have become recognized as the most common internal parasites of horses in developed countries. Disease symptoms associated with these infections have also been recognized at an increasing rate. Because of the continued emergence of drug resistance, a number of strategies for cyathostome control have been suggested which reduce the use of anthelmintics. These include the use of pasture vacuums, alternate grazing with other livestock such as sheep, goats or cattle, and pasture rotation methods. All of these approaches have difficulties in their implementation and have not been widely adapted. Because of this, additional approaches to augment the use of anthelmintics are being sought. One of these is the use of biological control strategies, particularly, the use of nematode-trapping fungi.

Nematode-trapping Fungi

Nematode-trapping fungi occur naturally throughout the environment, and more than 150 species have been identified. These fungi have evolved a wide variety of devices to trap or invade nematodes and use these animals as a source of nutrients. Fungal species which trap nematodes do so with adhesive areas along their vegetative hyphae, or with trapping devices which grow along these hyphae and snare nematodes in rings or net-like devices (above).   Some of these traps are contractile in nature. Once nematodes are trapped (below), fungal hyphae penetrate the worms cuticle and grow within the nematode. Other fungal species infect nematodes with spores. Once these spores enter the nematode, these endoparasitic fungi germinate producing an infection thallus which fills these worms, eventually resulting in the emergence of conidophores through the cuticle of the nematodes. These release more spores into the environment. Still other fungi are capable of penetrating the shells of nematode eggs. The nematode-trapping fungi do not discriminate between species or stages of nematodes found in the environment and thus readily trap the developing and infectious stages of parasitic nematodes of animals which are found in animal fecal pats, the soil and on pasture grass. The concept of utilizing nematode-trapping fungi for the control of animal parasites is thus in reducing or eliminating the infectious third-stage larvae (L3) of cyathostomes from pastures, reducing the potential of infection.

Researchers in Denmark at the Danish Institute of Experimental Parasitology in conjunction with Australian scientists at the CSIRO have identified nematode-trapping fungi that may be suitable as alternatives or aids in controlling gastrointestinal nematodes of livestock including horses. Their research strategy was to identify a nematode-trapping fungal species that was easily cultured and produced large numbers of spores which would be resistant to passage through the mammalian digestive tract. Once identified, these spores could be fed to individual animals, pass through the digestive tract, and be deposited in feces on pastures. In the feces, the spores would develop vegetative hyphae and trapping devices and remove the developing parasite larvae in and around the fecal pat. These initial investigations have been successful, and the common soil fungus Duddingtonia flagrans has been shown to meet all of these requirements in laboratory experiments. The spores do not germinate in the animal and have no effect on animals following ingestion. Spores in animal feces have been shown to destroy a wide range of parasitic nematode species including small strongyles of horses in laboratory cultures. These experiments have also been conducted under field conditions in the temperate climates of northern Europe, southern Australia, and the northern U.S. At this point the use of D. flagrans chlamydospores to aid in the control of nematode parasites of livestock has been patented by the Danish company Christian Hansen Biological Sciences (CHBS). The commercialization of this as a product, however, awaits further experimentation.

The Efficacy of Duddingtonia flagrans in Reducing Cyathostome L3 on Pasture in Southern Louisiana

Although D. flagrans is found throughout the U.S., including southern Louisiana, the effect of the semitropical climate on fungal development and nematode-trapping activity is unknown. Studies have recently been conducted in our laboratory to measure the effect of the nematode-trapping fungi D. flagrans on reducing strongyle L3 in equine fecal pats at different seasons of the year in this area. These studies were conducted in conjunction with Dr. Michael Larsen of the Danish Institute for Experimental Parasitology, using the Danish isolate of D. flagrans now being commercialized by CHBS. The experiments were designed to measure the effect of fungal spores on the development of small strongyle larvae in artificial fecal pats on pastures of the Veterinary Medical Research Farm. Three horses were used in these experiments. One of these horses was naturally infected with cyathostomes and served as a source of infected feces. Two other horses were treated with ivermectin, followed by daily treatments of fenbendazole at twice the normally recommended dosage for five days. We have shown that this experimental treatment regimen effectively removes all adult and migrating nematode species, including the cyathostome larvae within the mucosa. One of these treated horses served as a source of parasite-free feces. The other parasite-free horse was fed a barley grain mixture containing chymidiospores of D. flagrans. This horse received the fungus twice daily and served as a source of equine feces containing fungal spores that passed through the digestive tract. These three types of feces were used to make two types of equine fecal pats, those that contained strongyle parasite eggs without fungal spores and those with the same numbers of eggs without fungal spores. These two types of fecal pats were placed on a pasture in replicates of six each (Figure 3). The grass around these fecal pats was sampled for parasite larvae at two-week intervals over an eight-week period.
Fecal pats were placed on pasture at eleven 8-week periods throughout the year. These periods overlapped and included all seasons of the year. Comparisons of the number of cyathostome L3 collected from grass around fungal-treated and untreated pats were made. The fungus effectively reduced the number of L3 by an average of 88% and was effective at all seasons of the year in this study. These results suggest that feeding of these fungal spores on a regular basis would reduce pasture contamination with parasite larvae, reducing infections in horses maintained on these pastures.

The Potential for Parasite Control

Although results such as ours have generated considerable interest in the potential of using nematode-trapping fungi as an aid in livestock parasite control, a number of questions remain to be answered. It is clear that fungal spores would have to be fed to horses on a regular basis. Thus, this would only be suitable for animals maintained under management conditions where regular feeding was possible. It is not clear how much variation from the optimal spore number would allow for maximal killing of parasites in the fecal pat. Thus, for optimal control, horses would have to be fed individually. Further it is clear that this procedure will not remove internal parasites but only prevent reinfection. Thus, the use of these fungi must be coupled with the use of some effective anthelmintic treatment. Nonetheless a specific treatment regime can be envisaged which would reduce the use of anthelmintics and protect horses from infection with cyathostomes. Such a strategy would also take advantage of our knowledge of seasonal patterns of cyathostome transmission. In the Gulf Coast region, we have shown that the majority of cyathostome larvae occur on pastures during the cooler months of the year. Peak pasture L3 populations are found during the months of October through April. The numbers of L3 and potential of transmission decreases to extremely low levels during the hot summer months. Treatment of horses with an effective anthelmintic during September, prior to the transmission season, would remove internal parasites at a time when reinfection potential is low. Daily feedings of fungal spores would begin as the climate became more suitable for L3 survival on pasture and as strongyle eggs began to reappear in the horses feces following this treatment. The fungal spores developing in the feces would eliminate any L3 in the fecal pat that resulted from parasites that survived the initial treatment and would markedly reduce pasture contamination. Without reinfection it is likely that parasite burdens within the horses grazing these pastures would remain low. It is not clear how protective this effect would be, and the necessity and timing of a second anthelmintic treatment would have to be determined in future field experiments. If shown to be beneficial and commercially feasible, the use of nematode-trapping fungi would reduce the use of anthelmintics and the dangerous selection of worms that are resistant to even our most modern of endectocides.