Page 113 - September 2022

EQUINE HEALTH
Ascarids are some of the most common and damaging parasites of humans and many animals, so development of new therapies against this family of parasites would have major implications for human and livestock health.
then again at 5 to 6 months, depending on when the foal is weaned,” says Nielsen.
After that, the focus should be on strongyles and tapeworms. “After weaning, you might consider using an ivermectin product, perhaps in combination with praziquantel (for tapeworms) to address tapeworm infection before the end
of that calendar year. Some weanlings and short yearlings might have issues with tapeworms. You might also want to check fecal egg counts because we can’t be 100% sure the ascarids are all gone, in all of the foals. Sometimes weanlings and yearlings become reinfected with ascarids while also becoming infected with strongyles,” he says. There is no single dewormer that will work for all types of worms.
“In a weanling or yearling, we want to make sure we use a treatment regimen that reduces development of large strongyles. This is fairly easy because they have such a long life cycle. The large strongyle life cycle, particularly the bloodworm (S. vulgaris) is 6 to 7 months. As long as you treat in intervals shorter than 6 months with a drug that works, you can keep bloodworms at a very low level. The good news is that there is no resistance in large strongyles to the drugs we use,” says Nielsen.
This is why there was a dramatic decline in prevalence. In the 1960’s, about 80 to 100% of horses were infected with S. vulgaris, and now prevalence in intensely treated herds is zero or close to zero. “If we implement some of the current recommendations to not treat as intensively or
as heavily as before, to preserve efficacy of the drugs, there might be risk for re-emergence of these parasites unless you continue some basic treatments to keep these away,” Nielsen says.
Some farms have left horses untreated because they kept having zero egg counts; people figured the horses no longer needed dewormed. “This might be true for small strongyles, but you
can’t forget about the other strongyles. The
eggs look very much the same, and the large strongyles may be waiting in the background for opportunity to make it back,” he says. This has been documented in Denmark and Sweden.
During the yearling year, many horses may be treated three or four times. “At that age, we can primarily use ivermectin, but can also use other types of dewormers if they still work. You should only use a product if you have actually tested it by taking fecal samples afterward to see if there is reduced egg count. Otherwise, we are stuck with ivermectin and moxidectin for yearlings, an age where we can actually have a large burden
of encysted larvae in arrested development. It is important to treat for these encysted larvae in the fall, before going into winter,” says Nielsen.
One of the problems we face today is increasing resistance of worms to the drugs we use. “Even for the macrocyclic lactones (ivermectin, moxidectin) there are signs
of emerging resistance in small strongyles,
particularly the egg-reappearance period and the number of weeks it takes from the day of treatment until we have positive egg counts again. We completed a study a few years
ago, evaluating the efficacy of moxidectin and ivermectin, and for both drugs we found egg reappearance period to be only 4 weeks; the egg counts were positive again. This is quite a contrast from how it used to be. For moxidectin it used to be 16 to 22 weeks of egg suppression in the 1990’s when these products first came out. For ivermectin it used to be 9 to 13 weeks, and now for both of these drugs it’s just one month. We’ve watched these egg reappearance periods become shorter and shorter and realize that soon it will get to a point where these drugs are not effective anymore,” he says.
“The only thing that saves us is that the small strongyles are not very pathogenic, except in rare cases. Most horses are living happy lives even with hundreds of thousands of worms,” says Nielsen.
HOPE FOR THE FUTURE
For the past seven years, Nielsen has collaborated with researchers at the University
of Massachusetts, Virginia Tech and USDA, evaluating a bacterial dewormer. “We launched a pilot project a few years ago and called it ‘Let the Germs get the Worms.’ We were able to get that project done and used the data to get a USDA grant. We’ve been testing this product in various animals, including ruminants, swine and horses. It is a protein produced by free-living bacteria, and it kills worms. It is showing promise in vitro in the lab, and also in vivo—in the animals,” says Nielsen.
“In horses, these proteins are proving to have very good efficacy against ascarids. We are now working to see if they will have efficacy against strongyles as well. Since it is a protein, we need to coat it (so it will make it through the stomach and small intestine) so it doesn’t get degraded before
it reaches the large intestine where the strongyles hang out. Ascarids are in the small intestine, so the protein gets there first and there is apparently still enough of it left to kill the ascarids,” he says.
The published research showed it was highly effective against large roundworms in foals. Results of the research were published in the journal One Health. This research is focused on a free-living bacterium, Bacillus thuringiensis, which produces a family of crystal proteins to combat its competitors. Some of these proteins have potent activity against parasitic worms.
Ascarids are some of the most common
and damaging parasites of humans and many animals, so development of new therapies against this family of parasites would have major implications for human and livestock health.
To be therapeutically useful, however, any new therapies need to be safe, broadly active against this class of parasite, inexpensive, and able to be
produced in mass quantities. The researchers said they were unaware of any new drugs that meet these criteria in the commercial pipeline.
The recombinant (combined genetic material from two different sources) paraprobiotic (inactivated, non-viable microbial cells that confer a health benefit to the human or animal that consumes them) the researchers tested was dubbed Cry5B IBaCC, and they feel it has great promise to fill this void. In their study, they delivered Cry5B IBaCC to the test animals in the form of what they called a dead bacterial ghost; the bacterium was dead, but the protein crystals were still intact.
It proved highly effective against Ascaris suum infections in mice, and a single dose completely cleared A. suum in pigs. A single dose in 5 foals dropped their fecal egg counts of Parascaris equorum (the equine roundworm) to zero. These findings indicate that this treatment could make a significant contribution to long-term, safe, and effective control of ascarid parasites.
A pilot toxicology study in hamsters showed that five consecutive daily doses (200 mg Cry5B/kg body weight) was safe and non-toxic, with no deviation from normal histopathology and blood chemistry. The study team has not yet been able to get efficacy in horses against strongyle parasites from the crystal protein, however, since strongyles are hindgut parasites. When the protein is administered orally, it degrades before it reaches the large intestine, but there is still enough of it to work in the small intestine. Thus, the next step is to try to find the right formulation to protect it from degrading as it goes through the small intestine.
SPEEDHORSE September 2022 111
Courtesy Heather Smith Thomas