What are Cyathostomes, and Why Do We Care?
Cyathostomes, more frequently known as small strongyle worms, are the most common and ubiquitous parasite of the equine large intestine and are one of the biggest challenges in horse husbandry today. Because these parasitic worms seem to be developing resistance to most of the available dewormers, performing microscopic examinations of manure to count worm eggs before and after deworming, if needed, is essential to determining the effectiveness of those dewormers. Moxidectin (Quest) is the only dewormer without demonstrable small strongyle resistance in some areas of the country.

What makes them such a challenge?
Adult small strongyles in the cecum and ventral colon of the horse pass eggs during the grazing season. The eggs hatch and develop at 60-85 degrees F into Larval 1 and Larval 2 stages on pasture, and then into an infective Larval 3 (L3) stage. L3s have to eat but they will die if temps are over 85 degrees F and their metabolism speeds up. These larval stages are all encased in a protective sheath, and the L3 in its sheath can overwinter on pasture even in this climate.

Ingested L3 lose their protective sheath in the small intestine of the horse then move into the large intestinal crypts and migrate through the gut wall into the mucosa and submucosa of the cecum and ventral colon where the majority encyst. Most Cyathostomes remain in encysted form, some up to 30 months.

Late winter, early spring: Some encysted L3 develop into L4’s and move into the large intestinal lumen, by an unknown trigger. L4’s develop into L5’s and then mature into adults that begin to pass eggs. If adulticide dewormer is then used, there is an increased trigger for encysted L3s to migrate through the adult wall and develop into adults. At all times, however, the majority of Cyathostomes remain encysted.

Dealing with them
The only dewormers that can kill late stage L3 Cyathostome larvae as well as L4, L5, and adults are Quest (Moxidectin) and Panacur Power Pack, a double dose of fenbendazole given 5 days in a row. Ivermectin kills adults and some L4 and L5 larvae. No dewormer can kill the early encysted L3 larvae, so treatment of horses who have high levels of Cyathostomes must go on for as long as 30 months, with no additional contamination. That is hard to accomplish. Additionally, Cyathostomes are becoming resistant to many available dewormers, and the length of effectiveness of many dewormers is becoming shorter:

Dewormer Efficacy Periods
Pyrantel (Strongid) 4 Weeks
Anthelcide (Oxibendazole) 4-6 Weeks
Safeguard, Panacur (fenbendazole) 4-6 weeks 4-6 Weeks
Ivermectins 6-8 weeks 6-8
Moxidectin (Quest) 12-16

Larval Cyathostomosis
Larval cyathostomosis is a potentially life threatening disease that occurs when too many larvae migrate through the gut wall at one time. This crisis occurs in young (usually under 6) or immune compromised horses with a history of infrequent deworming (especially dewormers that only kill adult worms) and exposure to a numbers of horses with high worm loads out on pasture for the previous one or two years.

It is seasonal, typically occurring in the late winter/early spring timeframe. There is an eruption of large numbers of late L3 and L4 small strongyle larvae through mucosa of the cecum and ventral colon into lumen. An almost allergic reaction ensues with inflammation and swelling of the large intestinal wall causing pain and diarrhea, and an inability to absorb essential nutrients. Recent deworming with an adulticide may cause a worsening of clinical signs:

  • Loss of appetite
  • Colic with increased intestinal sounds and cramping, or occasional stasis of the gut
  • Depression
  • Diarrhea
  • Dehydration with dry mucous membranes and possible toxic line
  • Temp variable
  • CHECK PULSE CHARACTER IN FEET - Toxic shock is possible with resulting laminitis.
Lab work will rule out other causes of serious diarrhea and document low protein, low calcium, and stress.

Treatment will include:
  • oral agents to detoxify and sooth the gut, electrolytes and water.
  • IV fluids and IV plasma may be necessary
  • Corticosteroids will counter the almost allergic inflammatory activity
  • Pain relief as needed, avoiding nonsteroidal anti-inflammatory agents
  • Anti-spasmodics, i.e, Buscopan given IV or IM
  • Soft, easily digested feed like Equine Senior fed in small amounts frequently
  • Small amounts of alfalfa mix hay for buffering and added protein
  • Stomach ulcer medications as needed
  • A deeply bedded stall kept clean.
  • Moxidectin to kill as many larva as well as adult small strongyles, repeated at 12 week intervals
Adult Cyathostomosis
In some horses, larval small strongyles may just continue to encyst (a state of hypobiosis) in the mucosa, submucosa and crypts of the cecum and ventral colon and fail to erupt so that over time, massive amounts of cysts build up in the gut wall, basically reducing gut absorption capacity. This occurs in late summer after significant exposure to contaminated pasture. Remember that early L3 are not susceptible to dewormers of any kind.

Clinical signs include:
  • general weight loss, poor condition.
  • A fecal sent to the lab may be negative.
  • Anemia, signs of inflammation
  • Bile duct back up possible due to gut swelling with elevation of liver enzymes
  • Gastric ulcers may develop
Treatment is to deworm with moxidectin (reported to have less mucosal damage than Panacur Powerpack. No resistance yet reported. Pretreatment with corticosteroids is probably a good idea. Additionally, nutritional support such as vitamins etc is recommended, as is medication for cramping. As for feed, provide alfalfa mixed hay, senior pellets. Give 7.5 pounds of feed over maintenance to gain weight.

Recovery takes 2 months or more. Continue moxidectin every 12 weeks for 30 months.

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