Monday, August 16, 2010

Neck Thread Worms, Part 2


(throughout this article, click on the many links to read full articles that are cited)

One and a half years ago I did a post on Onchocerca cervicalis entitled "Neck Thread Worms and Midline Dermatitus". In spite of what I learned, I somehow avoided the sad fact that keeping Red comfortable from the effects of neck thread worms will require lifelong vigilance - an ongoing battle for the rest of his life.

Onchocerca cervicalis, AKA neck thread worms, do NOT go away. Adult neck thread worms are not touched by worming compounds and can live from 8 - 14 years. Every year when biting midge season arrives, their nasty life cycle starts over again with very uncomfortable consequences for your horse.

Yep. This totally sucks! However, the effects of neck thread worms CAN be controlled.

What the heck am I talking about? Onchocerca cervicalis is a member of the genus Onchocerca, the Filaroidea tribe, of the Nematoda Phylym. The common name of Onchocerca cervicalis is "Neck Thread Worms". It is estimated that 85% of older horses are infected with Onchocerca cervicalis (Gary Richard Mullen, Gary Mullen, Lance Durden, Medical and Veterinary Entomology, pl 178).

These nasty buggers live contented lives deeply embedded in the connective tissue of our horses. They do not show up in fecal counts as they never enter the digestive tract. Females may grow to be 30 cm. (think 11 inches). They coil and can often be felt as a bump under the skin of your horse ( It is NOT the adult O. cervicalis that causes most of the problems endemic to this parasite, although heavy infestations can cause swelling and pain. It is the 4th stage larvae that is responsible for skin and eye reactions.

The lifecycle of the neck thread worm depends upon the help of their best friend, the two winged biting midge (Culicoides gnats).
Thank you SO much, biting midges of the world. O. cervicalis depends on this vector both to complete its lifecycle, and to spread from horse to horse.

For a visual represenation of the lifecycle of Onchocerca cervicalis - Click HERE.

What are the symptoms of neck thread worms? Surprisingly, some horses are hosts to very large numbers of microfilariare without showing symptoms. Mordecai Siegal, et al, (UC Davis School of Veterinary Medicine Book of Horses, 1996, p. 161) suggests that symptoms arise once the horse develops a sensitivity to the dead microfilaraie. There are two different "patterns" of affliction:
  1. Lesions (itchy, crusty spots) develop around the eye and face. Bloody, itchy lesions in the center of the forehead are generally due to neck thread worms. There is evidence to suggest that some forms of uvitius are due to neck thread worms living within the eye.
  2. An inflamatory response involving the underside of the body, centered around the midline, resulting in intense itching. This is called "midline dermatitis".
Basically, your horse itches like crazy with itching centered along the midline. They will literally swoon when you do them the favor of scratching. Folks may see the condition and say your horse has sweet itch. This is a separate condition. O. cervicalis is often misdiagnosed as "sweet itch".

According to The U.C. Davis Book of Horses (pp. 161-162), the diagnosis of neck thread worms involves a thorough history, the finding of microfilariae in skin scrapings, and/or the response to treatment. Treatment may temporarily increase the discomfort of your horse as they often experience a severe, allergy-like response to the dead microfilaria. Ivermectin is usually the treatment of choice:

The dermatitis caused by Onchocerca microfilariae is 100% responsive to ivermectin at the regular dose. Disappearance of the skin lesions also validates the diagnosis, although complete resolution of the lesions might take up to a month. Midline edema within the first 48 hours after treatment was a common problem when ivermectin was first introduced. This was thought to be due to the acute death of the microfilariae. Relapses might occur within two to eight months because adult O. cervicalis continue to live happily in the nuchal ligament. They are not killed by any of the anthelmintics that are available and can continue to produce filariae, making repeat treatments necessary at appropriate intervals (although ivermectin might disrupt their reproductive success (The, PARASITE PRIMER PART 5—BOTS & BEYOND: LITTLE-KNOWN PARASITE ENEMIES, pp 22-23).

Treatment is NOT a one time event. Once your horse becomes infected, it will have repeated seasonal recurrences of microfilaria-induced itching. This is going to be linked to the arrival of biting midges and will continue throughout their season. This will differ depending upon your geographical location.

Does your horse seem to share the misery afflicting my Red Horse? Prepare your strategy of attack:

1. Keep records and take pictures. Talk to your vet. Your vet may be very familiar with Onchocerca cervicalis, or not. You might need to share information to help obtain an accurate diagnosis. Ask for a biopsy to see if microfilariae can be found in the skin scraping.

2. Develop a yearly treatment plan.
  • If there are neck thread worms in your horses' eyes BE VERY CAREFUL and have a plan with your vet to treat the secondary symptoms caused by the microfilariae die off.
  • Because O. Cervicalis depends on the biting midge for assistance in the completion of its life cycle, be prepared and worm with Ivermectin at the onset of midge, or gnat season. Consider worming with Ivermectin at 8 week intervals throughout the season. Consult with your vet about how to include this in your rotational worming schedule.
  • Be prepared to treat your horse's response to the die-off of microfillariae. Things may get worse for a few days. What started as many small, hard bumps up and down Red's neck opened, oozed, and finally went bald, all the while itching like crazy. Many articles recommend asking your vet to prescribe something to help reduce the intense itching.


Anonymous said...

Wow! That's scary stuff - we have lots of biting gnats but I've never been aware of this (although maybe I should have) - thanks for all the good information.

Sydney_bitless said...

WOW! Poor Red man. Have you tried MTG on his itchy spots?

One Red Horse said...

Kate, who would have thought that 85% of horses have neck thread worms? Who would have thought there was something so gross? Good thing many horses do not have a sensitivity to the microfilia.

Sydney, MTG is one of the products I use. I love it.

Anonymous said...

Thank you for posting this info Red! I am now worried! Both the mares have the midline dermatitis.

How do I find out the midge life cycle for where I live? (by state perhaps?).

One Red Horse said...

Maybe call your agricultural extenstion or google the life cycle of the Culicoides gnat.

Harold said...

I have a horse with the same problem I have had him 3 years I am in Tassy and purchased him in Melbourne and I dont know were he came before. I drench him every month and use preddy granules to help him. I still event and show jump with him he is a great little horse and I love him to bits but his blessed Onchocerca drives him and me mad

reflections said...

My horse has had a sore on her stomach for 1 1/2 years. I thought it was from the girth rubbing her. Have tried various creams,swat, etc to get rid of it. Nothing works. I found out about the thread worm from a equine chiorpractor. Looked it up and fround this site. Looks like the sore on this site. Going to call my vet tomorrow. My horse is a older mare. Thanks, for the information.

Anonymous said...

My friend's horse has just been diagnosed with this. Is there any chance you would be willing to talk to her. I forwarded your blog to her & she was impressed by the information you have gathered. She is really worried about her horse & would like to talk to someone with first hand knowledge. Thanks. Kelly.

One Red Horse said...

Hi Anon, sorry I just read your post. Is there a way to contact you?

Avalon said...

Hi one red horse I am currently in limbo unsure if i looking at an NTW infection or not and have been round in that many circles I'm tired and very down, I would love to be able to contact someone in the know is there anyway to contact you? My email is

Plaudit said...

My first horse got this. The vets couldn't help. Then her baby got it. I took four painful years out of my life and researched things. I found out about the Ivermectin back to back, plus 25.000 iu of vitamin A in fish liver oil form that I fed daily. I never took my horse off that form of vitamin A. I didn't have the problem anymore, and at Dressage shows everyone complimented me on my horse's coat:)Vitamin A helps them to resist worms, so does copper, but mine get that in sunflower seeds. And NO, I don't sell a thing. It works!!!!Vitamin A (Beta carotene) is the first thing to leave stored hay. Don't ever supplement Beta carotene, or the synthetics..dangerous!!!!That is the only supplement I use. I don't use a commercial form,I get the human pills, and drop it in their grain. Do not feed more, more is not better and will cause other problems. It can block calcium at High levels. Just food for thought, I would have given anything to know this back then!!!