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Deadly bloodworm: Your horse may show false negatives

blodorm
Photo: Canva Pro

What's the deal with these fertiliser tests? We spoke to Tina Phil, an expert in fertiliser testing and deworming. She tells us that worms and parasites are a serious problem, and unfortunately, not everything can be seen in a manure sample where only eggs are counted. Bloodworms, which used to be treated when all horses were regularly dewormed, have become an increasing problem and larval cultures should be performed to detect them.

In 1999, Denmark introduced compulsory prescription of deworming products. This was done because there was a lot of focus on parasites developing resistance to deworming. This means that a veterinarian must be able to diagnose a horse with worms before they can prescribe a cure. The prescription requirement applies throughout the EU (since 2008), but in many other countries deworming is freely available to all horse owners and a faecal sample is not required before a horse is dewormed.

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Who is Tina Pihl?

Tina Pihl is qualified as a veterinarian in 2002 and has a PhD in equine colic. She is an associate professor of internal medicine at the Copenhagen University Hospital for Large Animals and an expert in faecal samples, deworming and parasites. Tina Pihl has been researching equine bloodworm (Strongylus vulgaris) since 2015 and teaches and counsels in the field. As a horse owner herself, she knows the dilemmas that horse owners can face.

EGG COUNTING AND LARVICULTURE

A egg count shows how many parasite eggs the horse excretes in its faeces. An egg count can distinguish some parasite eggs, but not all can be seen on a faecal sample.

A larval culture is a process by which the eggs hatch. The manure is left for 14 days in a moist environment, during which time the eggs hatch into larvae, which can then be identified. In this way, the bloodworm can be diagnosed

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The picture to the left shows the larval growth process, which takes 10-14 days. It is a kind of humidity chamber where the eggs hatch and develop into larvae.
The picture on the right shows how the larvae are harvested. The fertiliser hangs in gauze at the top of the jar for 24 hours and the larvae crawl out into the water and fall to the bottom. They are collected from the bottom of the jar and examined under a microscope to identify which larvae were in the sample.

BLOODWORMS - THE INVISIBLE KILLER

One of the worms that can't be seen in egg tests is the bloodworm. And not to be taken lightly, it can actually lead to the death of the horse. The bloodworm has been the most common cause of colic in the past and is the most deadly of the worms and parasites horses can have.

The life cycle of the bloodworm is very long, which presents some challenges for testing and treatment. The horse can easily get sick and die from bloodworms without it showing up in a manure sample. Bloodworms have a six-month life cycle where they crawl around inside blood vessels and can cause blood clots and subsequently lay eggs in the gut, which will show up on a larval culture.

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"DURING THE PERIOD WHEN THE BLOODWORM IS CAUSING POTENTIAL DISEASE DAMAGE, WE CANNOT FIND EITHER LARVAE OR EGGS IN THE FERTILISER."

Bloodworms can be removed by deworming. However, if the horse has a low egg count, it will often not be dewormed and may still have bloodworms. "It's dangerous not to test for bloodworms." The experienced veterinarian tells us. But she maintains that if we test and treat when we find something, we can keep the incidence down.

RESISTANCE OR BLOODWORM

It is a puzzle when it comes to deworming legislation. Tina Pihl explains how, in the past, the Danish horse population rarely suffered illness or death due to bloodworms. This is because horses were dewormed about twice a year. Bloodworm researchers have seen a change since the changes in 1999 when it was decided that horses must be diagnosed before they can be dewormed. More horses are experiencing the consequences of bloodworms than before, but on the other hand, resistance does not develop as quickly as it does in other countries without prescription requirements.

"Bloodworms were completely forgotten because deworming was so effective as a treatment in the 80s and 90s. Then when I started seeing horses getting sick in the '10s, I didn't realise what disease it was at first."

explains Tina Pihl, and this was one of the reasons she chose to become a researcher in the field.

Resistance occurs when worms and parasites frequently encounter the medication and therefore change to survive the treatment. If full resistance develops, we can no longer treat our horses against worms and parasites. With no new dewormers on the market, it's important to take care of the ones we have and use them wisely.

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HIGH PROBABILITY OF FALSE NEGATIVE

Bloodworms can be detected by larviculture, but only at certain points in the bloodworm cycle. Tina Pihl recommends having a bloodworm larviculture done at least once a year, but preferably twice a year to avoid the horse having bloodworms. Because even if you do a larviculture, the result may be a false negative.

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"WE KNOW THAT THERE ARE HORSES THAT CAN HAVE BLOODWORMS WHERE WE CAN'T DETECT IT IN THE FERTILISER."

Emphasizes the researcher from the University of Copenhagen.
And that's why all horses in a paddock should be tested simultaneously and treated if bloodworms are found. Tina Pihl explains: "The international recommendations are that if you find one horse with bloodworms in a paddock, all horses in that paddock should be treated.

SYMPTOMS OF A HORSE THAT HAS FALLEN ILL WITH BLOODWORMS:

  • Mild colic
  • Fever
  • Peritonitis
  • Blood clots
  • The bowel may rupture and the horse will die

If the horse is operated on in time, the damaged piece of intestine can be removed and the horse has a good chance of survival. Tina Pihl emphasizes that not all horses will become ill with bloodworm, but it is worth trying to avoid it. As far as possible, this can be done by carrying out a larval culture twice a year at the same time as the egg count.

Tina and her research group are working to improve the diagnostic options for equine bloodworms. It will be interesting to see if the diagnostic options for bloodworms change in the future. But you can certainly do your part by testing your horse twice a year and being aware of any changes in your horse.

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