Echinococcus multilocularis in dogs; North America

Parasites are pretty gross in general…both the actual critters and the concept of them living in or on you. Some parasites are of pretty limited concern. Others….not so much.

If I had to make a list of “parasites I really don’t want”, Echinococcus multilocularis would be high on the list.

I won’t go into the full story about this parasite but here’s a bit of info to set the scene, if you’re not familiar with it.

• It’s a small tapeworm.
• Wild canids (e.g. foxes, coyotes) are the main definitive hosts. They can have adult parasites living in their intestinal tract, causing no problems. Those parasites shed eggs in their feces.
• When those eggs are ingested by intermediate hosts…mainly rodents…they develop nasty parasitic lesions in the liver and potentially elsewhere. That’s called alveolar echinococcosis (AE).
• If a wild canid eats an infected rodent, the life cycle continues.

Dogs can also be definitive hosts. They are nowhere near as commonly affected as wild canids, but they can have adult E. multilocularis in their gut and shed eggs. (They can also be intermediate hosts if they ingest eggs, developing nasty AE that is often fatal, but that’s a completely separate issue).

For us, the problem is that we can also develop AE if we inadvertently ingest parasite eggs from fecal contamination. It’s a very rare disease in people but it’s nasty. It looks like a tumour, acts like a tumour and has a prognosis like a tumour. It can be slow to develop and disease can be advanced by the time it’s diagnosed. Untreated, the mortality rate is very high. Treatment usually involves surgery to try to remove as much as possible with drugs after to suppress any leftover parasite (again..a lot like a tumour). Even with. treatment, the outcome can be poor.

Communication about diseases that are rare but serious is always hard. It’s important to remember that it’s rare so people don’t freak out. At the same time, rare things still occur and this isn’t something you want to have. So practical measures to avoid it are warranted.

Knowing more about the parasite is part of that.

A recent study I was involved with provides a comprehensive look at this parasite in dogs across North America (Evason et al, 2024).

Some highlights;

• E. multilocularis eggs were detected by PCR in 26/2,333,797 fecal samples from dogs that were submitted to an international diagnostic lab (Antech).
• That’s around 11 infections per million samples, highlighting the rarity. However, if you owned or had contact with one of those 26 dogs, there are some things to think about.
• 17 of the positive samples were tested by fecal flotation, a commonly used method for detecting parasites. Only 8/17 were positive. That’s not surprising since we know that fecal floats aren’t very sensitive, but it highlights the value of using PCR for detection of this parasite. Fecal floats also can’t differentiate E.multilocularis from Taenia, another type of tapeworm.
• Samples were from across Canada (n=10) and the US (n=16) (see the map below). Some were from areas where we’ve know that E. multilocularis is present in wild canids. However, it was more widespread and was found in areas where this parasite hasn’t really been on the radar.
• 25/26 infected dogs were treated with praziquantel. Post-treatment testing was performed on 25 dogs and all were negative. The untreated dog was lost to follow-up.

What do we do with this information?

As above, it’s hard to message ‘rare but really bad’. Some people will see that low incidence and will dismiss it. Some will hear what this parasite can do and freak out. Most people will be somewhere in between.

So, prevention approaches need to consider a few things.

1. Geographic risk factors
   • Is the parasite known to be there? Are there wild canids?
   • Realistically, since wild canid surveillance has been limited, we should probably assume that if wild canids are present, E. multilocularis probably is too.
2. Dog risk factors
   • Dogs get infected by eating infected rodents. As a result, dogs with outdoor access (especially unsupervised access) and dogs that are prone to eating anything (like my dog Ozzie) are at higher risk.
3. Owner risk aversion
   • Is the owner concerned about it? Is cost a major limitation?
   • Everyone’s different, and the key is giving them enough information to make an informed decision, even if the information we have is incomplete.

For more information about this parasite, check out the Echinococcus fact sheet in our Resources section.

Parasite photo credit: Alan R Walker. https://commons.wikimedia.org/wiki/File:Echinococcus-multilocularis-adult.jpg

Source link