Parasites are pretty gross in general – both the organisms themselves, and the concept of them living in or on you. Some parasites are of limited concern to people and pets, but others not so much… If I made a list of “parasites I really don’t want,” Echinococcus multilocularis (EM) would be high on the list.
Here’s a bit of information about EM if you’re not familiar with it. More information about EM and related Echinococcus parasites can be found on our infosheet on the Worms & Germs Resources – Pets page, or check out the EM infographic from OAHN.
- Echinococcus multilocularis is a vey small tapeworm.
- Wild canids (e.g. foxes, coyotes) are the natural definitive hosts. Adult parasites living in their intestinal tracts don’t cause them any problems, but they shed eggs that are passed in the feces.
- When those eggs are ingested by an intermediate hosts (usually small rodents), they develop into nasty budding parasitic cysts, usually in the liver but potentially elsewhere in the body as well. That condition is called alveolar echinococcosis (AE).
- If a wild canid eats an infected rodent with these cysts, the parasite matures into adult worms in the intestinal tract, and the parasite’s life cycle continues.
- Dogs can also be definitive hosts for EM, with adult parasites in their intestines and shedding eggs in their feces, though they’re nowhere near as commonly infected as wild canids. In a cruel and confusing trick of nature, dogs can also be intermediate hosts if they ingest enough parasite eggs, developing AE like rodents typically do, which is often fatal (but that’s a whole other issue).
The problem for people is that we can also develop AE if we inadvertently ingest parasite eggs from the feces of infected wild canids or dogs. It’s a very rare but very serious disease in people; the parasitic cysts look like a tumour, act like a tumour, and the condition had a prognosis like a malignant tumour. It usually develops very slowly, and can be quite advanced by the time it’s detected. Untreated, the mortality rate for AE in people is very high. Treatment usually involves surgery to try to remove as much of the cystic tissue as possible, and then life-long drug therapy to suppress any leftover parasites in the body. Even with. treatment, the outcome can be poor.
Communication about are rare but serious diseases is always tricky. It’s important to convey that it’s rare so people don’t freak out. At the same time, it is still a risk and this isn’t something anyone wants to have, so using a few practical measures to avoid it is completely warranted.
The first step in knowing more about the parasite and where to find it. A recent study with which I was involved with provides a comprehensive look at Echinococcus multilocularis in dogs across North America based on over two million laboratory tests from Antech Diagnostics (Evason et al, 2024).
Here are some of the highlights from this study:
- Echinococcus multilocularis DNA was detected by PCR in 26 of 2 333 797 fecal samples from dogs that were submitted by veterinary clinics in Canada and the US between March 2022 and July 2024. That’s approximately 11 infections per 1 000 000 samples, which highlights how rare the infection is in dogs; however, if you owned or had contact with one of those 26 dogs, there are some important things to think about.
- Seventeen (17) of the positive samples were also tested by fecal flotation, a commonly used method for detecting parasites and parasite eggs in feces, but the taenid-type eggs shed by EM were only detected in 8 of these 17 samples. That’s not surprising because we know that fecal floatation isn’t very sensitive for detecting eggs from this tapeworm, and it highlights the value of using PCR finstead. Fecal floatation also can’t differentiate EM from Taenia spp., which are other types of tapeworms whose eggs are visually identical to those of EM.
- Positive samples were submitted by veterinary clinics in both Canada (n=10) and the US (n=16) (see the map below). Some were in areas where we already knew EM is present in wild canids, but others were in regions where this parasite hasn’t really been on the radar – yet.
- Of the infected dogs, 25 of 26 were treated with praziquantel to eliminate the intestinal infection. Post-treatment testing was performed on these 25 dogs and all were negative. The untreated dog was lost to follow-up.
What do we do with this information?
As I mentions, it’s challenging to message “rare but really bad.” Some people will see the low incidence and therefore dismiss the risk. Some will hear what this parasite can do and freak out. Most people will be somewhere in between.
Preventative measures need to account for a few things:
- Geographic risk factors
- Knowing where the parasite is present in the wild canid population (the natural reservoir). At this point, since wild canid surveillance has been limited in many regions, we should probably assume that if wild canids are present, EM probably is too, at least in North America.
- 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.
- Owner risk aversion
- Everyone’s different, and the key is giving them enough information to make an informed decision, even if the information we have is incomplete. Cost of treatment and prevention is also an important consideration, especially for large dogs.
Photo credit: Alan R Walker (https://commons.wikimedia.org/wiki/File:Echinococcus-multilocularis-adult.jpg)
