Current Vet

In this episode of Current Vet, Dr. Lottie talks about toxocariasis, why parasite migration can cause some unexpected signs and the different routes of transmission that mean puppies can be born infected.

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Timeline:
00:00 Intro
01:52 Case
02:39 Aetiology & Pathogenesis
08:09 Clinical Signs
10:44 Zoonosis
12:49 Treatment
14:26 Prevention
15:12 Key Points
17:39 Outro

Recommended Reading:
  • The ESCCAP’s Guidelines on Worm Control in Dogs & Cats has a great overview on Toxocara spp. infections, lifecycles, diagnosis and prevention methods

References

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host
Dr. Lottie Wilkinson
Creator and host of the Current Vet podcast

What is Current Vet?

The podcast that makes veterinary medicine simple.

In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

Whether you’re cramming for exams or looking for a soundtrack for your dog walk, Current Vet will make vet med simple

Intro

Hello, hello, and welcome back to Current Vet, the podcast that makes veterinary medicine simple. I'm your host, Dr. Lottie, and today, we are talking about toxocariasis, which sounds a bit fancy, but of course, it's just the disease caused by infection with Toxocara parasites. It's a really interesting disease, and it involves many different aspects of vet med, including obviously the disease itself and then preventative care.

And because it's zoonotic, it's also importantly linked to public health. Before we get into this episode, I wanted to remind you of a really exciting launch we have coming up of our new website. It really is gonna be , an all-in-one platform for vet students, including topic revision, quiz, active recall testing.

We have non-clinical courses, and we have a very exciting first non-clinical course that we created in collaboration with one of our previous podcast guests. So I won't say any more on that, but if you've been listening to the podcast, you might have a guess of who it is And the big part for me on this website is that we have the progress trackers and stats trackers for the questions.

So if you want to do the questions and you test yourself, you can then see how well you're doing in relation to different topics, different species, and track your progress over time. This would've been so useful for me as a vet student, so I'm really hoping that you guys can try it out and use it for your own revision. As always, The link for the wait list is gonna be in the show notes, so if you're interested, sign up now, and we will send you early access information, and then we're going to be launching in the summer. But let's get back to the episode and get straight into our case.

Case

You're seeing a seven-week-old Labrador puppy, Archie, for his first vaccinations. His owner mentions that he's eating well, and he's playful, but he's developed quite a round belly in the past couple of weeks and has had a few bouts of diarrhea. They've also noticed the odd cough but haven't been too concerned

On exam, he is bright and friendly, but he's a little bit underweight from what you would want him to be. His coat is a little bit dull and you think he's looking a little bit small for seven weeks old. This is obviously not a crazy presentation for a puppy. We often see bouts of diarrhea. So let's talk about what Toxocara is and how it causes disease, and we'll keep coming back to Archie as we go.

Aetiology & Pathogenesis

So toxocariasis is a gastrointestinal infection with roundworms in the genus Toxocara. The two main species to be aware of in vet med are gonna be Toxocara canis, with the dog definitive host, and then Toxocara cati, with obviously a cat, feline- definitive host. These are nematode worms in the Ascarididae family, so they are classed as ascarids, and adult Toxocara live in the small intestine of the definitive host. And they're quite big worms. If you see them, they look like pale, kind of cream-colored spaghetti strands Toxocara eggs are the round, oval-shaped, thick-shelled brown eggs, and they have that kind of pitted or rough mammillated surface, as it's called.

These eggs are incredibly tough and survive in the environment for a long, long time. They resist most things that we're gonna normally rely on for cleaning and disinfection, so once environmental contamination has occurred, it becomes quite difficult to get rid of. So the life cycle of these worms is that the adult worms in the small intestine produce eggs, which are then passed in the feces as unembryonated, so they're gonna be non-infective eggs.
Once in the environment, the eggs embryonate, i.e., become infective over a couple of weeks, so like two to four weeks. And once they contain a stage two larva, they become infective. So a really important point for clients is that fresh feces are not immediately infective, but if they sit there, it can become a problem Dogs and cats
can become infected in three main ways. So first, there's gonna be ingestion of infective eggs, and that's usually from contaminated soil, from grass, so like the backyard or parks, maybe litter trays for cats, from dirty paws or fur, from toys, basically anything that could be contaminated They can then be infected if they eat a paratenic host.

So a paratenic host is a transport host. So the parasite can infect these paratenic hosts, but they don't mature and become adults in there. They just stay alive and stay infective. So small mammals like rodents or birds can ingest these Toxocara eggs. The larvae hatch and migrate into tissues and encyst in these paratenic hosts.

So then when a dog or cat eats that rodent or bird, they themselves become infected. So for cats that hunt or dogs that scavenge and have outdoor lifestyles, paratenic host transmission can be a major driver for infection. And then finally, we have vertical transmission in dogs

With Toxocara canis in dogs, larvae can be transmitted vertically across the placenta, so in utero from the dam into the puppies, and also through the milk once they start nursing once they're born. So puppies can be born infected or acquire infection whilst nursing in early life. So this is why Archie can have toxocara sis at seven weeks old even if he has never been outside.

Cats don't have the same transplacental route as dogs do, so kittens are infected once they are born in the environment by hunting exposure. But they can also be infected through nursing, so that milk is still gonna be a major source of infection for cats

So Archie acquires larvae either via infective eggs in the environment or more likely at this stage through his mum. The larvae are going to hatch in the small intestine, penetrate the intestinal wall, and enter into the portal circulation, and that's the circulation that goes via the liver. So they then migrate through the liver, they move to the lungs, and then they migrate into the airways.

As they migrate up through the airways, this triggers inflammation and eosinophilic recruitment. In young animals, these larvae are coughed up, swallowed, and then obviously return into the small intestine where they mature into adult worms. So that is going to give you two key groups of clinical signs.

So first we're going to get that GI disease from the adult worms in the small intestine, and then we're going to get respiratory signs from the larval migration. So that occasional cough that Archie's owners are reporting can directly relate to this GI parasite Something else that is worth mentioning is somatic migration in older animals.

So in many older animals, instead of completing the tracheal migration and becoming adult worms every time, larvae often migrate into other tissues like skeletal muscle and encyst there. So adult dogs and cats can be completely clinically normal, but still carrying infection that's still capable of it recrudescing, they can shed and spread the disease into the environment What clinical signs are we gonna see with infection?

Clinical Signs

So first up, obviously, those GI signs. We have pot-bellied appearance, especially in young puppies and kittens, diarrhea, we might see vomiting. You may see occasional adult worms in either the vomit or feces, and then we're gonna see poor growth and ill-thrift, especially in those young animals. These signs are due to nutrient competition from the worms when they're sitting in the intestines, mucosal irritation caused by the adult worms inhabiting the intestines, and they can also be from mechanical obstruction of the intestines if there is a really heavy burden.

We then have those respiratory signs, so coughing, tachypnea, and nasal discharge, and they are caused by that pulmonary migration phase.

As well as these two key signs, in really severe cases with heavy burdens, so particularly young puppies, we can also see intestinal obstruction or even intussusception On the other hand, adult cats and dogs are likely to be mildly infected or often completely asymptomatic

Diagnosis

Okay, so how are we gonna make sure that Archie has a Toxocara infection and not something else? First up is going to be a fecal flotation, and this is the most common diagnostic test. It works by using a high specific gravity solution. . You place a fecal sample in the solution, allow the parasite eggs to float to the top, which can then be collected and examined on a microscope slide, and the characteristic thick-shelled, pitted Toxocara eggs can be identified.

But it's important to remember that a negative result does not definitively rule out toxocariasis, And this is for several reasons. So firstly, eggs are only shed once adult worms are present and have started producing eggs. So flotations are gonna be negative throughout that prepatent phase, the migration phase of infection

And secondly, adult animals with dormant encysted larvae also won't be shedding eggs, so they will not be showing up as positive

Other things we can look for is potentially an eosinophilia on blood work, and we might be able to visualize adult worms in vomit or in faeces

if they are present. But in practice, the signalment, the history, and a positive response to treatment are often just as important as these test results

Zoonosis

I wanna take a little side note and talk about the zoonotic aspect of disease. There is a secret third type of host for Toxocara species, which is humans as accidental hosts. So if infective eggs are accidentally ingested by people, larvae can hatch and migrate through tissues and can encyst, but they are not going to mature - into adult worms because humans aren't the definitive host.

This causes three key clinical presentations. So the first one is visceral larva migrans, and this is when the larvae infect or encyst in organs. So they can be like the liver, the lungs, skeletal muscle. So we're gonna see signs like muscle aches, fever, lethargy, some weight loss, anorexia, and then signs related to the affected organs.

So if it's the lungs, we might see coughing and wheezing. The second key presentation in humans is ocular larva migrans, and this is when the larvae migrate to the eyes, and this can cause blurred or impaired vision, retinitis, uveitis, and sometimes even visible floaters in the eye. The third presentation is covert toxocariasis.

It's much more subtle, as the name suggests, but usually seen as general malaise, fatigue, headaches, and poor concentration. Infections are actually most common in children because they're gonna be playing outside in kind of sandpits or soil and other likely contaminated environments. And then, of course, as childrens do, they put dirty hands in their mouths and can accidentally ingest these eggs from the environment.

And this is where it's really important to explain the risks to owners, especially if they are going to public spaces like parks and sandpits with their dog, or if they have households with young children

Treatment

Okay, let's talk about treatment. So the aim of treatment is, of course, to eliminate adult worms, but also to reduce egg shedding, so reducing environmental contamination and minimizing the zoonotic risk. Anthelmintics are very effective against adult worms, but no drug , reliably clears all encysted larvae, which is why repeat treatments to target newly maturing worms is so important.

So common options are going to be fenbendazole. We have pyrantel pamoate, macrocyclic lactones like milbemycin or moxidectin

And imodepside in cats. In puppies and kittens, we can actually start treatment from as young as two weeks old, depending on the product used, and then it can be repeated every two to three weeks until they are a couple of months old. And then we continue that protocol every month until they're at least six months old.

Then they can transition to a normal deworming protocol routine. Adults should, of course, be on regular parasite prevention, especially breeding animals or those with outdoor access, because as we said, hunting and scavenging is such a risk factor for infection and transmission. In really heavy infections, supportive care like nutritional support, fluids, and gastroprotectants may also be required.

But generally, these infections respond well to treatment.

Prevention

Prevention is the most important part of managing toxocariasis, so key strategies are gonna include routine parasite prevention for dogs and cats, of course, early intervention and deworming in puppies and kittens, prompt removal of feces, preventing animal access to children's play areas or sandpits, and then, of course, educating clients about the clinical signs to look for, how best to prevent infections, and the implications that zoonotic risk can have for them and others in their household, especially when there are children.

Once eggs are in soil, they are extremely difficult to eliminate, so stopping contamination in the first place is gonna be the best thing

Key Points

So what are the key things that you need to know about toxocara sis? Toxocara sis is caused by Toxocara canis and Toxocara cati, with the greatest impact being in young animals, young puppies and kittens.

Puppies can actually be born infected due to vertical transplacental transmission, but this does not occur in cats. Larvae migrate through the liver and the lungs, which is going to explain those respiratory signs. While adult worms are the cause of GI disease, which is typically seen as vomiting, diarrhea, and abdominal distension.

Adult animals are typically asymptomatic, but they can still be carriers, and they can shed into the environment and contribute to contamination Fecal flotations are going to be most useful for diagnosis, but remember that a negative result does not rule out infection completely. Anthelmintics remove adult worms, but they are not super reliable for larval forms, especially encysted larvae, so repeat treatments are essential.

And then finally, prevention protects animals, it protects owners and public health. So hygiene, picking up feces, and limiting hunting or scavenging behaviors is really effective alongside routine deworming, of course.

Outro

And that is toxocariasis. Thank you so much for listening to this episode. If you found it helpful, follow or subscribe wherever you get your podcasts.

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