Canine Distemper

In this episode of Current Vet, Dr. Lottie talks about canine distemper, its incredibly varied presentation, the weird form that occurs in older dogs called ‘old dog encephalitis’ and how to approach diagnosis and treatment.

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Timeline:
00:00 Intro
00:53 Case
01:35 Aetiology & Pathogenesis
04:57 Clinical Signs
06:10 Diagnosis
11:30 Treatment
13:26 Prevention
15:31 Key Points
16:48 Outro

References

• Sykes, J.E. and Vandevelde, M. (2021) ‘22 - Canine Distemper Virus Infection’, in J.E. Sykes (ed.) Greene’s Infectious Diseases of the Dog and Cat (Fifth Edition). Philadelphia: W.B. Saunders, pp. 271–288. Available at: https://doi.org/10.1016/B978-0-323-50934-3.00022-7.
• Martella, V., Elia, G. and Buonavoglia, C. (2008) ‘Canine Distemper Virus’, Veterinary Clinics of North America: Small Animal Practice, 38(4), pp. 787–797. Available at: https://doi.org/10.1016/j.cvsm.2008.02.007.
• Beineke, A., Baumgärtner, W. and Wohlsein, P. (2015) ‘Cross-species transmission of canine distemper virus—an update’, One Health, 1, pp. 49–59. Available at: https://doi.org/10.1016/j.onehlt.2015.09.002.
• Beineke, A. et al. (2009) ‘Pathogenesis and immunopathology of systemic and nervous canine distemper’, Veterinary Immunology and Immunopathology, 127(1), pp. 1–18. Available at: https://doi.org/10.1016/j.vetimm.2008.09.023.

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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 veterinary medicine 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 carrying on our month of canine conditions, talking about canine distemper virus. It's a bit crap to learn about this one, as it can have so many different kind of presentations.

It can resemble so many different things. So we're gonna go through the condition today, talk about the virus itself, how it causes disease, and why it can cause that weird condition known as old dog encephalitis, even in dogs that have been fully vaxxed. I do just wanna say thank you so much to our lovely listener for requesting this topic, and if anyone has any topics that they wanna hear next, then just let us know.

Case

So let's start with our case. You've got a seven-year-old mixed breed dog presenting with progressive ataxia, occasional twitching of the limbs. The owner reports he has been a little bit off for a couple of weeks. There's some slight weight loss and no clear vaccination history because he has been recently fostered.

On exam, his mentation is quiet but responsive. He has a slight head tilt and mild proprioceptive deficits. You also notice subtle intermittent muscle twitching. So at this stage, distemper is probably not gonna be at the top of your list, and that is of course why we're going to fully talk about it today.

Aetiology & Pathogenesis

So what is canine distemper? Canine distemper virus is a morbillivirus. It's in the same family as measles. It's highly contagious. It's spread via aerosols and secretions, and it's maintained in populations not just by dogs, but also by wildlife reservoirs, so species like foxes, raccoons, skunks, mustelids, so that's gonna be your ferrets, mink, otters, and then other wild canids, so coyotes.

So even in areas with decent vaccination coverage, it is never going to fully disappear, and the virus can spill over into domestic dog populations again from these wildlife reservoirs. The virus usually infects the host via aerosolization or direct contact with contaminated body fluids, saliva, urine, and it enters via the respiratory tract and replicates in lymphoid tissues, so like the tonsils and lymph nodes.

Within the lymph system, the virus attacks lymphocytes, which causes- Immunosuppression, meaning the animal is more at risk of secondary infections, which can also confuse the clinical signs and diagnosis if we then have a concurrent disease.

From the lymph system, it can then spread systemically via the blood or lymphatics, and the virus targets epithelial tissues as it spreads through the body. So in the respiratory tract, it's going to infect the respiratory epithelium, causing coughing and nasal discharge. In the GI tract, it's gonna cause vomiting and diarrhea from gastroenteritis, and in the skin, it can cause hyperkeratosis, which is called hard pad disease.
The virus also spreads to the CNS, central nervous system, where it can cause neuronal demyelination, inflammation, and neuron damage. So neuro damage can actually be either acute or chronic. Acute neurological distemper occurs at a similar time as the systemic illness, and we're gonna see seizures, ataxia, paresis, paralysis, those sorts of signs.

On the chronic or delayed neuro disease presentation, this is gonna be that weird presentation known as old dog encephalitis. And this is essentially a chronic progressive encephalitis that can occur months or years after initial infection has occurred.

And it can occur in dogs with unknown or mild prior infection when they were younger, or even in dogs that have actually been vaccinated against the virus. So old dog encephalitis is caused by a persistent viral infection within the central nervous system, causing ongoing inflammation and demyelination.
We're gonna see progressive ataxia, myoclonus, which is that twitching that we saw in the case before, focal seizures known as gum chewing seizures, some behavioral changes, and we may also see generalized seizures.

Clinical Signs

So let's quickly just recap those clinical signs that we're gonna see in different stages of infection. So in the early phase, the immunosuppression phase We might see parexia, often biphasic parexia with a couple of days gap in between these two episodes, lethargy, inappetence, and anorexia.

And then dogs can recover here if the immune response is sufficient, or they can progress to further disease and further infection. So then we might see respiratory signs like mucopurulent nasal discharge, ocular discharge, coughing, dyspnea. We can see gastrointestinal signs, which might be vomiting, diarrhea, and weight loss.

Cutaneous signs, so that hyperkeratosis of the footpads, known as hard pad disease, and we can also see hyperkeratosis on the nose. And then the neurological signs. So in that acute case, we might see the seizures, ataxia, paresis, nystagmus, and then in the chronic cases, progressive ataxia, behavioral changes, myoclonus, and seizures, either focal or generalized.

Diagnosis

Okay, let's talk about diagnosis. Before we actually talk about the tests, it is important to know that treatment is often started based on a presumptive diagnosis. so in practice, you're , rarely gonna be sitting there with a definitive positive result before making treatment decisions. So instead, you're gonna be working off the signalment, so potentially an incomplete vaccination history or unknown vaccination history, and then the clinical signs of the progressive multisystemic GI, epithelial, and neuro signs.

By the time a dog is presenting with neurological signs, viral shedding might actually be quite inconsistent, and PCR results can give false negatives. And if it's the case of old dog encephalitis, there may be no clear history of earlier infection, as it could have happened months or even years ago.

So at that point, testing is gonna be less reliable. So if distemper is on your differential list, even without confirmation, we should be isolating the patient immediately and handling them as potentially dangerously infectious.

Okay, so sampling and diagnostic tests are best done in early stage disease. We can do a PCR on samples like conjunctival swabs or nasal swabs, also on whole blood, , urine or CSF, and it has a high sensitivity during viremic episodes, so that early stage disease.

But later on in the disease process, and when there is central nervous system infection, false negatives are more likely to occur. PCR is just gonna be detecting that viral genetic material and directly identifying the virus itself. We can then also do a hematology, biochemistry and early stage disease, it's likely gonna show lymphopenia.

We might see electrolyte derangements from the vomiting and diarrhea
as well as general signs of inflammation. And these are not gonna be diagnostic, but they will support a diagnosis.

Once neurological signs have developed, things become a bit trickier. So PCR can still be done on urine, conjunctival swabs, or CSF, but know that viral shedding can be reduced or actually just have stopped. So negative PCR results don't necessarily rule out distemper as a differential. We can do CSF analysis at this stage, and this can show a mononuclear pleocytosis, and this essentially just means that there's an increase in the mononuclear, white blood cells, so monocytes, lymphocytes, and macrophages.

We can also test the CSF for distemper virus specific antibodies or the viral antigens themselves with ELISA and PCR.

Know that the pleocytosis finding is nonspecific for the infection, but it does support or suggest an inflammatory CNS disease We could do serology to detect antibodies to CDV, but these are not gonna reliably distinguish between vaccine-induced antibodies or infection-induced antibodies. So there is a limited use for this in practice because the majority of the canine population is now vaccinated as distemper is a core vaccine.

We can do imaging, and MRI may show multifocal CNS lesions or demyelination. Again, this is not specific for distemper, and it's also not widely available or even if it is available, it's very expensive, so it may not be accessible for a lot of clients. However, it is useful for ruling out differentials. So if we have neurological disease and neoplasia, especially in an older dog, and neoplasia is a differential, it's useful for ruling out those differentials.

Other imaging methods like radiography can be used to assess the spinal cord, but this is not gonna be useful for looking at the brain. Finally, we also have postmortem testing. We can do histopathology on central nervous tissue, which can give a definitive diagnosis. Obviously, this is not going to be useful for diagnosis in a live patient,

but it's the most reliable way to get a definitive diagnosis. It's just obviously completely impractical for live diagnosis. But you'll see demyelination of the neurons and intracytoplasmic inclusion bodies in both epithelial cells and white blood cells.

So what are we actually gonna do in practice? We can run a PCR on urine or conjunctival swabs, consider a CSF analysis if that is possible, and use imaging to rule out differentials, so for example, neoplasia, if those are also high on your list. But most importantly, diagnosis is often based on the clinical presentation, based on the signalment, it's the neurological and multisystemic signs, and basically lack of a compelling alternative differential.

Treatment

There is no antiviral treatment available for distemper with proven clinical efficacy, so management is going to focus on supportive care. We're gonna use fluid therapy, nutritional support, and nursing care are gonna be really important.

We want to control seizures, so we can use anti-epileptic drugs, So in the acute setting, we'd use things like diazepam, or longer-term control could use phenobarbital or levetiracetam. We wanna manage secondary infections if they're present, and these are quite common because of that immunosuppression, so consider broad-spectrum antibiotics if there are signs of pneumonia or other bacterial infections.
And remember that we said that isolating the patient and trying to prevent spread to other dogs, but also other wildlife species, is really important. So isolating any suspected or confirmed cases is gonna be really key. Dogs with the non-neurological signs, so just those GI signs, potentially epithelial signs, the prognosis is variable.

Some do recover, some don't recover as well. But once that neurological involvement has progressed, the prognosis is typically guarded to poor. And then any dogs with the old dog encephalitis form, this is progressive, and it has a poor prognosis. So in our case, we are not gonna be trying to cure this infection.
We're gonna be controlling signs when possible, assessing progression of the disease, supporting quality of life, and making sure we're having conversations with owners about what our goals and aims of treatment are because we are not going for a cure.

Prevention

Okay, let's quickly talk about prevention. So unlike treatment, where our options are quite limited, prevention is very effective, and it's the main reason that distemper is now pretty uncommon in well-managed populations. So vaccination is going to be the key part of prevention in our dog populations. It is included in the DHPP vaccine, which is distemper, hepatitis, parvovirus, and parainfluenza.

And the standard protocol for dogs is Start at around six to eight weeks old. Repeat the vaccine every two to four weeks until they are around sixteen weeks old. They then get a booster. Some vaccine brands recommend at six months, some at twelve. And then adults typically get a booster every three years, kind of depending on the vaccine guidelines and their risk level.

Remember, though, even with good vaccination coverage in our dog populations, these wildlife populations of foxes and raccoons and ferrets can maintain the infection in the environment, so eradication is not gonna be realistic. So instead, we're gonna rely on herd immunity in our canine populations,

in our domestic dog populations, and basically hope that that provides enough protection that the disease reduces. If we suspect distemper, make sure we are immediately isolating these cases. Barrier nursing is really important, using dedicated equipment and dedicated staff for that dog and separating and reducing the infection risk to other animals in the building.

Anyone going near the dog needs to be in full PPE, and make sure that any bedding, any housing, any blankets or material that could possibly be contaminated are disinfected. CDV is actually very susceptible to disinfection, so this step is really key.

Key Points

So what are the key things that you should know about distemper before you go?

Canine distemper is a multisystemic disease that progresses from immunosuppression to epithelial disease, GI signs, respiratory signs, and neurological involvement. Neurological signs can appear at the time of infection, along with the rest of those GI and respiratory signs, or they can appear months to years later, and can be the only presenting feature in older dogs, and that is known as old dog encephalitis.

Diagnosis is typically presumptive, and remember that isolation and supportive care should be started as soon as distemper is suspected. There is no cure, so management is supportive, and there is a poor prognosis once neurological signs develop Old dog encephalitis is a chronic progressive form caused by persistent CNS infection, inflammation, and demyelination.

There is no treatment for this.

And then finally, disease is highly preventable with vaccination.

Outro

And that is it for today's episode on canine distemper. Thank you so much for tuning in. If you enjoyed this episode, give us a like, give us a rate, a follow, whatever you fancy. It really does help the podcast grow. And if you have any topics that you'd like to request, let us know in the comments or find us on TikTok and Instagram, @veterinaryvista, to tell us what you want to hear next.

Thank you again to our lovely listener who requested this episode, and we'll see you next time