Bovine Viral Diarrhoea (BVD)

In this episode of Current Vet, Dr. Lottie talks about bovine viral diarrhoea (BVD), the clinical signs that develop (not just diarrhoea!), how persistently infected calves are created and why treatment should NOT be attempted for these animals.

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Timeline:
00:00 Intro
00:36 Case
01:35 Aetiology & Pathogenesis
07:10 Clinical Signs
08:42 Diagnosis
11:02 Treatment
12:13 Prevention
13:28 Key Points
14:37 Outro

References

• Lanyon, S.R. et al. (2014) ‘Bovine viral diarrhoea: Pathogenesis and diagnosis’, The Veterinary Journal, 199(2), pp. 201–209. Available at: https://doi.org/10.1016/j.tvjl.2013.07.024.
• Lindberg, A.L.E. (2003) ‘Bovine viral Diarrhoea virus infections and its control. A review’, Veterinary Quarterly, 25(1), pp. 1–16. Available at: https://doi.org/10.1080/01652176.2003.9695140.
• Walz, P.H. et al. (2020) ‘Bovine viral diarrhea virus: An updated American College of Veterinary Internal Medicine consensus statement with focus on virus biology, hosts, immunosuppression, and vaccination’, Journal of Veterinary Internal Medicine, 34(5), pp. 1690–1706. Available at: https://doi.org/10.1111/jvim.15816.
• Grooms, D.L. (2004) ‘Reproductive consequences of infection with bovine viral diarrhea virus’, Veterinary Clinics: Food Animal Practice, 20(1), pp. 5–19. Available at: https://doi.org/10.1016/j.cvfa.2003.11.006.
• Wang, Y. and Pang, F. (2024) ‘Diagnosis of bovine viral diarrhea virus: an overview of currently available methods’, Frontiers in Microbiology, 15, p. 1370050. Available at: https://doi.org/10.3389/fmicb.2024.1370050.

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Creators and Guests

Host

Dr. Lottie Wilkinson

Creator and host of the Current Vet podcast

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Intro

Hello, hello, and welcome back to Current Vet. If you're new here, this is the podcast that makes vet new medicine simple. I'm your host, Dr. Lottie, and today's episode is all about bovine viral diarrhea, or BVD. Now, despite the name, BVD is not just about a dodgy tummy and diarrhea. It also involves immunosuppression, reproductive failure, and mucosal disease, which is the form of the condition that persistently infected animals get.

So let's dive straight into a case

Case

You're called out to see a 12-month-old beef heifer, Daisy. The farmer tells you that she has never quite been right. She's always been a little bit smaller than her peers, needed treating for pneumonia several times as a calf, and just hasn't done as well as the rest of the group.

Over the last week, things have taken a turn. She's become depressed, pyrexic, she's not eating, she's passing profuse diarrhea, she's drooling, and now there are these erosions visible on her gums and along the nasal planum. On exam, she's quite dehydrated, weak, and there are ulcerative lesions in her mouth, and the diarrhea is really foul smelling and watery.

She's losing weight rapidly. A couple of differentials that we're gonna consider are Salmonella, maybe a parasitic enteritis, and then our viral causes like malignant cattle fever and BVD

Aetiology / Pathogenesis

Okay, let's talk about what BVD actually is. So BVD is caused by bovine viral diarrhea virus, and it is a pestivirus in the Flaviviridae family. It's an enveloped positive sense RNA virus, which means it's actually quite fragile in the environment and doesn't survive well outside the host. But even though it's fragile, it spreads very efficiently from PI animals, and they're persistently infected animals.

There are two key species of BVD virus, and they're very imaginatively named BVDV-1 and BVDV-2. One
is more common globally, and two is less common, but sometimes associated with more severe outbreaks of disease. There are also two biotypes, and these are non-cytopathic and cytopathic

So in a normal non-pregnant cow, bovine viral diarrhea virus, usually enters via the oro-nasal route. It replicates in the tonsils and local lymph tissue and spreads via the bloodstream causing a viremia. The virus has a strong affinity for lymphoid tissue, bone marrow, and immune cells. It's gonna cause lymphopenia, impaired neutrophil function, and it is going to disrupt cytokine signaling, which causes a transient immunosuppression, so the immune system is not functioning as it should, but it's only transient, so it means it happens for a short-term period.

The infected animal may show mild signs like pyrexia, diarrhea, and lethargy or actually sometimes just nothing at all. Within one to two weeks, the immune system is going to clear the virus, clear the infection, and then protective immunity develops. But during that window of immunosuppression, the animal is a bit more likely to develop other infections, like enteric infections or respiratory disease

Occasionally, particularly with those BVDV2 strains like we said before, transient infections can be a little bit more severe. These animals could develop high fever, severe diarrhea, thrombocytopenia, and hemorrhagic syndrome, but this is far less common than the subtle just mild immunosuppression form.

If a pregnant cow becomes infected, the outcome for the fetus depends on the stage of gestation. So in early gestation, in that kind of period of zero to day forty, the embryo is likely to die. This can just be seen as the cow fails to conceive, they're a repeat breeder, or they have slightly extended calving intervals, so it's unlikely to be obvious that BVD was actually involved.

In mid-gestation, so around days forty to one hundred and twenty, the fetal immune system is not yet competent. It's not yet mature. So if they become infected with a non-cytopathic form of the virus, the fetus doesn't recognize the virus as foreign. Instead, immune tolerance is going to develop to the virus, so the calf is born infected in all its tissues.

It is unable to mount an immune response because the body sees it as a self-antigen, and they are persistently shedding the virus. So this is how those persistently infected animals are created, those PI calves If the fetus is infected in late gestation, so 120 or more days, by this point the fetus is immunocompetent.

So infection may cause abortion, can also cause congenital abnormalities like cerebellar hypoplasia or birth of a weak but a virus negative calf. So not all in utero infections produce PI calves. We can have different outcomes depending on the stage of gestation

So let's talk a little bit more about those PI calves. They are infected in every tissue. They shed the virus constantly in all body secretions, so saliva, nasal discharge, urine, faeces, milk, semen, anything. They do not produce antibodies because their immune system considers the virus to be a self cell, and they become a reservoir of infection that is going to maintain BVD within a cattle herd

If PI animals then become co-infected with a cytopathic strain of the disease, or if the infecting non-cytopathic strain mutates into a cytopathic form, the animal , is going to develop severe mucosal disease. So the cytopathic form of the virus destroys epithelial cells, which is gonna result in severe erosion and ulcerative lesions, profuse diarrhea, rapid deterioration of the animal, and then death
It's important to note that mucosal disease only develops in PEI animals because the immune system has learned to tolerate BVD virus, so it doesn't mount an immune response to infection

Clinical Signs

So what clinical signs are typically seen with BVD? So for transient infections in immunocompetent adult cattle, we can see that mild pyrexia, reduced appetite, transient diarrhea, or actually just nothing at all. The virus is typically cleared in a couple of weeks, and then immunity develops.

But remember that during that short infection window, BVDV causes immunosuppression, which is why we may also see signs of enteric or respiratory disease as well

Depending on the stage of gestation, pregnant cows may present with early embryonic death, repeat breeding, abortion, or Calves born may have congenital abnormalities. PI calves typically look very normal

So the first signs that they are infected often are that they are poorly growing. They're not developing the same as calves the same age. They are persistently ill, recurrent infections like pneumonia, and they have a much higher mortality rate It's almost invariable that PI animals develop mucosal disease before the age of two, and it is fatal.

So these signs of mucosal disease include that profuse diarrhea, oral and nasal ulceration, rapid weight loss, severe dehydration, and then death often quite quickly within a couple of days

Diagnosis

Let's go back to Daisy. So given her history and the clinical signs, BVD is high on your list. But how are we gonna confirm it? So with diagnosis, the most important thing is identifying PI animals because they are the biggest source of infection for the rest of the herd. We're not as bothered about finding those transiently infected immunocompetent animals because they recover typically without complication.

So we can use an antigen ELISA, and this is most often performed on ear notch samples, which is actually really useful and practical for farmers because they can take an ear notch sample while they are ear tagging their calves, and in this way can routinely test the whole herd, and this is gonna detect viral proteins.

We can also do a PCR, and this is gonna detect viral RNA. It's highly sensitive and can be performed on pooled blood samples to assess the wider herd status

If the cow's condition seems quite hopeless, irrespective of a definitive antemortem diagnosis, we can perform humane euthanasia and then confirm the virus on post-mortem samples

So say we have now identified a PI animal. We must assume that there are going to be more animals infected within the herd. So our next steps are gonna be to test all the calves in the cohort with an antigen ELISA or PCR to identify those hidden PI animals, or consider bulk milk tank samples in dairy herds or pooled blood samples to do antibody ELISAs and assess herd exposure If we're using antibody ELISAs in order to diagnose active infection, we need to see a fourfold increase in antibody levels between two consecutively taken tests, typically taken around three to four weeks apart, which is demonstrating that the infection is ongoing and an immune response has been mounted.

Remember, though, that PI animals are going to be antibody negative because they are immunotolerant to the virus. They do not mount an immune response . So antibody ELISAs are useless for detecting PI animals
Treatment

There is no antiviral treatment for BVD. Transient infections typically resolve with, resolve on their own, and we can give supportive care just to help them recover. We could do some fluids or nutritional support, but typically they're okay on their own. Secondary infections may need antibiotics, for example, if we think we have a bacterial pneumonia.

But then our PI animals cannot clear the virus, and mucosal disease is - almost always fatal, so treatment attempts are essentially useless Treatment in BVD is really therefore about herd level decision-making and preventing further spread rather than trying to cure individual cases. PI animals should be humanely euthanized because they will continue to deteriorate and then die anyway, generally before they reach two years old

, And they are also a huge ongoing source of infection for the rest of the herd because they continue to shed the virus in all body secretions, so keeping them in the herd poses a risk to all our other animals

Prevention

Prevention hinges on three key pillars. So number one is gonna be to identify and remove PI animals. So it's really non-negotiable, and without removal, eradication is going to be impossible. The second pillar is good biosecurity. We need to be testing all purchased animals, isolating them before introducing them to the herd, sourcing semen responsibly and from confirmed non-PI animals, screen the herd for PI animals either through PCR or antigen ELISA

The third pillar is then going to be vaccination. So vaccination helps to reduce the severity of infection and protect the calf during gestation

Modified live vaccines should not be used in pregnant cows, but inactive ones can be. Antibodies against the virus are then transferred in colostrum, which provides protection in the calf's early life. And many countries have structured eradication schemes that are based around mandatory testing, movement restrictions, removal of PI animals, and then ongoing monitoring and surveillance

Key Points

So before we finish, here are the essential things that you need to know about BVD. Bovine viral diarrhea virus is a pestivirus that causes immunosuppression, reproductive failure, and birth of persistently infected animals. PI animals are created when infection occurs in utero between around forty to a hundred days gestation, and these animals then shed the virus for life.

Mucosal disease occurs only in these PI animals and is almost always fatal. It causes severe mucosal ulceration, profuse diarrhea, rapid weight loss, and then invariably the animal is going to die before they reach two Diagnosis is gonna be focusing on identifying those PI animals using antigen ELISA or PCR tests, and there is no antiviral cure, so control depends on euthanasia of PI animals, really good biosecurity, and then vaccination.

Outro

And that is bovine viral diarrhea. If you found this helpful, follow or subscribe to Current Vet wherever you get your podcasts and share it with a friend or colleague who needs to learn about this topic too. If there's a topic you'd like us to cover next, find us on Instagram or TikTok @veterinaryvista or let us know what it is in the comments.

We'll see you next time