Cats have us wrapped around  their little toe beans.

It doesn’t matter how many times  they throw up a hairball in your bed or stand in the litter box just for  their pee stream to land on the wall, you’d do anything for your kitten. And for some, that includes bending the law a bit.

Not completely breaking it, like robbing  a bank to pay for that massive vet bill, but going into a grey area because your cat has a disease that has no  federally approved treatments. That’s the reality that people  with young cats diagnosed with Feline Infectious Peritonitis  or FIP have faced for years. Without intervention, a kitten with FIP will die.

But vigilantism may be paying off. That’s because some black  market treatments are working, to the point where the FDA changed its  position to save more sick kittens! [Intro music] If you haven’t heard of FIP, you know  its very distant relative, COVID-19. There’s a bunch of coronaviruses  that affect many different animals.

And the big scary one for cats is  feline enteric coronavirus or FECV. This isn’t FIP. We’ll get there in a minute.

Any cat can contract FECV, but kittens around 2 to 4 months old are usually the most vulnerable. If they’re hanging out around other cats’ feces, which they do if you have more  than one cat sharing a litter box, they can catch FECV. Or they can be exposed to it through  contact with any other contaminated object.

Since cats generally don’t perfectly disinfect  their paws after using the litter box, their entire environment is litterally  filled with potential contamination. Some estimates put exposure at around 80% of cats! Even grooming each other, which cats  love to do, risks spreading the virus.

Because of that, places with  lots of cats are at higher risk. But if you’re a cat person who’s never  heard of this super common virus, it might be because the  symptoms are often pretty mild. After exposure, cells in their  GI tract can become infected, but they might not have any symptoms.

Or they could experience some diarrhea, vomiting, loss of appetite, and brief respiratory symptoms. Then their immune systems  catch up within a week or two and take care of the situation. So FECV is no big deal. …unless  it mutates and becomes deadly.

For that to happen, your cat  needs to be unlucky in two ways: First, the cat's immune system fails  to initially fight off the virus, letting it replicate and replicate until oops! It got its copying a little wrong and mutated. The specific mutations can vary, but the end result is that the virus is no  longer limited to just hijacking GI cells.

It can now break into some immune cells too. But just because they can break in  doesn’t mean they can take over. For that, a cat needs to  be unlucky in a second way: their immune cells need to  be vulnerable to infection.

Which explains why only 5 to 10%  of feline coronavirus infections end up mutating and getting  cats sick with this super virus. Most of the time, kittens will be exposed to FECV. And often you won’t even know it’s happened.

Maybe you notice they’re a little sick,  but they get better in a week or so. It’s this 5 to 10% of doubly unlucky cats  that end up in a really dangerous situation. For them, the virus causes FIP.

And once you get FIP, it’s virtually 100% fatal. FIP isn’t a specific virus like  FECV, but a virus-related disease. It can happen weeks to years  after the initial FECV infection, usually hitting cats between 6  months and one-and-a-half years old.

And it totally throws their immune system off. Vulnerable immune cells are suddenly  shuttling virus all over the body. Immune signalling cells raise the  alarm, causing a lot of inflammation.

T cells and B cells are dying as  the virus starts to take over. It doesn’t look good. In fact, it can look like fluid  buildup in the cat’s abdomen and other cavities if they come down  with the “wet” presentation of FIP.

And that’s a bad thing, but at least  you can test that fluid for a clear diagnostic sign that the cat has FIP  and not some other mystery illness. If a cat develops the “dry” presentation of FIP, their organs become inflamed, especially  their kidneys, liver, and lungs. And they can develop  neurological and vision problems.

But those symptoms are harder to see  and can be caused by a lot of things, so the dry presentation can be really hard  for a vet to pinpoint what’s causing it. And if they don’t figure it out fast,  infected cats will develop fever, lethargy, organ failure, seizures,  shock, and die within about a week. The ideal solution to this devastating problem is to prevent it from taking hold in the first place.

And since convincing a cat to wash  their paws isn’t going to happen, experts have gone the vaccine route. There is a vaccine currently  available to prevent FIP. It’s just not very useful.

It can’t be given until the cat is 16 weeks old, which leaves up to two months when the kitten is low on maternal antibodies but not  yet old enough for the vaccine. If the kitten is exposed before vaccination, then they are either already  on track to develop FIP, or already developed immunity to FECV.  Either way, the vaccine does nothing. Also, the specific strain  of virus that the vaccine was developed against isn’t very common.

It only causes about one fifth of infections. So even if you bubble-boyed your cat for 16 weeks and got them vaccinated before exposure, there’s a good chance they’d still be susceptible. And good luck getting your  cat into or out of that bubble without some serious scratches on your arms.

Not everyone agrees on the efficacy  of the existing FIP vaccine. But it could be a good option if you know  your cat will be in a high risk environment in the future, and you can keep them  safe until they’re ready for the vaccine. For some, a not-so-confident  20% chance is worth taking over the close to 0% survival  rate if they develop FIP.

Sure, researchers could try  to whip up a better vaccine. But that might not be worth it because it  could ultimately make the disease worse. That’s because of something called  antibody-dependent enhancement.

When you’re making a vaccine against  a virus that can hijack immune cells, like mutant FECV can, the  antibodies from the vaccine need to be strong enough to neutralize the virus. If they aren’t, then the  virus can still replicate. Which means weak antibodies chauffeur the virus straight into vulnerable immune  cells, trojan horse style.

So, since vaccines aren’t the answer, and paw washing isn’t the answer,  we’re left with treatments. And as of the making of this video, no FDA approved treatments are available for FIP. Despite that, researchers and  a determined cat community have been exploring treatment options.

And their efforts have yielded  some promising results. But before we get into all of that,  it’s time for a quick ad break. This SciShow video is supported by KiwiCo.

As a kid, did you do the  baking soda volcano experiment? Or experiment with the potato battery? Those kinds of hands-on  science projects stay with you and can inspire a lasting interest  in science and scientific thinking.

That’s what KiwiCo is all about. KiwiCo is on a mission to  spark curiosity and empower the next generation of  innovators and problem-solvers. Their monthly crates deliver engaging,  hands-on projects that encourage kids to explore the principles of STEM, build critical thinking skills,  and express their creativity.

They sent us the Tinker crate! It’s got a little spider and a motion sensor and so when it senses motion  it drops down from the sky. Our director, Hiroka, put it together.

There were all of these,  like, circuits and things. It was kind of like a puzzle. So even an adult can have fun putting it together.

And you can use it to prank people  and it’s very cute and Halloweeny. For engaging projects that inspire  future scientists and engineers, click the link in the description  or use the code SCISHOW for 50% off your first monthly  crate at kiwico.com/SciShow. One potential FIP treatment option uses stem cells to give your cat’s immune system the  boost it needs to wipe out the virus.

In a 2025 case report, researchers administered five doses of stem cells to three cats with FIP. All the cats recovered within three weeks! So we know this can work.

But one cat did get reinfected later on. And, unfortunately, resuming  treatment wasn’t enough to save them. So you probably won’t see stem cell therapy as your vet’s first line of treatment anytime soon.

Especially since, in another  study, a vet attempted to treat two FIP afflicted cats with stem cells  and couldn’t save either of them. More popular and effective than stem cell  therapies are antivirals, such as GC376. Not the best name, but also not the  worst you’ll hear in this video.

This drug prevents an enzyme involved in viral transcription and replication  from doing those things. But it seems like this therapy is a mixed bag. A 2016 study found that GC376 could  effectively treat six out of eight cats.

But a 2017 study found that it could  only effectively treat 7 out of 20 cats. Plus it seems like GC376 might be bad for the kittens’ development when they’re  younger than four months old. On top of this, it’s not even a legal option.

Like I said earlier, none of these  drugs are FDA approved for treating FIP. Which means anyone selling, prescribing,  or administering GC376 takes a risk beyond the health of their cat. And, yes, people do take that risk.

But that might change soon,  because the manufacturer is already seeking FDA approval. Plus, there are other options to consider, like the popular antiviral  Remdesivir, AKA GS-5734. You might recognize this drug name because it’s an FDA-approved treatment for COVID-19.

As an antiviral, it works by mimicking  a nucleotide building block for viruses. When Remdesivir is scooped up  instead of a functional nucleotide, the viral genome gets a jam in its  metaphorical copy machine and stalls out. In a 2025 study, researchers  treated 29 cats with this antiviral.

And 25 survived! That’s an 86% survival rate. And additional studies ranged  from 77-88% survival rate.

Those numbers are so good that even though it’s not an FDA approved FIP treatment,  you can still get it legally in the US. Not technically for FIP, but it is FDA approved for other uses and vets are allowed  to prescribe it off-label for FIP. But that’s if you can afford it.

Remdesivir is super expensive, so it can  be hard for vets to get their hands on. And this has led people to the black market. Now, the modest success and risk of adulteration has so far deterred a lot of folks from  going there to get GC376 and Remdesivir.

But there’s another drug that draws  more people to the black market. It’s the main thing Remdesivir turns  into once it's in your body: GS-441524. This drug cuts out the middle  man, which might make it easier to get where it needs to be in your cat.

And it uses the same “jamming  the copy machine” principle. It’s been put to the test. A study in the journal “Viruses”  gave 18 cats with FIP a drug with GS-441524 as the active  ingredient, and all 18 cats survived.

Not all the studies looking  at this drug are that glowing, with others reporting survival rates of 75 to 80%. But studies showing its efficacy keep coming out, kind of turning GS-441524 into the  cat community's dirty little secret. It’s a secret because vets haven’t been  able to legally prescribe the drug.

So, through the desperation  of people who love cats, an underground network for the antiviral emerged. Facebook Groups formed, where you  could post anonymously requesting the medication and connect with  suppliers who source the drug and send it out in unlabeled vials. Advice about dosing and treatment protocols is also shared in DMs to avoid culpability.

Drug manufacturers mainly in China became major suppliers for websites  selling this medication. And people pay thousands of dollars for the stuff. This is not the recommended  way to treat anything because there’s no quality control, no  professional oversight of the dose or concentration you’re administering,  and no guarantee that you’ll even get a product after sending an  unlicensed stranger all that money.

If it sounds sketchy, that’s because it is! But for these folks, you do what  it takes to save your kitties. It’s like you’re living one of those  “Arms of an Angel” commercials.

And, to reduce risk, many  still worked with their vets for monitoring and supportive treatments. Just like in the studies, a  lot of cats have recovered! The people who tried it for  their cats were sold on it, and vets were coming around to the drug too.

As the hype increased, it wasn’t long before the cat was out of the bag  and regulators took notice. In a 2024 statement, the FDA pretty  much gave this antiviral their blessing. Well, as much as they could unofficially.

It’s still not a legally approved treatment, but they aren’t going to go after anyone  for prescribing or using it for FIP. At this point, it seems like  the FDA is ready to approve it. The problem is they can’t approve it, because the company who owns the patent  hasn’t submitted it for approval as a veterinary medication, and they don’t  seem very motivated to do it soon either.

You gotta be kitten me! But with this pinkie swear from the  FDA, you no longer need the black market to get this medication. Vets can now safely get it from the pharmacy.

This is a huge win for cats! So many kittens that had  incredibly bleak prognoses before will be able to get treatment that  gives them a legitimate chance of survival, without the countless risks of the black market. But it’s not the end of this feline fairytale.

The antiviral alone probably  isn’t the perfect solution. Researchers are advocating for several of the therapies we mentioned to be used together. A 2025 study argued that using GS-441524 followed by stem cell therapy offered  the best outcomes for these cats.

Another study combined GS-441524 and GC376, and after four weeks of this combined treatment, only one cat out of forty six died. That’s practically a 98% survival rate. Ultimately, even with patents standing in the way, it was only a matter of time  before a determined community figured out how to help  their ailing feline friends.

I guess the saying is true. Cats always land on their feet. [♪ OUTRO]