Of all the domesticated animals, cats are certainly the most polarizing; it’s pretty hard to find someone who is just ambivalent towards cats. When you stop and think about it, they are an odd animal to have in our homes. Nowadays, we have them as companions, but it seems strange that several thousand years ago, we permanently befriended a small, self-absorbed carnivore. They can have serious attitude and seem to run on their own schedules. This may be because they are probably unique among domestic animals in that nobody intended for them to become pets, they just hopped on the anthropocene bandwagon early. They really haven’t changed much from their wild ancestors. You don’t see wolves catching Frisbees or bison merrily trotting into the milking parlor, but often you’d be hard pressed to tell the difference between a completely wild cat and a domestic one. They’ve retained a lot of the features that make them… well, cats.

This strange phenomenon of pet cats begets a controversial question: should they be allowed to go outside? Weird question, right? It might seem perfectly reasonable to have cats come in and out of the house as they please, just like they’ve done for thousands of years. But the balance between the needs of an essentially wild animal and living in a 21st century home are a bit more complicated than appear at first blush. If you suddenly had a murderous wild caveman living on your couch, would it be okay to dress him in a tux and take him to prom? What are we supposed to do with our little self-domesticated mesopredators? Keep them indoors? Let them roam free? Is a life outdoors a basic right for cats fulfilling their ancestral needs, or is it a dangerous and fragile place from which they should be sheltered? Confine them to fluorescent lighting and bad carpeting for their own safety? Or pitch them out into a violent world so they don’t scratch up our La-Z-Boy? Does anyone actually know? Ok, that’s enough rhetorical questions for one paragraph (or is it?).

Let’s stay focused

This is a hot topic, and as usual more vociferance is used than science in the debate. Before going too far into it, I want to make it clear that this discussion can easily stray into the tangential and philosophical. This isn’t about whether or not cats want to go outdoors (most of them really do), it’s really about what the consequences are when they do or don’t. I have my own opinions about whether or not they should, but that’s not really the aim of this post (don’t worry, I’ll give it to you later just to be annoying). My focus will specifically be on the health of pet cats when they roam outdoors. Let’s make a science-based argument to Hank the Cat about whether or not he should run his Senate election campaign from the living room couch. Scientific findings in this field are emerging, nuanced,* often surprising, and open to passionate interpretation. With that said, let’s dive into it.

The risks of being an outdoor cat

The first and most intuitive question is, “do indoor cats live longer?” Impossibly, no scientific answer to this exists. Despite authoritative claims on millions of cat websites, I could not track down a source on this.** No one has actually compared longevity between indoor and outdoor cats. Claims that outdoor felines live years shorter than indoor ones are like the Cheshire cat: they appear out of nowhere, and then vanish if you try looking at them too closely. The closest thing I could find was one study looking at causes of mortality in French cats that stated outdoor cats usually lived about 10 years, and that indoor cats could live up to 20 or more years. But no direct comparison reported in the data, just that statement. We don’t have a study where that single variable has been examined.

So we have to concede that science does not have a clear answer to this frequently asked (and answered) question. What the crap, science? Now don’t get me wrong, it’s pretty obvious that there are many more dangers outside than inside, and that the expected lifespans of outdoor cats would be shorter. We actually have a fair amount of data about what happens to cats when they do venture out into the world, and oh boy what a scary world it is.

The biggest risk for outdoor cats appears to be trauma, specifically, road traffic accidents. Getting hit by a car, as you might expect, is deleterious to your health. There may be several thousand cats killed by cars every year in cities like Baltimore (I’m not sure if this was supposed to be commentary on how they drive on the East Coast or not). But it’s not just roads; in a fantastical study where researchers put cameras on cats and watched them roam about (they call it the “KittyCam”, but I would have suggested “Purr-veilance”), they found cats were engaging in all sorts of “risky behaviors”. These cats were acting like stoned teenagers: drinking strange liquids, hanging out with unfamiliar conspecifics, climbing trees and buildings, and entering storm drains (creepy, who knows what’s down there).

None of the cats in that study came to unsavory fates, but they were lucky to have not been eaten by a clown. Or anything else for that matter. Being on the smaller side of the carnivore spectrum, cats are definitely considered edible-eligible by other animals. Where I live, we have coyotes everywhere. They’re bold. I swear I’ve seen them ordering lattes at the local coffee shop. And they do in fact kill a fair number of cats (although whether or not these are pets or strays is hard to determine). Although less data is available, many other animals such as mountain lions, wolves, and birds of prey have probably tasted cat. Heck, I bet a pelican has tried once or twice (trigger warning: don’t click on those links, seriously). Cats themselves are not always cat lovers, and territorial spats leading to serious injury are pretty common.

For pure, murderous, non-accidental cruelty, however, nothing comes close to people. As the 1987 nature documentary Predator shows us, man is the most dangerous hunter, especially when mud-encrusted and harriedly tossing grenade-spears around. And indeed, a recent study identified cases of gunshot, poisonings, burnings, and asphyxiation in about 0.25% of cats from the general population. While these data didn’t directly compare outdoor and indoor lifestyle, cats that never leave the home are probably not getting shot very often. Sadly, there are a lot of assholes out there that like to shoot free-roaming cats.

Whether by tooth, tire, or firearm, it is unsurprising that most of the traumatic deaths are young, dumb kitties prowling about. In a study of English cats, accidental deaths start off as a big proportion, but then sharply drop off after five years of age (going from the #1 cause to the #6 cause). We can’t say directly that all of these deaths are from being outdoors since the researchers didn’t categorize that, but it is interesting to note that about 90% of cats in the UK are allowed outside. Other studies looking at feline mortality in other ways have confirmed the live-fast-die-young phenomenon. Accidental death may be responsible for half of young cats deaths.

Infectious disease also kills the young, although again we don’t know if these were outdoor cats or not. Not every infectious disease requires being outside to get, but there are plenty where it is an established risk factor such as feline retroviruses, mycoplasmosis, tularemia, plague, and rabies. I couldn’t find direct evidence (i.e. I got lazy), but I bet worms, ectoparasites, bite abscesses, fungal infections, and other infectious diseases are overrepresented in outdoor cats.

The last risk that is commonly mentioned for outdoor cats is poisoning, but this is a bit tricky since both indoor and outdoor poisons exist. There are definitely potential toxins outside, but you could make a reasonable argument that there are more potential poisons inside the house. No one, to my knowledge, has compared the risk of toxicity for indoor and outdoor cats.

Social costs of outdoor cats

Maybe you know a cat owner who is okay with the chance of some horrible fate befalling their pet. After all, that’s how they live in the wild, right? They may have a fatalistic stance, but outdoor cats have consequences beyond how satiated the local coyotes are. Is there a greater cost to feline libertarianism?

Unfortunately, yes. One of the most controversial topics regarding outdoor cats is their effect on wildlife, and cats can be devastating to small wildlife. It is difficult to know exactly how much of the carnage rests on the shoulders of pet cats versus feral ones, however, there is good documentation that owned cats hunt native wildlife species. That kitty door in your kitchen is home base for a killing spree.

After that endangered pocket gopher gets eaten by Garfield, it gets turned into feces, and there are no port-a-potty’s for cats. They can really turn out the turds (an estimated 1.2 million tons per year in the U.S.). This is bad, because cat feces are a major source of exposure for a parasite named Toxoplasma gondii. You may have heard about this as a serious risk to pregnant women and immunocompromised people, but domestic cats pooping al fresco might be very damaging to wildlife as well. In addition to toxoplamosis, there is a long list of bad bugs that can cats can spread. It should be said that feral cats are probably a much bigger factor in disease transmission than pets, but pets probably contribute a little, especially if they are unvaccinated and not on parasite preventatives.

How healthy are indoor cats?

Okay, okay, okay, so it’s dangerous for free-roamin’ felines, and they’re not exactly environmental stewards. From one Cat to the others, “just remember there’s a lot of bad and beware”. If you keep Mittens indoors, it’s gonna be hard (but not impossible) for her to catch rabies or get hit by a car, but that doesn’t mean there aren’t going to be tradeoffs. It would seem to stand to reason that keeping a cat indoors should keep it healthier, and that is probably true, however there is going to be an increased risk for other conditions. There is a mixed but growing body of evidence that when we keep cats indoors, they are at higher risk for some chronic diseases like obesity, hyperthyroidism, cystitis, dental lesions, and diabetes. I see these every day in general practice, where they are significant causes of morbidity and sometimes mortality (although less direct than a speeding Buick).

Why would this happen? Well, just like taking a tuber-gathering bipedal ape out of the wild and shoving an endless supply of cheeseburgers into his mouth comes with its own set of health consequences, you can’t expect cats living an inactive lifestyle to be perfectly healthy. Importantly, many of these conditions are not a direct result of being indoors, but related to too much food and lack of exercise (e.g. obesity and diabetes) and stress (e.g. cystitis). These are common conditions that reduce quality of life in pet cats, but they can be prevented or mitigated to some degree. This simplification doesn’t necessarily explain all the conditions (e.g. we still don’t know what causes feline hyperthyroidism), but it is likely that lifestyle plays a more important role in overall health than an unidentifiable toxin that only indoors cats are exposed to. Controlling diet and removing stressors can help to reduce the incidence of many of these conditions. Cats are gonna cat, and if you mess with what makes them a cat you can expect problems. As one source on what a cat wants put it, “the consensus seems to be that cats appear to benefit from appropriate access to resources, control of interactions with owners, and a tolerable intensity of conflict”. I love that. How many other domestic animals do you think require a tolerable intensity of conflict?

Conclusion: There’s chickadee blood on your hands

It probably is true that on average outdoor cats are not going to live as long as their indoor brethren, and most at risk are going to be young cats since, like young organisms everywhere, they are making stupid decisions. Outdoor cats can meet some gruesome ends, with a lot depending on where you live. Outdoor cats in Manhattan face a different set of dangers than those living next door to Alf. In addition to putting their own lives in peril, these free-roamers cause completely unnecessary and often severe damage to public and wildlife health. It’s probably better for everyone to just have your kitten stay inside, but there is a tradeoff. Increased risk of some chronic conditions comes with the safety of a sedentary life. These diseases are to some degree preventable with appropriate care. I know a lot of people like having cats because they seem easy to take care of, but that convenience might be hiding a true cost. If it means you let your cat outside, you’re exposing it to life-threatening dangers (and there’s chickadee blood on your hands). If it means you keep your cat indoors but don’t control its diet or pay attention to its health, don’t be surprised if medical problems develop.

My own opinion? Thanks for asking! It may not be worth much salt because I’m not currently a cat owner, but growing up I lost a cat to the great outdoors, and I’ve had plenty of patients disappear or show up with horrific injuries. I also happen to love wildlife and would like to do as much as possible to preserve it, so I’m unapologetically on the side of permanent confinement for pet cats. I think that cats can be perfectly happy and healthy without roaming free, but it does take more work. But if you want a pet that doesn’t take any work and won’t hurt anybody (unless chucked at their head), get a pet rock! If you’ve got an indoor cat, try keeping it physically and mentally healthy with controlled diet and stress (hey this is good advice, I wonder if it would work for people?). Go out for walks on a leash or build a “cat-io” (don’t be self-conscious, haters gonna hate). You’ll find your indoor cat is happier and more inclined to perform cute behavior for your cat video festival submission (sorry, cats are not allowed to attend).


* A major caveat to most of this research is that it’s often hard to distinguish between pet cats that are allowed outside, and feral cats that have gone permanently Alexander Supertramp.

** Please let me know if you have any better information.

Posted by Greg Bishop

Greg Bishop is a veterinarian in Southern California who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.