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There are some (feral) outdoor kittens in my yard that have taken a liking to my indoor cat (and vice versa) through "interactions" through a sliding glass door. They have also smelled each other via the glass door being slightly cracked (such that they can smell but not touch each other). Ignoring run-away and fighting considerations considerations (as these won't happen), is it safe to allow my indoor cat to come into physical contact with the outdoor cats? I ask specifically from the perspective of disease transmission. My cat is up to date with his vaccinations and I give him anti-flea medication... is this sufficient to allow them to interact? Or is transmission of something too high a risk? I can be certain that the outdoor cats aren't going to like attack/bite him as they are kittens and he is significantly larger than they are, so transmission would come from just touching or licking each other.

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    Could you clarify what "up to date with his vaccinations" means in this context? It is not uncommon for vets to opt for a different vaccination regime for indoor-only cats, e.g. vaccinate only against diseases that are more easily tracked indoors by humans and as such carry the highest risk for the cat. Does he have core vaccinations, or does he also have vaccinations typically only recommended for outdoor cats? Are you living in an area that has elevated risk, e.g. close to large forest with varied wildlife, are you in an area with a risk for rabies, etc.?
    – bgse
    Oct 21, 2022 at 14:00

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excellent and important question. I am trap-neuter-return certified and frequently handle feral cats, bringing them into my own home (where I have indoor cats) where they await+recover from their spay/neuter surgeries before being released back outside.

TL;DR: It is not "safe" to let your indoor cats interact with outdoor cats of unknown health status, meaning that even through a cracked door there is absolutely a level of risk you are exposing your cat to. Some of the viruses your cat could be potentially exposed to are very detrimental to your cats health and are untreatable. I'd suggest just letting them enjoy each other's company through the glass door but with no further contact. When I bring feral/outdoor cats into my own home I have to take a number of precautions to ensure not only that my indoor cats have 0 interaction with them but also deeply sanitizing everything the outdoor cats came into contact with.

Longer version: No vaccine or flea medication is 100% effective. And there are viruses cats may carry that actually have no vaccine and no cure for treatment. Sadly, feral/outdoor cats can have a host of contagious disease, fleas, lice and parasites that are transmittable to other cats and have large health consequences.

Responsible rescuers in the US (where I'm based so can speak on) do not let indoor cats come into contact with outdoor cats or any objects an outdoor cat might have touched until that outdoor cat has tested negative for two specific viruses, has been dewormed, treated for fleas/lice, and quarantined for at least 2 weeks.

The viruses of concern: Specifically, there are two contagious disease of note that feral cats might have - FIV and FELV - that rescuers in the United States take protective measures against to insure the safety of all cats.

FIV stands for Feline Immunodeficiency Virus. There is no vaccine for FIV and there is no cure for it, so your cat is not protected from this virus. It is only passed through bite wounds, which you say you are not concerned about, but the risks that FIV poses to your cats life and health are severe and I would say it is likely not worth it. FIV creates immune deficiency in cats meaning "which allows normally harmless bacteria, viruses, protozoa, and fungi found in the everyday environment to potentially cause severe illnesses" (Cornell Feline Health Center). While cats who have FIV can live normal lives (and are worthy of adoption and love!), this virus is severe and it can make them much more likely to suffer fatal or near fatal consequences from minor illnesses.

FELV stands for Feline Leukemia Virus. While there is a vaccine for FELV, it isn't considered a core vaccine in cats so your cat might not actually be vaccinated against this virus. And again, no vaccine is 100% effective. FELV also create immune deficiency in cats and can cause cancer. It is progressive and eventually fatal. FELV can be passed through saliva, mucus, mutual grooming, feces, urine, and bite wounds. There is no treatment for FELV. Rescues and vets actually suggest that pet owners who have an FELV+ cat should not introduce new cats to their household unless they are also FELV+.

Finally, outdoor cats commonly have feline calicivirus. This is a virus that causes respiratory infections (like a cat cold!) but it can also cause painful sores in the cats mouths. Your cat likely has been vaccinated against this if you take them to the vet regularly (vaccinations are usually annual or every 3-years depending on which type your vet uses). While less severe than the viruses above, calicivirus is extremely contagious and airborn just like the human cold. This is likely the virus they'd be most susceptible to. Once infected, some cats can become carriers of the virus for life. In these instances it is often dormant and doesn't interfere with infected cats living a normal life, but it can flare up and these cats then become sources of infection for other cats.

A final suggestion: As I think my answer has illustrated, outdoor cats can have a number of health problems that are detrimental to their lives. Cats also reproduce incredibly quickly. If you have feral kittens in your neighborhood, they can quickly grow to be an incredibly large feral colony. I recommend searching for "trap-neuter-return" services in your area. Local rescues may be able to help TNR these kittens. At least in the US, TNR programs also vaccinate the feral cats they spay/neuter so this would help stop some of those viruses I illustrated above from getting a foothold in your area.

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  • Great answer, regarding FeLV I'd add that studies suggest the virus can even survive for some time on clothes and tracked inside by humans if they come into contact with infected cats, so I would absolutely suggest vaccinating indoor cats too, especially given they might slip outside at some point and the disease is highly contagious. We have a FeLV+ cat living in our home with two negative companions (vaccinated), and I'm dying a thousand deaths each year when they are re-tested before updating the vaccination.
    – bgse
    Oct 26, 2022 at 8:12
  • The other solution would be to going beyond trap/neuter/release, paying for a full health workup, and trying to convince the fluff balls that living with humans is a good thing. Ir considering your cat an outdoor cat that spends most of its life indoors and accept the nontrivial health risks which comes with that decision. But, yeah, youngsters are often disease vectors; ask any elementary school teacher.
    – keshlam
    6 hours ago

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