Bats and Rabies

by Brian Dunning
November 12, 2024

With a fatality rate of 100% to both humans and animals who contract it, rabies is one of the world's deadliest viral infections. It is transmitted through the saliva, either by biting or by consuming food that another animal has spat out. And what animal is among the most feared transmitters of this disease? In many places, it's bats. A bat coming in the night and sinking its teeth into you is one of the most fearsome prospects many people can conceive of, and so today we're going to turn our skeptical eye to bats and rabies. Is this something that you or your animals need to worry about?

This episode was suggested in part by a tragedy at our next door neighbor's house, here in the central Oregon high desert. These are all horse properties, and our neighbor noticed bats in the attic of her barn. Fearful for her horses' safety, she had them all vaccinated for rabies. Well, we got a call in the middle of the night. Their horse trailer was unavailable and they needed ours to transport one of their horses to the equine hospital. I hooked up, raced over there, we loaded the horse, and drove him to the hospital. Within the next day or two, two more of their horses also had to be transported. Long story short, two of the three sick horses had to be euthanized. It was a very hard week, and really sad. I still miss the one guy who used to run along the fence and say hi to us every time we'd come home.

It turns out what had happened was that batch of rabies vaccine was contaminated. Luckily the manufacturer did compensate our neighbor very well, not that that's much comfort. Horses are particularly prone to adverse reactions from vaccines, and even in the best of conditions, the rabies vaccine is the riskiest, and is usually required to be administered by a veterinarian. The risk is not just for the normal immune response that the vaccine is intended to provoke, but a severe, life-threatening reaction requiring medical intervention, such as colic or anaphylaxis. In this case, all three horses developed equine colic, which is very horrible.

But this made me wonder: Were the horses ever truly at risk of rabies from the bats? And by the same token, are humans or other animals also at the same risk? It sounded like a job for Skeptoid.

The disease is really as bad as diseases get. When you get infected by the virus, either through a bite or some other vector, the virus moves slowly through your system. During this phase of the disease, it can actually be stopped and cured through the administration of a vaccine and also human rabies immune globulin, a treatment which is nearly 100% successful. However, over the course of anywhere from ten days to several months, the virus will eventually reach the spinal cord or brain. Once it does, the disease is 100% fatal and nothing can be done except palliative care. Although a handful of people have been known to survive the infection through history — albeit with severe neurological damage — their numbers are statistically insignificant. If rabies gets to your brain, you will die. As your brain tissue is destroyed, you may experience aggression, delirium, seizures, and paralysis for weeks, followed by coma, followed by death.

Fortunately human infections are vanishingly rare in the United States, with an average of about three infections per year; although some 60,000 Americans receive that post-exposure treatment, mostly precautionary, after a bite from some animal whose rabies status is unknown. Globally, however, 70,000 people still die each year, 99% of them from dog bites. The reason for the big difference is the prevention efforts in the United States, mainly identifying, quarantining, and euthanizing infected animals.

I happen to be a big fan of bats. I even have a bat box here on the property, which is a house designed to attract them to move into — though mine has not yet become occupied. I've encountered bats while exploring old mines countless times, and one thing I've come to learn is that they will never, ever, no way, actually run into you when they fly. Their echolocation is too good and they're too fast. So a bat is not going to accidentally bite or scratch you. To catch rabies from a bat, it would have to deliberately land on you and bite you.

So do bats do that? There are some 45 species of bats in the United States. The subfamily of bats that subsist on mammalian blood is called — appropriately — vampire bats. So these are mainly the ones it's possible to contract rabies from. How many vampire bats are in the United States?

Zero. You and your animals are completely safe from bat bites in the United States.

Actually, one bat from one Mexican vampire bat species was found in Texas, once — as of this writing. One single bat. So in the United States, bats biting people, horses, or cattle is a nonexistent problem. But there are three species of vampire bats in Mexico. There are a lot in Central America. There are even more in South America. In fact, in parts of South America, vampire bat rabies is actually the leading cause of death of cattle. Horse deaths from vampire bat rabies began to be reported the moment Cortes brought the first horses to the American mainland in 1519.

Clarification: People dying from rabid bat bites in the United States may be a statistically nonexistent problem, but it still happens; there are outliers and exceptions in every data set. In most years, it happens to fewer than three Americans, usually from attempting to catch and handle a bat, then failing to seek treatment. Americans are approximately 200 times as likely to be struck by lightning than to catch rabies from a bat bite. —BD

Horses in the United States can still get rabies — and a few cases are reported every year, usually in the low double digits, so it's rare but real. It's just not from bat bites. The typical disease vector is a rabid raccoon or coyote or some other animal getting tangled up in the horse's feet, and a lot of kicking and biting and scratching ensues. Since very few raccoons or coyotes actually have rabies (when they get it, they die quite quickly), this is a pretty unusual combination of events.

Bats can still transmit rabies to horses and cattle without biting them, so our neighbor was not completely off base in having her horses vaccinated. Sometimes bats will spit out their mouthful of insects, and that might land on something that the horse will eat. Bat feces and urine can also transmit rabies if it gets on a horse's food. So there are still plausible mechanisms. And so, even though equine rabies is very rare in the United States, the rabies vaccine is still recommended as an annual shot.

Australia is another country that has no vampire bats. Australia also has no rabies, but its four species of flying fox bats do still represent a danger to people, horses, and cattle. That's because the flying foxes carry different viruses: Hendra disease and lyssavirus (very similar to rabies). Few humans catch these, because you'd have to try and handle one of the flying foxes for it to bite you. Horses and cattle that catch these diseases usually do so by eating either feces or fruit spit out by the bats.

You may wonder that since there is an equine vaccine for rabies, is there also a human vaccine, and if so, why don't we just give it to everyone especially in those countries with thousands of annual deaths? There is a rabies vaccine for humans, used in two different therapies. One, pre-exposure vaccination, is given to people who have a high risk of exposure, for example people who work with wild animals or in certain laboratories or who are traveling to a high-risk place. The other is the post-exposure vaccination already discussed, given to people who, say, were just bitten by an animal suspected of being rabid.

There are a number of reasons the vaccine is not given to everyone as part of the normal course of childhood vaccines. First, exposure to rabies is extremely rare; the vaccine is almost never needed. Second, it's really expensive. And third, it doesn't provide lifelong protection so you can't do a one-and-done.

You may have heard on the Radiolab podcast about an experimental treatment for people whose rabies has already reached the brain. Since rabies is symptom-free until it progresses far enough to be fatal, people often don't know they have it until it's too late. The treatment is called the Milwaukee Protocol, and has been adapted for use in Brazil under Brazilian conditions where it's called the Recife Protocol. There are a lot more cases of human rabies in Brazil than in the United States, mostly caused by dogs, but also by bats and a few other animals. Both protocols consist of putting the patient into a medically induced coma and then giving them the post-exposure vaccine treatment, but also adding a whole huge raft of other drugs and compounds. So far, almost all patients who are so treated still die — and what's more concerning is that the very few patients who have survived, including the case study featured on Radiolab, did not develop rabies antibodies; indicating that they likely did not have rabies to begin with. In 2015, the Pediatric Infectious Disease Journal declared the Milwaukee Protocol to be no longer valid. One journal article concluded:

Despite initial hope and enthusiasm for the Milwaukee protocol in the treatment of rabies, subsequent trials of this regimen have failed. Serious concerns over the current protocol recommendations are warranted in light of a weak scientific rationale. The recommendations for therapeutic coma, NMDA receptor antagonists, and the screening/prophylaxis/treatment of cerebral vasospasm are supported by little to no scientific evidence in the literature. The recommendations made by the protocol warrant serious reconsideration before any future use of this failed protocol. Unfortunately, we do not have an alternative protocol to put forward for therapy of patients with rabies.

And so, since we have no solution to this problem, we are reminded once again that we need to have a good and accurate understanding of the problem if we ever hope to develop a good solution.

So in summary, let us review our accurate understanding of the problem. Rabies from bats is not something people in the United States have to worry about really at all, and horses and cattle need to worry about rabid raccoons and coyotes far more than about the bats. But since the equine rabies vaccine is readily available and rabies is a horrible way to die, horse owners are not being at all unreasonable to include the rabies vaccine in their horses' annual course. So despite the unexpected and extremely rare case of the vaccine being contaminated, my neighbor was right to do it, although her discovery of bats was probably not a relevant motivation. But for people living in other parts of the world where rabid vampire bats are absolutely a real threat, obviously they have much more reason to stay current on the vaccines.

In any event, please don't catch rabies.

By Brian Dunning
Follow @BrianDunning