Why you shouldn't salt a leech that's sucking your blood

The sight of a swollen, slimy leech clamped to your skin sucking away at your blood may evoke a wave of panic and disgust. But resist the temptation to pour salt on it, as folk wisdom recommends, because that could cause the leech to vomit into the wound, posing unnecessary health risks, suggest biologists behind a new exhibit on bloodsucking animals.

Sebastian Kvist, curator of invertebrates, and Doug Currie, senior curator of entomology at the Royal Ontario Museum, are the creators of Bloodsuckers, which opens at the Toronto museum Saturday. The interactive exhibit, which runs until March 22, 2020, includes better advice for how to remove leeches and ticks. 

It also features live specimens and explores the biology of bloodsucking animals, their use in medicine, and their appearances in stories through the ages.

But why wait for the exhibit? 

Kvist and Currie shared with CBC News some things you need to know about bloodsucking animals, from leeches to ticks to vampiric moths and bloodthirsty fish known for swimming up people's genitals.

Why shouldn't you salt a leech?

Kvist says that when leeches get stressed while feeding, they can vomit some of the blood back into the open wound.

"It'll bring with it a bit of bacteria, normally, and so you can get a minor infection from it," he says.

"A really good way of stressing a leech is pouring salt or  lemon on it or burning it with a cigarette butt, so we tell people not to do that anymore."

Instead, he recommends using a fingernail or a credit card to break the seal between the leech's mouth and your skin quickly, "so it doesn't have time to regurgitate blood."

Another option is to wait until the leech is done feeding so it will fall off on its own. That takes about 30 or 45 minutes. Kvist cautions, however, that because leeches' saliva contain powerful blood thinners that can cause you to bleed slowly for longer than usual — up to 36 hours.

Why do bloodsuckers often go for the groin?

Because leeches are slow eaters, they are pretty careful about finding a nice spot to feed where they won't be disturbed, Kvist says. 

"It tries to nestle in maybe between toes or any kind of crevice on your body — groins or armpits and stuff like that are what they normally like."

The ROM exhibit also features live specimens of another bloodsucker with a reputation for going for the groin. The candiru, which is also known as the toothpick fish or vampire fish of the Amazon River feeds on blood from fish gills, which it locates by homing in on ammonia that the gills expel, Kvist says.

Ammonia is also a component of human urine, which may be why doctors have reported the fish being found lodged inside human urethras, he adds. (Although he notes this hasn't been verified in the scientific literature.)

Which bloodsucking animals pose the greatest health risks?

Leeches are not known to transmit any diseases to humans. Nor are black flies.

One key feature of bloodsucking animals that can transmit diseases is that they have multiple blood meals over their lives, says Currie.

That includes ticks, which can carry Lyme disease, Rocky Mountain spotted fever, and tick paralysis. 

"The good news is it takes 24 to 48 hours [of sucking on your blood] before they have the capacity to transmit a disease," Currie says. If you remove them before that, you're probably fine.

That's because they're extremely slow feeders. It takes them weeks or days to complete a blood meal of up to 600 times their body mass. 

"It's really spectacular. They can be quite disgusting looking," Currie says.

The other requirement for disease transmission is that the microbes that cause the disease must be able to survive local conditions.

Mosquitoes can carry West Nile and Eastern equine encephalitis in Canada, but are theoretically capable of carrying a wider range of diseases, such as Dengue fever. It's just that the microbes that cause those diseases can't survive our current climate, Currie says.

"The concern is as the climate warms, and [the] disease agent works its way north, there is the potential for some of our local mosquitoes to transmit it." 

What are some bloodsucking animals we might know?

Many people are familiar with bloodsucking insects, vampire bats and even sea lampreys, but here are some bloodsuckers you may not have heard of:

• In southeast Asia, there are vampire moths that suck blood from a range of animals, including humans. While only the females are bloodsuckers among many biting flies, in moths, it's the males that are out for blood. Currie says it's thought that the males take the salt from the blood and present it to the females as a "nuptial gift" — kind of like the moth version of flowers or chocolate.
• The Atlantic Ocean is home to a vampiric snail that sucks blood from the gills of fish, Kvist says. Since it's not very speedy, it applies both a local anesthetic and some kind of sedative while it's feeding.
• There are also some species of blood-feeding birds. They include vampiric ground finches in the Galapagos that peck at other birds and drink their blood, and oxpeckers in sub-Saharan Africa that eat blood-filled ticks, lice and mites, but also slurp from wounds in the skin of animals like rhinos and wildebeests.

Why do scientists think bloodsuckers should be viewed in a more positive light?

Many bloodfeeders are key components of the food web.

With black flies, while the adult females may feed on larger animals like us, the larvae are an abundant food for other aquatic animals, Currie says: "And so without them we certainly wouldn't have the fish that we'd like to go fishing for. It feeds waterfowl and plays a really important role in our northern ecosystems."

Obviously, leeches have been important in medicine - they're still used to relieve blood congestion after surgery to reattach fingers and toes, and their powerful blood thinners were key to making the first human kidney dialysis possible in 1921, Kvist says.

In general, he says the adaptations of blood sucking animals are biologically extraordinary.

"I think that the animals being able to take your blood — your very life force — without you knowing it or without you noticing it is quite spectacular."

Currie hopes the exhibit will give people the information they need to live "in harmony" with bloodsuckers.

"I don't think we're going to get anyone to love bloodsuckers any more than they already do, but I think what we do want to enlighten the public about is they are interesting in their own right," he says.

"They are important components of ecosystems in which they live, and really they have inspired us in a lot of ways through art and culture."