Bed Bugs Won't Give You Chagas Disease (probably)

Bed Bugs Won't Give You Chagas Disease (probably)

It causes Chagas disease, also known as “American trypanosomiasis.” Over 8 million people are infected with Chagas disease, mostly in Latin America.
CDC/ Dr. Mae Melvin.

Bed Bugs Won't Give You Chagas Disease (probably)

CDC/ Dr. Mae Melvin.

Nobody likes bed bugs, but one thing we could always say was "at least they don't transmit any human diseases!" That's still true, although some research published this week is causing some concern.

A new paper in The American Journal of Tropical Medicine and Hygiene reported that bed bugs might be able to host and transmit Chagas Disease, a chronic heart disease caused by the blood parasite Trypanosoma cruzi. That sounds scary, but the experts I talked to all agree no one should panic.

A lot of alarming things can be found in bed bugs; over 45 different potential human disease agents are known, including bacteria, viruses (Hepatitis B), worms, and protozoans (like the trypanosomes in the new paper). So far, none of them are transmitted to humans by bed bugs. This new research doesn't change that.

Here's a run-down of the new research, and what you need to know.

Chagas Disease: the Poop Connection. ————————————

A Kissing Bug (Triatoma sanguisuga) , a larger cousin of the Bed Bug.

CDC/James Gathany

In areas where Chagas Disease occurs, you get infected when a kissing bug with the parasite in its gut first bites you, and then poops into the wound it makes.

Kissing bugs are a relative of bed bugs, and they both feed the same way – they stick a beaky straw into your skin and slurp up your blood. Kissing bugs poop after they feed, and if the bug is infected, it releases the parasite onto your skin.

Breaks in the skin (like the one the bug just made to drink your blood) or mucus membranes allow the Chagas parasite easy access to your blood. "Kissing bugs" get their name from their habit of biting near the lips and eyes. Which are mucus membranes.

Scratching makes it more likely that you get the disease, because you create an opening in the skin and then rub the feces into the wound. Insult, meet injury.

To recap: the bug bites you on the face; then poops on your face; and then you scratch the wound (on your face? Did I mention it's on your face?) and infect yourself.

But Wait, There's More ———————-

How do these kissing bugs get infected in the first place? They feed on small mammals and pick up the Chagas parasite. There is another source of Chagas disease infection in kissing bugs: coprophagy. Literally: shit-eating.

Baby kissing bugs don't have an important bacterial symbiont they need in their gut to help them digest food and create essential vitamins. They fill up with the bacteria by eating their parent's feces. Or the feces of any other adult kissing bug. If the adult insects also happen to be infected with the Chagas disease parasite, then the young kissing bugs pick that up as well.

The Proof is in the Pooping —————————

In the laboratory, Chagas parasites will live in a wide variety of insects, including bed bugs. But unless the infected insects also poop out the parasite into a wound, transmission of the disease is unlikely. That's what the new research was designed to test.

Bed bugs are guilty of a lot of things, but they are not shit eaters. They feed exclusively on blood, so that is the only way for a bed bug to pick up the Chagas parasite. The researchers were able to successfully infect bed bugs with the Chagas parasite by feeding them on infected mice. They also demonstrated that if you collected bed bug feces and rubbed them into scraped areas of mouse skin, they could infect the mice with Chagas disease.

Lastly, the researchers also measured how long it took for bed bugs to poop after a blood meal, and found the Time-to-pooping by kissing bugs and bed bugs were similar.

Dr. Michael Levy, an author on the new study, summed it this way: "We knew that [bed bugs] probably could transmit the parasite. After our work we know they really can transmit it. We don't know if they are transmitting it." In other words, they have demonstrated a potential way for the disease to be transmitted, but it's not yet a reality outside the laboratory.

Since I'm at the Entomological Society of America Annual Meeting this week, I'm conveniently surrounded by bed bug experts. I tracked down Dr. Zach Adelman, who is giving a presentation on bed bugs and infectious disease transmission, and asked him about the new paper. We talked about "vector competence," a term that describes how well an insect can acquire, maintain, and then transmit an infectious disease.

Thankfully, bed bugs seem to be incompetent disease vectors. "Should we be frightened? The infection rates of [kissing bugs] are very high even in the absence of an infected host because of their coprophagy. That's why kissing bugs are such great vectors; they maintain high levels of infection for long periods of time without any human involvement," said Adelman. "But bed bugs don't do that. They only feed on blood. So what are the Chagas infection rates of bed bugs going to be? My guess is the prevalence will be very low."

David Lilly, University of Sydney, suggested that while the new study was really interesting, the infection risk for humans by bed bugs was probably low, based on his knowledge of bed bug behavior. "I feed bed bugs all the time... They don't hang around on the host after feeding, so the chances of bed bugs defecating on skin are low."

To see what he was talking about, look at this photo of bed bugs feeding on entomologist Lou Sorkin:

Note the posture of these feeding bed bugs. Even though they have access to a hand, the bugs are basically standing on the paper and extending their proboscis to push their beak into the skin. (The reddened skin is a birthmark, not a reaction to the bed bugs).

Lou Sorkin, ANMH

The bed bugs in this photo could walk onto his hand, but they don't. They are staying safe on their piece of paper and just reaching over, in kind of a John Travolta/Saturday Night Fever posture. Bed bugs eat and run, which means a Chagas-infected bed bug would be unlikely to poop on a human.

In some of the more unusual conversations I've had with scientists, the odds of actually being pooped on by a bed bug are a point of contention. While entomologists I talked to here at the meeting felt the chances were low, Levy disagrees, saying "bed bugs are dangerous poopers."

We Need More Basic Ecology ————————–

One thing all the scientists agreed on was we need more basic ecological research to understand what happens in nature, and what the actual level of threat is for bed bugs and Chagas. We can do a lot of things in the laboratory, but that doesn't always translate to the external world.

Levy said "Bed bugs do not cause Chagas disease, they can only carry it. If you have bed bugs in your house you are not at risk of infection unless the disease is already present... There is much we don't know about the behavior and ecology of both the Chagas parasite and bed bugs—perhaps there is some detail about the two that we are yet to uncover that will prove them incompatible, and that the threat is empty."

To further understand the relationship and assess if infection is a real fear, he hopes to collect bed bugs in the houses of individuals with Chagas disease in Peru, to and see if they are indeed picking up the parasite.

So, remain calm. Levy also said, and I agree: "Cities and health agencies need to start to treat bed bugs as a public health issue rather than simply a pest." You are far more likely to be injured by misusing pesticides to try to exterminate bed bugs, or trying to kill them with fire. If you live in an area where you are concerned about Chagas (only 6 human cases have been found in the US so far), here's some tips on keeping kissing bugs out of your house.

R. Salazar et al. 2014. Bed Bugs (Cimex lectularius) as Vectors of Trypanosoma cruzi. American Journal of Tropical Medicine and Hygiene. DOI: 10.4269/ajtmh.14-0483