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'Probability of Kissing Bugs Spreading Disease'

March 22, 2018
Boulder Bug

Recently, the media lit up with the news that kissing bugs are loose in the United States and are spreading a horrific, tropical sickness called Chagas disease. Like most sensational news stories, the panic is overblown, and the actual risk of contracting the disease here in the Midwest is practically nonexistent.

Nevertheless, I have received many telephone calls and physical samples at the diagnostic laboratory asking me to identify insects and provide assurances that kissing bugs have not invaded. In every case, these sightings and samples have turned out to be common household insects that, although they bear a slight resemblance to the kissing bug, are completely harmless.

The most commonly submitted look-a-likes are boxelder bugs - they are especially common during the fall. They enter and reside in homes throughout the winter, so may be especially easy to confuse with the rare kissing bug. Boxelder bugs may be a nuisance but they don't bite or harm people. In any case, the kissing bug story is valuable in that it promotes awareness and vigilance when it comes to insects inside our homes.

In contrast to boxelder bugs, kissing bugs are slightly bigger, about a half-inch long and their flat abdomen is somewhat cup-shaped with orange-red to yellowish horizontal markings. Most importantly, kissing bugs belong to a family of insects that have a characteristically long, slender head with a very sharp rostrum (or beak) that is normally tucked underneath the head. This beak is a modified mouthpart that the insects use to pierce and suck fluids out of their prey.

Several species belong to the kissing bug group called Triatoma. Most are quite common in central and south America, but we have always found one or two members of this group living throughout the lower half of the United states. We most often call them cone-nosed bugs, or bloodsucking conenoses - not nearly as endearing names as kissing bugs.

Triatomine bugs are unique because they feed on the blood of mammals (including humans), birds, and reptiles. These bugs feed at night, so it's uncommon to actually see the insect. By day, they hide in brush piles, under tree bark, or under rocks. Outdoor pet kennels and rodent dens are common hiding places. However, the adults are attracted to artificial lights during the summer and early fall. When they come to house lights, they sometimes take up residence in furniture or in the house structure. If they find food sources, they are happy to lay eggs and make families.

Not all kissing bugs behave exactly the same. While no one would choose any of these bugs for a family pet, one particular species has two particularly bad behaviors. First, it is attracted to feed on humans. Second, it has a despicable habit of defecating either right before or right after it feeds. This is definitely bad behavior. Where or why it picked up this nasty habit nobody seems to know.

What we do know, however, is that this behavior is at the root of what causes Chagas disease to spread.

Here is how it works. The bug finds its way into a human home and sets up residence. It becomes active at night and, because it feeds on blood, has learned to key in on the carbon dioxide that is exhaled by a sleeping person. The bug bites the person, most often near the face and lips (which is why it is called a kissing bug). At the same time, it very inconsiderately deposits its feces right on the skin where it is feeding. Then, the victim accidentally (by itching or scratching) smears the pathogen into his or her mouth or nose or into the open wound created by the bite and becomes infected.

It seems like a simple process. All that is required is the disease pathogen, a vector (the bug), and a host person. When all three of these are brought together, bam, we have a chance of getting Chagas disease.

But the real question is: What are the chances of actually getting Chagas disease in the United States?

When we discuss chances and odds, we are really talking about probability. We can't predict many events with total certainty. The best we can say is how likely they are to happen, using the idea of probability. And to talk with a scientist about probability is a dangerous thing - scientists never say never.

Even when the probability of something occurring is extremely low (even miniscule), such as, "Will the moon fall out of the sky tomorrow?" scientists will never say never. Bear that in mind when you ask a scientist about the chances of a Chagas disease epidemic in the United States. They will never say it is impossible, even though they may say that the probability is extremely low.

The reason they are comfortable saying "extremely low" is because of conditional probability. Conditional probability simply means that certain outside conditions affect probability and in the case of determining if Chagas disease will become a problem in the United States, it is those outside conditions that really affect the probability.

For example, we know that the main species of kissing bug responsible for transmitting Chagas disease is seldom found here. It is better adapted for more tropical climes. And even in South and Central America (where it is commonly found), it only becomes a problem when it enters human homes and resides there in large numbers.

Homes in the United States are constructed with windows, walls, and doors that keep most insects out. We normally have air conditioning so that we can keep doors and windows closed or screened, especially at night when lights are on. That is not always so outside the United States. So, the probability of encountering Chagas disease spreading kissing bugs in United States is low. Add to that the fact that only a small percentage of these bugs carry the parasite that causes Chagas disease, and the probability becomes even lower.

Further, to become infected, the bug must have fed upon a rodent or another host animal that already has the parasite in its blood. So even if a kissing bug happens to hitchhike into your state, and find a way inside your house, it still must also have fed on an infected mammal host to come infected.

The parasite itself also has a low probability of survival. Once acquired, it must reproduce inside the cell bodies of the kissing bug and its young develop sufficiently to move onto the rectal cell wall, invade the feces there and be excreted with the feces.

What's more, the feces must be deposited in sufficient quantities on the host's skin and close enough to where it can be smeared (by chance) into a wound or across mucous membranes. Even if the pathogen makes it into the body of a human host, it must there cross a network of proteins that line and protect the exterior of the cells in order to contact and invade the host cells.

All considered, it is fair to say that odds of this all coming together are not good. Even in places with large kissing bug infestations, human health and disease experts say that that only 1 in 1,000 to 2,500 bites result in infection.

So, why do scientists say the probability of a Chagas disease outbreak in the United States is very low? Let's recap. First, Chagas disease is spread mainly by only one species of kissing bug and that species is rarely found in the United States and almost never occurs in homes here. Second, the transmission of the disease requires previously infected bugs whereas, the reservoir of Chagas disease pathogens here is very low. Third, considering that it's actually not easy to catch Chagas disease even if bitten by an infected bug, it is safe to say that Chagas disease probably will not become epidemic in the United States.

Still, scientists will never say never. At least the probability of them never saying never is high.

More information is available from the Centers for Disease Control and Prevention (CDC)

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