FOCUS ON PUBLIC HEALTH: Disease-Transmitting Pests

As a professional that deals with insects on a daily basis, you know how to control termites, ants and cockroaches. Your customers are happy when you get rid of the many kinds of insects and pests that invade their homes and businesses. If you asked your customers why they want you to control such pests, what do you think they’d say? Would they tell you that insect pests are nasty and dirty? Indeed, no one wants ants in the pantry or cockroaches crawling across their dinner plates.

When a human sees crawling insects, a natural response is to feel disgust or fear. Our ancestors probably experienced those same reactions as well. But why would people fear a creature many times smaller than themselves? In some cases we have learned that an insect can cause harm, such as when we’re bitten by a deer fly. But what about cockroaches? They do not bite or sting. The same may also be said about most ants. There are a few ant species that may bite or sting, but for the most part, ant workers go about their business of collecting food for the colony

Perhaps our brains are overreacting and there is nothing to fear from most insects that we see in our homes, grocery stores or restaurants. But, then again, perhaps not.

TRANSFER EFFECT. Honey bees pick up pollen from one flower and transfer it to another. Butterflies and wasps also transfer pollen between flowers, ensuring that orange and apple trees will produce fruit each year. And though people may not think of cockroaches and ants the same way they think of bees or wasps, these insects too pick things up from one place and transfer them to another.

Flies are known to transfer disease, but crawling insects have gone relatively unnoticed in the cycle of insect-borne disease transmission. In fact, crawling insects live in environments teeming with germs of all kinds: fungi, viruses and bacteria.

Most of the germs that crawling insects contact are harmless and we do not need to worry about them. However, some of these germs are deadly or may result in serious human disease. We know that some cockroaches carry leprosy and have been held responsible for cases of human pneumonia. But crawling insects aren’t “born” with human pathogens — they encounter them in the environment. The germs stick to the cuticle of the insect or are eaten and passed through feces.

In today’s world it’s not likely that an insect will encounter leprosy while crawling around. However, it is likely that a crawling insect will encounter a food-borne pathogen. Even in seemingly sterile environments there are always some types of bacteria present.

In food-handling areas, two bacteria of major importance are Salmonella and Escherichia coli (E. coli). Both Salmonella and E. coli can live for long periods of time on surfaces such as floors, counters or walls. Additionally, Salmonella and E. coli live in and on “food” animals, such as pigs, chickens and cows. All it takes is one ant to encounter a single bacterium to cause a problem. The bacterium may be transferred back to the nest, which provides a warm, humid environment for growth. The bacterium (singular) then becomes bacteria (plural).

Ants are social insects and they will transfer bacteria to each other, perhaps eventually infecting the entire colony. Then, in the process of foraging, the ants can easily transfer bacteria to food or animals that may become food. The risk of bacterial transmission by crawling insects exists even in residential kitchens. People throw plastic foam containers and plastic wrapping from chicken or beef products into the garbage. Ants (and cockroaches) are attracted to such a food source and could pick up either Salmonella or E. coli from the blood or tissue that remains on the wrapping. Once bacteria enters the ant or cockroach colony, it can easily survive and replicate.

SALMONELLA. Today, we are especially concerned about specific bacteria. The group of bacteria known as Salmonella is extremely important in cases of food-borne illness. There is one type of Salmonella poisoning that is generally passed from person to person via the fecal-oral route. If you’ve heard of Typhoid Mary, then you may know that she carried Salmonella in her gut and caused outbreaks of disease everywhere she went. Most Salmonella is not passed from person to person though, but through contaminated food stuffs or through environmental contamination. Insects can encounter Salmonella in the environment and transfer it to previously clean surfaces, processing equipment, food animals or to prepared food.

E. COLI. Another bacteria about which we are particularly concerned is given the rather unusual name E. coli 0157:H7. The bacterium E. coli 0157:H7 causes severe disease and death in humans. The number/letter combination 0157:H7 is a specific scientific designation for a strain of E. coli that causes hemorrhagic colitis and can lead to a hemolytic uremic syndrome (HUS) in children or thrombocytopenic purpura (TTP) in the elderly. Illnesses resulting from infections with E. coli 0157:H7 are on the rise and routes of transmission are of special concern. One method of transmission is from crawling insects that have the bacteria on their cuticle or in their feces.

The bacterium E. coli 0157:H7 does not make insects sick. The antigens that are present on the surface of the bacteria cell (O antigens) or on the pili (H antigens) are especially designed for causing illness in our intestinal tracts. It takes about 10 or fewer cells of E. coli 0157:H7 to cause illness in a healthy person.

But, as intimidating as E. coli 0157:H7 is, don’t confuse it with your general, run-of-the-mill brand of E. coli. Most E. coli is harmless or even beneficial to us. We all carry normal E. coli around in our intestines, which helps us make certain vitamins and prevents other bacteria from making us sick.
 
CONCLUSION. To date, I have found both Salmonella and E. coli on beetles, ants, silverfish and cockroaches — all crawling insects. I am continuing my investigations to determine the prevalence of E. coli 0157:H7 on insects. In any case, when communicating the importance of insect control, the mitigation of disease is an important factor. While people are concerned with mosquitoes spreading encephalitis fevers, the risk of disease transmission by crawling insects must also be addressed. In the end, perhaps the primeval repulsion some people feel when they see a crawling insect is justified and part of the overall survival strategy employed by the human race.

The author is a Ph.D. student at the University of Florida specializing in urban entomology. She can be reached at dbranscome@pctonline.com.