Bed Bugs: Vectors of Human Disease?

Bed bugs, Cimex lectularius, have been in the news a lot lately, mostly because of increasing reports of infestations, but also because of a few sensational biting events wherein the victims displayed horrific bite marks on their bodies. Bed bugs have been known as a human parasite for thousands of years, but scientific studies of the medical entomology of these insects are recent and limited. International travel, immigration, changes in pest control practices, and insecticide resistance seem to be contributing to a recent resurgence of this blood-sucking insect in developed countries. Bed bug infestations have been reported increasingly in homes, apartments, hotel rooms, hospitals, and dormitories in the United States since 1980 (Table 1).

The insect family Cimicidae contains species commonly called bed bugs, bat bugs and swallow bugs. Bed bugs are found in temperate regions worldwide. The 91 species in this family are wingless, obligate hematophagous ectoparasites that feed on bats, birds and mammals. The word Cimex is derived from the Roman designation for bug, and lectularius from the Latin name for couch or bed. Only two species readily feed on humans, Cimex lectularius and C. hempterus, but others may rarely do so as well.

Adult bed bugs are oval shaped, flat, and approximately 5 millimeters long. They resemble unfed ticks or small cockroaches and are easily visible, even to the untrained eye. Adults are reddish brown (chestnut) in color while immature bed bugs resemble adults, are much smaller, and may be light yellow. They have a pyramid-shaped head with prominent compound eyes, slender antennae, and a long proboscis tucked backward underneath the head and thorax. After a blood meal, the bugs may increase in length by 30 to 50 percent and in weight by 150 to 200 percent.

Bed bugs sense and seek warmth, a trait that helps them locate warm-blooded hosts. They generally avoid light, hide during the day, and feed at night. Hiding places are usually within five feet of suitable hosts and include seams in mattresses, crevices in box springs, backsides of headboards, spaces under baseboards or loose wallpaper, and even behind hanging pictures.

There are five nymphal stages through which the insects pass to reach adulthood. At each nymphal stage, insects must take a blood meal in order to complete development and molt to the next stage. The insects take five to 10 minutes to ingest a full blood meal. Adult bed bugs have an average life span of six to 12 months and can survive a year without feeding. When their preferred human hosts are absent, they may take a blood meal from any other warm-blooded animal, including pets.1, 2

Dispersal of human-associated bed-bug species generally depends on human hosts for movement from one location to another. This may occur by way of furniture, clothing, suitcases, used mattresses, and other personal possessions. Bugs may also migrate from one apartment or hotel room to another through holes in walls, water pipes, or gutters.

VECTORS OF HUMAN DISEASE. Transmission of more than 40 human diseases has been attributed to bed bugs, but there is little evidence that such transmission has ever occurred (Table 2). Older scientific literature reported (sometimes with little supporting evidence) that bed bugs may be vectors of plague, yellow fever, tuberculosis, relapsing fever, leprosy, filariasis,3 kala azar (leishmaniasis),4 cancer,5 smallpox,6 yellow fever, and Chagas’ disease (Trypanasoma cruzi).7, 8 Some of this confusion arises from the fact that bed bugs are blood-sucking insects and may ingest disease germs during feeding. However, just because an insect can "take up" a germ, doesn’t mean it can later transmit that organism to another host. The possibility of HIV and hepatitis B (HBV) transmission by bed bugs also has been carefully investigated. HIV can be detected in bed bugs up to eight days after ingestion of highly concentrated virus in experimental blood meals. However, no virus replication has been observed within the insects, nor has virus been detected in bed bug feces.9, 10 Mechanical transmission of HIV failed using a system of feeding bed bugs through artificial membranes.10

The best candidate for human disease transmission by bed bugs is hepatitis B virus (HBV) because bed bugs collected from huts in a HBV-endemic area in northern Transvaal, South Africa, were hepatitis B surface antigen (HBsAG) positive,11 as were samples collected from Egypt and the Ivory Coast.12, 13 The author notes here that hepatitis surface antigen is a "piece" of the virus that is often used by health care professionals as a marker for HBV infection, but its presence doesn’t necessarily mean that the entire, live virus is present. Anyway, HBsAG also has been shown to persist in bed bugs for more than seven weeks after experimental feeding, but no replication of the virus itself was detected in the insects.14 Polymerase chain reaction (PCR) assays have detected HBV DNA in bed bugs and their excrement up to six weeks after feeding on infected blood.15, 16 Despite these findings, a two-year bed bug eradication project in Gambia had no effect on rates of HBV infection, despite dramatic reduction of bed bug numbers.17

No study to date has demonstrated bed bug "vector competence," the ability to acquire, maintain, and transmit an infectious agent, and an attempt to demonstrate vector competence for HBV failed in an experiment with chimpanzees.18 In that experiment, bed bugs were fed HBV-infected blood through an artificial membrane. Two weeks later, analysis of the bugs showed that about 50 percent of the insects contained virus. These insects were then fed on chimpanzees, but no infections or seroconversions (production of antibodies) resulted in the primates. When the same blood used to infect the insects was later injected into the chimpanzees, they rapidly developed HBV infection.18

Even though evidence for disease transmission by bed bugs is equivocal, issues of vector competence, reactions to insect bites, embarrassment, and mental anguish have been the basis for lawsuits against landlords and lodging corporations.19, 20

BITE REACTIONS. A variety of clinical reactions to bed bugs have been reported in the medical literature (Table 1), but basically, they range from: (1) no reaction, to (2) cutaneous (skin) reactions, to (3) systemic (all over) reactions.Local skin reactions have been reported to be uniphasic or multiphasic in timing and are similar to those previously noted to other insect bites and stings. A view of each type of reaction follows:

Cutaneous Reactions. It seems that the usual response to a bed bug bite appears to be no reaction with a barely visible punctum at the location of the bite. In my experience, medical attention is sought most commonly for 2-5 mm-sized itchy, red spots at bed bug feeding sites, one per insect. These usually itch intensely and if not scratched too much, resolve within a week or so. The size and itching associated with these common reactions may increase over time in some individuals who experience repeated bites. There are no data to establish how frequently common reactions occur.

Some patients experience complex cutaneous reactions. Reports of these have included itchy wheals around a central bite punctum, papular urticaria, blisters and hives-like lesions at bite sites usually noted upon arising in the morning. In some cases, these reactions evolve into itchy papules or nodules that when scratched may become infected (like impetigo) and persist for weeks. Secondary infection also may result in folliculitis, cellulitis, or an eczematoid dermatitis.

Cutaneous reactions occurring after bed bug bites appear to reflect a person’s immune responses to bed bug salivary proteins.21 One report supports an immunologic response to salivary proteins as the basis for some cutaneous reactions to bed bug bites. A hotel guest noticed isolated macules (flat, not raised spots) after her first visit to a hotel. After a second stay at the same hotel one year later, macules again appeared but this time evolved over three days into crops of erythematous nodules with blisters.22 She was skin tested with a C. lectularius salivary gland extract a year after the second hotel stay. Dermal injection of increasing concentrations of the extract resulted in a dose-related increase in the size of wheal and flare reactions that occurred within 20 minutes of injection.

Systemic Reactions. There are a few reports of systemic reactions from bed bug bites, for instance asthma, generalized urticaria, and anaphylaxis.23-25 Some authors have suggested that generalized urticaria (hives all over) from bed bug bites is not unusual.26 However, the descriptions of the "urticaria" in some reports suggests the medical condition, erythema multiforme.27 One man staying in a hotel awakened during the night with severe itching and urticaria on his arm and neck; bed bugs were found in the room.28 He developed angioedema and hypotension, was hospitalized, and thought to have suffered a heart attack. Eight months later, after an experimental bed-bug bite, he developed a wheal at the bite site and generalized itching that required epinephrine administration to resolve his symptoms. A home evaluation of another man who had asthma revealed bed bugs in his bedding and an intradermal allergy skin test with an extract of bed bugs was positive.25 When his bedding was changed, the asthma attacks ceased.

TREATMENT OF REACTIONS. Pest management professionals should never offer medical advice or treatment to customers that have been bitten by bed bugs, but an understanding of the treatment for local and generalized reactions is helpful when interacting with customers. A review of each follows:

Local Reactions. The treatment of common and complex cutaneous reactions is symptomatic and not evidence based (Table 3). Bite sites should first be washed with soap and water. If lesions are intensely itchy, topical application of over-the-counter or prescription anti-itch agents or intermediate potency corticosteroids may be helpful. Sites that appear to be secondarily infected may benefit from systemic antibiotics as appropriate.

Generalized Reactions. Systemic reactions to bed bug bites should be treated as insect induced anaphylaxis.29 This is a serious situation and treatment includes intramuscular epinephrine first and antihistamines and corticosteroids where appropriate. Patients with previous generalized reactions should be instructed by medical personnel in the use of an epinephrine auto-injector device to be kept available when traveling and also referred to an allergist.

CONCLUSION. With the increased incidence of bed bugs in many parts of the United States and the world, this hard-to-control pest isn’t going to stop generating newspaper headlines any time soon. Therefore, it’s critical PMPs not only have a thorough understanding of the biology and control of bed bugs, but also understand the medical and health effects of this ubiquitous pest that is making such a comeback in the pest management industry.

The author is an associate extension professor at Mississippi State University. He can be reached at jgoddard@giemedia.com.

References cited