Remember when you were a child, getting tucked in by your parents, and they would say, "good night, sleep tight, don't let the bed bugs bite"? Were you ever really concerned about them biting? They never did. Times have changed since then. Bed bugs are back, and are a growing problem in western nations. Now that bed bugs are back, it is important for humans to become reacquainted with bed bugs.
Bed bugs, or cimicids, are small, blood sucking parasites, with the most common human bed bugs being Cimex lectularius. It is theorized that C. lecturlarius evolved in caves shared by bats and humans, as they are pests to both (Mullen and Durden 2009). Historically, their presence has been noted as far back as 200 B.C. in ancient Greece (William and Anthon 1847). In 77A.D, they were mentioned by Pliny the Elder in Rome as having medicinal value, and as late as the 18th century in France by Jean-Étienne Guettard recommending using bed bugs to treat hysteria (William and Anthon 1847). Most others saw bed bugs as an obvious pest. There were many traditional methods used to repel and kill bed bugs, including the use of a variety of plants, fungi, and insects (Wolff and Wolff 1804). This did very little to control their infestation on a large scale, as prior to the 1940's, bed bugs were very common human pests. An example of this can be seen from a report of the UK Ministry of Health, which mentioned that in many areas, every house was infested with bed bugs (Boase 2004). The bed bug problem was finally contained, specifically in western industrialized nations, post World War II with the arrival of pesticides like DDT (Adler 2010).
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Cimicids are strictly hematophores, with blood being their only source of nutrients (Usinger 1966). Bed bugs are hemimetabolous and undergo five immature life stages before morphing into adult form (Bonnefoy et al. 2008). Larvae require a blood meal in order to morph into the next instar. Studies have shown that there is a minimum meal size to allow for stage progression in C. lectularius (Tawfik 1968), Hesperocimex sonorensis (Ryckman 1958), and S. antennatus (Overal and Wingate 1976). Without this meal, the larvae die within a matter of days (Usinger 1966). Feeding by C. lectularius occurs when hosts are minimally active, for example, while sleeping, and bed bugs leave their refugia to feed for 10 to 20 minutes, returning to their refugium shortly afterward (Usinger 1966). C. lectularius feeds once per week in laboratory settings (Usinger 1966), but it has been more recently reported that at some stages they can survive for 18 months without a meal (Yancy 2011). To aid in digestion, cimicids harbor microbial symbionts that live in structures called mycetomes (Usinger 1966, HypÅ¡a and Aksoy 1997). In-between feedings, bed bugs remain concealed inside the refugium.
Another notable adaption is traumatic insemination (Usinger 1966). Most research on bed bugs has focused on their unique mating system of traumatic insemination (Stutt and Siva-Jothy 2001). During mating, the male pierces the female's abdominal wall and sperm are injected into the mesospermalege, part of the female's paragenital system through which the sperm travel (Usinger 1966). The right genital clasper of the male is a highly modified claw-like structure that performs the penetration (David 1956). Females also possess a normal reproductive tract that functions during the laying of eggs, but males do not use this tract for insemination. Traumatic insemination has been found to result in reduced reproductive success in females, thus the process is probably a male reproductive strategy to prevent females from mating further (Stutt and Siva-Jothy 2001). In addition, feeding behavior seems influence mating, as C. lectularius males are more sexually interested in females who have recently fed (Stutt and Siva-Jothy 2001), likely because blood meals are important for adult females to produce eggs (Usinger 1966). Bed bugs do not lay eggs until the female has mated, unlike other insects that produce eggs regardless of whether mating has occurred (David 1956).
Bed bugs have a relatively narrow amount of hosts, all of which live enclosed spaces like caves and buildings. This may be because they have high body temperatures and need a stable thermal environment to survive. Human hosts are most commonly parasitized by C. lectularius and C. hemipterus, though these species can survive on birds, bats, and rabbits in the laboratory (Usinger 1966). Wild C. lectularius has been found on birds (Marshall 1981) and bats (Usinger 1966). Bats, swifts, swallows, and humans have the potential to coexist, which allows for C. lectularius to switch between hosts (Usinger 1966).
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Preferred biting sites of cimicids lack hair or feathers, have a thin epidermis, and have plentiful peripheral blood supply. On humans, these bite locations range depending on the individual. Cimicids feed on hairless sites such as the wings, forearms, feet, and penis of bats (Overal and Wingate 1976, Ueshima 1968). Birds are bitten on the featherless base of the legs and near the eyes (Platt 1975). Bed bugs have been known to inject anesthetic into hosts to hide their presence (Yancy 2011). Additionally, bed bug saliva contains vasodilatory substances to increase blood flow to bite sites (Valenzuela et al. 1995); however, the allergic response of the host that follows is probably caused by other components of the saliva. The strength of the reaction varies from no response to death, though death is relatively rare in human hosts (Thomas et al. 2004). Very few people are insensitive to bed bug bites, and affected areas typically display rashes and cause discomfort (Usinger 1966).
Studies suggest C. lectularius detects human hosts from as far away as 1.5 m through the use of heat cues, kairomones, and/or CO2 (Marx 1955). Temperature sensors are present on the antennae and are capable of sensing differences of 1â-¦C to 2â-¦C (Sioli 1937). Kairomones of the hosts include dried human sweat, sebaceous gland material, and dried ear secretions (Sioli 1937). Despite the variety of detection methods, human hosts can remain undetected from bed bugs in rooms for several weeks (Marx 1955). Once a bed bug is engorged, previously attractive cues become neutral or repellent prompting the bug to leave the risky host environment after feeding (Aboul-Nasr and Erakey 1968).
Bed bugs have the potential to act as vectors and their bites can lead to secondary infections. Feeding wounds may allow other infections to enter a host, but this occurs rarely (Ter Poorten and Prose 2005). Bed bugs are also capable of carrying the infectious particles of typhus, kala-azar, anthrax, plague, relapsing fever, tularemia, Q fever, hepatitis B virus, and HIV (Burton 1963). Transmission of these infectious agents is highly unlikely. In 2001, Silverman et al., found that despite the fact that both HIV and hepatitis B can persist within the C. lectularius gut for several weeks, no viral replication or documented infections have been recorded. Cimicids also harbor trypanosomes including Trypanosoma cruzi (Bower and Woo 1981), which causes Chagas disease. Although cimicids are capable of transmitting trypanosomes to bats, studies have shown transmission of trypanosomes from bed bugs to bats was low, and trypanosomes do not replicate in bed bugs (Bower and Woo 1981). Dangers of transmission can occur if these infectious agents evolve to replicate within bed bugs.
Bed bugs can also have detrimental fitness effects on hosts. This has been extensively studied in cliff swallows. One study found that parasitized swallow chicks had slower feather growth than non-parasitized chicks (Chapman and George 1991). Another study found that long-term survival of adult cliff swallows cleared of parasites was 14% higher than for those untreated from the same colony (Brown et al. 1995). Furthermore, it was found that eradicating bugs from nests increased cliff swallow chick survival by 50% to 100%, and led to higher body mass in nestlings (Brown and Brown 1996).
Natural bed bug population sizes are poorly documented, and range from a few individuals per bat roost or bird nest (Platt 1975, Usinger 1966), to several hundred per nest (Grubb et al. 1986), or many thousand per cave (Marshall 1981, Overal and Wingate 1976). Cimicid populations in human households range from 4 to 221 cimicids per house (Newberry and Jansen 1986) and up to 5000 per mattress (Lewis 1949). Recorded infestation rates of C. lectularius in human populations' range depending on which year the study was taken, as post 1940 population sizes were small and more recent population sizes are larger. It has been determined that larger host colonies tend to support higher densities of cimicids both in humans (Matheson 1941) and swallows (Brown and Brown 1996). That being said, population turnover in residents is a more important indicator to infestation rates (Boase 2001, Boase 2004) with traveling backpackers, immigrants, and the homeless being examples of sources of infestation (Doggett et al. 2004, Hwang et al. 2005, Lewis 1949, Newberry and Jansen 1986).
Dispersal can occur both actively and passively. Bed bugs can remain motionless in a refugium for 35 days at room temperature before they disperse (Johnson 1941), but first instars walk readily and female adults are slightly more active walkers than males (Johnson 1941). Walking is initiated most likely by hunger, but research suggests that it is not necessarily directly related to feeding (Johnson 1941), and can result in dispersal to seek out new hosts. Another feature of bed bugs that suggests dispersal is actively sought is that the attachment sites differ between dispersing and feeding individuals (Brown and Brown 1996). The aforementioned Brown and Brown found that 0.11% of 40,827 adult cliff swallows carried adult swallow bugs on their feet rather than in their feathers. Passive dispersal is the most important way for cimicids to reach new hosts. Bed bugs can be transported by humans in clothing, luggage, and furniture (Boase 2001, Doggett et al. 2004, Lewis 1949), and because of human host range, new sites have been rapidly colonized by human-associated bed bugs (Hwang et al. 2005). Bat and swallow-associated bugs have also been reported as rapidly colonizing new sites when transported in fur or feathers of their hosts (Brown and Brown 1996).
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Working theories on the resurgence of bed bugs include human complacency, increased international travel, and increased resistance to pesticides (Adler 2010). The reasoning for human complacency is simple in that people, primarily in the western world, have not had to deal with bed bugs for some time, and thus do not recognize them or know how to control infestations. International travel has increased since WWII due to advances technologies, decreased cost of travel, and increased global business (Boase 2001). This is clearly seen as many bed bug infestations have been found in hotel rooms (Yancy K.B., 2011). Most evidence to the resurgence comes from recent research, which suggests that bed bug pesticide-resistance is increasing dramatically. Bed bugs are developing resistance to pesticides including DDT and organophosphates (Steelman et al. 2008). Pesticide resistance is also increasing more in areas where there is a lot of travel. One study found that New York City bed bugs were 264 times more resistant to 1% deltamethrin, in contact bioassay, than an insecticide-susceptible population collected in Florida (Yoon et al. 2008). Moreover, another study looked at bed bug population genetics using mitochondrial DNA marker. Researchers found high levels of genetic variation within the bed bug populations, which suggests that bed bug populations did not undergo a genetic bottleneck one would expect from post WWII insecticide control (Szalanski et al. 2008). It is likely that the bottleneck did not occur as populations may have been maintained on other hosts, such as birds and bats, thus given fewer barriers to gene flow there may be a rapid increase of resistant populations.
There is a potentially huge economic impact on human hosts as cimicid infestations result in multimillion-dollar damage in the hospitality industry, poultry industry, and private and communal households (Davies 2004). Costs primarily arise from cost of pest control, as well as damage to social reputation specifically in the hotels (Boase 2001, Davies 2004), replacement of infested infrastructure (Hwang et al. 2005), and monetary reparation by guests (Marshall 2004). The poultry industry losses are due to a series of complex consequences including a loss of productivity due to allergic reactions of workers, reduced egg value due to bug fecal spots on the eggs, and lower egg production from parasitized chickens (Axtell 1999). As bed bug infestation rates are on the rise in the United States, actions are being taken to halt this resurgence. Recently, state lawmakers in Pennsylvania have gotten involved with the bed bug epidemic by writing a new bill that specifically creates rules for landlords and hotels (McCormick 2011). Currently, 15 other states have legislation like it (McCormick 2011). The bill targets owners or landlords of multi-unit buildings by making it law that they must keep a bed bug free environment (McCormick 2011). The bill also targets hotels, whose guests must have a written certificate that their room is bed bug free (McCormick 2011). Violators of these laws can see fines anywhere from $300 to $1000, but lawmakers have put in place provisions to protect landlords from tenants that are the cause infestation (McCormick 2011). Additionally, grants have been handed out to researchers in at both Rutgers (Mims 2011) and University of Missouri-St. Louis (Walker 2011) in order to determine potential ways to battle bed bugs.