Travel back in time just 25-30 years and you would be hard pressed to find many people who recognised a bed bug. Pest control technicians would have known but would mostly say that this ectoparasite, or more accurately micro-predator, was a pest of the past, largely eliminated by use of modern insecticides. How wrong could they be? Bed bugs were simply undergoing a downturn in fortune prior to a resurgence and rapid spread of insecticide resistant populations starting in the early 1990s.
These days, patients with bed bug bites are quite likely to turn up in dermatology clinics as confusing immune reactions often unresponsive to routine treatment. In some places the bed bug is considered the number one biting nuisance.
What are bed bugs?
Cimex lectularius, the temperate bed bug is found throughout the cooler parts of the world, which means it primarily occurs in developed countries. In upland areas in Africa and parts of southern Asia it overlaps with, Cimex hemipterus, which replaces it in the tropics. Also there are other species associated with birds and bats that may sometimes invade houses and bite people, so encouraging house martins, swallows, and pigeons to nest on buildings could result in visits from their ectoparasites. However, in reality it is the human species that causes most problems.
Like all true bugs of the order Hemiptera they are hemi-metabolous and have five nymphal stages between the hatching egg and the adult bug. In this family of bugs the adults are wingless so only size differentiates juveniles from adults. All stages, other than newly emerged, 1mm long, colourless first stage nymphs, are medium to chestnut brown in colour, flattish, and oval in outline growing to 5-6mm as adults. The insects feed on blood taken from the host through the tubular mouthparts of their rostrum, which folds back under the thorax when not in use. The bugs feed about once a week, less often in colder conditions, and take relatively enormous blood meals that distend the abdomen into a taught sausage shape. They then quickly excrete the excess water to enable them to re-enter the crevice where they usually hide. As a result droplets of the watery faeces, blackish with the digested remains of haemoglobin from previous meals, are deposited around the threshold of the harbourage. These dry to form hard black spots that are diagnostic of the places where the bugs hide.
A structural pest
Despite its name the bed bug is mainly a structural pest, inhabiting various cracks and crevices in the vicinity of a host’s bed, including cracks in the bed frame and headboard, behind skirting boards, picture frames, and wallpaper, as well as in other pieces of furniture and electrical items like radios and wall sockets. Bugs can find their way onto the mattress where they hide in the welted seams around the edges and under daisy buttons.
The life cycle is prolonged, depending upon their ability to gain access to blood meals with sufficient frequency. Given ideal conditions this could be as short as 5-6 weeks from emergence to maturity but if conditions are unsuitable and hosts infrequent it could take as long as a year to complete. Either way, once the bugs become adults they can live for a year or more and even for up to several months if starved. Having fed, adult female bugs are supplied with nutrients necessary to form a batch of eggs, which they lay more or less continuously at an average rate of about 3-5 daily throughout their lives. The eggs resemble large (1mm long) head louse eggs that are inserted into cracks or folds within the harbourage and glued into position, where the empty shells remain after hatching.
The insects emerge at night and detect their hosts using a combination of odour and warmth. Exhaled carbon dioxide plays a role but bed bugs are particularly sensitive to aldehydes such as nonanal and heptanal as well as some alcohols. Having located the host they crawl over the bed until they find an area of exposed skin, preferably in contact with a surface the bugs can stand while feeding, resulting in rows or clusters of bites from several bugs feeding at once. Not all victims are sleeping. My first encounter with wild bugs was in a cinema in Bangladesh, where bugs lived in the wooden arms of the seats, so when the house lights went down more than one “main feature” started – dinner-time for the unseen occupants, explaining why early 20th century cinemas were often known as “bug-hutches”.
Bites and more bite reactions
Bed bugs improve their feeding rate and prevent blood clotting in their mouthparts by injecting anticoagulant saliva as they feed. This saliva contains nearly 100 polypeptides that inhibit clot formation, assist vasodilation or digestion. But, however efficiently the insects feed, some of this is always left behind in the tissues. Initially, as with exposure to any haematophagous insect, the naive victim shows no reaction, explaining how bugs can live undetected in hotels and hostels because those bitten mostly move on before they show an immune response. Only after repeated biting does a reaction occur in essentially the same sequence observed with other biting species.
Although most people exhibit a classic progression through the immune response to bug bites a few exhibit variant or anomalous reactions. Despite the complex mix of salivary proteins that results in physiological differences between species, there are sufficient similarities between bed bugs and culicine mosquitoes that people sensitised to one may respond to the other. However, bed bugs can causing distinctively severe reactions in some people. Whether this is because they are wasteful of saliva during feeding, or the antigens in their saliva are particularly immunogenic, is not clear.
Reports of severe and unexpected reactions to bed bug bites are not new. In 1949 an outbreak of “tram-legs” was identified in 36 women who travelled on Amsterdam trams. The condition consisted of large bullae horizontal on the backs of their thighs, just where the leg comes in contact with the edge of the seat. After a series of tests to see if the patients were allergic to any of the seat components it was found that one tram hosted numerous bugs. Most victims do not progress beyond the basic maculo-papular reactions likely to be found following any insect bite. Where more complex presentations occur most have never been investigated for their specific cause, although sensitised patients show both circulating IgG and IgE to a variety of species-specific salivary components. One reported case was misdiagnosed as erythema multiforme before bugs were found in the apartment. In another case a systemic anaphylactiform response occurred, the cause of which was confirmed by a prick test using extracts from a bug taken from the home. But the most common severe reaction is a pronounced localised or even systemic bullous hypersensitivity that not only may take longer than usual to develop but may also progress to a localised, necrotising vasculitis, which in one case has been shown due to the nitrophorin in the bug saliva.
Could bed bugs be vectors of disease?
Other than causing annoying and severe irritation, bed bug bites are relatively harmless and there is no confirmed infection risk beyond pyoderma following excoriation of bite reactions. However, there is a moderately extensive literature that shows some pathogens, such as hepatitis B virus particles, can survive intact within the bug gut but no evidence they can be passed on. More recent studies have shown that Bartonella quintana can survive for more than 2 weeks in the gut and be passed in the faeces, in much the same way as the pathogen would be passed by its normal vector the clothing louse. Also bed bugs have been shown to be potentially competent vectors of South American trypanosomes, Trypanosoma cruzi, although it is believed the risks of passing the parasites on are low because this species of bug rarely defecates on their host while feeding, unlike triatomine bugs which are the recognised vectors for the parasites. Nevertheless, T cruzi infection (Chagas’ disease) is increasingly prevalent outside South America due to human migration so there is always a slight risk that some bugs may exhibit anomalous behaviour and thereby become competent vectors.
Getting rid of bed bugs
The bed bug is not a pest that is easily dealt with. Household crawling insect sprays are mostly ineffective, either through low dosing or resistance to the insecticide. Consequently, elimination of infestation is a professional task. However, avoidance of being attacked is a practical proposition that requires awareness and vigilance. Always check around the bed structure, walls, and fitments in hotels and guest houses for signs of “spotting” where the bugs have defecated because, if you don’t you could be taking some unwelcome guests back home in your luggage.
Revised extract from Dermatology in practice 2016; 22(3): 74–77