Research undertaken in the USA demonstrates that bed bugs, like the triatomines, can transmit Trypanosoma cruzi, the parasite that causes Chagas disease, one of the most prevalent and deadly diseases in the Americas.
In a study published online on 17 November 2014 in the American Journal of Tropical Medicine and Hygiene, senior author Dr Michael Z Levy, assistant professor in the department of Biostatistics and Epidemiology at the University of Pennsylvania’s Perelman School of Medicine, and researchers at the Universidad Peruana Cayetano Heredia in Peru, conducted a series of laboratory experiments that demonstrated bi-directional transmission of Trypanosoma cruzi(T. cruzi) between mice and bed bugs.
The significance of this study is that, to date, there has only been debate, rather than evidence, as to whether bed bugs transmitted any disease – so have always been classified as simply a nuisance pest. As such, bed bugs neither attracted the level of public health concern, nor research funding.
Their published press release explains:
The role of the bloodsucking triatomine bugs as vectors of Chagas disease which affects six to eight million worldwide, mostly in Latin America, and kills about 50,000 a year has long been recognised.
The insects infect people not through their bite but faeces, which they deposit on their sleeping host, often around the face, after feeding.
Bed bugs, on the other hand, are usually considered disease-free nuisances whose victims are left with only itchy welts from bites and sleepless nights.
In the first experiment run at the Zoonotic Disease Research Center in Arequipa, Peru, the researchers exposed 10 mice infected with the parasite to 20 uninfected bed bugs every three days for a month. Of about 2,000 bed bugs used in the experiment, the majority acquired T. cruzi after feeding on the mice. In a separate experiment to test transmission from bug to mouse, they found that nine out of 12 (75%) uninfected mice acquired the parasite after each one lived for 30 days with 20 infected bed bugs.
In a third experiment, investigators succeeded in infecting mice by placing faeces of infected bed bugs on the animal’s skin that had either been inflamed by bed bug bites, or scraped with a needle. Four out of 10 mice (40%) acquired the parasite by this manner; 1 out of 5 (20%) were infected when the skin was broken by the insect’s bites only. A final experiment performed at the Penn bed bug lab in Philadelphia demonstrated that bed bugs, like triatomines, defecate when they feed.
“We’ve shown that the bed bug can acquire and transmit the parasite. Our next step is to determine whether they are, or will become, an important player in the epidemiology of Chagas disease,” Levy said. “There are some reasons to worry – bed bugs have more frequent contact with people than kissing bugs, and there are more of them in infested houses, giving them ample opportunity to transmit the parasite. But perhaps there is something important we don’t yet understand about them that mitigates the threat.”
T. cruzi is also especially at home in the guts of bed bugs. “I’ve never seen so many parasites in an insect,” said Renzo Salazar, a biologist at the Universidad Peruana Cayetano Heredia and co-author on the study. “I expected a scenario with very low infection, but we found many parasitesthey really replicate well in the gut of the bed bugs.”
Other investigators have shared this suspicion. In 1912, just three years after Carlos Chagas described the transmission of the disease by kissing bugs, French parasitologist ámile Brumpt recounted that he had infected almost 100 bed bugs exposed to an infectious mouse, and then used them to infect two healthy mice. Decades later an Argentine group replicated his work. These experiments, largely ignored during the recent bed bug resurgence, missed one key point.
“Mice can hunt and eat bed bugs,” said Dr Ricardo Castillo-Neyra, DVM, co-author and postdoctoral fellow at the Universidad Peruana Cayetano Heredia and Penn. “The older studies were almost certainly only documenting oral transmission of the parasite. Our work shows for the first time that bed bugs can transmit the parasite when their faeces are in contact with broken skin, the route by which humans are usually infected.”
“There have always been triatomine bugs and cases of Chagas disease in the U.S., but the kissing bugs we have here don’t come into homes frequently like the more dangerous species in South and Central America do,” Levy said. “I am much more concerned about the role of bed bugs. They are already here – in our homes, in our beds and in high numbers. What we found has thrown a wrench in the way I think about transmission, and where Chagas disease could emerge next.”
Equally worrying is the invasion of bed bugs into areas where Chagas disease is prevalent, especially in countries where traditional insect vectors of the parasite have been nearly eliminated, Levy said. In these areas, bed bugs will be repeatedly exposed to T. cruzi, and could re-spark transmission where it had been extinguished.
“Bed bugs are harder to kill than triatomines due to their resistance to common insecticides.” Levy said. “No one is prepared for large scale bed bug control. If the parasite starts to spread through bed bugs, decades of progress on Chagas disease control in the Americas could be erased, and we would have no means at our disposal to repeat what had been accomplished.”
Often referred to as a silent killer, Chagas disease is hard to diagnose in its early stages because the symptoms are mild or absent. The parasites are hidden mainly in the heart and digestive muscle and over time can cause cardiac disorders and sometimes digestive or neurological problems. In later years, the infection can lead to sudden death or heart failure caused by progressive destruction of the heart muscle. Although there are some drugs to treat Chagas disease, they become less effective the longer a person is infected.
The long asymptomatic period of Chagas disease complicates surveillance for new outbreaks of transmission. In Arequipa, Peru, thousands became infected with the parasite before a case appeared in the hospital. The same could happen in cities in the United States if the parasite were to emerge in the bed bug populations, the authors say.
“Carlos Chagas discovered T. cruzi in triatomine insects before he saw a single case of the disease,” Levy said. “We need to learn from his intuition–check the bugs for the parasite.”