PHILADELPHIA--The bed bug may be just as dangerous as its sinister cousin, the triatomine, or "kissing" bug. A new study from Penn Medicine researchers in the Center for Clinical Epidemiology and Biostatistics demonstrated 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.
The role of the bloodsucking triatomine bugs as vectors of Chagas disease--which affects 6 to 8 million worldwide, mostly in Latin America, and kills about 50,000 a year--has long been recognized. The insects infect people not through their bite but feces, 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 a study published online this week in the American Journal of Tropical Medicine and Hygiene, senior author Michael Z. Levy, PhD, 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 T. cruzi between mice and bed bugs.
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 9 out of 12 (75 percent) 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 feces 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 percent) acquired the parasite by this manner; 1 out of 5 (20 percent) 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 parasites--they really replicate well in the gut of the bed bugs."
Bed bugs and kissing bugs are distant cousins but share many striking similarities. Both insects hide in household cracks and crevices waiting for nightfall and the opportunity to feed on sleeping hosts. They are from the same order of insects (Hemiptera) and both only feed on blood. (One main difference is their size: kissing bugs are five times as big as a bed bug). With so much in common, it seemed logical to the authors that the kissing bug's most infamous trait, the transmission of T. cruzi, is also shared by the bed bug.
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 Émile 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 Ricardo Castillo-Neyra, DVM, PhD, coauthor 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 feces are in contact with broken skin, the route by which humans are usually infected."
More people in the U.S. are infected with T. cruzi now than ever before. The Centers for Disease Control and Prevention estimates that the number of Chagas disease cases in the U.S. today could be as high as 300,000.
"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."
Other co-authors of the study include Aaron W. Tustin, Katty Borrini-Mayorí and César Náquira.