Bac­te­r­ial Sym­bionts Halt The Dis­ease Bur­den of Dengue Virus

by Rupin­der Kaur, Sarah R. Bor­den­stein, and Seth R. Bor­den­stein

Amidst a global surge and fo­cus on COVID-19 in­fec­tions, other dis­eases that have had and will con­tinue to have im­pact on hu­man health can­not be for­got­ten. Dengue, for ex­am­ple, con­tin­ues to be a ma­jor mosqui­to-borne vi­ral in­fec­tion, im­pact­ing more than 100 mil­lion peo­ple per year. Den­gue can in­flict joint pain, rash, and fever lead­ing to se­vere ill­ness with no spe­cific treat­ment. In the ab­sence of an ef­fec­tive vac­cine and mos­quitoes de­vel­op­ing re­sis­tance to in­sec­ti­cides, the dis­ease calls for an al­ter­na­tive so­lu­tion with a long-last­ing im­pact. A non-profit re­search con­sortium called World Mos­quito Pro­gram (WMP) last month re­ported a dra­matic drop in Dengue cases and relat­ed hos­pi­tal­iza­tions in Yo­gyakarta, In­done­sia via the de­ploy­ment of mos­qui­toes car­ry­ing a bac­te­r­ial sym­biont called Wol­bachia, prov­ing the ef­fi­cacy of this bio­con­trol sys­tem to im­prove hu­man health.

Fig­ure 1. Wol­bachia can be found nes­tled against sper­matids, the pre­cur­sors of sperm, in arthro­pod testes. A cross sec­tion shows the in­ti­mate as­so­ci­a­tion be­tween Wol­bachia and sper­matid tails. Ar­rows in­di­cate re­gions of con­tact be­tween Wol­bachia and sper­matid mem­branes; 'ax' de­notes fla­gel­lar ax­onemes; 'md' de­notes mi­to­chon­dr­ial de­riv­a­tives. Source

Wol­bachia are Gram-neg­a­tive, ma­ter­nally-trans­mit­ted sym­biotic bac­te­ria that in­habit a wide va­ri­ety of hosts in­clud­ing in­sects, spi­ders, mites, and ter­res­trial isopods, as well as fi­larial and plant ne­ma­todes. It was in the early 1920s that grad­u­ate stu­dent Mar­shall Her­tig, along with his PhD men­tor Dr. Simeon Burt Wol­bach (Har­vard Uni­ver­sity, Cam­bridge, Mass­a­chu­setts), first de­scribed a bac­terium, later named Wol­bachia, in the go­nads of var­i­ous in­sects (Fig­ure 1). Thirty years later, Hannes Laven (Jo­hannes Guten­berg Uni­ver­sity, Mainz, Ger­many) dis­cov­ered that some geogra­phically iso­lated C. pip­i­ens mos­qui­toes failed to breed, pro­ducing few or no prog­eny due to a phe­nom­e­non named cy­toplasmic in­com­pat­i­bil­ity (CI). In 1967, he es­tab­lished that an in­com­pat­i­bil­ity fac­tor trans­mit­ted from moth­ers to off­spring is re­spon­si­ble for the fail­ure. Jan­ice Yen and Ralph Barr (Uni­ver­sity of Cal­i­for­nia in Los An­ge­les, Cal­i­for­nia) in 1971 pro­posed that Wol­bachia are the ul­ti­mate incompati­bility fac­tor caus­ing CI, and it was later dis­cov­ered that Wol­bachia-car­ry­ing (sym­bi­otic) male sperms in fact get mo­dified. This sperm mod­i­fi­ca­tion pre­vents the nor­mal de­vel­op­ment of fer­til­ized em­bryos not carry­ing ma­ter­nal Wol­bachia (aposym­bi­otic), re­sult­ing in em­bry­onic lethal­ity. CI‑causing Wol­bachia strains self­ishly use this re­duc­tion in aposym­bi­otic host fit­ness to their fa­vor, as sym­bi­otic fe­males are com­pat­i­ble and can suc­cess­fully re­pro­duce with both sym­bi­otic and aposym­bi­otic males, pro­viding a rel­a­tive fit­ness ben­e­fit to sym­bi­otic fe­males that rapidly drive Wol­bachia through the po­pulation.

In 2008, two in­de­pen­dent re­search groups from Aus­tralia and the United King­dom made a ground­breaking dis­cov­ery that wMel Wol­bachia from Drosophila melanogaster flies block the pro­liferation of nat­ural vi­ral pathogens to pro­vide re­sis­tance to the host. Re­searchers all over the world then ex­panded the in­ves­ti­ga­tion to mos­qui­toes and showed that Wol­bachia are also refrac­tory to sev­eral other med­ically im­por­tant mos­quito-borne viruses (ar­boviruses) such as dengue, chikun­gunya, West Nile, and Zika. These find­ings led to an idea that the ap­pli­ca­tion of a CI-based drive could spread Wol­bachia into wild mos­quito pop­u­la­tions to make them re­frac­tory to arbo­viruses and limit the virus spread to hu­mans. Dengue‑sprea­d­ing Aedes ae­gypti mos­qui­toes, how­ever, do not nat­u­rally carry Wol­bachia, pos­ing a chal­lenge to drive Wol­bachia into wild mos­quito pop­u­la­tions. Lever­ag­ing mi­croin­jec­tion tech­niques, wMel Wol­bachia was in­jected into A. ae­gypti eggs and es­tab­lished with suc­cess­ful trans­mis­sion to the next gen­er­a­tions. Fur­ther, to im­ple­ment the idea into the field, a non-profit Elim­i­nate Dengue Pro­gram was es­tab­lished that is now called WMP, sup­ported by the Bill and Melinda Gates Foun­da­tion and sub­se­quently oth­ers. The­o­ret­i­cal mod­el­ing pre­dicts the es­tab­lish­ment of Wol­bachia in A. ae­gypti pop­u­la­tions un­der most epidemio­logical set­tings and thus the spread of the pathogen-block­ing trait should markedly curb dengue trans­mis­sion. Us­ing pop­u­la­tion re­place­ment strat­egy (PRS), sym­bi­otic males and fe­males are re­leased to prop­a­gate and es­tab­lish Wol­bachia in the tar­get pop­u­la­tion that even­tu­ally re­duces the vec­tor com­pe­tence and thus dis­ease bur­den in hu­mans (Fig­ure 2). The de­ploy­ment of mos­qui­toes in­fected with the wMel strain of Wol­bachia has been a re­sound­ing suc­cess. In North­ern Aus­tralia, for ex­am­ple, the re­lease of ten sym­bi­otic mos­qui­toes per house per week for 10 weeks sta­bly inte­grated Wol­bachia into wild A. ae­gypti pop­u­la­tion at fre­quen­cies above 80–90%. Im­por­tantly, the bac­te­ria have not been lost from the mos­quito pop­u­la­tion in any of the ar­eas af­ter stop­ping the re­leases, high­light­ing suc­cess of the method. WMP started their field tri­als pri­mar­ily in Aus­tralia in 2011 and, with the ad­vance­ment of PRS ef­forts, re­ported no lo­cal dengue trans­mis­sion in the area of Townsville, Aus­tralia. This led them to es­tab­lish field sites and col­lab­o­ra­tions in many trop­i­cal coun­tries with fre­quent cases of dengue in­fec­tion in­clud­ing In­done­sia, Brazil, Colom­bia, Viet­nam, Sri Lanka, Mex­ico, and some Pa­cific Is­lands. Data con­firmed that rise in Wol­bachia-car­ry­ing mos­quitoes as­so­ciates with pro­tec­tion of the res­i­dents from dengue trans­mis­sion.
 

Fig­ure 2. a) In pop­u­la­tion re­place­ment stra­tegy, CI-in­­­duc­ing Wol­bachia spread through­out un­in­fected tar­get popula­tions, re­placing the na­tive species with pa­thogen-block­ing, Wol­bachia-in­fected mos­­quitoes that are no longer ca­pa­ble of trans­mit­ting viruses. b) In­com­pat­i­ble in­sect tech­nique en­tails re­lease of CI-caus­ing sym­bi­otic male mos­qui­toes that upon mat­ing with wild-type aposym­bi­otic fe­males cause em­bry­onic lethal­ity, thus re­duc­ing the size of dis­ease-trans­mit­ting mos­quito pop­u­la­tions. Source

An al­ter­na­tive strat­egy that has been suc­cess­fully used for Dengue con­trol is In­com­pat­i­ble In­sect Tech­nique (IIT) also known as Pop­u­la­tion Sup­pres­sion, in which sym­bi­otic males that are in­ca­pable of pro­ducing vi­able off­spring af­ter mat­ing with aposym­bi­otic fe­males are re­leased, caus­ing mos­quito pop­u­la­tion break­down. Along with WMP, mul­ti­ple or­ganizations are now de­ploy­ing IIT in­clud­ing Mos­qui­t­o­Mate (Ken­tucky, USA), Ver­ily (Cal­i­for­nia, USA), Com­mon­wealth Sci­en­tific and In­dus­trial Re­search Or­ga­ni­za­tion (Aus­tralia), and Singapore's Na­tional En­vi­ron­ment Agency (Sin­ga­pore). By re­leas­ing sym­bi­otic male mos­qui­toes, their ef­forts have also proven suc­cess­ful, caus­ing 95% pop­u­la­tion re­duc­tion in Cal­i­for­nia, more than 80% re­duc­tion in Aus­tralia, and 90% re­duc­tion in Sin­ga­pore.

In the most re­cent study, WMP pro­vided a "gold stan­dard" trial prov­ing the ef­fi­cacy of PRS in Wol­bachia con­trol mea­sures. They re­leased sym­bi­otic mos­qui­toes in cen­tral Yo­gyakarta, In­done­sia over a num­ber of years. By se­lect­ing 12 wMel-in­fected mos­quito re­lease and non-re­lease sites, they sur­veyed and com­pared the propor­tions of pa­tients with dengue in­fec­tion and dengue-re­lated hos­pi­tal­iza­tions from re­lease and con­trol sites. This com­par­a­tive and con­trolled method yielded a 77% sig­nif­i­cant re­duc­tion in the in­ci­dence of dengue cases and 86% re­duc­tion in hos­pi­tal­iza­tions among a com­mu­nity that en­dures fre­quent dengue out­breaks. Re­searchers have also re­ported a sim­i­lar re­duc­tion in cases of den­gue as well as chikun­gunya in an ur­ban area near Rio de Janeiro, Brazil.

Every year, ev­i­dence for Wol­bachia-based ef­fi­cacy to con­trol dengue and re­lated mos­quito-borne viruses is rapidly ac­cu­mu­lat­ing, high­light­ing the enor­mous po­ten­tial of this ap­proach to fight ar­bo­viral dis­eases at a global scale. That day is not far when gov­ern­ments and the World Health Orga­nization will ap­prove this mi­cro­bial ally for even broader and more sig­nif­i­cant ap­pli­ca­tions.

 

Drs. Rupin­der Kaur, Sarah R. Bor­den­stein, and Seth R. Bor­den­stein are bi­ol­o­gists in the De­part­ment of Bio­lo­gical Sci­ences, Van­der­bilt Uni­ver­sity, Van­der­bilt Mi­cro­biome Ini­tia­tive in Nashville, Ten­nessee, USA. The labo­ratory en­deav­ors to un­der­stand and dis­sem­i­nate the prin­ci­ples that shape in­ter­ac­tions be­tween an­i­mals, mi­crobes, and viruses and the ba­sic and trans­la­tional out­comes of these in­ter­ac­tions. The lab team has stud­ied Wol­bachia for more than 25 years and di­rects the world­wide sci­ence ed­u­ca­tion se­ries Dis­cover the Mi­crobes Within! The Wol­bachia Project that en­gages stu­dents world­wide in bio­di­ver­sity, biotech­nol­ogy, and bioinfor­ma­tics on the Wol­bachia sym­bio­sis.