Malaria Parasite Transmission Flies High when Mosquitos Are Around

A new study pub­lished in Evol­u­tion Let­ters has tested the idea that para­sites can evolve the abil­ity to time their invest­ment in trans­mis­sion to match the activ­ity of their vec­tors. Luke Turn­er reports:

One of the biggest chal­lenges that all organ­isms face is sur­viv­ing in their dynam­ic and con­stantly chan­ging sur­round­ings. Adapt­ing to fluc­tu­ations in factors such as food avail­ab­il­ity, tem­per­at­ure or day length requires beha­vi­our­al and physiolo­gic­al change. This is no dif­fer­ent for para­sites, which sur­vive by gain­ing nutri­ents at the expense of a host spe­cies. A vari­ety of physiolo­gic­al alter­a­tions have been observed in para­sites due to changes in their envir­on­ment, both over short (cir­ca­di­an) and long-term (sea­son­al) peri­ods. This vari­ation in physiology is par­tic­u­larly inter­est­ing because it changes the parasite’s dynam­ic with its host, dir­ectly affect­ing para­site epidemiology.

The ulti­mate goal of any para­site is to increase its trans­mis­sion to more hosts. Para­sites could use envir­on­ment-induced changes in host beha­viour and physiology to their advant­age, profit­ing from favour­able con­di­tions when they arise. When a para­site is trans­mit­ted through a vec­tor (i.e. an inter­me­di­ate organ­ism that car­ries the para­site to its host), this the­ory is known as the ‘Hawk­ing hypo­thes­is’. There is lots of evid­ence that para­sites are able to respond to changes in the envir­on­ment in order to max­im­ise their vec­tor trans­mis­sion. In Wucher­er­ia ban­crofti para­sites, for example, their trans­miss­ible ele­ments (micro­fil­aria) are trans­ferred between the lungs and peri­pher­al blood of an infec­ted host. In areas where the para­site is trans­mit­ted through a night-bit­ing mos­quito vec­tor, micro­fil­aria are released into the blood at night, where­as in places where the mos­quito vec­tor bites dur­ing day­light hours, W. ban­crofti micro­fil­aria are more abund­ant in the blood through­out the day. This indic­ates that para­sites may be able to induce physiolo­gic­al changes with­in a host in response to a change in the abund­ance of the parasite’s vec­tor, lead­ing to an increased like­li­hood of transmission.

A sim­il­ar pro­cess occurs with­in hosts that are infec­ted by mal­aria, a wide­spread and dev­ast­at­ing dis­ease which is trans­mit­ted by mos­quito vec­tors. From the point of view of the para­site caus­ing mal­aria, it would be most bene­fi­cial to max­im­ise their infec­tious­ness around the time that mos­qui­tos com­monly bite. Des­pite this being observed in nat­ur­al sys­tems, it is not yet clear wheth­er para­sites have the abil­ity to coordin­ate their infec­tious­ness with when mos­qui­tos feed on their hosts.

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The mos­quito – a vec­tor for mal­aria.                                       Photo: https://pixabay.com/en/mosquito-insect-schnake-fly-animal-83639/

New research car­ried out by Pigeau­lt et al. (2018), pub­lished in Evol­u­tion Let­ters, aimed to test wheth­er a para­site which causes mal­aria can align its invest­ment in trans­mis­sion with the beha­viour of their vec­tors. Using Plas­modi­um rel­ictum as the para­site spe­cies, the research found evid­ence that two sep­ar­ate trans­mis­sion strategies have evolved in response to envir­on­ment­al vari­ation in mos­quito pres­ence. By cre­at­ing a the­or­et­ic­al mod­el, it was iden­ti­fied that when mos­qui­tos are act­ive in a reg­u­lar daily pat­tern, a time-vary­ing strategy can emerge in the para­site. This was con­firmed in the exper­i­ment­al spe­cies, where an increase in para­site infec­tion of Culex mos­qui­tos dur­ing the late after­noon coin­cided with the peak in bit­ing activ­ity of the mos­qui­tos. This provides sup­port for the ‘Hawk­ing hypo­thes­is’ and sug­gests that the Plas­modi­um para­site can adapt to the daily tim­ing of mos­quito pres­ence in order to increase its own transmission.

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The para­site lin­eage used in Pigeau­lt et al. (2018), Plas­modi­um relictum

A second strategy that the para­site adopts is a more imme­di­ate reac­tion to the pres­ence of a bit­ing mos­quito, which evolves when the abund­ance of vec­tors is less pre­dict­able. These plastic responses are triggered as soon as a mos­quito bites the infec­ted bird host, res­ult­ing in the imme­di­ate growth of the para­site and invest­ment in trans­mis­sion, and there­fore an increased chance of infect­ing the vec­tor as it feeds on the bird. This strategy is imple­men­ted in both the short-lived acute phase of the parasite’s infec­tion, when trans­mis­sion to mos­qui­tos is very high, but also in the chron­ic phase which lasts for months and does not lead to high infec­tion rates in mos­qui­tos. Dur­ing the long chron­ic phase, this plastic trans­mis­sion strategy is effect­ive at allow­ing the para­site to respond to sea­son­al vari­ation in mos­quito abund­ance, react­iv­at­ing its trans­mis­sion when they appear in the envir­on­ment. On the oth­er hand, when in its highly infect­ive acute phase, the para­site can react to unex­pec­ted and sud­den changes in the abund­ance of mos­qui­tos, increas­ing its trans­mis­sion when they begin feed­ing on the host in high frequencies.

Des­pite this strong sup­port for the idea that Plas­modi­um para­sites can imple­ment physiolo­gic­al changes in them­selves to match the beha­viour of their mos­quito vec­tor, it is import­ant to con­sider altern­at­ive explan­a­tions for this pat­tern. It is pos­sible that daily nat­ur­al fluc­tu­ations in the host bird’s immune sys­tem affects the infec­tious­ness of the para­site, lead­ing to an increase in trans­mis­sion dur­ing the late after­noon. There may also be daily vari­ation in the pro­duc­tion of immune com­pounds with­in the mos­quito, res­ult­ing in dif­fer­ences in how eas­ily the para­site can infect the vector.

Non­ethe­less, this research provides excit­ing insights into the adapt­ive cap­ab­il­it­ies of para­sites that cause mal­aria. It demon­strates how they are able to align their invest­ment in vec­tor trans­mis­sion with daily pat­terns of mos­quito abund­ance, while also ini­ti­at­ing imme­di­ate responses to mos­quito bites when their pres­ence is less pre­dict­able. With mal­aria affect­ing the lives of anim­als and humans around the world and caus­ing dev­ast­at­ing symp­toms, this research takes a step towards under­stand­ing the vari­able trans­mis­sion of the Plas­modi­um para­site. Fur­ther­more, it provides hope for con­trolling the spread of human mal­aria in the future.

 

Luke Turn­er is a MSc Sci­ence Com­mu­nic­a­tion stu­dent at the Uni­ver­sity of Shef­field. The ori­gin­al study is freely avail­able to read and down­load from Evol­u­tion Let­ters here.