The Economist - 04.12.2021

86 Science & technology The Economist December 4th 2021

Trypanosomes

A nasty sting

A

s recent events have made abun­

dantlyclear,newviraldiseasesinpeo­

pleoftenstartasspilloversfrominfections

affectingotherspecies.Butvirusesarenot

theonlypathogenstodoso.Leishmania­

sis,sleepingsicknessandChagas’disease,

threepotentiallylethalillnessescausedby

single­celled creatures called trypano­

somes,areprobablyinthiscategory,too.

Notonlyaretheyspreadbyinsects(sand

flies,tsetsefliesandkissingbugsrespec­

tively),theypresumablyoriginatedinin­

sects, too (though notnecessarily their

current vectors)—for most known try­

panosomesareinsectparasites.Thatraises

thequestionofhowtheyleaptthespecies

barrier.AstudyjustpublishedinthePro-

ceedingsoftheRoyalSociety, byEvanPalm­

er­YoungofAmerica’sDepartmentofAgri­

culture,suggeststheanswermaybe“bees”.

Dr Palmer­Young’s starting­point was

theobservation,madea fewyearsago,that

Crithidia mellificae, a trypanosome once

thoughtexclusivetohoneybees,wasturn­

ingupinmammals.Marmosets(atypeof

NewWorldmonkey),coatis(smallcarni­

voresrelatedtoraccoons),fruitbats,crab­

eatingfoxesandocelotsareallnowknown

tohostit.Hethereforewonderedifthere

wassomethingspecialaboutthisparticu­

lartrypanosomethatallowedtheseleaps

intomammalstohappen—andwhether,if

there was, its adaptation to honeybees

mightbethatsomething.

Twothoughtsoccurredtohim.Onewas

that,thoughmostinsectsareectothermic

(meaning  their  body  temperatures  vary

with their surroundings), bees, like mam­

mals,  often  generate  extra  heat,  and  also

benefit from the proximity, in their hives,

of their neighbours. This keeps their body

temperatures at mammal­like levels in the

mid­to­upper thirties. 

Dr  Palmer­Young’s  other  thought  was

that  honeybee  guts  are  more  acidic  than

those  of  most  insects  (this  helps  them  to

digest  nectar  and  pollen).  Indeed,  they

have pHs which match those of mammali­

an digestive tracts. He therefore speculated

that  mammalian  heat,  or  acidity,  or  both,

may create a barrier to trypanosome infec­

tion of mammals which parasites adapted

to bees can easily overcome.

To  test  this  idea,  he  and  his  colleagues

looked at C. mellificae, and also at a second

honeybee  trypanosome  parasite,  Lotmaria

passim.  For  comparison,  they  studied  two

strains  of  Crithidia fasciculata,  a  trypano­

some common in mosquitoes that is close­

ly related to C. mellificae. 

They cultured all four types of trypano­

some in flasks, and then exposed samples

of  each  either  to  a  range  of  temperatures

from  20­41°C  at  constant  acidity,  or  to  a

range of acidities from pH 2.1 (very acidic)

to  pH  11.3  (very  alkaline)  at  constant  tem­

perature.  While  doing  so  they  monitored

the parasites’ population­growth rates. 

As Dr Palmer­Young had theorised, both

of  the  honeybee  parasites  tolerated  the

sorts  of  temperatures  common  in  hives.

But they did not tolerate them equally well.

The  growth  rate  of  C. mellificae,  a  species

well established in bees, peaked at 35.4°C.

That  of  L. passim,  which  was  not  reported

in bees until 2014 and is thought to be a re­

cent arrival, peaked at 33.4°C. Both strains

of C. fasciculata, however, could cope only

with  lower  temperatures.  Their  growth

rates peaked near 31°C. The acidity experi­

ments similarly revealed that both bee par­

asites thrived at pH 5.2, the level of acidity

in  honeybee  digestive  tracts,  whereas  the

mosquito parasites needed a more alkaline

pH of around 7.5 to grow well.

All told, both honeybee gut­acidity and

hive  temperature  create  a  barrier  to  try­

panosomes  that  C. mellificae has  largely

overcome, that L. passim is just about cop­

ing  with,  and  which  the  mosquito  para­

sites cannot endure. How this happened is

unclear.  What  is  clear,  though,  from  the

mammalian  spillovers  of  C. mellificae,  is

that  it  can  lead  to  a  parasite  also  able  to

thrive in mammals. 

Whether that was a path once taken by

the trypanosomes which cause leishmani­

asis,  Chagas’  and  sleeping  sickness  re­

mains  to  be  seen—as  does  how  their  re­

spective  modern  vectors  fit  into  the  pic­

ture.  Dr  Palmer­Young’s  finding  suggests,

though, that monitoring hivesfordiseases

which could spill over into humanswould

be an endeavour worth pursuing.n

Bees may be a missing link in the

chain leading to three nasty diseases

Disease incubators?

a recent series of test flights in partnership

with the French Civil Aviation Authority, it

was  found  that  during  simulated  engine

failures this set­up could provide the pilot

with 30 seconds of electric­only power.

That  might  not  seem  much  to  shout

about, but it is enough to give the pilot of a

single­engine  helicopter  a  much  better

chance of coping with an engine failure, by

executing a tricky emergency­landing pro­

cedure known as autorotate. This involves

maintaining  a  steady  rate  of  descent,  so

that the air passing up through the rotors is

sufficient to keep them spinning in a way

that provides enough lift for the helicopter

to “glide” to a safe landing—much as a sy­

camore seed gently windmills down to the

ground.  After  further  development  work,

Airbus plans to add the hybrid backup sys­

tem to new helicopters. 

The  problem  with  adding  bits  of  kit  to

an  aircraft,  however,  is  that  it  can  reduce

flying efficiency by increasing weight, thus

requiring  more  fuel  to  be  burned.  But  in

this case the hybrid system can also be em­

ployed  to  boost  a  helicopter’s  perfor­

mance, and to do so by an amount that will

more  than  overcome  the  weight  penalty,

says  Tomasz  Krysinkski,  Airbus  Helicop­

ter’s head of research and innovation. 

The  performance  gain  comes  about  in

two  ways.  First,  the  electric  motor  can,  in

normal flight, provide the rotor with addi­

tional  torque  when  a  bit  of  extra  power  is

required. Torque is a twisting force which

electric motors are particularly good at de­

livering,  and  they  deliver  it  much  more

rapidly  than  combustion  engines  can

manage.  It  is  this  occasional  boost  from

the electric system that helps reduce emis­

sions. Once it delivered, the battery would

then  be  topped  up  again  in  flight,  using

power from the engine.

The  torque  boost  also  enables  the  sec­

ond  important  benefit.  This  comes  about

because  performance  and  safety  affect  an

aircraft’s  certified  maximum  take­off

weight.  By  improving  both,  the  hybrid

backup  system  will  allow  a  helicopter  to

carry  heavier  loads.  The  increase  should

amount  to  an  additional  passenger,  says

Mr  Krysinkski,  which  would  greatly  in­

crease a helicopter’s earnings potential. 

To improve the system yet further, Air­

bus  is  working  on  extending  the  electric­

only  flight  time  to  several  minutes.  This

would still be a long way from what evtols

can  manage,  at  least  with  existing  battery

technology.  Helicopters  can,  though,  fly

much  longer  missions  than  evtols,  and

are  more  versatile.  They  can  carry  heavier

loads  and  are  often  required  to  operate  in

extreme weather conditions—as when un­

dertaking  air­sea  rescues—and  in  hazard­

ous  circumstances  such  as  firefighting.

Hybrid  technology  can  helpwithall  that,

too, and so should keep choppersairborne

for a few more years to come.n