Transmission
Despite all their accumulated knowledge and skills no
malariologists could explain how the parasite spread
from one human to another. The clues were, however,
in place. Over the centuries, circumstantial evidence had
accumulated that suggested that mosquitoes might
somehow be connected with malaria and by 1883 the
American physician, Albert King, had assembled the
mass of evidence that was to become known as the
mosquito-malaria doctrine [33]. Between 1884 and 1897,
Laveran, Manson (who in 1877 had demonstrated that
the filarial worms responsible for lymphatic filariasis
were transmitted by mosquitoes [34]), and the Italian
malariologists, had become increasingly convinced that
mosquitoes were involved in the transmission of
malaria. Thereafter opinions differed with some observers,
including Manson, believing that humans became
infected by drinking water contaminated by infected
mosquitoes while others thought that the infection was
acquired by inhaling dust from dried-up ponds in which
infected mosquitoes had died, in other words, variations
on the water and ingestion and air and inhalation theories
proposed by Tommasi-Crudeli and Klebs in 1879.
Manson also toyed with the idea that transmission
might be mechanical, i.e. the parasites were passively
carried from host to host on the proboscis of a
mosquito.
By 1894 Manson, who had spent much of his working
life in Taiwan and was then in his 50s and in an
established medical practice in London, turned his
attention to the possibility of mosquito transmission of
malaria but, as he was unable to go to malarious countries
himself, he needed someone to carry out the
necessary investigations and experiments for him. His
colleague-to-be was an unlikely choice, Ronald Ross
(Figure 2).
Ronald Ross 1857-1932. In 1897 Ronald Ross working
in India discovered that culicine mosquitoes transmitted
the avian malaria parasite Plasmodium relictum
and suggested that human malaria parasites might also
be transmitted by mosquitoes. Later, when working in
Sierra Leone in 1899, he demonstrated that the human
malaria parasites were indeed transmitted by anopheline
mosquitoes. In the meantime, however, several Italian
scientists had already shown that this was the case.
Ross then aged 37 was an established army surgeon
working in India who did not believe that malaria was
caused by a blood parasite but thought that it was an
intestinal infection. Throughout the second half of 1894,
Manson worked on Ross, showed him blood slides containing
malaria parasites and convinced him that incriminating
a mosquito vector of malaria was a goal worth
aiming for. Ross returned to India and over the next
four years Manson directed operations at a distance and
we are fortunate to have an almost complete collection
of the letters that passed between the two men [32].
This was not an easy relationship partly because Ross’s
first priorities were his military commitments and these
inevitably delayed the work he was doing with malaria
and partly because, from time to time, Ross seemed
more interested in writing poetry and novels. Nevertheless,
the cooperation reached a satisfactory conclusion
but later ended in acrimony