Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 76

fourth clutch (Lorenz and Koella, 2011). More
rapid death and lower fecundity may therefore
occur in those mosquitoes old enough to be
infectious.
Blanford et al . (2005) studied the impact of
6-h exposures to B . bassiana repeated every 3
days of An . stephensi infected with the murine
malaria parasite Plasmodium chabaudi . Daily
mortality rates 11 days after exposure were
markedly higher in the co-infected mosquitoes
compared to those mosquitoes exposed solely to
fungi. The proportion of mosquitoes that carry
infectious malaria parasites is low (thought not
to exceed 20% of the female vector population;
Beier et al. , 1999), because, as described earlier,
only a small proportion of mosquitoes survive
the EIP. Thus, in both cases where older or
malaria-infected mosquitoes are more suscept-
ible to infections, any mutations conferring
resistance to fungi would only be benefi cial to a
few mosquitoes whereas impacts on malaria
transmission could be substantial. Such a
disproportionate ef ect of fungi is predicted to
result in little or no emergence of resistance,
even when low costs of resistance are considered
(Read et al. , 2009). Reducing the lifespan
specifi cally of malaria-infected mosquitoes
might result in selection of faster developing
Plasmodium parasites (Michalakis and Renaud,
2005). However, if this were the case, it might be
expected that chemical insecticides would exert
the same selective pressure, if not more so. To
date, there has been no evidence that more rapid
development has occurred following intro-
ductions of chemical control.

also demonstrated reduced lifespan and hindered
replication of the dengue virus (Dong et al. ,
2012).
Taking this a step further, fungi have also
been genetically transformed to express
antipathogenic molecules. Recombinant M .
anisopliae isolates were constructed to express
either a salivary gland midgut peptide (SM1),
previously shown to block the sporozoite and
salivary gland attachment in anophelines, or a
single chain antibody, which agglutinates
sporozoites, or scorpine, an antimicrobial toxin.
Anopheles gambiae mosquitoes were exposed to
transformed fungi 11 days post-infection with P .
falciparum . Sporozoite counts for mosquitoes
exposed to spores of fungi expressing SM1,
agglutination antibodies, or scorpine were
reduced by 71%, 85% and 90%, respectively.
Fungi expressing both scorpine and SM1
resulted in a 98% reduction in sporozoite load.
This was achieved with just seven spores and
was shown to be rapid and long-lasting (Fang et
al. , 2011; Rasgon, 2011).
If fungi are deployed to target the pathogen
with no or limited ef ects on mosquito survival,
the pressure for mosquitoes to develop fungal
resistance would be further reduced. In fact, on
the contrary, malaria infection of mosquitoes
has been shown to impose a fi tness burden on
mosquitoes (Ferguson and Read, 2002).
Whether the pathogen itself may develop
mechanisms to escape the toxins released by
fungi has yet to be investigated.
The choice of isolate used for mosquito
control is therefore crucial in ensuring that the
desired lethal or sub-lethal ef ects are achieved.
Isolates not only vary in their own inherent
virulence properties, but these traits may be
modifi ed depending on the insect they are
targeting. For example, strains shown to be
highly virulent to orthopteran hosts, which
include locusts and grasshoppers, may only
have sub-lethal ef ects on Diptera ('true fl ies')
(Blanford et al. , 2005). Other factors infl uencing
virulence are how the isolate was cultured
(Alves et al. , 2002b; Fargues et al. , 2002; Shah et
al. , 2005; da S. Pereira et al. , 2009; Darbro et al. ,
2011) and environmental factors such as
temperature and humidity (Luz and Fargues,
1998, 1999). Culture methods can also af ect
toxin production within a fungal strain.
Rigorous

Isolates that target the pathogen
Fungal invasion of insects may also have a direct
impact on the pathogen being transmitted.
Dissections carried out on a subsample of
mosquitoes surviving in a malaria-infected
cohort showed no dif erence in prevalence or
load of oocysts, but only 8% of those exposed to
B . bassiana had sporozoites, compared to 35% of
those not exposed to fungus (Blanford et al. ,
2005). Combined with reduced survival of the
mosquito host, only 0.4% of those mosquitoes
exposed at the start of the experiment were
calculated to be able to transmit malaria
compared to 31% of controls. Similarly, studies
on St . aegypti adults exposed to B . bassiana have and well-designed screening