Experimental evidence of size/age-biased infection of Biomphalaria glabrata (Pulmonata: Planorbidae) by an incompatible parasite species: consequences for biological control.

Because the digenetic trematode Plagiorchis elegans can elicit a rapid, severe and permanent suppression of the reproductive output in the snail Biomphalaria glabrata, it is considered as a potential biological control agent of human schistosomiasis. This assumption however is derived from laboratory experiments that are poor approximations of what occurs in a natural ecosystem. In order to recreate conditions that resemble those found in nature, we exposed B. glabrata as individual populations composed of a young, juvenile and adult snails to various concentrations of P. elegans eggs to assess the probability of encountering the parasite eggs by the different snail sizes/age groups. We demonstrated that within populations composed of different size/age classes, larger/older snails displayed the negative effects typical of exposure to P. elegans, whereas smaller individuals appeared relatively unaffected, particularly at lower levels of exposure. These findings coupled with the difficulty of producing large quantities of parasite eggs suggest that P. elegans has limited efficiency as a biological control agent of human schistosomiasis.

Plagiorchis elegans (Trematoda) and incompatible snail hosts: implications for snail life history traits and biocontrol of human schistosomiasis.

We examined the effect of Plagiorchis elegans on egg production and survival on Bulinus truncatus and Helisoma trivolvis trivolvis. Both species are incompatible hosts for P. elegans. Helisoma t. trivolvis occurs sympatrically with P. elegans; B. truncatus does not. Overall, P. elegans had no effect on survivorship or egg production in H. t. trivolvis or on the survivorship of B. truncatus. Its effect on egg production in B. truncatus was transitory; egg production was reduced by 50% for 5 wk following exposure but returned to normal thereafter. Neither egg production nor survivorship was affected in adult H. t. trivolvis. Egg production ceased at 14 wk post-exposure (PE), but resumed when the snails were paired. Young H. t. trivolvis also produced eggs after exposure, but later than the adults and only after they had been paired with another snail. This suggests that a need for periodic cross-fertilization in H. t. trivolvis rather than the effect of the parasite is responsible for the cessation of egg production in this species. Survivorship in young H. t. trivolvis was significantly higher in exposed snails between wk 7 to 10 PE than in controls.

Efficacy of pyrethroid-treated nets against malaria vectors and nuisance-biting mosquitoes in Tanzania in areas with long-term insecticide-treated net use.

OBJECTIVE: To measure pyrethroid susceptibility in populations of malaria vectors and nuisance-biting mosquitoes in Tanzania and to test the biological efficacy of current insecticide formulations used for net treatment. METHODS: Anopheles gambiae Giles s.l., An. funestus Giles s.l. and Culex quinquefasciatus Say were collected during three national surveys and two insecticide-treated net (ITN) studies in Tanzania. Knockdown effect and mortality were measured in standard WHO susceptibility tests and ball-frame bio-efficacy tests. Test results from 1999 to 2004 were compared to determine trends in resistance development. RESULTS: Anopheles gambiae s.l. and An. funestus s.l. were highly susceptible to permethrin (range 87-100%) and deltamethrin (consistently 100%) in WHO tests in 1999 and 2004, while Culex quinquefasciatus susceptibility to these pyrethroids was much lower (range 7-100% and 0-84% respectively). Efficacy of pyrethroid-treated nets was similarly high against An. gambiae s.l. and An. funestus s.l. (range 82-100%) while efficacy against Cx. quinquefasciatus was considerably lower (range 2-100%). There was no indication of development of resistance in populations of An. gambiae s.l. or An. funestus s.l. where ITNs have been extensively used; however, susceptibility of nuisance-biting Cx. quinquefasciatus mosquitoes declined in some areas between 1999 and 2004. CONCLUSION: The sustained pyrethroid susceptibility of malaria vectors in Tanzania is encouraging for successful malaria control with ITNs. Continued monitoring is essential to ensure early resistance detection, particularly in areas with heavy agricultural or public health use of insecticides where resistance is likely to develop. Widespread low susceptibility of nuisance-biting Culex mosquitoes to ITNs raises concern for user acceptance of nets.

Occurrence of the leucine-to-phenylalanine knockdown resistance (kdr) mutation in Anopheles arabiensis populations in Tanzania, detected by a simplified high-throughput SSOP-ELISA method.

BACKGROUND: Molecular markers of insecticide resistance can provide sensitive indicators of resistance development in malaria vector populations. Monitoring of insecticide resistance in vector populations is an important component of current malaria control programmes. Knockdown resistance (kdr) confers resistance to the pyrethroid class of insecticides with cross-resistance to DDT through single nucleotide polymorphisms (SNPs) in the voltage-gated sodium channel gene. METHODS: To enable detection of kdr mutations at low frequency a method was developed that uses polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA)-based technology, allowing rapid, reliable and cost-effective testing of large numbers of individual mosquitoes. This was used to assay mosquitoes from sites in lower Moshi, Tanzania. RESULTS: Sequence-specific oligonucleotide probes (SSOP) were used for simultaneous detection of both East and West African kdr mutations with high specificity and sensitivity. Application of the SSOP-ELISA method to 1,620 field-collected Anopheles arabiensis from Tanzania identified the West African leucine-phenylalanine kdr mutation in two heterozygous individuals, indicating the potential for resistance development that requires close monitoring. CONCLUSION: The presence of the West African kdr mutation at low frequency in this East African population of An. arabiensis has implications for the spread of the kdr gene across the African continent.

Kinases, intracellular calcium, and apolipophorin-III influence the adhesion of larval hemocytes of the lepidopterous insect, Galleria mellonella.

Based on the results from the use of selective inhibitors and activators, active protein kinase A, protein tyrosine kinase, and protein kinase C (PKC) isoforms decreased the adhesion of larval Galleria mellonella hemocytes to glass slides. The protein kinase A inhibitor at all concentrations increased granular cell adhesion only whereas protein tyrosine kinase elevated both granular and plasmatocyte attachment at the lowest concentration. Active, Ca(2+)- and lipid-dependent PKC isoforms limited plasmatocyte and granular cell adhesion whereas PKC that was inhibited by selected compounds (with differed modes of PKC inhibition) enhanced hemocyte attachment. The granular cells were more sensitive to the PKC inhibitors than were plasmatocytes. Phospholipase C and its diacylglyceride product were necessary to reduce hemocyte adhesion and maintain PKC activity. Extracellular Ca(2+), possibly transported through L-channels, was required for plasmatocyte attachment. In contrast, lowering the levels of cytosolic Ca(2+) was associated with decreased PKC activity and was required for hemocyte adhesion.

The impact of selective oviposition and infection with Plagiorchis elegans on Aedes aegypti pre-imago population dynamics at optimal food availability.

Progressive changes in the attraction of waters harboring pre-imago populations of Aedes aegypti exposed to different levels of the entomopathogenic digenean Plagiorchis elegans to ovipositing conspecific females were assessed under conditions of optimal food availability. The impact of ovipositional preference and parasitic infection on population structure and development was investigated. Probabilities that larvae progress from one stage to the next or die within 24 h were calculated for all life stages. Exposure to P. elegans cercariae did not significantly affect the attractiveness of larval-holding waters. Ovipositional preference increased significantly with growing bio-mass of the larval population, with the event of pupation and, in some cases, with late instar mortality. Exposure to various levels of the parasite significantly increased mortality of all instars, but most of the damage caused by the parasite occurred in the form of increased pupal mortality and decreased adult emergence. Exposure to the parasite significantly reduced the number of adults produced yet did not impair larval development. Thus, larval recruitment into environments containing P. elegans remains high, the structure of larval populations remains relatively normal, but few adults are produced.

Apolipophorin-III affects the activity of the haemocytes of Galleria mellonella larvae.

Apolipophorin-III (apoLp-III) impaired the adhesion of plasmatocytes and a granular cell-subpopulation of larval Galleria mellonella to glass slides. The protein bound to haemocytes, limited the responses of the plasmatocytes to Bacillus subtilis and increased the percentage of a subgroup of granular cells with adhering bacteria. The total number of bacteria adhering to all the haemocytes on the slides declined. Injections of apoLp-III slowed bacterial removal from the haemolymph without affecting total haemocyte counts and impaired haemocyte attachment to glass slides. Purified apoLp-III bound to B. subtilis. ApoLp-III in serum bound to bacteria within 5 min, peaked at 15 min and was either shed or dissociated by 60 min. ApoLp-III bound to B. subtilis lowered the adhesion of the bacteria to the haemocytes and slowed the removal of the bacteria from the haemolymph.