The influence of larval density, food stress, and parasitism on the bionomics of the dengue vector Aedes aegypti (Diptera: Culicidae): implications for integrated vector management.

New larval control strategies for integrated vector management of Aedes aegypti are in high demand, including the use of biological control agents. Exposure of Aedes aegypti to parasites, starvation, and overcrowded conditions during larval development reduces the probability of survival to eclosion, can directly affect fitness parameters such as adult size and fecundity, and can affect the size, provisioning, and viability of eggs produced by females. We compared these parameters after exposing larvae to 1) abundant food at low larval densities, 2) food deprivation and high larval density, and 2) infection with the endoparasite Plagiorchis elegans, an entomopathogenic digenean trematode. Female mosquitoes that eclosed from larval conditions of starvation and overcrowding were smaller and laid fewer and smaller eggs than controls. The proportion of females to complete an oviposition cycle was reduced in the P. elegans-infected treatment group. Parasite load was negatively correlated with wing length and egg size. Infection of Ae. aegypti with P. elegans has sublethal effects and may reduce population-level reproductive output, but one-time low-density P. elegans exposure does not have sufficient effect on Ae. aegypti fitness parameters to be considered a viable biocontrol option.

Plagiorchis elegans (Trematoda) induces immune response in an incompatible snail host Biomphalaria glabrata (Pulmonata: Planorbidae).

Plagiorchis elegans has been shown to decrease the fecundity and survivorship of the incompatible snail host Biomphalaria glabrata. Furthermore, a prior infection with P. elegans was shown to render the snails resistant to the compatible parasite Schistosoma mansoni. Here, we test the hypothesis that infection with P. elegans stimulates the immune system of B. glabrata. Our findings indicate that infection by P. elegans significantly increased the number of free hemocytes in the hemolymph of B. glabrata by an average of ~60%. Furthermore, this immuno-stimulated state lasted from the first day post-infection (PI) to some time between 7 and 21 days PI. This is one of the few reported examples of a parasite stimulating the immune response of an incompatible host.

Entomological evaluation of malaria vectors at different altitudes in Hai district, northeastern Tanzania.

Entomological monitoring in four villages situated along an altitude transect in the Hai District of Northeastern Tanzania identified Anopheles arabiensis Patton as the principal vector of malaria and detected seasonal changes in vector behavior. Over a 13-mo sampling period, 10,557 mosquitoes were collected with CDC light traps, pyrethrum spray catches, and pit traps of which 5,969 (56.5%) wereAn. arabiensis, 762 (7.2%) wereAnopheles funestus Giles s.l., 3,578 (33.9%) were culicines, and 248 (2.3%) were nonvector anophelines. Vector densities declined rapidly with increasing altitude, demonstrating a 50% decrease in annual human biting rate for every 86-m rise in altitude. Light traps were found to be more efficient than spray catches for the collection of An. arabiensis. This observation was attributed to increased exophily of this species, most notably in the wet season, and is supported by seasonal changes in the human blood index and fed/gravid ratio. These results indicate that spray catches may underestimate the abundance of exophilic vectors such as An. arabiensis and that entomological monitoring may require more than one collection method, especially at low vector densities. The annual entomological inoculation rate (EIR) decreased sharply with increasing altitude, with large variation around the estimate at low vector densities. Increased transmission because of unpredictable short rains at low altitudes and spatial clustering of infective mosquitoes may contribute to elevated EIR estimates.

Effects of Plagiorchis elegans (Digenea: Plagiorchiidae) infection of Biomphalaria glabrata (Pulmonata: Planorbidae) on a challenge infection with Schistosoma mansoni (Digenea: Schistosomatidae).

Prior exposure of Biomphalaria glabrata to the eggs of an incompatible digenean, Plagiorchis elegans, rendered this snail host less suitable to a compatible species, Schistosoma mansoni. Although P. elegans failed to develop patent infections in B. glabrata, it reduced the production of S. mansoni cercariae by 88%. Concomitantly, host attributes such as reproduction, growth, and survival were compromised. The effect of P. elegans infection was most severe among snails that, in addition, had developed patent schistosome infections. Although few S. mansoni cercariae were produced, egg production by B. glabrata was only 4% of control values. Furthermore, no doubly infected snails survived for more than 3 wk after patency, whereas controls experienced no mortality during the same time period. The above effects were attributable to the establishment and persistence of P. elegans sporocysts in the tissues of the incompatible snail host. Their indirect antagonistic interaction with thelarval stages of S. mansoni may be mediated, in part, through their long-term stimulation of the host's internal defense mechanisms. These findings are discussed with a view to use P. elegans and other plagiorchiid digeneans as agents in the biological control of snails and snail-borne diseases.

Effects of Plagiorchis elegans (Trematoda: Plagiorchiidae) infection on the carbohydrate metabolism of fourth instar Aedes aegypti (Diptera: Culicidae).

Infection of fourth-instar Aedes aegypti (L.) with the entomopathogenic digenean Plagiorchis elegans (Rudolphi) alters the carbohydrate metabolism of the insect. Within 24 h of cercarial penetration, total body extracts of infected fourth instars exhibited decreased trehalase activity, increased trehalose-6-phosphatase activity, and a concomitant accumulation of trehalose when compared with uninfected larvae. The amounts of glucose, glycogen and lipids, and the activity of glycogen phosphorylase a were similar in extracts of infected and control larvae. The predominant fatty acids, in both control and infected larvae, were C 18:0, C 18:1, and C 18:3. There were no significant differences in the types or proportions of fatty acids found in control and infected larvae. Parasitic infection is discussed in terms of impaired trehalose metabolism.

Interaction of Xenorhabdus nematophilus (Enterobacteriaceae) with the antimicrobial defenses of the house cricket, Acheta domesticus.

Fifth instar Acheta domesticus nymphs exhibited a decline in total hemocyte counts during the first hour of exposure to dead Xenorhabdus nematophilus; the bacterial level in the hemolymph also declined during this time. Thereafter bacterial numbers in the hemolymph increased as the level of damaged hemocytes increased. The bacteria lowered phenoloxidase activity in vivo by initially reducing the number of hemocytes containing prophenoloxidase and later by inhibiting enzyme activation. Preincubating X. nematophilus in hemolymph with active phenoloxidase in vitro accelerated the removal of the bacteria from the hemolymph in vivo which may be due to modification of the bacterial surface by serine proteases. Lysozyme activity increased in bacteria-injected insects in parallel with an increase in counts of damaged hemocytes; most of the enzyme was located in hemocytes. Lipopolysaccharides of X. nematophilus caused changes in hemocyte counts and phenoloxidase and lysozyme levels comparable to whole bacteria. Lipopolysaccharides also slowed the removal rate of the bacteria from, and accelerated bacterial emergence into, the hemolymph.

Interaction of hemocytes and prophenoloxidase system of fifth instar nymphs of Acheta domesticus with bacteria.

The hemocytes to which bacteria adhere were defined and the contribution of the prophenoloxidase system of fifth instar nymphs of Acheta domesticus to adhesion were examined. The physicochemical parameters affecting hemocyte and phenoloxidase activity were determined. Both plasmatocytes and granular cells responded to bacteria, the latter cells entrapping the microorganisms on filopodial extensions. The optimum pH for hemocyte adhesion to glass slides was 6.5, the granular cells being the most sensitive hemocyte type. Although hydrophobic resin beads and positively-charged beads favoured hemocyte attachment, these parameters did not contribute to differential bacterial adhesion to hemocytes. Activation of phenoloxidase was neither enhanced nor inhibited by 0.1 and 1 mg/ml of laminarin or zymosan nor by dead Bacillus subtilis. However, live B. subtilis activated the enzyme and dead Xenorhabdus nematophilus inhibited enzyme activation. Serine protease components of the prophenoloxidase system had opsonic properties for B. subtilis but not for X. nematophilus. Phenoloxidase activity was enhanced by Ca(2+) and Mg(2+) and inhibited by SO(2-)(4).

Plagiorchis elegans (Digenea: Plagiorchiidae) infections in Stagnicola elodes (Pulmonata: Lymnaeidae): host susceptibility, growth, reproduction, mortality, and cercarial production.

Eggs of Plagiorchis elegans were readily ingested by Stagnicola elodes of all ages, but were more infective to immature than mature snails. Infection enhanced the growth of the host in a dose-dependent manner. The number of cercariae released by immature snails increased with the age of the snail host; mature snails yielded fewer cercariae. Heavily infected snails tended to die prematurely, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails that acquired the infection as juveniles never produced eggs. Actively reproducing snails ceased egg laying within days of infection and never recovered. All parasite effects on the growth and reproduction of the snail host first manifested themselves during the early stages of infection, long before the development of daughter sporocysts and cercariae, and are therefore attributable to the effects of mother sporocysts. The study provides insight into how this natural enemy of mosquito larvae may be established in natural snail populations by means of strategically timed introductions of parasite eggs, with a goal of maximizing cercarial production for the biological control of sympatric mosquito larvae.

Oviposition attraction and repellency of Aedes aegypti (Diptera: Culicidae) to waters from conspecific larvae subjected to crowding, confinement, starvation, or infection.

As the biomass of Aedes aegypti (L.) larvae increased in relation to the volume of rearing waters, oviposition attraction of these waters to conspecific, gravid females first rose to a peak and then declined. Further increases in biomass rendered waters strongly repellent. Comparable responses were elicited by a decrease in the volume of rearing waters or an increase in the relative size or number of mosquito larvae. Low volumes of water reduced oviposition attraction and increased repellency, whereas larger volumes increased attraction. Excessively large volumes diluted attraction to neutrality. Constraints imposed by the physical dimensions of the larval environment which interfered with the normal postural movements and behaviors of the larvae also induced repellency, independent of rearing volume. Titration of repellent waters revealed that infection with the digenean Plagiorchis elegans (Rudolphi) generated the most powerful repellent effect, whereas crowding or starvation induced significantly weaker responses. At no time did dilution of repellent waters restore attractive properties. Repellents, even at minute concentrations, overrode attractants. The density-dependent action of oviposition attractants and repellents may help to maintain larval populations near optimal levels through their influence on recruitment. Oviposition repellency induced by sublethal infections with P. elegans may maintain population levels below the carrying capacity of the environment. Persistence of oviposition attraction and repellency varied inversely with temperature. The magnitude of the repellent effect induced by larvae appears to be a good indicator of their probability of survival and may be of selective significance. Repellency may deflect ovipositing females away from sites close to human habitation and may lead to new, and perhaps more effective, method, to control mosquitoes and the diseases they transmit.

Effects of Plagiorchis elegans (Digenea: Plagiorchiidae) infection on the reproduction of Biomphalaria glabrata (Pulmonata: Planorbidae).

Infection with the digenean parasite Plagiorchis elegans dramatically reduced the reproductive output of Biomphalaria glabrata exposed to the parasite as juveniles or adults. The total number of eggs produced by infected snails was reduced to approximately 7 and 13% of control values, respectively. Parasitic castration was attributed to the presence of mother sporocysts that readily established in the tissues of this incompatible host. Infection did not result in the production of cercariae but significantly shortened the life span of juvenile and adult B. glabrata by approximately 23 and 10%, respectively. Plagiorchis elegans also castrated its compatible host, Stagnicola elodes.

Serum composition of Aedes aegypti (Diptera: Culicidae) larvae and the production of an oviposition repellent are influenced by infection with the entomopathogenic digenean Plagiorchis elegans (Trematoda: Plagiorchiidae), starvation, and crowding.

Subjecting Aedes aegypti (L.) larvae to conditions that induced the production of oviposition repellency also reduced their wet and dry weights and the concentration of total serum carbohydrates, amino acids, and proteins. Thus, infection with metacercariae of the entomopathogenic digenean Plagiorchis elegans (Rudolphi), starvation for 7 d, or crowding for 2 d reduced larval dry weights by as much as 32, 20, and 23%, respectively, and wet weights by 20, 14, and 11%, respectively. Total serum carbohydrates declined by as much as 36, 21, and 29% for infected, starved, and crowded larvae, respectively, amino acids by 39, 48, and 44%, and protein concentrations by 72, 63, and 62%, respectively. Repellency dilution titers were correlated inversely with movement of the mouth parts and gut. Incubation of infected, starved, and crowded larvae in 0.01 g/liter glucose greatly reduced the level of repellency of their waters, whereas adding glucose to repellent waters had only minor effects. Results indicate that the induction of repellency is associated strongly with nutritional depletion effects.

The effects of temperature and light on the survival, development, and infectivity of Plagiorchis elegans eggs.

Plagiorchis elegans eggs were incubated at 0, 4, 10, 20, or 30 C under a 12-hr:12-hr (L:D) photoperiod for 120 days. Approximately one-quarter of the eggs had already initiated the process of embryonation when passed with the feces of the hamster (Mesocricetus auratus), the experimental definitive host. Eggs failed to embryonate at 0 C and died within 2 days. Incubation at 4 C allowed full embryonation, but the mean number of embryonated eggs per day (1.32+/-0.15) and the mean number of eggs available over the course of incubation (egg days) (219.00+/-2.24) remained low. These values increased progressively as incubation temperatures rose and reached levels of 3.59+/-0.30 and 1,467.50+/-4.56, respectively, at 20 C. Although incubation at 30 C further increased the mean number of eggs per day (5.45+/-0.56), the mean number of egg days declined sharply to 735.25+/-4.71, suggesting that elevated temperatures enhanced embryonation but lowered the survival of embryonated eggs. This was also reflected in the infectivity of eggs over time. Embryonated eggs incubated at 10 and 20 C remained infective significantly longer than eggs incubated at higher or lower temperatures. Incubation in constant light yielded significantly lower mean numbers of embryonated eggs per day (1.86+/-0.09) and mean number of egg days (96.25+/-0.99) than incubation in constant darkness (2.23+/-0.17 and 701.50+/-2.86 eggs, respectively) but did not affect the infectivity of embryonated eggs. The data suggest that at moderate temperatures and shielded from light, fully embryonated eggs of P. elegans may survive in the aquatic environment for a period of months. Such eggs may serve as a source of infection for populations of Stagnicola elodes and ensure the sustained production of the highly entomopathogenic cercariae required for mosquito control.

Oviposition responses of Aedes aegypti and Ae. atropalpus (Diptera: Culicidae) females to waters from conspecific and heterospecific normal larvae and from larvae infected with Plagiorchis elegans (Trematoda: Plagiorchiidae).

Ovipositing Aedes aegypti (L.) females were attracted to waters in which uninfected Aedes atropalpus (Coquillett) larvae had been reared, but were repelled by waters from larvae of either species infected with the entomopathogenic digenean Plagiorchis elegans (Rudolphi). In contrast, Ae. atropalpus females were attracted to or repelled by waters from conspecific uninfected and infected larvae, respectively, but did not respond to corresponding waters from Ae. aegypti larvae. The sensitivity of ovipositing females of both species to repellents and attractants is discussed in terms of possible selective pressures exerted by unstable breeding environments.

Plagiorchis elegans: emergence, longevity and infectivity of cercariae, and host behavioural modifications during cercarial emergence.

We investigated changes in the behaviour of Stagnicola elodes associated with the emergence of Plagiorchis elegans cercariae. Within 15 min of the reduction in light intensity, which triggered the onset of cercarial emergence, infected snails moved to the top of the water column and remained there for 2-3 h. Seventy-nine percent of all cercariae that emerged from the snail did so during this period. Uninfected snails showed no such behavioural changes following the change in light intensity. Cercariae were released in a dense cloud around the snail at the water surface and dispersed passively. Within 3-4 h more than 80% of all cercariae had settled in the bottom 5 cm of the water column. The infectivity of cercariae increased from less than 20% upon emergence from the snail to greater than 75% 4-6 h post-emergence, and then declined steadily to below 5% by 24 h post-emergence. Cercarial longevity was greater than 30 h and exceeded the period of infectivity. This may be related to steadily falling endogenous glycogen levels. A delay in attaining maximum infectivity may represent an adaptive mechanism allowing time for cercarial dissemination, thus reducing superinfection, and subsequent parasite-associated mortality, of second intermediate hosts.

Enteral and parenteral phases of Trichinella nativa and Trichinella pseudospiralis in the deer mouse, Peromyscus maniculatus.

Trichinella nativa and Trichinella pseudospiralis infections in a wild rodent host, the deer mouse (Peromyscus maniculatus), were characterized. Forty-six percent of 400 inoculated T. nativa were recovered on day 4 postinoculation (PI); 77% and 23% were found in the small and large intestines, respectively. Thirty-one percent of the worms recovered on day 4 remained in the large intestine beyond day 20 PI. Worms were embedded in the mucosa of the small intestine, cecum, and colon. Females recovered from the small and large intestines had statistically indistinguishable in vitro larval releases. Distension of the cecum and passage of loose stools were associated with the presence of worms in the large intestine. The ability of T. nativa to establish and thrive in the large intestine of deer mice was confirmed following intracecal implantation of first-stage larvae. On day 4 PI, 35% of 400 inoculated T. pseudospiralis were recovered, with 91% and 9% found in the small and large intestines, respectively. Although T. pseudospiralis established in the large intestine of deer mice, few worms remained beyond day 20. Females recovered from the small and large intestines had statistically indistinguishable in vitro larval releases. Although higher establishments of T. nativa (56% vs. 46%) and T. pseudospiralis (52% vs. 35%) were observed in CD-1 mice than in deer mice on day 4 PI, neither was able to colonize the large intestine of the former. The large intestine may be a more important habitat for adult trichinae than previously recognized.

Factors affecting the acquisition of Plagiorchis noblei (Trematoda: Plagiorchiidae) cercariae by black fly (Diptera: Simuliidae) larvae and the effect of metacercariae on host survival.

Larvae of 4 species of black flies, Prosimulium mixtum, Simulium vittatum, S. decorum and Stegopterna mutata, were exposed in the laboratory to cercariae of the digenean Plagiorchis noblei in flowing water. Prevalence and intensity of infection of all species varied directly with exposure intensity and decreased with increasing water velocity; prevalence increased with larval size. Infection levels for P. mixtum were higher than for the 3 other species. Mortality among infected larvae of all species was 3 times that of controls. Exposure of black fly larvae to P. noblei cercariae may adversely affect their survival.

Impact of the entomophilic digenean Plagiorchis noblei (Trematoda: Plagiorchiidae) on the survival of Aedes provocans under field conditions.

Under field conditions, exposure of Aedes provocans to Plagiorchis noblei cercariae reduced the number of pupae produced to about one-third of control values. Larvae bearing more than 2 metacercariae rarely survived, and pupal mortality was almost 3 times higher among exposed individuals than controls. Adult females exposed to cercariae as pre-imagos experienced a 32% reduction in longevity. The life span of males was not affected. The potential of entomophilic digeneans as biological agents in the control of mosquitoes is discussed.

Plagiorchis noblei (Plagiorchiidae) in Aedes aegypti: parasite acquisition and host mortality in trickle infections.

The prevalence and intensity of experimental infections of Aedes aegypti with the digenean Plagiorchis noblei increased significantly with the level of trickle exposure to cercariae. Daily exposure to doses of 16 cercariae/day yielded a mean infection intensity of 13.0 metacercariae; doses of 1 cercaria/day resulted in only 2.4 metacercariae per infected mosquito larva. The prevalence of infection rose from 46% at an exposure of 1 cercaria/day to 99% at 16 cercariae/day. Host mortality rose concomitantly from 25% to 88%. Host mortality and parasite acquisition were independent of environmental temperatures (21-29 C), despite the fact that developmental times, and consequently the number of daily exposures, were more than 50% greater at the low end of the temperature scale. This may be attributable to low activity of mosquito larvae and the resulting decrease in the number of encounters with cercariae.

The effects of single exposures of Aedes aegypti larvae and pupae to Plagiorchis noblei cercariae in the laboratory.

The mortality of Aedes aegypti pre-imagos harboring metacercariae of Plagiorchis noblei Park, 1936, is governed by the stage of development of the host at the time of infection and the location of the parasite in the insect body. First and second instar larvae generally succumbed to infection, regardless of site. Infections of the head and thorax of third and fourth instar larvae were generally lethal or gave rise to imparied adults. However, older instars frequently survived abdominal infections. Pupae showed greater tolerance to cephalic, thoracic and abdominal infections and generally emerged as adults. Again, many such infected adults were impaired.

Responses of the fish ectoparasite Salmincola edwardsii (Copepoda) to stimulation, and their implication for host-finding.

The short-lived infective copepodid stages of the copepod Salmincola edwardsii, ectoparasitic on brook trout, Salvelinus fontinalis, are under strong selective pressure to evolve efficient host-finding mechanisms. In laboratory experiments, we quantified the effects of visual and mechanical stimulation on the activity of the copepodids. We found that shadows passing above the copepodids and shock waves passing through the water generated marked increases in the rate and length of upward swimming bursts, which resulted in more time spent suspended in the water column and greater distances travelled. However, we found no strong evidence for a relationship between the magnitude of the copepodids' response and the intensity of stimulation. Although these responses are not host-specific, they allow the parasites to conserve their limited energy stores by increasing their host-finding activity only when potential hosts are nearby.