Relationship of hemolymph phenol oxidase and mosquito age in Aedes aegypti

Mosquito ageing modulates the development, virulence and transmission potential of pathogens

Host age variation is a striking source of heterogeneity that can shape the evolution and transmission dynamic of pathogens. Compared with vertebrate systems, our understanding of the impact of host age on invertebrate-pathogen interactions remains limited. We examined the influence of mosquito age on key life-history traits driving human malaria transmission. Females of Anopheles coluzzii, a major malaria vector, belonging to three age classes (4-, 8- and 12-day-old), were experimentally infected with Plasmodium falciparum field isolates. Our findings revealed reduced competence in 12-day-old mosquitoes, characterized by lower oocyst/sporozoite rates and intensities compared with younger mosquitoes. Despite shorter median longevities in older age classes, infected 12-day-old mosquitoes exhibited improved survival, suggesting that the infection might act as a fountain of youth for older mosquitoes specifically. The timing of sporozoite appearance in the salivary glands remained consistent across mosquito age classes, with an extrinsic incubation period of approximately 13 days. Integrating these results into an epidemiological model revealed a lower vectorial capacity for older mosquitoes compared with younger ones, albeit still substantial owing to extended longevity in the presence of infection. Considering age heterogeneity provides valuable insights for ecological and epidemiological studies, informing targeted control strategies to mitigate pathogen transmission.

Higher temperature accelerates the aging-dependent weakening of the melanization immune response in mosquitoes

The body temperature of mosquitoes, like most insects, is dictated by the environmental temperature. Climate change is increasing the body temperature of insects and thereby altering physiological processes such as immune proficiency. Aging also alters insect physiology, resulting in the weakening of the immune system in a process called senescence. Although both temperature and aging independently affect the immune system, it is unknown whether temperature alters the rate of immune senescence. Here, we evaluated the independent and combined effects of temperature (27°C, 30°C and 32°C) and aging (1, 5, 10 and 15 days old) on the melanization immune response of the adult female mosquito, Anopheles gambiae. Using a spectrophotometric assay that measures phenoloxidase activity (a rate limiting enzyme) in hemolymph, and therefore, the melanization potential of the mosquito, we discovered that the strength of melanization decreases with higher temperature, aging, and infection. Moreover, when the temperature is higher, the aging-dependent decline in melanization begins at a younger age. Using an optical assay that measures melanin deposition on the abdominal wall and in the periostial regions of the heart, we found that melanin is deposited after infection, that this deposition decreases with aging, and that this aging-dependent decline is accelerated by higher temperature. This study demonstrates that higher temperature accelerates immune senescence in mosquitoes, with higher temperature uncoupling physiological age from chronological age. These findings highlight the importance of investigating the consequences of climate change on how disease transmission by mosquitoes is affected by aging.

Sex, age, and parental harmonic convergence behavior affect the immune performance of Aedes aegypti offspring

Harmonic convergence is a potential cue, female mosquitoes use to choose male mates. However, very little is known about the benefits this choice confers to offspring performance. Using Aedes aegypti (an important vector of human disease), we investigated whether offspring of converging parental pairs showed differences in immune competence compared to offspring derived from non-converging parental pairs. Here we show that harmonic convergence, along with several other interacting factors (sex, age, reproductive, and physiological status), significantly shaped offspring immune responses (melanization and response to a bacterial challenge). Harmonic convergence had a stronger effect on the immune response of male offspring than on female offspring. Further, female offspring from converging parental pairs disseminated dengue virus more quickly than offspring derived from non-converging parental pairs. Our results provide insight into a wide range of selective pressures shaping mosquito immune function and could have important implications for disease transmission and control.

Age influences the thermal suitability of Plasmodium falciparum transmission in the Asian malaria vector Anopheles stephensi

Models predicting disease transmission are vital tools for long-term planning of malaria reduction efforts, particularly for mitigating impacts of climate change. We compared temperature-dependent malaria transmission models when mosquito life-history traits were estimated from a truncated portion of the lifespan (a common practice) versus traits measured across the full lifespan. We conducted an experiment on adult female Anopheles stephensi, the Asian urban malaria mosquito, to generate daily per capita values for mortality, egg production and biting rate at six constant temperatures. Both temperature and age significantly affected trait values. Further, we found quantitative and qualitative differences between temperature-trait relationships estimated from truncated data versus observed lifetime values. Incorporating these temperature-trait relationships into an expression governing the thermal suitability of transmission, relative R₀(T), resulted in minor differences in the breadth of suitable temperatures for Plasmodium falciparum transmission between the two models constructed from only An. stephensi trait data. However, we found a substantial increase in thermal niche breadth compared with a previously published model consisting of trait data from multiple Anopheles mosquito species. Overall, this work highlights the importance of considering how mosquito trait values vary with mosquito age and mosquito species when generating temperature-based suitability predictions of transmission.

Geriatric Invertebrates

Invertebrates are becoming more popular and, as collections age, clients may seek veterinary intervention where the welfare of the animal must be considered. This article covers aging in many invertebrate species but with a focus on species likely to be seen in general practice. Supportive care may be an option to prolong life, but euthanasia must be considered for invertebrates with age-related unmanageable conditions.

Anopheles gambiae larvae mount stronger immune responses against bacterial infection than adults: evidence of adaptive decoupling in mosquitoes

BACKGROUND

The immune system of adult mosquitoes has received significant attention because of the ability of females to vector disease-causing pathogens while ingesting blood meals. However, few studies have focused on the immune system of larvae, which, we hypothesize, is highly robust due to the high density and diversity of microorganisms that larvae encounter in their aquatic environments and the strong selection pressures at work in the larval stage to ensure survival to reproductive maturity. Here, we surveyed a broad range of cellular and humoral immune parameters in larvae of the malaria mosquito, Anopheles gambiae, and compared their potency to that of newly-emerged adults and older adults.

RESULTS

We found that larvae kill bacteria in their hemocoel with equal or greater efficiency compared to newly-emerged adults, and that antibacterial ability declines further with adult age, indicative of senescence. This phenotype correlates with more circulating hemocytes and a differing spatial arrangement of sessile hemocytes in larvae relative to adults, as well as with the individual hemocytes of adults carrying a greater phagocytic burden. The hemolymph of larvae also possesses markedly stronger antibacterial lytic and melanization activity than the hemolymph of adults. Finally, infection induces a stronger transcriptional upregulation of immunity genes in larvae than in adults, including differences in the immunity genes that are regulated.

CONCLUSIONS

These results demonstrate that immunity is strongest in larvae and declines after metamorphosis and with adult age, and suggest that adaptive decoupling, or the independent evolution of larval and adult traits made possible by metamorphosis, has occurred in the mosquito lineage.

Mosquito age and avian malaria infection

Background

The immune system of many insects wanes dramatically with age, leading to the general prediction that older insects should be more susceptible to infection than their younger counterparts. This prediction is however challenged by numerous studies showing that older insects are more resistant to a range of pathogens. The effect of age on susceptibility to infections is particularly relevant for mosquitoes given their role as vectors of malaria and other diseases. Despite this, the effect of mosquito age on Plasmodium susceptibility has been rarely explored, either experimentally or theoretically.

Methods

Experiments were carried out using the avian malaria parasite Plasmodium relictum and its natural vector in the field, the mosquito Culex pipiens. Both innate immune responses (number and type of circulating haemocytes) and Plasmodium susceptibility (prevalence and burden) were quantified in seven- and 17-day old females. Whether immunity or Plasmodium susceptibility are modulated by the previous blood feeding history of the mosquito was also investigated. To ensure repeatability, two different experimental blocks were carried out several weeks apart.

Results

Haemocyte numbers decrease drastically as the mosquitoes age. Despite this, older mosquitoes are significantly more resistant to a Plasmodium infection than their younger counterparts. Crucially, however, the age effect is entirely reversed when old mosquitoes have taken one previous non-infected blood meal.

Conclusions

The results agree with previous studies showing that older insects are often more resistant to infections than younger ones. These results suggest that structural and functional alterations in mosquito physiology with age may be more important than immunity in determining the probability of a Plasmodium infection in old mosquitoes. Possible explanations for why the effect is reversed in blood-fed mosquitoes are discussed. The reversal of the age effect in blood fed mosquitoes implies that age is unlikely to have a significant impact on mosquito susceptibility in the field.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0912-z) contains supplementary material, which is available to authorized users.

Aging in personal and social immunity: do immune traits senesce at the same rate?

How much should an individual invest in immunity as it grows older? Immunity is costly and its value is likely to change across an organism's lifespan. A limited number of studies have focused on how personal immune investment changes with age in insects, but we do not know how social immunity, immune responses that protect kin, changes across lifespan, or how resources are divided between these two arms of the immune response. In this study, both personal and social immune functions are considered in the burying beetle, Nicrophorus vespilloides. We show that personal immune function declines (phenoloxidase levels) or is maintained (defensin expression) across lifespan in nonbreeding beetles but is maintained (phenoloxidase levels) or even upregulated (defensin expression) in breeding individuals. In contrast, social immunity increases in breeding burying beetles up to middle age, before decreasing in old age. Social immunity is not affected by a wounding challenge across lifespan, whereas personal immunity, through PO, is upregulated following wounding to a similar extent across lifespan. Personal immune function may be prioritized in younger individuals in order to ensure survival until reproductive maturity. If not breeding, this may then drop off in later life as state declines. As burying beetles are ephemeral breeders, breeding opportunities in later life may be rare. When allowed to breed, beetles may therefore invest heavily in “staying alive” in order to complete what could potentially be their final reproductive opportunity. As parental care is important for the survival and growth of offspring in this genus, staying alive to provide care behaviors will clearly have fitness payoffs. This study shows that all immune traits do not senesce at the same rate. In fact, the patterns observed depend upon the immune traits measured and the breeding status of the individual.

Vector competence of Aedes aegypti mosquitoes for filarial nematodes is affected by age and nutrient limitation

Mosquitoes are one of the most important vectors of human disease. The ability of mosquitoes to transmit disease is dependent on the age structure of the population, as mosquitoes must survive long enough for the parasites to complete their development and infect another human. Age could have additional effects due to mortality rates and vector competence changing as mosquitoes senesce, but these are comparatively poorly understood. We have investigated these factors using the mosquito Aedes aegypti and the filarial nematode Brugia malayi. Rather than observing any effects of immune senescence, we found that older mosquitoes were more resistant, but this only occurred if they had previously been maintained on a nutrient-poor diet of fructose. Constant blood feeding reversed this decline in vector competence, meaning that the number of parasites remained relatively unchanged as mosquitoes aged. Old females that had been maintained on fructose also experienced a sharp spike in mortality after an infected blood meal ("refeeding syndrome") and few survived long enough for the parasite to develop. Again, this effect was prevented by frequent blood meals. Our results indicate that old mosquitoes may be inefficient vectors due to low vector competence and high mortality, but that frequent blood meals can prevent these effects of age.

Patterns of phenoloxidase activity in insecticide resistant and susceptible mosquitoes differ between laboratory-selected and wild-caught individuals

Background

Insecticide resistance has the potential to alter vector immune competence and consequently affect the transmission of diseases.

Methods

Using both laboratory isogenic strains and field-caught Culex pipiens mosquitoes, we investigated the effects of insecticide resistance on an important component of the mosquito immune system: the phenoloxidase (PO) activity. As infection risk varies dramatically with the age and sex of mosquitoes, allocation to PO immunity was quantified across different stages of the mosquito life cycle.

Results

Our results were consistent in showing that larvae have a higher PO activity than adults, females have a higher PO activity than males, and PO activity declines with adult age. We obtained, however, a marked discrepancy between laboratory and field-collected mosquitoes on the effect of insecticide resistance on PO activity. In the laboratory selected strains we found evidence of strong interactions between insecticide resistance and the age and sex of mosquitoes. In particular, 7 and 14 day old esterase-resistant adult females and acetylcholine-esterase resistant males had significantly higher PO activities than their susceptible counterparts. No such effects were, however, apparent in field-caught mosquitoes.

Conclusions

Combined, the field and laboratory-based approaches employed in this study provide a powerful test of the effect of insecticide resistance on PO-mediated immunity. The use of laboratory-selected insecticide-resistant strains is still the most widely used method to investigate the pleiotropic effects of insecticide resistance. Our results suggest that the outcome of these laboratory-selected mosquitoes must be interpreted with caution and, whenever possible, compared with mosquitoes captured from the field.

Infection of Anopheles gambiae mosquitoes with entomopathogenic fungi: effect of host age and blood-feeding status

Physiological characteristics of insects can influence their susceptibility to fungal infection of which age and nutritional status are among the most important. An understanding of host–pathogen interaction with respect to these physiological characteristics of the host is essential if we are to develop fungal formulations capable of reducing malaria transmission under field conditions. Here, two independent bioassays were conducted to study the effect of age and blood-feeding status on fungal infection and survival of Anopheles gambiae s.s. Giles. Mosquitoes were exposed to 2 × 10¹⁰ conidia m⁻² of oil-formulated Metarhizium anisopliae ICIPE-30 and of Beauveria bassiana I93-825, respectively, and their survival was monitored daily. Three age groups of mosquitoes were exposed, 2–4, 5–8, and 9–12 days since emergence. Five groups of different feeding status were exposed: non-blood-fed, 3, 12, 36, and 72 h post-blood feeding. Fungal infection reduced the survival of mosquitoes regardless of their age and blood-feeding status. Although older mosquitoes died relatively earlier than younger ones, age did not tend to affect mosquito susceptibility to fungal infection. Non-blood-fed mosquitoes were more susceptible to fungus infection compared to all categories of blood-fed mosquitoes, except for those exposed to B. bassiana 72 h post-blood feeding. In conclusion, formulations of M. anisopliae and B. bassiana can equally affect mosquitoes of different age classes, with them being relatively more susceptible to fungus infection when non-blood-fed.

Age-associated mortality in immune challenged mosquitoes (Aedes aegypti) correlates with a decrease in haemocyte numbers

Mosquitoes vector pathogens. One aspect that has been overlooked in mosquito-pathogen relationships is the effect of host age on immune competence. Here, we show that there is age-associated mortality following immune challenge with Escherichia coli. This mortality correlates with a decrease in haemocyte numbers (blood cells) and a decreased ability to kill E. coli. Although the number of haemocytes decreases, the available haemocytes retain their phagocytic ability regardless of age, and we estimate that individual granulocytes can phagocytose approximately 1500 E. coli. Moreover, transcription profiles for cecropin, defensin and gambicin in E. coli challenged mosquitoes do not change with age, indicating that the increased susceptibility is not attributed to fewer humoral antimicrobial peptides. These results suggest that a contributing factor for the age-associated mortality is the decrease in circulating haemocytes, which reduces the overall phagocytic capacity of mosquitoes. To our knowledge, this is the first report detailing an age-associated decline in the immunological capabilities of mosquitoes following challenge with an infectious agent. These data also call for caution in the analysis and interpretation of experimental results when mosquito age has not been closely monitored. Lastly, a model for haemocyte function is presented.

Physiology of immunity in the water flea Daphnia magna: environmental and genetic aspects of phenoloxidase activity

In an attempt to understand the ecological correlates of immunocompetence in Daphnia magna (Crustacea, Cladocera), we tested for variation in immune function in relation to feeding conditions, host conditions, and host genotype. We investigated both phenotypic (environmental dependent and condition dependent) as well as genotypic aspects of the prophenoloxidase activating system (Pro-POAS), which has been described as a key factor in invertebrate immunity. Daphnia magna is an ideal study system to disentangle phenotypic and genetic variation because females can reproduce clonally. Well-fed Daphnia showed higher phenoloxidase (PO) activity than Daphnia kept at a low food level. Wounding provoked a higher level of PO activity, indicating that the Pro-POAS was condition dependent. Further, we found clonal variation in PO activity among four clones of D. magna isolated from four different populations. The same four clones were tested for their resistance to the bacterial pathogen Pasteuria ramosa. High resistance corresponded to high PO activity. Our results suggest adaptive variation in PO activity and suggest that its expression is costly. These costs may influence the evolution of the PO activity level and the maintenance of its genotypic variation.

Involvement of peroxidase in chorion hardening in Aedes aegypti

Peroxidase activity is detectable in Aedes aegypti ovaries, containing developing eggs, at 24 h following blood feeding, and peak peroxidase activity is reached at 36-48 h after the blood-meal. Peroxidase is associated with the chorion layer in mature eggs and the majority of the enzyme is released from the chorion layer by treating the isolated chorion fraction with SDS/urea. Analysis of the SDS/urea solubilized chorion proteins using SDS-PAGE with tropolone/H2O2 or dopa staining verified the presence of both peroxidase and phenol oxidase in the released chorion proteins. The molecular weight of chorion peroxidase is about 61,000 Da as determined by SDS-PAGE analysis. Incubation of the solubilized chorion proteins with tyrosine and H2O2 produces dityrosine, and hyrolysis of hardened egg chorion results in the detection of dityrosine and trityrosine in the chorion hydrolysate. Data suggest that chorion peroxidase is involved in the hardening of the mosquito egg chorion by catalyzing the formation of ditryrosine through tyrosine residues on structural proteins. The overall hardening of the A. aegypti egg chorion includes both peroxidase-mediated chorion protein crosslinking through dityrosine formation and phenol oxidase-catalyzed chorion melanization.