Mosquito age and avian malaria infection

Abstract

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.

Fig. 1

Schematic representation of the experimental design. Black bird uninfected blood meal; red birds Plasmodium-infected blood meal; h3, h7, h17: haemolymph sampling from 3, 7 and 17-day old mosquitoes, respectively. Haemolymph was sampled immediately before the blood meal on a separate sample of mosquitoes (haemocyte extraction is destructive). Young, old-unfed and old-blood fed mosquitoes are colour marked and fed from the same bird (n = 5 different birds)

Fig. 2

Haemocyte counts in the haemolymph of young (grey lines) and old (blue lines) mosquito cohorts across the three different time points since emergence in Block 1 (a, b, c) and Block 2 (d, e, f) of the experiment. Half of the mosquitoes in the old cohort were blood fed on Day 7 (see Fig. 1). Blood-fed females are represented by a dashed line, unfed females by a full line (when dashed and full lines overlap only the latter is shown). Total haemocytes (a, d) are the sum of the total number of granulocytes (b, e) and oenocytoids (c, f). Due to a problem with an anticoagulant solution, in Block 2 only the haemocytes of old-unfed and old-fed mosquitoes could be quantified

Fig. 3

Infection prevalence (% mosquitoes containing ≥1 oocyst) in young (7-day old mosquitoes) and in old (17-day old) mosquitoes that were either allowed a previous uninfected blood meal (old-blood fed) or not. Figure represents results for Block 1 (a) and Block 2 (b)

Fig. 4

Mean oocyst burden (NB oocysts across mosquitoes containing ≥1 oocyst) in young (7-day old mosquitoes) and in old (17-day old) mosquitoes that were either allowed a previous uninfected blood meal (old-blood fed) or not. Figure represents results for Block 1 (a) and Block 2 (b)