‘We like it wet’: a comparison between dissection techniques for the assessment of parity inAnopheles arabiensisand determination of sac stage in mosquitoes alive or dead on collection

Abstract

BackgroundThe determination of parous rates in mosquitoes, despite numerous shortcomings, remains a tool to evaluate the effectiveness of control programs and to determine vectorial capacity in malaria vectors. Two dissection techniques are used for this. For one, the tracheoles of dried ovaries are examined with a compound microscope and in the other the follicular stalk of ovaries is examined, wet, with a stereomicroscope. The second method also enables the sac stage of parous insects (which provides information on the duration of the oviposition cycle) and the mated status of insects to be determined. Despite widespread use the two techniques have not previously been compared.MethodsWe compared the two dissection techniques using Anopheles arabiensis, collected with a tent-trap in Eritrea. The paired ovaries were removed in water and one was examined by each method. From a separate set of dissections from Tanzania, we also determined if the sac stages of Anopheles gambiae s.l. (83% of 183 identified by PCR being Anopheles arabiensis the remainder being A. gambiae) that were alive on collection were different to those that died on collection and what the implications for vectorial capacity estimation might be.ResultsSeven per cent of the dry ovaries could not be classified due to granulation (yolk) in the ovariole that obscured the tracheoles. The sensitivity of the dry dissection was 88.51% (CI [79.88–94.35%]) and the specificity was 93.55% (CI [87.68–97.17%]) among the 211 ovaries that could be classified by the dry technique and compared to the ovaries dissected wet. 1,823 live and 1,416 dead from Furvela tent-traps, CDC light-trap and window-trap collections were dissected ‘wet’ from Tanzania. In these collections parous insects were more likely to die compared to nulliparous ones. The proportion of parous mosquitoes with ‘a’ sacs (indicative of recent oviposition) was significantly greater in insects that were dead (0.36) on collection in the morning compared to those that were alive (0.12) (Chi square 138.93, p < 0.001). There was a preponderance of newly emerged virgin insects in the outdoor collection (Chi sq = 8.84, p = 0.003).ConclusionsIn anophelines the examination of mosquito ovaries using transmitted light in a ‘wet’ dissection is a more useful and informative technique than examination of dry ovaries. In order to correctly estimate the duration of the oviposition cycle mosquitoes should be dissected as soon as possible after collection. Younger insects were more likely to attempt to feed outdoors rather than indoors.

Autodissemination of pyriproxyfen suppresses stable populations of Anopheles arabiensis under semi-controlled settings

“…Furthermore, the likelihood of collecting older (parous) mosquitoes was 4 times higher in PPF-exposed population compared to non-exposed. Contrary to adult based interventions that cause a shift to younger (nulliparous) mosquitoes due cumulative impacts on adult survival [41], the high proportion of parous mosquitoes in this study, indicates that the autodissemination technique, like other larviciding approaches, has a higher impact on younger mosquitoes (inhibition of emergence) and a limited impact on the survival of adults [42]. This imply that the surviving adult mosquitoes can promote successive transfer of PPF to aquatic habitats.…”

Fipronil and ivermectin treatment of cattle reduced the survival and ovarian development of field-collected Anopheles albimanus in a pilot trial conducted in northern Belize

“…Mosquito collections were conducted for two consecutive nights prior to each cattle feed. To estimate potential changes in the age structure of the mosquitoes used during the experiment, all mosquitoes that had not blood-fed at the conclusion of each feeding trial were dissected and scored as either nulliparous or parous, based on the presence (= nulliparous) or absence (= parous) of tracheolar coiling or skeins on the surface of the ovaries [23–25]. Parity rate was expressed as the percentage of parous mosquitoes.…”

“…Blood meal digestion was scored as either negative (no traces of blood in the midgut) or positive (blood present in the midgut). Ovarian development was scored as fully gravid (ovaries with fully developed ovarioles), half gravid (ovaries enlarged but ovarioles not fully developed), or not gravid (small ovaries with no ovariole development) [23–25].…”

Novel sampling methods for monitoring Anopheles arabiensis from Eritrea

“…Greatest numbers of host-seeking females were collected in the sentinel tent-trap run for 23 consecutive nights during the experiment (geometric mean of 162 per night 95% CI [117.13-224.87]). The majority of host-seeking parous mosquitoes had large follicular sacs, indicating a rapid return to host-seeking after oviposition (Charlwood et al, 2018a). Total numbers collected declined during the experiment whilst at the same time the parous rate among the 281 mosquitoes dissected (Charlwood et al, 2018a) increased from 21% to 56% (Fig.…”