The nature of individual heterogeneity shape outbreak dynamics

It is now common-place that pathogen transmission during an outbreak can be more heterogeneous than what is commonly assumed, and that it can have major consequences on their dynamics. However, previous studies did not explore the impact of the different biological sources of heterogeneity while controlling for the resulting heterogeneity in the number of secondary cases. In this study, we explore the role of individual variation in infection duration and transmission rate on parasite emergence and spread in a population. We simulate outbreaks using a custom stochastic SIR model, with and without evolution of the parasite. We show that for a given mean, the variance in the number of secondary cases is the main driver of the outbreak probability, with or without evolution, while it does not play a role on the outbreak dynamic once it emerged. On the opposite, a smaller and more realistic variance in the infection duration causes a faster outbreak. It is therefore useful to take into consideration more realistic distributions when modelling infectious diseases outbreaks.

A novel assay to isolate and quantify third-stage filarial larvae emerging from individual mosquitoes

BackgroundMosquitoes transmit filarial nematodes to both human and animal hosts, resulting in worldwide health and economic consequences. Transmission to a vertebrate host requires that ingested microfilariae develop into infective third-stage larvae capable of emerging from the mosquito proboscis onto the skin of the host during blood feeding. Determining the number of microfilariae that successfully develop to infective third-stage larvae in the mosquito host is key to understanding parasite transmission potential and to developing new strategies to block these worms in their vector.MethodsWe developed a novel method to efficiently assess the number of infective third-stage filarial larvae that emerge from experimentally infected mosquitoes. Following infection, individual mosquitoes were placed in wells of a multi-well culture plate and warmed to 37 °C to stimulate parasite emergence. Aedes aegypti infected with Dirofilaria immitis were used to determine infection conditions and assay timing. The assay was also tested with Brugia malayi infected Ae. aegypti.ResultsApproximately 30% of Ae. aegypti infected with D. immitis and 50% of those infected with B. malayi produce emerging third-stage larvae. Once D. immitis third-stage larvae emerge at 13 days post infection, the proportion of mosquitoes producing them, and the number produced per mosquito remain stable until at least day 21. The prevalence and intensity of emerging third-stage B. malayi were similar on days 12-14 days post infection. Increased uptake of D. immitis microfilariae increases the fitness cost to the mosquito but does not increase the number of emerging third-stage larvae.ConclusionsWe provide a new assay with an associated set of infection conditions that will facilitate assessment of the filarial transmission potential of mosquito vectors and promote preparation of uniformly infectious L3 for functional assays. The ability to quantify infection outcome will facilitate analyses of molecular interactions between vectors and filariae, ultimately allowing for the establishment of novel methods to block disease transmission.Graphical Abstract

Lack of efficacy of triclabendazole against Fasciola hepatica is present on sheep farms in three regions of England, and Wales

The liver fluke Fasciola hepatica is a parasitic trematode that has a major impact on livestock production and human health. Control of F hepatica is difficult and relies on anthelmintics, particularly triclabendazole, due to its efficacy against both adult and juvenile stages of the parasite. Emergence of triclabendazole-resistant F hepatica populations has been reported in a number of countries, including the UK, but the overall prevalence and distribution of triclabendazole resistance is unknown. In this study, the authors established the presence of reduced efficacy of triclabendazole in sheep flocks in England and Wales, using a validated composite faecal egg count reduction test. Seventy-four sheep farms were sampled from Wales, southwest, northwest and northeast England between Autumn 2013 and Spring 2015. F hepatica eggs were detected in samples from 42/74 farms. Evidence of a lack of efficacy of triclabendazole was detected on 21/26 farms on which the faecal egg count reduction test was completed, with faecal egg count reductions ranging from 89 per cent to 0per cent. Regression analysis suggested that both prevalence of F hepatica and lack of efficacy of triclabendazole were spatially correlated, with higher faecal egg counts and lower percentage reductions on farms located in the northwest of England, and Wales. Overall, the results show that reduced efficacy of triclabendazole is present across England and Wales, with a complete lack of therapeutic efficacy observed on 9/26 farms.

Octopamine mimics the effects of parasitism on the foregut of the tobacco hornworm Manduca sexta

The parasitic braconid wasp Cotesia congregata lays its eggs inside the body of the larval stage of its host, the moth Manduca sexta. The Cotesia congregata larvae develop within the hemocoel of their host until their third instar, when they emerge and spin cocoons and pupate on the outer surface of the caterpillar. From this time until their death approximately 2 weeks later, the Manduca sexta larvae show striking behavioral changes that include dramatic declines in spontaneous activity and in the time spent feeding. Coincident with these behavioral changes, it is known that octopamine titers in the hemolymph of the host become elevated by approximately 6.5-fold. Octopamine is an important modulator of neural function and behavior in insects, so we examined hosts for neural correlates to the behavioral changes that occur at parasite emergence. We found that, in addition to the changes reported earlier, after parasite emergence (post-emergence), Manduca sexta larvae also showed marked deficits in their ability to ingest food because of a disruption in the function of the frontal ganglion that results in a significant slowing or the absence of peristaltic activity in the foregut. This effect could be produced in unparasitized fifth-instar larvae by application of blood from post-emergence parasitized larvae or of 10(−6)mol l(−1)d,l-octopamine (approximately the level in the hemolymph of post-emergence larvae). In contrast, blood from parasitized larvae before their parasites emerge or from unparasitized fifth-instar larvae typically had no effect on foregut activity. The effects of either post-emergence parasitized blood or 10(−6)mol l(−1) octopamine could be blocked by the octopamine antagonists phentolamine (at 10(−5)mol l(−1)) or mianserin (at 10(−7)mol l(−1)).

Cold Tolerance of Hickory Shuckworm (Lepidoptera: Tortricidae) Larvae and Associated Parasites

Hickory shuckworm larvae, Cydia caryana (Fitch), in pecan shucks, were killed by extended periods of freezing temperature. While storage at −7°C killed 88% of larvae after 32 days of exposure, −18°C killed 85% after only 1 day and 100% by 8 to 16 days of exposure. Larvae exposed to cold temperatures prior to −18°C exposure appeared to possess enhanced cold resistance but were still killed after 32 days of exposure. Survival of the ichneumonid parasite, Calliephialtes grapholithae (Cresson), which comprised 92% of the total of all parasites emerging from pecan shucks, was not detectably affected by exposure to cooling (3–5°C) for up to 5 wks. Thereafter, when emergence began to decrease, about half of the parasites in the extended cooling treatment were viable for at least 10 wks. Conversely, 3 other parasites Phanerotoma fasciata Provancher (Braconidae), Macrocentrus instabilis Muesebeck (Braconidae), and Lixophaga mediocris Aldrich (Tachinidae), comprised the remaining 8% of total parasite emergence and survival was severely affected by extended cooling (3–5°C).

Discrimination by the Pupal Parasite Spalangia cameroni (Hymenoptera: Pteromalidae) Between Live and Freeze Killed House Fly (Diptera: Muscidae) Pupae

Live house fly pupae were suitable as hosts for Spalangia cameroni Perkins at all age classes tested. However, no parasite emergence occurred from house fly pupae freeze-killed when 12 h old and very limited emergence occurred for pupae freeze-killed when 132 h old. Furthermore, significantly more parasites emerged from hosts that were alive when parasitized when compared with freeze-killed hosts parasitized under similar conditions. In choice experiments, S. cameroni exhibited a strong preference for live hosts over freeze-killed hosts at all parasite-to-host ratios. It does not appear that freeze-killed hosts will be useful as a survey tool or as a method for field propagation of S. cameroni as they are for other species of pteromalids.

BENOMYL INHIBITION OF COTESIA (= APANTELES) MARGINIVENTRIS1 SURVIVAL IN FOUR LEPIDOPTEROUS HOSTS2

Four noctuid larvae, Heliothis zea (Boddie), Spodoptera exigua (Hubner), Pseudoplusia includens (Walker), and S. ornithogalli (Guenee) were reared on semi-synthetic diet in which varying concentrations of the carbamate fungicide benomyl were incorporated. These larvae were exposed to parasitization by Cotesia (= Apanteles) marginiventris (Cresson), and the effect of the fungicide on parasitism and host survival determined. Successful parasite emergence from H. zea, S. exigua, and P. includens was significantly reduced (P = 0.01) as benomyl concentration increased. Parasite emergence from S. ornithogalli was reduced, but a significantly significant dosage response was not evident. Implications of these findings, including a possible deleterious effect on natural biological control following benomyl application, are discussed.

A Note on Temperature and Postdiapause Development of the Larch Sawfly and Its Parasite Bessa harveyi (Tnsd.)

Some information of interest on the postdiapause development of the larch sawfly, Pristiphora erichsonii (Htg.), and its tachinid parasite Bessa harveyi (Tnsd.) has been obtained in the course of laboratory rearings. Sawfly cocoons were placed in cold storage at 2 to 3°C. in September or October and held until the following year, usually late March, when they were incubated at 10 or 15°C. They were then held at constant temperatures until emergence was complete. Sawfly and parasite emergence was recorded daily.