A community-level investigation of the yellow fever virus outbreak in South Omo Zone, South-West Ethiopia, 2012–2014

BackgroundA yellow fever (YF) outbreak occurred in South Omo Zone, Ethiopia in 2012-2014. This study aimed to analyse historical epidemiological data, to assess the risk for future YF outbreaks through entomological surveillance, including mosquito species identification and molecular screening for arboviruses, and finally to determine the knowledge, attitudes and current preventative practices within the affected communities.Methodology/Principal FindingsFrom October 2012 to March 2014, 165 cases and 62 deaths were reported, principally in rural areas of South Ari region (83.6%), south-west Ethiopia. The majority of patients were 15-44 years old (74.5%) and most case deaths were males (76%). Between June and August 2017, 688 containers were sampled from across 177 households to identify key breeding sites forAedesmosquitoes.Ensete ventricosum(“false banana”) was identified as the primary natural breeding site, and clay pots outside the home as the most productive artificial breeding site. Entomological risk indices from the majority of sites were classified as “high risk” for future outbreaks under current World Health Organization criteria. Adult trapping resulted in the identification of members of theAedes simpsonicomplex in and around households. Screening of adult females revealed no detection of yellow fever virus (YFV) or other arboviruses. 88% of 177 participants had heard of YF, however many participants easily confused transmission and symptoms of YF with malaria, which is also endemic in the area.Conclusions/SignificanceStudy results emphasise the need for further entomological studies to improve our understanding of local vector species and transmission dynamics. Disease surveillance systems and in-country laboratory capacity also need to be strengthened to facilitate more rapid responses to future YF outbreaks.Author SummaryDespite the availability of a highly effective vaccine, yellow fever virus (YFV) remains an important public health problem across Africa and South America due to its high case-fatality rate. This study aimed to assess and reduce the risk for future outbreaks. During this study, historical data analysis was conducted to understand the epidemiology of the recent outbreak in 2012-2014. Entomological surveillance was also carried out, including both mosquito species identification and molecular screening for arboviruses, as well as a household survey to understand the knowledge and attitudes towards yellow fever (YF) within the affected areas and to assess community-level practices for YF prevention. We found a high abundance ofAedes simpsonicomplex in the context of low vaccination coverage. Community knowledge and practice levels were low for reducing potential breeding sites, highlighting the need for increased dissemination of information to community members on how to reduce their risk of exposure to mosquito vectors of arboviruses.

Estimates of mortalities of larvae and pupae of the Aedes simpsoni (Theobald)(Diptera: Culicidae) complex in Uganda

Studies were carried out to estimate and compare mortalities of larvae and pupae of Aedes (Stegomyia) simpsoni sensu lato breeding in axils of Colocasia esculenta, Xanthosoma sagittifolium (Araceae) and Musa spp. (Musaceae) in different locations in Uganda and also to ascertain whether there are any differences in their dynamics that could be used to separate the species. The probability of larvae surviving from one day to the next (40·5 – 99·8%), and the proportions dying in each instar (0·97 – 77·3%) and dying daily in all the four instars (3·9 – 46·3%) varied between seasons at each site and between sites and species of plants. Estimates of larval and pupal mortality were generally highest for fourth instar [52·4 ± 6·0% (mean ± SE)] and lowest for the second instar larvae (16·0 ± 2·6%). There was higher larval instar mortality in axils of X. sagittifolium (96·9%) and Musa spp. (82·2 – 96·5%) than in C. esculenta (47·1 – 88·4%).

Mark-release-recapture studies on three anthropophilic populations of Aedes (Stegomyia) simpsoni comple(Diptera: Culicidae) in Uganda

Multiple mark-release-recapture experiments were carried out on females of three anthropophilic populations of Aedes simpsoni (Theobald) complex at Nkokonjeru and Bwamba in Uganda to determine and compare the population size, survival rates and longevity of the adult stages. There were high recapture rates (26.23%, 2 7.42% and 28.28%) at the different sites. The population sizes, estimated by Jolly's stochastic method for the three sites (465, 561 and 675 female mosquitoes) were not significantly different, neither were the survival rates of the females (0.6026, 0.7145 and 0.8152). Other parameters estimated for the three populations were also similar. Population size estimates by Jolly's stochastic model and the simple Lincoln Index methods were not in agreement. The other method over-estimated population size. Since the estimated parameters for the Nkokonjeru population are similar to those of the Bwamba populations, there is a potential for the transmission of yellow fever virus to the human population at Nkokonjeru, as at Bwamba.

Studies on some of the physical and biological factors affecting the abundance of the Aedes simpsoni (Diptera: Culicidae) complex-larvae and pupae in plant axils

Some physical and biological factors affecting the abundance of larvae and pupae of the Aedes simpsoni (Theobald) complex, in leaf axils of different plants at several locations in Uganda, were investigated during the rainy and dry seasons. The mean number of axils per plant, axils containing larvae and pupae, and the mean number of larvae and pupae per ml of axil water all varied between seasons and locations. The percentage of axils containing larvae and pupae increased with volume of water in the axils of the different plants. The mean number of larvae and pupae per axil containing water also increased with water content of the axils. There was more water in the axils during the rainy than during the dry seasons. The mean temperature of water was lower in Xanthosorna sagittifoliurn (20.4°C) and Colocasia esculentum (22.1°–22.7°C) than in those of banana (25.0°–27.2°C) and this affected developmental rates of larvae and pupae. The indices of association between larvae and pupae of the A. simpsoni complex and Malaya taeniarostris (Theobald) did not indicate competition and there was some habitat segregation between them. The volume and temperature of water in the axils appeared to be the major factors affecting larval and pupal abundance of A. simpsoni in the axils.

Host preference in Aedes simpsoni (Theo.) (Diptera, Culicidae) with special reference to the anthropophilic and non-anthropophilic forms in Uganda

Precipitin tests of the blood-meals of the non-anthropophilic population of Aedes simpsoni (Theo.) in Bwayise showed that at least 62% of the meals were mammalian; none was certainly avian or reptile. The remaining 38% were mostly too far digested for identification. Of the positive mammalian feeds, 84% were of rodents and only 5%; were of primates, probably human. In laboratory choice experiments, approximately two-thirds of a sample of the Bwayise strain fed on rodent in preference to monkey. Precipitin tests on blood-meals of the Bwamba strain of Ae. simpsoni confirmed earlier findings that this strain is strongly anthropophilic. At least 86% of wild-caught individuals had fed on mammals, 65% of these specifically identified: 28% of the feeds were of rodents and 72% were of primates, mostly human. During a five-month study at Bwayise in 1969, eleven species of rodents were caught; 91% of the rodents diurnally active were Arvicanthis niloticus which contributed 38% of the total collection and was twice as active during the afternoon as in the morning. The remaining species were predominantly nocturnal. A close relationship was found between the times of activity of A. niloticus and the biting cycle of Ae. simpsoni. In feeding tests using the three commonest rodent species, Ae. simpsoni selected A. niloticus as often as, or more often than, the nocturnal rodent species. This preference was maintained over one generation reared in culture. It is suggested that A. niloticus is the major natural host of the Bwayise strain by reason of the synchrony between rodent and mosquito activity rhythms. The Bwayise strain does not normally feed upon wild primates, so explaining further the almost total absence of human cases of yellow fever in areas of Uganda where this mosquito is non-anthropophilic.

Observations on the Mosquito Fauna of Mombasa

SummaryRecords are given of the species of mosquitos captured at Mombasa in the years 1943–44 and 1949–52. They were caught by four different methods, namely: (1) day-time net catches in the bush, (2) window-trap catches, (3) diurnal (bait) biting catches in the bush and (4) twenty-four hour (bait) biting catches in three different situations. Sixty-six species were taken in the day-time net catches and nine others which did not figure in the net catches were taken in one or other of the remaining three methods.The habits of the commonest species, as indicated by these catches, are discussed in some detail.Culex pipiens fatigans Wied. occurred in large numbers in biting catches, net catches and window-trap catches. In 24-hour catches most biting occurred after midnight, but, in the window traps, activity was greatest between 18 and 21 hr.Aëdes simpsoni (Theo.) was not taken in window traps or houses. Seasonal occurrence, as indicated by larvae in banana axils, net and biting catches in the bush, is discussed. Twenty-four hour catches confirmed earlier findings that this mosquito bites almost solely during daylight.Aëdes pembaensis (Theo.) occurred throughout the year with little seasonal variation; this is thought to be connected with its breeding in crab-holes. It is often found in fair numbers in houses when these are near the breeding grounds. Window-trap catches show that most activity takes place at night though some also occurs during the day. The biting rhythm as shown by 24-hour catches in houses, bush and on tree platforms is discussed.Aëdes albicosta (Edw.). Previously this mosquito may have been confused with Aëdes albothorax (Theo.) and Aë. lineatopennis (Ludl.). It can occur at times in great numbers when it may bite viciously both indoors and outside.Anopheles gambiae Giles and A. funestus Giles were not evident in large numbers. In window traps most activity took place at night although some also occurred during the day. In 24-hour catches, A. gambiae fed mostly just before dawn though biting occurred also from midnight onwards. A. funestus appeared suddenly and in fair numbers in window traps but was absent for much of the year.Aëdes aegypti (L.), though fairly common, is recorded only in Tables and in one figure as its bionomics have already been discussed elsewhere.Notes are given on other species, which include Culex duttoni Theo., C. nebulosus Theo., C. tigripes Grp., Aëdes metallicus (Edw.), Aë. albocephalus (Theo.), Eretmapodites subsimplicipes Edw. and E. quinquevittatus Theo.

The Genus Musa Linn. and its Role in the Breeding of Aedes (Stegomyia) simpsoni (Theo.) on the Kenya Coast

SummaryThe varieties of the banana plant (Musa) on the Kenya coast, with particular reference to Aëdes simpsoni (Theo.), a species concerned with the transmission of yellow fever in East Africa, are described, and the species of mosquito larvae found in their leaf axils are recorded. A. simpsoni was virtually the only species present.Details are given of the proportion of leaf axils that contain water and the percentage of these that produce larvae. Breeding was found to take place in all varieties, but to a much lesser extent in that known as Kibungala than in the others. Although there was often more water in the lower and upper axils than in the middle ones they produced less larvae individually than did the latter. Larvae were also found in old axils at the base of the stems in spite of the fact that these sites seldom appear to provide direct access for an ovipositing mosquito. Water was present throughout the year in the upper axils only.It was found that the water content of axils varied from day to day and from morning to evening during dry periods and that the axils were often apparently dry for days at a time although the larvae were able to survive. It is suggested that the larvae are able to survive in the water film at the base of the axils, but that such unfavourable conditions may account for the retarded growth that has been observed.The relationship between water content and number of larvae per axil is discussed, and the conclusion is reached that unless all measurements of water content for all varieties are made simultaneously each day, a comparison of average water content becomes impossible, and therefore no connection can be established from the data so far available.

The Mosquitoes of Bwamba County, Uganda. II.—Biting Activity with special Reference to the Influence of Microclimate

1. The work was carried out in Bwamba County, Uganda, a district where human yellow fever has recently occurred. It consisted mainly of 24-hour catches in banana plantations and rain forest combined with climate observations.2. It was found that forest was cooler and moister by day than a banana plantation while the plantation was cooler and moister than the open air. By night, however, the climate of all three environments was strikingly similar.3. Methods used in making 24-hour catches and large scale routine catches are described. Trials with trap nets showed these to be less efficient than hand catching.4. Experimental catches showed that Aëdes simpsoni, the only species known to be a vector of yellow fever in Bwamba, bites mainly in coffee gardens, maize fields and the thinner parts and edges of banana plantations. It is scarce in fully exposed situations. It occurs within the edges of primeval rain forest. It prefers human blood to that of goats, fowls, and monkeys, and attacks the head and shoulders selectively.