Comparative larvicidal property of leaf extract of Chromolaena odorata L (Composidae) and Chlopyrifos (organophosphorus compound) on Simulium larvae

OBJECTIVE

To assess the phytochemical properties of Chromoleana odorata in the control of blackflies to help check the problem of environmental pollution from the use of chemical insecticides.

METHODS

Dried pulverized leaves of Chromolaena odorata (100 g) were soaked in distilled water for 24 hours and the extract was filtered, marc pressed and evaporated over water bath. Stock solution of the dried mass (1 g) was concentrated by dissolving in 100 mL of water. Serial dilutions (100 mg/mL, 10 mg/mL, 1 mg/mL, 0.1 mg/mL, 0.01 mg/L, and 0.001 mg/mL) of both the extract and Chlopyrifos (organo phosphorus) were prepared and toxicity was tested on the larvae of Simulium species.

RESULTS

The chemical compound recorded 100% larval mortality at all concentrations while the plant extract recorded 100% larval mortality in three (100, 10, and 1 mg/mL) of its diluent concentration. The percentage of larval mortality by chemical compound and plant extract was statistically insignificant (t=0.2456, P>0.05), but the mortality rate was significant within 30 min interval of exposure time (t=3.756, P<0.05). The LC50 of the plant extract was determined at about 0.001 mg/mL concentration. This showed that the plant extract had an appreciable larvicidal property close to that of the chemical compound.

CONCLUSION

This result indicates the usefulness of plant materials in the control of obnoxious insect vectors of mn. It also opens additional approach to the management of environment from toxic chemicals that are non-biodegradable and cause environmental imbalance in the ecosystem.

[Taxonomic composition and landscape-biotopical distribution of black flies (Diptera, Simuliidae) in the Khemchik river basin (Western Tuva)]

Taxonomoc composition and altitude distribution of black flies have been investigated in the top and middle stream of the Khemchik river, Western Tuva. Similarity with adjacent territories by the species composition is revealed. A dependence of the blackfly abundance on the stream characteristics (water temperature, flow velocity, grading of soil, electric conductivity of water, and presence of epibioses), as well as on the hydrobiology characteristics of the localities is shown. The comparison of blackfly complexes from the upper part of the Sayano-Shushenskoe reservoir and the Todzha depression, Tuva, is carried out.

Toxicity of fipronil and its enantiomers to marine and freshwater non-targets

Fipronil is a phenylpyrazole insecticide used in agricultural and domestic settings for controlling various insect pests in crops, lawns, and residential structures. Fipronil is chiral; however, it is released into the environment as a racemic mixture of two enantiomers. In this study, the acute toxicity of the (S,+) and (R,-) enantiomers and the racemic mixture of fipronil were assessed using Simulium vittatum IS-7 (black fly), Xenopus laevis (African clawed frog), Procambarus clarkii (crayfish), Palaemonetes pugio (grass shrimp), Mercenaria mercenaria (hardshell clam), and Dunaliella tertiolecta (phytoplankton). Results showed that S. vittatum IS-7 was the most sensitive freshwater species to the racemic mixture of fipronil (LC50 = 0.65 microg/L) while P. pugio was the most sensitive marine species (LC50 = 0.32 microg/L). Procambarus clarkii were significantly more sensitive to the (S,+) enantiomer while larval P. pugio were significantly more sensitive to the (R,-) enantiomer. Enantioselective toxicity was not observed in the other organisms tested. Increased mortality and minimal recovery was observed in all species tested for recovery from fipronil exposure. These results indicate that the most toxic isomer of fipronil is organism-specific and that enantioselective toxicity may be more common in crustaceans than in other aquatic organisms.

Larval description of Simulium (Notolepria) cuasiexiguum and Simulium (Chirostilbia) obesum and new records of black fly species (Diptera: Simuliidae) in the states of São Paulo and Minas Gerais, Brazil

We describe the larval stage of Simulium cuasiexiguum Shelley, Luna Dias, Maia-Herzog & Lowry and Simulium obesum Vulcano and provide new distribution records of four black fly species in the State of São Paulo (Simulium cuasiexiguum, Simulium metallicum s.l. Bellardi, Simulium stellatum Gil Azevedo, Figuer6 & Maia-Herzog and Simulium oyapockense s.l. Floch & Abonnenc) and three species in the State of Minas Gerais (Simulium duodenicornium Pepinelli, Hamada & Trivinho-Strixino, Simulium obesum and Lutzsimulium pernigrum Lutz).

Abundance and diversity of human-biting flies (Diptera: Ceratopogonidae, Culicidae, Tabanidae, Simuliidae) around a nickel-copper smelter at Monchegorsk, northwestern Russia

In the summers of 2001 and 2002, we quantitatively sampled human-biting flies in twelve sites located 1.6 to 63 km from a large copper-nickel smelter at Monchegorsk on the Kola Peninsula, Russia. We collected 429 specimens of three species of Ceratopogonidae, 92 specimens of seven species of Culicidae, 76 specimens of seven species of Tabanidae, and 4,788 specimens of 19 species of Simuliidae. Culicoides chiropterus was for the first time reported from the Kola Peninsula. Catches of Culicidae and Simuliidae decreased near the smelter, presumably due to the combined action of toxicity of pollutants, pollution-induced forest damage, and decline in vertebrate density. An abundance of Ceratopogonidae and Tabanidae, the size of the most common black fly species, Simulium pusillum, and the diversity of all families did not change along the pollution gradient.

[Blackflies (Diptera: Simuliidae) of the Genera Hellichiella Rivosecchi et Cardinali, 1975 and Boreosimulium Rubzov et Yankovsky, 1982]

The genus name Boreosimulium Rubzov et Yankovsky, 1982 was considered for a long time as a junior synonym of the genus name Hellichiella Rivosecchi et Cardinali, 1975. The status of Boreosimulium as a separate generic (or subgeneric) name has been confirmed in the monograph by Adler, Currie and Wood (2004). In the present paper the indentification keys for the Boreosimulium and Hellichiella (subfam. Simuliinae) species are given. Also the keys for the genera Stegopterna Enderlein, 1930 and Greniera Doby et David, 1959 (subfam. Prosimuliinae) being similar in some characters to the above genera are provided. The lists of species of these genera, together with their distribution ranges (according to the data obtained to the end of 2004) are added.

Balancing food availability and hydrodynamic constraint: phenotypic plasticity and growth in Simulium noelleri blackfly larvae

Organisms through phenotypic plasticity can cope with multiple changed environmental conditions. Theory predicts that animals in streams and rivers should be able to balance demands of the needs to obtain food efficiently and to adjust response to hydrodynamic variability. This study examined effects of variations in food availability and current velocity on the feeding structure and growth rate of Simulium noelleri blackfly larvae. The larvae developed larger labral fans and more rays under slow current and low food regimes than in fast current and high food conditions. In both fast and slow current regimes, growth rates were higher and development periods to the final-instar stage were shorter in high food treatments. The estimated flux rates of food particles through labral fans under high food treatments for both fast and slow current regimes were higher than those under low food treatments. Although both food and current velocity appeared to have selected for flexibility of feeding structure and growth rate, food availability was a more important factor for phenotypic and developmental plasticity than current velocity. The results indicate a strong link between environmental changes in food availability and current velocity, phenotypic plasticity, and growth rate of S. noelleri. This study suggests that plasticity of ecomorphs with macroevolutionary significance may play a role in the early evolutionary stages of blackfly larvae.

[Landscape and mass-attack of Simuliidae-Diptera on man and domestic animals]

Mass-attack of Simuliidae-Diptera on man and domestic animals should not be considered only as a consequence of quality of running freshwater, but also as an effect of the actual state of the surrounding area. The territory, in fact, impacts the qualitative properties of a water body and can trigger adverse phenomena, especially so when modified by significant anthropic activities. In different locations the native settings have been replaced by deforestation, intensive farming and zootechnological activities. This paper sets forth the results of some investigations carried out further to a mass attack of Simuliidae-Diptera on man at different sites of the national territory. These results have been re-assessed taking into account the areas of interest and the modifications that they have undergone.

Physico-chemical indices of breeding sites of Simulium damnosum in the lower Cross River Basin, Nigeria

To determine the factor influencing the abundance development and distribution of Simulium damnosum (S.I) immature stages in the lower Cross River Basin, Nigeria, the physico-chemical characteristics of water at breeding sites were analysed longitudinally over a 12 month period. Analysis of the water samples showed that water current correlated positively and significantly (p < 0.05), with dissolved oxygen (r = 0.95 vs 0.10); conductivity (r = 0.95 vs 0.81); hydrogen ion concentration (r = 0.03 vs 0.46); and biochemical oxygen demand (r = 0.80 vs 0.64); from Agbokim Waterfalls and Afi River respectively. The result indicates that these parameters and low ionic concentration are the most essential requirement for the development of S. damnosum pre-imaginal stages. Amplitudes of annual variability as measured by co-efficient of variation varied between the parameters. From the trends in hydrological variables it was deduced that the integrated impact of precipitation, input of surface runoff and municipal effluents played an overriding role in determining the absolute levels and temporal pattern in the water quality attributes. Of the 3578 pre-imaginal stages of S. damnosum collected, 52.52% and 47.4% were collected from Agbokim Waterfall and Afi River respectively. There was a significant difference (p < 0.05) in the number of pre-imaginal stages collected from both sites. The highest number of pre-imaginal stages of S. damnosum collected were at the peak of rain (July-September) indicating that S. damnosum in the River Basin is a wet season breeder. The need for long term laboratory colonization of blackfly immature stages becomes imperative especially how these parameters could be exploited in control programme through the testing of larvicides without adverse ecological damage to the aquatic habitat.

Review of blackfly (Diptera: Simuliidae) control in South Africa

The medical, veterinary and economic importance of blackflies in South Africa, and the historical development of blackfly control programmes in various South African rivers, are reviewed in this paper. In 1996 it was estimated that blackflies can cause more than R 88 million damages per annum along the middle and lower Orange River where Simulium chutteri is considered the main pest species. A clear link between the construction of dams and the spread of the blackfly problem was shown. Four phases characterize the development of blackfly control in South Africa: (1) during the 1960s blackflies in the Vaal River were controlled with DDT; (2), during the 1970s and into the 1980s blackflies were controlled using water-flow manipulation; (3) when used at strategic times, water-flow manipulation could be used to enhance the effect of natural predator populations; and (4) during the 1990s the organophosphate temephos and toxins produced by the bacterium Bacillus thuringiensis var. israelensis were tested for their efficacy against blackflies. The larvicides temephos and B. thuringiensis proved to be effective and are still used in several control programmes. The latest research focuses on the factors that influence adult blackfly survival and annoyance, as well as the development of methods that can be used to protect sheep from blackfly attacks.

Larval biometry of Simulium rubrithorax (Diptera: Simuliidae) and size comparison between populations in the states of Minas Gerais and Roraima, Brazil

The number of larval instars of Simulium (Hemicnetha) rubrithorax Lutz (Diptera: Nematocera) was determined using the lateral length of the head capsule. In this study 1,035 larvae, of different sizes, were measured (639 from the state of Roraima and 396 from the state of Minas Gerais). A frequency distribution analysis was carried out on the measurements of the lateral length of the head capsule to determine the number of larval instars. The limits of each instar were defined by the lower frequency of the measurements falling in a range of values, by the presence of the "egg burster" that characterizes the first larval instar, and by the developmental stage of the gill histoblast. The determination of the instar number was tested using a Student's t-test (p < 0.05), the Dyar rule and the Crosby growth rule. The results indicate the existence of 7 larval instars for this species, although this result was not in accordance to the Crosby rule. Last-instar larvae from two widely separated geographical populations (Roraima and Minas Gerais), collected in habitats with different water temperature were compared and no differences (p > 0.05) were observed between them.

[Features of larval development in Simulium buissoni Roubaud, 1906, and S. sechani Craig and Fossati, 1995 (Diptera: Simuliidae) in the Marquesas Archipelago (French Polynesia)]

Two anthropophilic species are known from the Marquesas archipelago (French Polynesia): Simulium buissoni Roubaud, 1906 and S. sechani Craig & Fossati, 1995. The need to control them requires good knowledge about their larval growth, larvae being the only stage actually susceptible to control. A biometric study based on cephalic measures of the two species, shows that they both have seven larval instars. Size variations following species and sampling sites are examined, as are variations of the sex of the two last larval instars. A recolonization experiment, after treatment of a part of a river, allows us to quantify larval growing length of S. buissoni, which is about 12 days. These data allow us to determine the optimal treatment frequency.

An epizootic of patent iridescent virus disease in multiple species of blackflies in Chiapas, Mexico

Simulium blackfly larvae (Diptera: Simuliidae) were collected from rivers and streams at 500-1500 m a.s.l. in Chiapas State of southern Mexico. Among 45 sites surveyed over an area of 2300 km2 (around 15 degrees 15'N 92 degrees 20'W), some Simulium larvae from three sites were opalescent violet-blue, interpreted as patent infection with invertebrate iridescent virus (IIV). Dissection confirmed the presence of putative Iridovirus particles, 130nm diameter, but no IIV isolates were obtained from homogenates injected into Galleria mellonella (L) larvae (Lepidoptera: Pyralidae). All Simulium with patent IIV infection died before metamorphosis, whereas approximately 60% of asymptomatic Simulium survived to adulthood in the laboratory. During 1997, standard monthly samples from two parallel rivers 42-50 km north-west of Tapachula comprised the following species proportions (and rates of patent IIV infection): 41.8% (47%) Simulium mexicanum Bellardi complex, 31.3% (31.4%) S. rubicundum Knab, 10.1% (13.1%) S. paynei, 6.5% (2.9%) S. callidum (Dyar & Shannon), 6.3% (5.1%) S. ochraceum Walker complex, 3.1% (0.7%) S. downsi Vargas et al., 0.7% S. samboni Jennings and 0.2% S. metallicum Bellardi complex, showing a strong correlation between blackfly abundance and the prevalence of patent infection. An epizootic of IIV in January and February (infection rates 41-100%) was followed by absence of larvae (March-August) until the end of the rainy season, when numbers collected on nylon strings rose to approximately 1/cm with patent IIV infection rates of 0-12.5% during September-December. Further investigations are underway to isolate this IIV and assess its potential usefulness for biological control of Simulium pests and vectors of onchocerciasis.

Onchocerca-Simulium complexes in Venezuela: can human onchocerciasis spread outside its present endemic areas?

The compatibility between sympatric and allopatric combinations of Onchocerca volvulus-anthropophilic species of Simulium was studied in the north-eastern focus of human onchocerciasis as well as in a densely populated locality of the Amazonas State in Venezuela. The objectives were to test the conjecture that local adaptation exists between the parasite and its vectors (the Onchocerca-Simulium complex hypothesis), and assess the possibility of the infection spreading from its present distributional range. For the homologous combination, O. volvulus-S. metallicum cytospecies E in Anzoátegui State (north-eastern focus), parasite yield was 45% in contrast to 1% for the heterologous, southern parasite-S. metallicum infection. This was significantly lower than the parasite yield (4-10%) expected after allowing for the effect of density-dependent limitation of infective larval output described in this paper for S. metallicum. The population of S. exiguum s.l. from southern Venezuela allowed no larval development beyond the L1 stage of either northern or southern parasites. Mechanisms for such refractoriness probably operate at the level of the thoracic muscles, not affecting microfilarial uptake or migration out of the bloodmeal. The parasite yield of southern O. volvulus in S. oyapockense s.l. flies biting man at Puerto Ayacucho (Amazonas) was about 1%, in agreement with the figures recorded for highly compatible sympatric combinations such as O. volvulus-S. ochraceum s.l. in Guatemala. No infective larval development of the northern parasite was observed in southern S. oyapockense. These results, together with considerations of typical worm burdens in the human host, presence/absence of armed cibaria in the simuliids, parasite-induced vector mortality, and fly biting rates, suggest a lower potential for onchocerciasis to spread between the northern and southern endemic areas of Venezuela than that between Amazonian hyperendemic locations and settlements outside this focus with high densities of S. oyapockense s.l.

Distribution of the larvae of blackflies (Diptera: Simuliidae) at Doi Inthanon National Park, northern Thailand

The larvae of seventeen Simulium species consisting of thirteen known species, three unnamed species and one new species were distributed in various localities with different habitats at 400-2,460 meters above sea level in Doi Inthanon National Park, Chiang Mai Province, northern Thailand. Simulium caudisclerum, S. sp G and S. sp H were restricted to the high altitude of 2,460 m at the cool Ang Kha habitat of the summit of Doi Inthanon. On the other hand, S. nakhonense and S. rudnicki occurred only at low altitudes from 400 m-700 m. Some Simulium species such as S. fenestratum, S. asakoae, S. inthanonense and S. sp J were found in the warmer localities at 1,010 m-1,280 m altitudes. Moreover, S. feuerborni and S. rufibasis were widely distributed species at different altitudes from 700 m-2,460 m and 1,010 m-2,300 m respectively. Therefore, the distribution and abundance of Simulium species seem to correlate with altitudes as well as micro-habitat factors such as water temperature and water velocity.

Effects of Polydispyrenia simulii (Microspora; Duboscqiidae) on development of the gonads of Simulium pertinax (Diptera; Simuliidae)

The effects of the infection caused by the microsporidium Polydispyrenia simulii in gonads of the simuliid Simulium pertinax were evaluated. This blackfly species is the main target of control programmes in southeastern Brazil. Infected and noninfected blackflies were reared from larval stage to adults using a laboratory rearing system. There was no significant difference between the oogenic development and dimensions of ovarian follicles of healthy females compared to those reared from infected larvae. Otherwise, there was a significant difference between the fecundity of healthy females and those reared from infected larvae. In males, there was an evident reduction of motility of spermatozoids of males reared from infected larvae. The results obtained suggest that the microsporidium P. simulii have an important effect on the natural population dynamics of S. pertinax.

Effects of preservation methods, parasites, and gut contents of black flies (Diptera: Simuliidae) on polymerase chain reaction products

Molecular analysis of biological specimens usually requires extraction of high-molecular weight DNA free of foreign DNA contaminants. DNA was extracted from black flies at different life stages that had been preserved by 4 methods: larvae and adults in ethanol, larvae in Carnoy's solution, adults on card-points, and adults hand-swatted and sun-dried. Using specific primers for the mitochondrial ND4 gene, a 257-bp amplicon was obtained from specimens preserved by ethanol, card-point mounting, and sun-drying. Successful amplification often required DNA dilutions > or = 1:20 (< 1-10 ng). DNA from specimens preserved in Carnoy's solution (ethanol: acetic acid, 3:1) yielded degraded DNA, resulting in fewer successful amplications. Parasitic nematodes and, to a lesser extent, gut contents resulted in extra products when amplified with randomly amplified polymorphic DNA (RAPD) primers. Sufficient DNA was extracted from the head of a larva for a successful polymerase chain reaction (PCR), eliminating the need to remove the contaminating gut and parasites.

Effects of constant temperatures on oviposition and immature development of Simulium bidentatum (Diptera: Simuliidae), a vector of bovine Onchocerca (Nematoda: Onchocercidae) in central Kyushu, Japan

Laboratory developmental studies were carried out on Simulium bidentatum (Shiraki) under constant air and water temperatures (7, 10, 13, 16, 19, 22, 25, and 28 degrees C). The median time required for female flies to complete oocyte maturation after blood feeding decreased from 8 to 1.5 d and the median time from blood feeding to oviposition decreased from 16 to 3 d as air temperature increased from 7 to 28 degrees C. Air temperatures from 16 to 22 degrees C were most suitable for oviposition. An average of 61.4% of eggs hatched. Egg developmental period decreased from 17 to 4.5 d when water temperature increased from 7 to 19 degrees C and decreased to 3.5 d when water temperature was between 22 and 28 degrees C. Larval and pupal development occurred at water temperatures > or = 13 degrees C, but was most successful from 16 to 22 degrees C. Seven (in 16 lots) or 6 instars (in 1 lot) were indicated by using the Dyar rule. We conclude that S. bidentatum is adapted to moderate weather conditions in spring and autumn and that approximately 7 generations of this species occur per year in Oita, Japan.

Biological control program against simuliids in the State of São Paulo, Brazil

In Brazil, the use of biological vector-control methods has been largely confined to experimental research, with little or no application of such techniques by public institutions responsible for implementing control programs. The notable exceptions have been the black fly control program carried out by the Health Secretariat in the State of São Paulo. Since the 1980s, São Paulo's "Superintendência de Controle de Endemias" has been conducting studies on the viability of using Bacillus thuringiensis (H-14) for simuliid control, and the results have been so encouraging that the agency has now incorporated this method into its Simuliid Control Program.

Age determination in the blackfly Simulium woodi, a vector of onchocerciasis in Tanzania

Age-dependent changes in the ovarioles of Simulium woodi (Diptera: Simuliidae) and the age structure of a wild population of this species in the Amani hills of north-eastern Tanzania were studied using the ovarian oil injection technique. Contrary to previous theories, egg sacs degenerate completely and do not form dilatations. The physiological age of the females could be determined only by gonotrophic dilatations formed from degenerating follicles during gonotrophic cycles. In individual females, the proportions of ovarioles with degenerating follicles were 3-54% (mean 22%) in the first cycle and 8-61% (mean 36%) in the second gonotrophic cycle. Agonotrophic degenerating follicles occurred in 2% of ovarioles in 12% of females. Additional information with regard to the degree of parity is given by the level of granulation in the basal body of each ovariole, a group of six to eight cells in the calyx wall enclosed by the end of the ovariolar sheath. Their granulation progressively increased in intensity following each subsequent ovulation.

A mid-arm interchange as a potential reproductive isolating mechanism in the medically important Simulium neavei group (Diptera: Simuliidae)

The Simulium neavei group is medically important as a vector of human onchocerciasis and is unique among blackflies because of the attachment to freshwater crabs during parts of the larval and pupal life cycle stages. Detailed larval salivary gland polytene chromosome maps are presented for two taxa designated S. neavei Amani form A and S. neavei Amani form B, which are probably synonymous with the previously described morphospecies, S. nyasalandicum and S. woodi, respectively. Simulium neavei Amani form B differs from S. neavei Amani form A by three paracentric inversions, a large deletion and a chromocentre, while the latter taxon differs from the former one by a mid-arm interchange. The fixed mid-arm interchange (1-tr) between two related taxa is unique among the Simuliidae. Detailed banding analysis of S. neavei Amani form B shows that, relative to S. neavei Amani form A, sections 16a, 16b, and 17c of the short arm of chromosome one (IS) have been inserted into section 69/70 of the short arm of chromosome three (IIIS). Assignment of directionality for this interchange inferred because of the near universality in the Simuliidae of a chromosome III long arm to short arm ratio of 2:1. We propose that the mid-arm interchange acted as a primary isolating mechanism because of the segregational load on F1 heterozygous interchange progeny. The magnitude of the segregational load would be dependent on the frequency of pairing and crossing-over between the transposed interstitial segment and its original counterpart in the chromosome complement. Nonpairing in the transposed region and normal random segregation would presumably lead to a 50% of gametes carrying the duplicated or deficient transposed segment, resulting in a segregation load of 50%. However, crossing-over in the paired interstitial region may greatly increase segregational load as result of multivalent formation at metaphase one (MI) and the generation of dicentric bridge(s) and acentric fragment(s) upon cell division. Such meiotic disjunction irregularities could lead to meiotic arrest, gametic loss and (or) cell restitution, and the eventual formation of polyploid (lethal) zygotes.

New techniques for rearing black flies from pupae (Diptera: Simuliidae)

Simple techniques are described for collecting black fly pupae from streams using plastic strips and for rearing large numbers of adult black flies in inexpensive enclosures made of chicken wire and cloth mesh netting, and 2 methods are described for rearing adult black flies individually from pupae. The first method of rearing individual black flies uses 1.5-ml microcentrifuge tubules and the second uses easy-to-construct rearing chambers that provide moisture for the developing pupa and water for the adult to imbibe. Instructions for assembly are provided. Specimens obtained using these rearing chambers are of museum quality.

Comparison of the sensitivity of different geographical isolates of Onchocerca volvulus microfilariae to ivermectin: effects of exposure to drug on development in the blackfly Simulium ornatum

Four geographical isolates (Ghana forest, Ghana savannah, Cameroon forest, Guatemala) of Onchocerca volvulus microfilariae (mf) and O. lienalis mf (UK) were examined for their sensitivity to ivermectin by incubation in vitro in drug followed by assessing their ability to develop in the blackfly Simulium ornatum after intrathoracic injection. Parasites were incubated for 30 min in ivermectin (10(-6) to 10(-9) M), which resulted in a concentration dependent decrease in the numbers of parasites surviving and developing in the insect; there were significant reductions in parasite recoveries from all isolates in the 10(-6) M to 5 x 10(-8) M ivermectin groups, but no significant effect was seen following incubation in concentrations of 10(-8) M and below. Experiments consistently demonstrated that the 4 isolates of O. volvulus were similarly sensitive to ivermectin (in the 10(-7) M ivermectin groups there was a reduction of 76.3% to 85.1% in numbers of infective larvae, and 60.9% to 85.5% in numbers of all larval stages, compared to controls); O. lienalis mf were significantly more sensitive (100% reduction in infective larvae, 98.7% reduction in all larval stages). This baseline information on drug sensitivity and techniques should prove useful for examining populations of O. volvulus for possible development of drug resistance in the future.

Distribution of onchocerciasis in selected river basins of four west African countries

Detailed epidemiological mapping of eight river basins was conducted in four West African countries as part of an effort to identify the communities at risk of onchocercal blindness to guide a programme of large scale ivermectin distribution, attempting control of eye disease. The results show a surprising variability of pattern in the geographical distribution of intensity of onchocerciasis infections in the communities of the river basins investigated. These patterns were at time very different from what was expected on the basis of the available entomological and demographic information. The technique of detailed mapping proved very useful in achieving an excellent coverage of the communities at risk of onchocercal blindness where, so far, satisfactory treatment coverage has been attained for four consecutive yearly treatments.

Onchocerciasis in British cattle: a study of the transmission of Onchocerca sp. in north Wales

The transmission of bovine Onchocerca spp. in Britain was investigated by studying the Ceratopogonidae and Simuliidae which attacked bait cattle between April and October at two sites in North Wales, where either Onchocerca lienalis and O. gutturosa (Cynwyd), or O. lienalis alone (Pentrefoelas) occurred. Based on the relative abundance of each species, their seasonal variation in abundance and preferential feeding sites on cattle, the presence of Onchocerca sp. infective larvae and the development of patent infections in bait cattle, vectors were identified as Simulium ornatum s.1. at Pentrefoelas and S. reptans at Cynwyd (0.5% and 1.5% of parous flies infective, respectively). There was no significant difference between the lengths of the infective larvae from vectors at either site (Pentrefoelas: mean = 543.5 microns, SD = 29.45; Cynwyd: mean = 550.86 microns, SD = 35.26; p > 0.1), which were consistent with descriptions of O. lienalis. The identity of a vector for O. gutturosa was not determined and the role of certain candidates is discussed. In this respect, the absence of Culicoides nubeculosus was notable. Transmission of Onchocerca sp. was limited to a short period of about 8 weeks, by a combination of vector abundance and parasite uptake from dermal skin layers, and a mean daily temperature requirement of 15 degrees C or more for complete parasite development in the vector. The effect of this temperature restriction in relation to the geographical distribution of the two species is discussed.

[Entomology of onchocercosis in Soconusco, Chiapas. 6. Quantitative studies of the transmission of Onchocerca volvulus by 3 species of Simuliidae in a community with high endemicity]

In Guatemala, there is no doubt about the participation of Simulium ochraceum as vector of Onchocerciasis. However, in Mexico practically there are not studies focussed to determine the role of this species in the transmission. The objective of the present investigation was to determine which of the 3 species of Simulium founded in the Soconusco region of Chiapas, is the main, and which were secondary in the transmission of Onchocerciasis in that area. The locality of Morelos, in the Huixtla "municipio" of Chiapas, localized a 1200 meters over the sea level (mosl) were selected to carried out the present study. According to own parasitological studies, this locality is considered as highly endemic (more than 66% prevalence). From March 1979 to March, 1981, we performed captures of Simulium sp caught on human bait. Quantitative studies and of transmission potentials were also performed. The following results were obtained: a) Absolute black flies densities (nulliparous and pariparous) and infected black flies (with L1 and L2 larvae): S. ochraceum, S. metallicum and S. callidum, in that order was the distribution of densities. Infected black flies were obtained in the 3 species. However, in despite of an irregular distribution in all the year, it was possible to identify 2 peaks of maximum infection of S. ochraceum in March, 1979 and March 1981. b) Monthly bite densities and infective bites.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on environmentally-induced colour variation in Simulium sirbanum (Diptera: Simuliidae) using a portable rearing system

Previous studies on the morphological identification of adult female Simulium damnosum s.l. have involved use of colour characters. To determine the degree to which these qualitative characters are influenced by the physical environment, a portable rearing system was constructed. Wild-caught female flies and flies emerging from pupae maintained at ambient temperatures or above showed entirely pale antennae, fore-coxae, wing-tufts and hairs on the scutellum and ninth abdominal tergite, as is normal for S. sirbanum. Adults reared from larvae at below ambient temperatures were also pale, except for the antennae which were dark in all specimens. Adult females emerging from pupae maintained at below ambient temperatures were entirely pale up to the fourth day of emergence and mostly with dark antennae. The biological significance and the implications to the morphological identification of the savannah species of the S. damnosum complex in West Africa are discussed.

Distribution of Simulium yahense after larviciding activities in the southern extension of the Onchocerciasis Control Programme

The distribution range of Simulium yahense in the Eastern Operational Area of the Onchocerciasis Control Programme increased after larviciding treatments were started in 1988, to control important Simulium damnosum s. 1. vectors of onchocerciasis. A distribution map indicating the extended range is drawn and the importance of this expansion of S. yahense to disease transmission discussed.

A temporary focus of savanna species of the Simulium damnosum complex in the forest zone of Liberia

Savanna species of the Simulium damnosum complex, the major vectors of the blinding savanna type of onchocerciasis, were considered to be rare in Liberia, until the dry season of 1988. In 1988 they became a serious nuisance, biting people at the Bong iron ore mine within the rain forest zone. S. damnosum s.str. and S. sirbanum were found breeding, in association with S. adersi a non man-biting savanna species, in a stream emerging from the mine's tailings pond. The local forest species were extremely rare in this stream. The water of the stream was characterized by an increased hardness and higher temperatures in comparison with those of natural watercourses in the area. The mass occurrence of flies was probably related to expansions of the tailings ponds, when rich nutrition was provided for the blackfly larvae by the decaying forests submerged by the rising water levels. The phenomenon did not recur in 1989 when only a few savanna flies were caught, and none were seen in the dry season of 1990. Although it is not known why the artificial environment of the tailings ponds was so attractive for the savanna species the events clearly demonstrated that savanna flies seasonally invading the area, possibly aided by the northeasterly harmattan winds, can become established in the rain forest zone if suitable conditions are met. No infections with Onchocerca volvulus were found in more than 1000 flies caught by vector collectors but, after experimental infection with the local forest strain, a few parasites developed to the infective stage. As yet there is no evidence that the occurrence of savanna flies in the rain forest zone of Liberia was of epidemiological significance.

[The fat body of prepupal and pupal blackflies and its utilization by the female during its life cycle]

The study performed revealed lipids, proteins and carbohydrates in the content of trophocytes which formed greater assemblies (lipid-protein-glycogen complexes, lipid-protein and protein-glycogen granules). The prepupal stage off stream Simulium noelleri was featured by neutral lipids and granules consisting of protein and glycogen. Mature pupae and emerged females had acid lipids and lipid-protein-glycogen complexes. Prepupal stages of the river S. morsitans, Schönbaueria pussilla sp. 1, Sch. pussilla demonstrated acid and neutral lipids that formed lipid-protein granules. The aforementioned granules were not infrequent in the river population of S. noelleri but in some of them protein-glycogen granules were revealed. The different correlations of lipid-protein and protein-glycogen granules were documented in the pupae of the studied species and emerged females. Gradual consumption of lipid accumulations was noted in the development of females ovaries in all species studied. Besides, the greater complexes were the first to be consumed, than followed proteins, carbohydrates and finally lipids.

The development of Onchocerca ochengi in surrogate temperate Simuliidae, with a note on the infective larva

Laboratory reared nulliparous female flies of six temperate species of Simuliidae were examined for their susceptibility to infection with Onchocerca ochengi by intrathoracic injection of cryopreserved skin microfilariae obtained from cattle in Mali. Three species (S. equinum, S. ornatum and S. erythrocephalum) supported development to the infective stage, one species (S. variegatum) allowed partial development and the remaining two species (S. reptans and S. aureum) were insusceptible to infection. The most suitable surrogate vectors were S. equinum and S. ornatum which had survival rates of 44% and 49%, proportions of microfilariae developing to third stage larvae of 6.4% and 3%, and infection rates with infective larvae of 13.5% and 14% respectively. O. volvulus infective larvae, produced by intrathoracic microfilarial injection in S. ornatum, were 586-760 microns (mean 687 microns) long and were significantly shorter (p less than 0.02) than the O. ochengi infective larvae (645-880 microns, mean 756 microns). No constant differences in the posterior or anterior morphology, or in the acid phosphatase staining patterns between O. ochengi and O. volvulus, were seen. These results raise the possibility that the presence of O. ochengi in a population of infective larvae from vector flies in endemic onchocerciasis zones might be identifiable on the basis of their length alone.

Vector density, parasite prevalence, and transmission of Onchocerca volvulus in Guatemala

Transmission of Onchocerca volvulus at 4 locations with different prevalences of human onchocerciasis in the Atitlán region of Guatemala is described in relation to vector density and infection rates. The percentages of residents with skin biopsies positive for microfilariae of O. volvulus at these locations were 13.8%, 33.3%, 65.4%, and 89.6%. The following variables associated with transmission were calculated from our observations (the values are presented in an order that corresponds with the above prevalence rates): frequency of third-stage larvae (calculated on an annual basis) in parous Simulium ochraceum, 0, 0.004, 0.005, and 0.004; estimated daily biting density of S. ochraceum, 23, 24, 254, and 1,509 flies per day; and estimated annual infective biting density (based on S. ochraceum), 0, 18, 185, and 1,101 potentially infective bites per year. The frequencies of third-stage larvae are very small compared with those observed in Africa, and suggest that transmission of O. volvulus in Guatemala depends on high vector density. Locations with low, and perhaps tolerable, levels of onchocerciasis (less than 15% of female residents with skin biopsies positive for microfilariae) have mean daily biting densities for S. ochraceum of less than or equal to 24 flies, and infected residents normally have mean microfilarial densities of less than or equal to 3 microfilariae per mg of skin. Stratification of prevalence rates by age group proved useful for assessing current transmission within a village.

Biting activity of black flies in Guatemala: parity rates and differences between localities and habitats

Host-seeking activity of anthropophilic black flies at 4 localities in Guatemala with different prevalence rates of onchocerciasis was assessed using human attractants and semimonthly catches over one year. Density of host-seeking Simulium ochraceum was greatest at the locality with the highest incidence of onchocerciasis and very reduced at the 2 localities with low levels of human infection. The overall percentage of parous host-seeking S. ochraceum at the 4 localities ranged from 41%-49%. Host-seeking activity appeared to be concentrated near streams containing immature stages, and few females dispersed as far as 3 km away. S. metallicum was the second most frequently captured species; however, at the locality with the highest prevalence of onchocerciasis, its host-seeking density was much less than that of S. ochraceum. For S. metallicum, the overall percentage of parous females ranged from 28%-34% at the 4 localities. S. metallicum were consistently taken in much greater numbers in coffee cultivation areas than in housing areas. Host-seeking S. callidum and S. gonzalezi also were captured.

Seasonality of adult black flies and Onchocerca volvulus transmission in Guatemala

Transmission of Onchocerca volvulus, principally by Simulium ochraceum, was studied over a 14-month period in the Atitlán region of Guatemala. Semimonthly catches of black flies were made on human volunteers at 4 localities with different prevalences of human onchocerciasis. Host-seeking activity of S. ochraceum reached its greatest magnitude in the early dry season (October-January) but then declined rapidly and was lowest during the late dry season (February-May). The frequency of O. volvulus larvae in parous host-seeking S. ochraceum also varied seasonally, and this variation was most pronounced for third stage larvae. At a hyperendemic locality, the highest frequency of this larval stage occurred during the February-March period (0.0142 in 1979 and 0.0095 in 1980). From June-January, the frequency of third stage larvae in S. ochraceum was less than 0.003. The frequency of early first stage larvae exhibited the least seasonal variation, ranging from 0.0354 in August-September to 0.0628 in April-May. The transmission rate of O. volvulus by S. ochraceum also varied seasonally. At the same hyperendemic locality, infective biting density of S. ochraceum attained its greatest magnitude in February-March. The survival rate of female S. ochraceum from one gonotrophic cycle to the next was estimated from the ratio of flies with early first stage larvae to those with infective stage larvae. These rates varied seasonally and ranged from 0.2132 to 0.3974, with the highest rates occurring in the late dry season.