Suppressive action of Samorin on the cyclical development of pathogenic trypanosomes in Glossina morsitans centralis

Male Glossina sexually sterilized by gamma-irradiation are as efficient vectors of trypanosomiasis as fertile males. An attempt was made, using isometamidium chloride (Samorin), to interfere with the cyclical development of trypanosomes in sterile males, destined for use in the sterile insect release (SIR) method of tsetse eradication. The infection rate with mature Trypanosoma congolense Broden was effectively reduced in sterile male Glossina morsitans centralis Machado, when the flies were fed on an infected goat 2 days after they were fed as tenerals on an in vitro bloodmeal containing 8 micrograms Samorin/ml blood. The infection rates with mature T.vivax Ziemann and T.brucei brucei Plimmer & Bradford were completely suppressed at this drug dose. Whensterile teneral males were fed on a bloodmeal containing 12 micrograms/ml Samorin and given the infected bloodmeal 10 days later, infections by mature T.vivax, T.congolense and T.b.brucei were completely suppressed. Hence in the management of a tsetse eradication programme utilizing the SIR method, it is recommended that the sterile teneral male tsetse should, prior to release, be given a bloodmeal containing Samorin at 12-15 micrograms/ml blood. This will effectively suppress future disease transmission.     

The sunset activity of tsetse flies: a light threshold study on Glossina morsitans

ABSTRACT. At dusk, tsetses fly from their day-time resting sites on tree trunks to spend the night on leaves and twigs. At the end of the day in the laboratory, they show a few minutes of heightened activity which apparently represents this behaviour. This occurs immediately after lights-out in a square-wave LD cycle, but just before the end of a 30-min artificial 'dusk' which mimics the natural change in light intensity at sunset. The activity is triggered when the declining light falls to a mean value of c. 350 mW m^-2. Accurate, 24-h light measurements made in Zimbabwe show that in tsetse 'bush' this intensity occurs close to sunset. Neither the initial photophase light intensity (900 and 2500 mW m^-2 were tested) nor the rate of dimming affect the critical value, which is also the same whether arrived at within c. 10 min in a 'logarithmic dusk' or within c. 20 min in a 'linear dusk'. In newly-emerged or recently fed flies, however, it is lower ( c. 50–100 mW m^-2); and when a similar activity burst is induced by a 'dusk' at midday (i.e. at the flies' phase of minimum activity), the threshold is c. 200 m W m^-2. It is concluded that this short behavioural programme is primarily a direct response to a specific, low light intensity, but that the threshold is modified by circadian and other physiological inputs.     

Prevalence of enlarged salivary glands in wild populations of Glossina pallidipes in Kenya,with a note on the ultrastructure of the affected organ

Enlarged salivary gland was found to be widespread among wild populations of Glossina pallidipes in Kenya. The incidence of this abnormality varied from 0.9% in Meru National Park in Central Kenya to 5.4% in the Shimba hills area on the Kenya coast. Ultrastructurally, the enlarged glands were multinucleated with lumen reduced substantially in size. A large number of viruses filled both the lumen and the broken pieces of epithelial cytoplasm. In some cases Trypanosoma brucei trypanosomes were seen in the lumen of the enlarged glands. The epithelial cytoplasm was heavily vacuolated. Comment is made on the suitability of the diseased flies as transmitters of T. brucei.     

The effect of host blood in the in vitrotransformation of bloodstream trypanosomes by tsetse midgut homogenates

Abstract. Midgut homogenates prepared from Glossina morsitans morsitans , that had previously been fed on different host blood samples, were tested for their abilities to transform bloodstream Trypanosoma brucei into procyclic (midgut) forms in vitro. Compared to rat and goat blood samples, eland blood had the least capacity to support trypanosome transformation, whereas buffalo blood showed intermediate capacity. Fractionation of rat blood showed the importance of the cellular portion since both rat and eland red blood cells (RBCs) supported the process. Virtually no transformation was observed in rat and eland plasma or serum fractions. Suspending rat blood cells in eland plasma led to a reduction in parasite transformation rates. Further experiments showed that the RBC membranes were also capable of supporting the process. These results clearly show the important role played by blood, especially the red blood cells, in the transformation of bloodstream trypanosomes. In addition, the low transformation rates observed in eland blood is due to an inhibitory factor(s) present in the plasma fraction.     

Insecticide-treated cattle for tsetse control: the power and the problems

Trypanosomiasis control increasingly involves financial input from livestock owners and their active participation. If control is carried out on smaller scales than in the past, methods such as aerial and ground spraying and sterile insect techniques will have reduced application. There will be increased reliance on trypanocidal drugs, and bait methods of tsetse control--where flies are attracted to point sources and killed. If drug resistance develops, cheap and simple bait methods offer the only means of disease control that might be applied, and paid for, by stockowners themselves. The methods have been effective in some circumstances, but not in others, and it is important to understand the reasons for the successes and the failures. Analysis is presented of the results of two Tanzanian tsetse control campaigns involving the use of insecticide-treated cattle. Between 1991 and 1996, following the introduction of widespread dipping in the Kagera Region, trypanosomiasis declined from >19000 cases to <2400 and deaths from >4000 to 29. On four ranches in the region, tsetse have been almost eliminated and trypanosomiasis prophylaxis is no longer used. Similarly aggressive use of pyrethroids on Mkwaja Ranch in Tanga Region has not had such dramatic effects. Tsetse and trypanosomiasis are still common, despite high levels of prophylaxis and the deployment of approximately 200 odour-baited targets. The difference in the results is attributed to a combination of the much smaller area covered by treated animals at Mkwaja, a greater susceptibility to re-invasion and a more suitable habitat for the flies. A better understanding of the dynamics of the use of insecticide-treated cattle is needed before we can predict confidently the outcome of particular control operations.     

Nutritional stress affects the tsetse fly's immune gene expression

Tsetse-transmitted trypanosomiasis poses a serious threat to human and animal health in sub-Saharan Africa. The majority of tsetse flies ( Glossina spp.) in a natural population will not develop a mature infection of either Trypanosoma congolense or Trypanosoma brucei sp. because of refractoriness, a phenomenon that is affected by different factors, including the tsetse fly's immune defence. Starvation of tsetse flies significantly increases their susceptibility to the establishment of a trypanosome infection. This paper reports the effects of nutritional stress (starvation) on (a) uninduced baseline levels of gene expression of the antimicrobial peptides attacin, defensin and cecropin in the tsetse fly, and (b) levels of expression induced in response to bacterial ( Escherichia coli ) or trypanosomal challenge. In newly emerged, unfed tsetse flies, starvation significantly lowers baseline levels of antimicrobial peptide gene expression, especially for attacin and cecropin. In response to trypanosome challenge, only non-starved older flies showed a significant increase in antimicrobial peptide gene expression within 5 days of ingestion of a trypanosome-containing bloodmeal, especially with T. brucei bloodstream forms. These data suggest that a decreased expression of immune genes in newly hatched flies or a lack of immune responsiveness to trypanosomes in older flies, both occurring as a result of fly starvation, may be among the factors contributing to the increased susceptibility of nutritionally stressed tsetse flies to trypanosome infection.     

Patterns of genetic diversity and differentiation in the tsetse fly <Emphasis Type="Italic">Glossina morsitans morsitans</Emphasis> Westwood populations in East and southern Africa

Genetic diversity and differentiation within and among nine G. morsitans morsitans populations from East and southern Africa was assessed by examining variation at seven microsatellite loci and a mitochondrial locus, cytochrome oxidase (COI). Mean COI diversity within populations was 0.63 ± 0.33 and 0.81 taken over all populations. Diversities averaged over microsatellite loci were high (mean number of alleles/locus ≥7.4; mean H _E ≥ 65%) in all populations. Diversities averaged across populations were greater in East Africa (mean number of alleles = 22 ± 2.6; mean h _e = 0.773 ± 0.033) than in southern Africa (mean number of alleles = 18.7 ± 4.0; mean h _e = 0.713 ± 0.072). Differentiation among all populations was highly significant (R _ST = 0.25, F _ST = 0.132). Nei’s G _ij statistics were 0.09 and 0.19 within regions for microsatellites and mitochondria, respectively; between regions, G _ij was 0.14 for microsatellites and 0.23 for mitochondria. G _ST among populations was 0.23 for microsatellite loci and 0.40 for mitochondria. The F, G and R statistics indicate highly restricted gene flow among G. m. morsitans populations separated over geographic scales of 12–917 km.     

Sex pheromone of the tsetse species, Glossina austeni: isolation and identification of natural hydrocarbons, and bioassay of synthesized compounds

Copulatory responses of male Glossina austeni (Newstead) (Diptera: Glossinidae), that were elicited after contact with frozen female tsetse, were not observed after solvent washing of cuticular lipids. Chromatographic analysis of extracts from laboratory-reared and field-collected G. austeni females yielded natural hydrocarbons that were highly stimulatory to males. Most of this activity was produced by compounds in the alkene fraction. Gas chromatograms (GC) contained five natural alkenes; these were separated by preparative GC for bioassays conducted in Tanzania. The two major alkenes were identified using gas chromatography-mass spectrometry (GC-MS) to be 13,17-dimethyltritriacont-1-ene and 13,17-dimethylpentatriacont-1-ene, after the samples had undergone derivatization using dimethyl disulphide and saturation with deuterium. These alkenes and natural alkanes were quantified from G. austeni of both sexes from laboratory and field samples to confirm that their presence was consistent in this species. Trials of synthetic samples resulted in the order of biological activity for the stereoisomers of 13,17-dimethyltritriacont-1-ene as follows: S,R-33:1 > R,S- 33:1 > S,S-33:1 > R,R-33:1. Dose-response data showed an ED(50) at 5 microg per treated, solvent-washed male decoy. Of the four stereoisomers of 13,17-dimethylpentatriacont-1-ene, R,R-35:1 showed the most activity. This is the first report of alkene-induced sexual activity in males of the genus Glossina.     

User-friendly models of the costs and efficacy of tsetse control: application to sterilizing and insecticidal techniques

An interactive programme, incorporating a deterministic model of tsetse (Diptera: Glossinidae) populations, was developed to predict the cost and effect of different control techniques applied singly or together. Its value was exemplified by using it to compare: (i) the sterile insect technique (SIT), involving weekly releases optimized at three sterile males for each wild male, and (ii) insecticide-treated cattle (ITC) at 3.5/km(2). The isolated pre-treatment population of adults was 2500 males and 5000 females/km(2); if the population was reduced by 90%, its growth potential was 8.4 times per year. However, the population expired naturally when it was reduced to 0.1 wild males/km(2), due to difficulties in finding mates, so that control measures then stopped. This took 187 days with ITC and 609 days with SIT. If ITC was used for 87 days to suppress the population by 99%, subsequent control by SIT alone took 406 days; the female population increased by 48% following the withdrawal of ITC and remained above the immediate post-suppression level for 155 days; the vectorial capacity initially increased seven times and remained above the immediate post-suppression level for 300 days. Combining SIT and ITC after suppression was a little faster than ITC alone, provided the population had not been suppressed by more than 99.7%. Even when SIT was applied under favourable conditions, the most optimistic cost estimate was 20-40 times greater than for ITC. Modelling non-isolated unsuppressed populations showed that tsetse invaded approximately 8 km into the ITC area compared to approximately 18 km for SIT. There was no material improvement by using a 3-km barrier of ITC to protect the SIT area. In general, tsetse control by increasing deaths is more appropriate than reducing births, and SIT is particularly inappropriate. User-friendly models can assist the understanding and planning of tsetse control. The model, freely available via http://www.tsetse.org, allows further exploration of control strategies with user-specified assumptions.     

Contribution of a parasitoid species to multiplication and transmission of a multiple nucleopolyhedrovirus in caterpillars

Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and Microplitis pallidipes are both used as biocontrol agents of the beet armyworm (Spodoptera exigua). However, it has not been determined how beet armyworms respond when these agents interact. Here, we studied the effects of M. pallidipes on virus multiplication and transmission using quantitative detection of SeMNPV. Our results indicated that parasitoids promoted virus multiplication in caterpillars (10⁵ copies per caterpillar) and that it was more advantageous when the M. pallidipes oviposited one day prior to infection with NPV. Interestingly, SeMNPV was transmitted by M. pallidipes in four ways. Transmission efficiency was higher for parasitoids whose body surfaces were contaminated with NPV, and for parasitoids ovipositing on NPV-infected caterpillars, than for those emerging from NPV-infected caterpillars, or feeding on mixtures of honey, water and NPV. Our study reveals that parasitoids do affect the proliferation and transmission of NPV in caterpillars and suggests that M. pallidipes could be used to strengthen the effectiveness of SeMNPV as a biocontrol agent.