Negative density‐dependent dispersal in tsetse (Glossina spp): red flag or red herring?

A deterministic model of the distribution of tsetse flies (Glossina spp) was used to assess the extent to which the efficacy of control operations would be affected by three different modes of density dependence in per capita adult dispersal: (i) density‐independent dispersal which has been commonly adopted in previous models, (ii) positive density‐dependent dispersal which has occasionally been discussed in the tsetse literature, (iii) negative density‐dependent dispersal (NDDD). The last has recently been suggested, from genetic studies, to change the dispersal rate of tsetse by up to 200‐fold, thereby posing a severe risk for the success of tsetse control operations. Modelling outputs showed that NDDD poses no such risk, provided the mean daily dispersal of tsetse is below about 1 km, which is greater than any rate actually recorded in the field or indicated by the genetic studies. NDDD can be problematic only if tsetse disperse at rates that appear highly unlikely, or even impossible, on energetic grounds. Under some circumstances these high rates would help rather than hinder the control officer. NDDD is not necessary to explain the results of control operations, and not sufficient to explain the results of successful control programmes.     

Molecular identification of bloodmeal sources and trypanosomes in Glossina spp., Tabanus spp. and Stomoxys spp. trapped on cattle farm settlements in southwest Nigeria

The interactions of host, vector and parasite in bovine trypanosomiasis transmission cycles in southwest Nigeria are not yet well understood. Trypanosoma (Trypanosomatida: Trypanosomatidae) species infection prevalences and bloodmeal sources were determined in transmitting vectors of the genera Glossina (Diptera: Glossinidae), Tabanus (Diptera: Tabanidae) and Stomoxys (Diptera: Muscidae) collected using Nzi traps in cattle settlements in southwest Nigeria. Sequenced cytochrome B mitochondrial DNA segments obtained from vector digestive tracts identified bloodmeal sources from eight host species, namely human, cattle, hippopotamus, giraffe, gazelle, spotted hyena, long‐tailed rat and one unidentified species. Overall, 71.1% [95% confidence interval (CI) 63.0–78.1], 33.3% (95% CI 21.9–47.0) and 22.2% (95% CI 16.2–29.9), respectively, of Glossina, Tabanus and Stomoxys flies were positive for trypanosomes. The observed trypanosome species were Trypanosoma vivax, Trypanosoma congolense, Trypanosoma brucei, Trypanosoma evansi, Trypanosoma simiae and Trypanosoma godfreyi. Trypanosome DNA was more prevalent in tsetse (34.8% Tr. vivax, 51.1% Tr. b. brucei, 5.2% Tr. congolense, 4.4% Tr. simiae and 24.4% mixed infections) than in other flies and the main determinants in all flies were seasonal factors and host availability. To the best of the present group's knowledge, this is the first report of Trypanosoma species in Tabanus and Stomoxys flies in Nigeria. It indicates that vector control programmes should always consider biting flies along with tsetse flies in the control of human and animal trypanosomiasis.     

Two hytrosaviruses, MdSGHV and GpSGHV, induce distinct cytopathologies in their respective host insects

Recently, a new virus family (Hytrosaviridae) was proposed for double-stranded DNA viruses that cause salivary gland hypertrophy in their dipteran hosts. The two type species, MdSGHV and GpSGHV, induce similar gross pathology and share several morphological, biological, and molecular characteristics. This histological study revealed profound differences in the cytopathology of these viruses supporting their previously proposed placement in different genera.     

Activation of three species of tsetse (Glossina spp.) in response to host derived stimuli

Recordings were made of the activation of hungry Glossina morsitans morsitans Westwood, G. pallidipes Austen, and G. austeni Newstead in response to odours from ox breath and ox urine, and a moving visual stimulus, in a wind tunnel. The spontaneous activity of G.m.morsitans was very low (less than 4% of males and 2% of females active per min during control periods). That of G.austeni and G.pallidipes was in the region of 20% except for G.pallidipes females when in excess of 40% were active during control periods. Addition of ox urine odours to the airstream had no effect on activity in any of the species investigated but addition of ox breath odours to the airstream significantly increased activity of G.pallidipes and of G.m.morsitans, although for the latter only approximately 12% of flies were active. For G.austeni the addition of ox breath odours resulted in a significant increase in activity of females but not of males. The moving visual stimulus resulted in a significant increase in the activity of both sexes of G.austeni and G.m.morsitans but no change in the activity of G.pallidipes. The low level of spontaneous activity and the low response to ox breath odours in a strain of G.m.morsitans maintained in the laboratory since 1969 was compared with a new colony of this species which originated from puparia collected in Zimbabwe in 1991. No differences in either spontaneous activity or the response to ox breath odour was recorded, but females from the new colony were significantly more responsive to a moving visual stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationships between protease activity, host blood and infection rates in Glossina morsitans sspp. infected with Trypanosoma congolense, T.brucei and T.simiae

Abstract. Midgut protease activity in Glossina morsitans centralis and G. m. morsitans , at 48h post bloodmeal averaged 1.8IU of trypsin-like activity. These two tsetse subspecies differ in their susceptibility to trypanosome infection. Except for low levels in flies fed on waterbuck blood (0.7IU), activity did not differ in flies fed a variety of host bloods (goat, pig, cow, buffalo, eland) and trypanosome species ( Trypanosoma congolense, T.brucei, T.simiae ). Protease activity was also not correlated with infection rates, despite large differences in infection rates among experiments. Nevertheless, addition of 0.06M D(+)-glucosamine to parasitaemic blood resulted in a three-fold reduction in protease activity, coincident with a large increase in infection rate. This effect did not occur when parasites or D(+)-glucosamine were added alone to the bloodmeal, suggesting that the effect was due to metabolism of D(+)-glucosamine by parasites.     

Cattle-tsetse contact in relation to the daily activity patterns of Glossina morsitans submorsitans in The Gambia

Abstract. The daily flight activity patterns of one of the main vectors of animal trypanosomiasis in West Africa, Glossina morsitans submorsitans , were assessed using four different methods. Results from all the methods showed that there was some flight activity nearly every hour in all seasons but they differed in the level of contact between grazing cattle herds and G.m.submorsitans. In the late dry season, trap data indicated that there was negligible activity from midday to late afternoon, whereas observations of tsetse contact with cattle herds or hand-net collections on herd followings showed no fall in attack rates on the cattle by G.m.submorsitans.
Differences between trap and animal-baited collection data may be attributable to the type of G.m.submorsitans sampled by each method. Male G.m.submorsitans captured by traps were more fat depleted than those caught on ox-baited flyrounds or by hand-net collections on herd followings. All methods showed that male G.m.submorsitans were most fat depleted in the late dry season and least in the early dry season. It was concluded that the traps were mainly sampling the spontaneous flights of G.m.submorsitans. Hunger and endogenous rhythms increase the likelihood of spontaneous flights towards dusk, particularly in conditions such as those at midday in the very hot, late dry season. However, the presence of cattle herds in infested habitats probably activated nearby G.m.submorsitans and the continual movement through the grazing areas ensured contact with tsetse throughout grazing.
The data indicated that strategic management of herd grazing times cannot eliminate the risk of trypanosomiasis transmission occurring, irrespective of the harshness of the dry season climate. An assessment of the level of this risk could only be measured suitably by collecting tsetse using animal-baited methods, not from trap data.     

Reciprocal regulation of fat content and flight activity in male tsetse flies (Glossina palpalis)

Abstract. To disentangle cause and effect in previously observed relationships between fat content and flight activity in male tsetse ( Glossina spp.), three groups of flies were fed at different intervals to raise their fat content to different levels before their flight activity was recorded. The greater the mean daily blood intake, the higher the fat content and the greater the subsequent spontaneous flight activity, thereby using up almost all of the fat reserves before the next blood meal. It is proposed that although male flies would benefit from maximum food intake to permit maximum flight associated with mate-seeking, they do not in fact feed as often as possible either in the field or the laboratory. This is explicable if energy acquisition is constrained by an additional mortality risk associated with feeding.     

The effect of wind speed on the flight responses of tsetse flies to CO2: a wind-tunnel study

Abstract. Female Glossina morsitans morsitans Westwood were video-recorded in a wind-tunnel as they entered, in cross-wind flight, a broad plume of CO₂ (a component of host odour). At a wind speed that corresponds with peak catches in the field (c. 0.6 ms-¹) odour produced both significant upwind turning responses (in-flight anemotaxis) and kinetic responses (reduced flight speed and increased sinuosity (m-¹). At a wind speed of c. 0.2 ms-¹ flies displayed anemotactic, but not kinetic, responses to odour. At very low wind speeds (0.1ms-¹) neither upwind turning responses nor kinetic responses to odour were detected. The results are discussed with regard to current theory of host-location by tsetse.     

Odour movement, wind direction, and the problem of host-finding by tsetse flies

ABSTRACT. Movement of host odour was modelled in natural tsetse habitats with smoke and ultra-light 7-cm-long wind vanes; the speed and direction of the air movements were analysed from video recordings thereof. Wind of <1 ms^-1 did not move in straight lines, since large packets of air (>10 m across) often changed direction together. The rate of this change of direction (meander) correlated negatively with windspeed. In open woodland with a shrubby understorey (in which windspeed was reduced by a factor of >5 from that above the canopy, to ax 0.3 m s^-1), this wind meander fell by 2d? s^-1 change of direction for each 0.1 m s^-1 increase in windspeed (r²=0.96). Over open ground without shrub cover, the meander fell by 0.5d? s^-1 per 0.1 m s^-1 increase in windspeed (r²=0.85). In both situations, such meandering virtually ceased in winds of > 1 m s^-1. In woodland, the relationship between the direction of air movement near the surface of bare earth (one potential tsetse landing site) and that c. 0.5 m above ground level (flight height) was often weak (r²=0.2-0.4), but this problem would be reduced if the fly averaged the ground-level wind for at least 30 s. Odour (smoke) travelling from a source 15 m ‘upwind’ over open ground arrived at a notional tsetse fly for 80% of the time from a direction within 10d? of the true source direction. In typical tsetse woodland, however, the ‘odour’ arrived from all directions (including >90d? away from the source), with only a 30% bias towards the true source direction (±10d?). Evidently, tsetse must navigate up odour plumes by means that get round these difficulties-simple, moth-type upwind anemotaxis alone seems unlikely to be adequate.     

Hypertrehalosaemic activity in corpus cardiacum-corpus allatum-aorta complex and adipokinetic response of Glossina morsitans

Abstract. Extracts from the corpus cardiacum-corpus allatum-aorta (CC-CA-A) complex of Glossina morsitans morsitans Westwood contain a hypertrehalosaemic factor when assayed in Periplaneta americana L. and in G.morsitans. A slight though significant decrease, followed by an increase, in haemolymph total carbohydrate occurs when tsetse are flown for 1 h. When assayed in Locusta migratoria L., the extracts have no adipokinetic activity, but L.migratoria corpus cardiacum extract produces an adipokinetic response in the female tsetse. It is suggested that the neurosecretions contained in the tsetse CC-CA-A complex contain a hypertrehalosaemic factor whose role is to mobilize glycogen.