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Onderstepoort, South Africa

An equation, strongly reminiscent of Fisher's equation, is used to model the response of tsetse populations to proposed control measures in the vicinity of a game reserve. The model assumes movement is by diffusion and that growth is logistic. This logistic growth is dependent on an historical population, in contrast to Fisher's equation which bases it on the present population. The model therefore takes into account the fact that new additions to the adult fly population are, in actual fact, the descendents of a population which existed one puparial duration ago, furthermore, that this puparial duration is temperature dependent. Artificially imposed mortality is modelled as a proportion at a constant rate. Fisher's equation is also solved as a formality.The temporary imposition of a 2% day-1 mortality everywhere outside the reserve for a period of 2years will have no lasting effect on the influence of the reserve on either the Glossina austeni or the G. brevipalpis populations, although it certainly will eradicate tsetse from poor habitat, outside the reserve. A 5km-wide barrier with a minimum mortality of 4% day-1, throughout, will succeed in isolating a worst-case, G. austeni population and its associated trypanosomiasis from the surrounding areas. A more optimistic estimate of its mobility suggests a mortality of 2% day-1 will suffice. For a given target-related mortality, more mobile species are found to be more vulnerable to eradication than more sedentary species, while the opposite is true for containment. © 2010 Elsevier Inc. Source


Verhoef F.A.,University of Pretoria | Venter G.J.,Onderstepoort Veterinary Institute | Weldon C.W.,University of Pretoria
Parasites and Vectors | Year: 2014

Background: Culicoides imicola Kieffer and Culicoides bolitinos Meiswinkel (Diptera: Ceratopogonidae) are both of veterinary importance, being vectors of Schmallenberg, bluetongue and African horse sickness (AHS) viruses. Within South Africa, these Culicoides species show a marked difference in their abundances according to altitude, with C. imicola highly abundant in lower altitudes, but being replaced as the dominant species by C. bolitinos in cooler, high-altitude regions. Methods. The thermal physiology of field collected adults of each species was determined to evaluate whether it could account for differences in their distribution and abundance. Critical thermal maxima (CTmax) and minima (CTmin), as well as upper and lower lethal temperatures (ULT and LLT) were assessed after acclimation temperatures of 19C, 24C and 29C. Critical thermal limits were determined using an ecologically relevant rate of temperature change of 0.06C.min-1. Results: Significant differences in CTminand CTmaxwere found between acclimation temperatures for C. imicola and C. bolitinos. In C. bolitinos, the LLT of individuals acclimated at 24C was significantly improved (LLT50= -6.01C) compared with those acclimated at the other temperatures (LLT50= -4C). Acclimation had a weak (difference in LLT50of only 1C) but significant effect on the LLT of C. imicola. When CTmin, CTmax, LLT and ULT were superimposed on daily maximum and minimum temperature records from locations where each tested Culicoides species is dominant, it was found that temperatures frequently declined below the CTminand LLT of C. imicola at the location where C. bolitinos was dominant. Conclusions: The distribution and abundance of C. imicola is likely directly constrained by their relatively poor tolerance of lower temperatures. Results for C. bolitinos suggest that the adult phase is hardy, and it is hypothesised that the thermal biology of other life stages could determine their range. © 2014 Verhoef et al.; licensee BioMed Central Ltd. Source


Monanema joopi n. sp. is described from blood drawn from the heart of the murid Acomys (Acomys) spinosissimus in South Africa. It is characterised by a non-bulbous cephalic extremity, shared with only one of its five congeners, and a cylindrical tail with caudal alae and a spicular ratio of 2.7 in the male. As is typical for the genus, microfilariae are skin-dwelling. They are 185 to 215 micrometres long and have no refractory granules beneath their sheath. A key to the species of Monanema is presented and an amended generic description, based on the six currently known species, is proposed. Species of Monanema are primarily lymphatic and the low intensity of infection with M. joopi n. sp. in blood from the heart, might suggest that not all adults settle in the heart cavities. One might also consider that other, more susceptible rodents serve as hosts for this parasite as well. To date, the geographic range of Monanema includes North America, Africa and Australia, each with representatives of a different lineage. Given the present hypotheses on the evolutionary origin and subsequent migrations of rodents, we expect the origin of Monanema to be in the Palearctic-Oriental region. Source


Von Teichman B.,Onderstepoort Biological Products Ltd. | Engelbrecht A.,Onderstepoort Biological Products Ltd. | Zulu G.,Onderstepoort Biological Products Ltd. | Dungu B.,GalVmed Doherty Building Pentlands | And 2 more authors.
Vaccine | Year: 2011

Two modified live attenuated vaccines against the disease Rift Valley fever (RVF) have been tested for safety and efficacy in young calves. The RVF Smithburn vaccine produced in South Africa and used successfully to prevent and control the disease in endemic sub-Saharan countries was compared to the candidate vaccine RVF Clone 13. Five sero-negative calves per vaccine group were vaccinated with a single dose of each vaccine and tested for antibody response. All vaccinated calves were challenged with a highly virulent RVF virus together with five unvaccinated calves used as control of the challenge. Protection was confirmed in all vaccinated animals as they did not show any clinical signs typical of RVF. A good neutralizing antibody response was induced post-vaccination and no viraemia could be detected post-challenge in calves of both vaccine groups. All non-vaccinated control animals showed clinical symptoms of RVF, high viraemia and were euthanized. This study reported the first case of blindness in cattle resulting from virulent RVF virus infection in unvaccinated calves used as negative controls. © 2011 Elsevier Ltd. Source


The hypothetical impact of aerial spraying on tsetse fly populations is investigated. Spray cycles are scheduled at intervals two days short of the first interlarval period and halted once the last of the female flies that originated from pre-spray-deposited pupae have been sprayed twice. The effect of temperature on the aerial spraying of tsetse, through its reproductive cycle and general population dynamics, is of particular interest, given that cooler weather is preferred for the settling of insecticidal droplets. Spray efficacy is found to come at a price due to the greater number of cycles necessitated by cooler weather. The extra cost is argued to be worth while. Pupae, still in the ground at the end of spraying, are identified as the main threat to a successful operation. They are slightly more vulnerable at the low temperature extreme of tsetse habitat (16°C), when the cumulative, natural pupal mortality is high. One can otherwise base one's expectations on the closeness with which the time to the third last spray approaches one puparial duration. A disparity of anything close to the length of a spray cycle advocates caution, whereas one which comes close to vanishing should be interpreted as being auspicious. Three such key temperatures, just below which one can anticipate an improved outcome and just above which caution should be exercised, are 17.146°C, 19.278°C and 23.645°C. A refinement of the existing formulae for the puparial duration and the first interlarval period might be prudent in the South African context of a sympatric Glossina brevipalpis-Glossina austeni, tsetse population. The resulting aerial spraying strategy would then be formulated using a G. brevipalpis puparial duration and a G. austeni first interlarval period. © 2010 Elsevier B.V. Source

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