Biology subjects

Hargrove, J. W.

Publications and source records attributed to Hargrove, J. W..

10 recordsLinked to original sources

bioRxiv2026

Creating complete life histories of individual female tsetse (Glossina spp) to study the effects of meteorological conditions on fly size in Zimbabwe

Combining novel methodologies with ovarian dissection, we estimated life histories for ca. 90,000 individual female Glossina pallidipes and G. m. morsitans sampled from 1988-1999 in Zimbabwes Zambezi Valley. Using temperature-dependent development rates we stepped back through each flys life, fixing dates of successive pregnancies, adult emergence, pupal period, pregnancy and oogenesis. This enabled modelling of relationships between wing and egg lengths, and conditions prevailing when these lengths were being determined. Egg lengths increased with maternal wing length, were shorter in primiparous flies but changed little with age thereafter. G. pallidipes egg lengths were positively related to NDVI and negatively to temperature (R2 = 0.68), for variables averaged over the period of oogenesis for each fly, and then averaged again across weekly cohorts of flies. G. m. morsitans mean egg lengths, pooled by month, showed the same pattern (R2 = 0.53). Pooled mean wing lengths increased with NDVI and decreased with temperature prevailing while flies were developing in the ovaries and uterus; R2= 0.66 (G. pallidipes) and R2= 0.56 (G. m. morsitans). The models - fitted using flies captured after November 1991 - gave good predictions, with no further modeling, for egg and wing lengths of flies captured between September 1988 and November 1991. The models facilitate true predictions of future changes in fly size based on readily available meteorological data, benefiting vector and disease control efforts in predicting likely changes in tsetse population densities and distribution. Selection against small individuals in the hot-dry season is not restricted to teneral mortality continuing for some weeks after emergence. NDVI, measures of wetness and temperature can indirectly impact tsetse size, mortality and population density by affecting vertebrate host density and, thereby, the probability of tsetse locating and feeding on a host. Our methodology impacts numerous areas of vector biology and control.

zoology↗

bioRxiv2025

Modelling framework to demonstrate elimination of a vector population: tsetse elimination in Chad

Every year, over 700 000 people, particularly children under five, die from vector-borne diseases worldwide. Effectively controlling current endemics and preventing new outbreaks requires an integrated approach that can lead to the elimination of both vectors and diseases. In the last two decades, integrating medical interventions and vector control has significantly reduced the incidence of Gambian Human African Trypanosomiasis (gHAT), with the World Health Organization validating eight countries as having eliminated the disease as a public health problem. However, elimination of the tsetse vector has not been confirmed, leaving the possibility of re-emergence. We developed a five-step modelling framework to assess vector elimination by calculating: (i) the probability of vector capture; (ii) the probability of observing a series of zero catches, even without actual elimination; (iii) the probability of natural elimination; (iv) the probability of failing to detect a rebound; and (v) the reinvasion risk. Our case study is g-HAT in Mandoul, Chad and the elimination of G. fuscipes fuscipes. We used vector control from 2014 to 2025 with no tsetse detected since 2018. We cannot yet conclude, with more than 90% confidence, that tsetse has been eliminated from Mandoul, nor that any remnant population will be naturally eliminated. However, since vector control was stopped in April 2025, we estimate that with continued sampling over the next two years, and no tsetse detected, elimination could be demonstrated with 99% confidence. Our multi-step modelling framework can be applied to other vectors, providing policymakers with clear guidelines for ongoing and future efforts. Significance StatementThe World Health Organisation has set the elimination of transmission of several neglected tropical vector-borne diseases, including human African trypanosomiasis (sleeping sickness), as a target for 2030. We show that deliberate elimination of tsetse, the vector, is feasible and can be demonstrated. We draw on our large-scale intervention in Mandoul, Chad where 3000 insecticide-treated Tiny Targets were deployed between 2014 and 2025, with no tsetse detected since 2018. While small undetected remnant populations cannot be entirely excluded, they would rapidly rebound in the absence of control, rendering them detectable. If no tsetse are caught over the next two years, it will confirm elimination. This illustrates a pathway for assessing and achieving vector elimination as a cornerstone of disease eradication.

ecology↗

bioRxiv2023

Modelled impact of Tiny Targets on the distribution and abundance of riverine tsetse

BackgroundThe insecticide-treated baits known as Tiny Targets are one of the cheapest means of controlling riverine species of tsetse flies, the vectors of the trypanosomes that cause sleeping sickness in humans. Models of the efficacy of these targets deployed near rivers are potentially useful in planning control campaigns and highlighting the principals involved. Methods and principal findingsWe produced a simple non-seasonal model of the births, deaths, mobility and aging of tsetse, and we programmed it to simulate the impact of seven years of target use against the tsetse, Glossina fuscipes fuscipes, in the riverine habitats of NW Uganda. The models outputs matched reality in showing that: (i) good control can be achieved despite the degradation of targets, (ii) local elimination of tsetse is impossible if invasion sources are not tackled, and (iii) with invasion and target degradation it is difficult to detect any effect of control on the age structure of the tsetse population. ConclusionsDespite its simplifications, the model can assist planning and teaching, but allowance should be made for any complications due to seasonality and management challenges associated with greater scale. Author summaryWe produced a simple model of the population dynamics of the riverine tsetse fly, Glossina fuscipes fuscipes, to simulate the field results of controlling this insect for seven years using Tiny Targets, i.e., artificial insecticide-treated baits, in NW Uganda. The model is potentially useful in planning tsetse control and illustrating the principles involved. Thus, it confirmed that targets can give good control, even if the targets degrade so much that they need replacement after six months. It showed that reinvasion can limit severely the efficacy of control campaigns and mask the changes in the age structure of the population. We stress the need to consider the possibility of seasonal complications and problems of managing large scale operations.

ecology↗

bioRxiv2022

Identification of the area sampled by traps: a modelling study with tsetse

BackgroundSampling with traps provides the most common means of investigating the abundance, composition and condition of tsetse populations. It is thus important to know the size of the area from which the samples originate, but that topic is poorly understood. Methods and principal findingsThe topic was clarified with the aid of a simple deterministic model of the mobility, births and deaths of tsetse. The model assessed how the sampled area changed according to variations in the numbers, arrangement and catching efficiency of traps deployed for different periods in a large block of homogeneous habitat subject to different levels of fly mortality. The greatest impacts on the size of the sampled area are produced by the flies mean daily step length and the duration of trapping. There is little effect of trap type. The daily death rate of adult flies is unimportant unless tsetse control measures increase the mortality several times above the low natural rates. Formulae are suggested for predicting the probability that any given captured fly will have originated from various areas around the trap. ConclusionsFormulae for predicting the probability that any given captured fly originated from various areas around the trap are produced. Using a mean daily step length (d) of 395m, typical of a savannah species of tsetse, then any fly caught by a single trap in a 5-day trapping period could be regarded, with roughly 95% confidence, as originating from within a distance of 1.3km of the trap, that is from an area of 5.5km2. Authors SummaryWe produced a simple, deterministic model to highlight important principles in the neglected matter of the probability that any trapped tsetse will have originated from various sizes of area around the trapping site. The modelling was kept simple by envisaging the use of just one trap, or a group of only five traps, evenly spaced inside a circular area within a large block of homogeneous habitat and operated for no more than 30 days. In deriving formulae for the sampled area, we found it appropriate to consider only the flies mean daily step length, and the number of days duration of trapping. The type of trap employed was unimportant. The daily mortality of the adult tsetse population had little effect unless the death rates imposed by control measures were several times greater than the natural rates.

ecology↗

bioRxiv2022

Improved models for the relationship between age and the probability of trypanosome infection in female tsetse, Glossina pallidipes Austen

Between 1990 and 1999, at Rekomitjie Research Station, Zambezi Valley, Zimbabwe, 29,360 female G. pallidipes were dissected to determine their ovarian category and trypanosome infection status. Overall prevalences were 3.45% and 2.66% for T. vivax and T. congolense, respectively, declining during each year as temperatures increased from July - December. Fits to age-prevalence data using Susceptible-Exposed-Infective (SEI) and SI compartmental models were statistically better than those obtained using a published catalytic model, which made the unrealistic assumption that no female tsetse survived more than seven ovulations. The improved models require knowledge of fly mortality, estimated separately from ovarian age distributions. Infection rates were not significantly higher for T. vivax than for T. congolense. For T. congolense in field-sampled female G. pallidipes, we found no statistical support for a model where the force of infection was higher at the first feed than subsequently. The long survival of adult female tsetse, combined with feeding at intervals [≤] 3 days, ensures that post-teneral feeds, rather than the first feed, play the dominant role in the epidemiology of T. congolense infections in G. pallidipes. This is supported by estimates that only about 3% of wild hosts at Rekomitjie were harbouring sufficient T. congolense to ensure that tsetse feeding off them take an infected meal, so that the probability of ingesting an infected meal is low at every meal.

zoology↗

bioRxiv2022

Improved estimates of abortion rates in tsetse (Glossina spp)

Abortion rates were assessed for 170, 846 tsetse (154,228 G. pallidipes and 19,618 G. m. morsitans) sampled in Zimbabwe in 1988 - 1999. Abortions were diagnosed for flies where the uterus was empty and the largest oocyte awaiting ovulation was <82% of the expected mature length. Of tsetse caught in odour-baited traps, 0.64% (95% ci: 0.59 - 0.69) of G. pallidipes and 1.00% (0.76-1.29) of G. m. morsitans were diagnosed as having suffered a recent abortion. For flies from artificial refuges, abortion rates were higher, 2.03% (1.77 - 2.31) and 2.28% (1.85 - 2.79) for the two species, respectively. Abortion rates decreased with increasing wing fray and contrary to laboratory findings, did not increase in the oldest flies; they were highest in the hottest months and years, and increased with decreasing adult wing length. Percentages of tsetse with empty uteri, regardless of abortion status, were significantly higher than the estimated abortion percentages. For tsetse from traps, 4.01% (95% ci: 3.90 - 4.13) of G. pallidipes and 2.52% (2.14-2.95) of G. m. morsitans had empty uteri; for flies from artificial refuges the percentage were 12.69% (12.07 - 13.34) and 14.90% (13.82 -16.02), respectively.

zoology↗

bioRxiv2020

Apparent negative density-dependent dispersal in tsetse (Glossina spp) is an artefact of inappropriate analysis

Analysis of genetic material from field-collected tsetse (Glossina spp) in ten study areas has been used to predict that the distance ({delta}) dispersed per generation increases as effective population densities (De) decrease, displaying negative density dependent dispersal (NDDD). This result is an artefact arising primarily from errors in estimates of S, the area occupied by a subpopulation, and thereby in De, the effective subpopulation density. The fundamental, dangerously misleading, error lies in the assumption that S can be estimated as the area ([S]) regarded as being covered by traps. Errors in the estimates of{delta} are magnified because variation in estimates of S is greater than for all other variables measured, and accounts for the greatest proportion of variation in{delta} . The errors result in anomalously high correlations between{delta} and S, and the appearance of NDDD, with a slope of -0.5 for the regressions of log({delta}) on log(e), even in simulations where dispersal has been set as density independent. A complementary mathematical analysis confirms these findings. Improved error estimates for the crucial parameter b, the rate of increase in genetic distance with increasing geographic separation, suggest that three of the study areas should have been excluded because b is not significantly greater than zero. Errors in census population estimates result from a fundamental misunderstanding of the relationship between trap placement and expected tsetse catch. These errors are exacerbated through failure to adjust for variations in trapping intensity, trap performance, and in capture probabilities between geographical situations and between tsetse species. Claims of support in the literature for NDDD are spurious. There is no suggested explanation for how NDDD might have evolved. We reject the NDDD hypothesis and caution that the idea should not be allowed to influence policy on tsetse and trypanosomiasis control. Author summaryGenetic analysis of field-sampled tsetse (Glossina spp) has been used to suggest that, as tsetse population densities decrease, rates of dispersal increase - displaying negative density dependent dispersal (NDDD). It is further suggested that NDDD might apply to all tsetse species and that, consequently, tsetse control operations might unleash enhanced invasion of areas cleared of tsetse, prejudicing the long-term success of control campaigns. We demonstrate that NDDD in tsetse is an artefact consequent on multiple errors of analysis and interpretation. The most serious of these errors stems from a fundamental misunderstanding of the way in which traps sample tsetse, resulting in huge errors in estimates of the areas sampled by the traps, and occupied by the subpopulations being sampled. Errors in census population estimates are made worse through failure to adjust for variations in trapping intensity, trap performance, and in capture probabilities between geographical situations, and between tsetse species. The errors result in the appearance of NDDD, even in modelling situations where rates of dispersal are expressly assumed independent of population density. We reject the NDDD hypothesis and caution that the idea should not be allowed to influence policy on tsetse and trypanosomiasis control.

zoology↗

bioRxiv2019

The weakest link: uncertainty and sensitivity analysis of extinction probability estimates for tsetse (Glossina spp) populations

BackgroundA relatively simple life history allows us to derive an expression for the extinction probability of populations of tsetse, vectors of African sleeping sickness. We present the uncertainty and sensitivity analysis of extinction probability for tsetse population, to offer key insights into parameters in the control/eradication of tsetse populations. MethodsWe represent tsetse population growth as a branching process, and derive closed form estimates of population extinction from that model. Statistical and mathematical techniques are used to analyse the uncertainties in estimating extinction probability, and the sensitivity of the extinction probability to changes in input parameters representing the natural life history and vital dynamics of tsetse populations. ResultsFor fixed values of input parameters, the sensitivity of extinction probability depends on the baseline parameter values. For example, extinction probability is more sensitive to the probability that a female is inseminated by a fertile male when daily pupal mortality is low, whereas the extinction probability is more sensitive to daily mortality rate for adult females when daily pupal mortality, and extinction probabilities, are high. Global uncertainty and sensitivity analysis showed that daily mortality rate for adult females has the highest impact on the extinction probability. ConclusionsThe strong correlation between extinction probability and daily female adult mortality gives a strong argument that control techniques to increase daily female adult mortality may be the single most effective means of ensuring eradication of tsetse population. Author summaryTsetse flies (Glossina spp) are vectors of Trypanosomiasis, a deadly disease commonly called sleeping sickness in humans and nagana in livestock. The relatively simple life history of tsetse enabled us to model its population growth as a stochastic branching process. We derived a closed-form expression for the probability that a population of tsetse goes extinct, as a function of death, birth, development and insemination rates in female tsetse. We analyzed the sensitivity of the extinction probability to the different input parameters, in a bid to identify parameters with the highest impact on extinction probability. This information can, potentially, inform policy direction for tsetse control/elimination. In all the scenarios we considered, the daily mortality rate for adult females has the greatest impact on the magnitude of extinction probability. Our findings suggest that the mortality rate in the adult females is the weakest link in tsetse life history, and this fact should be exploited in achieving tsetse population control, or even elimination.

zoology↗

bioRxiv2019

Extinction probabilities, times to extinction, basic reproduction number and growth rates for tsetse (Glossina spp) populations as a function of temperature

Increases in temperature over recent decades have led to a significant reduction in the populations of tsetse flies (Glossina spp) in parts of the Zambezi Valley of Zimbabwe. If this is true for other parts of Africa, populations of tsetse may actually be going extinct in some parts of the continent. Extinction probabilities for tsetse populations have not so far been estimated as a function of temperature. We develop a time-homogeneous branching process model for situations where tsetse flies live at different levels of fixed temperatures. We derive a probability distribution pk(T) for the number of female offspring an adult female tsetse is expected to produce in her lifetime, as a function of the fixed temperature at which she is living. We show that pk(T) can be expressed as a geometric series: its generating function is therefore a fractional linear type. We obtain expressions for the extinction probability, expected number of female offspring per female tsetse, and time to extinction. No tsetse population can escape extinction if subjected, for extended periods, to temperatures outside the range 16 {degrees}C - 32{degrees}C. Extinction probability increases more rapidly as temperatures approach and exceed the upper and lower limits. If the number of females is large enough, the population can still survive even at high temperatures (28{degrees}C - 31{degrees}C). Small decreases or increases in constant temperature in the neighbourhoods of 16{degrees}C and 31{degrees}C, respectively, can drive tsetse populations to extinction. Further study is needed to estimate extinction probabilities for tsetse populations in field situations where temperatures vary continuously.\n\nAuthor summaryTsetse flies (Glossina spp) are the vectors of the African sleeping sickness. We derived an expression for the extinction probability, and mean time to extinction, of closed populations of the flies experiencing different levels of fixed temperatures. Temperatures play a key role in tsetse population dynamics: no tsetse populations can escape extinction at constant temperatures < 16{degrees}C > 32{degrees}C. The effect of temperature is more severe if tsetse populations are already depleted. Increasingly high temperatures due to climate change may alter the distribution of tsetse populations in Africa. The continent may witness local extinctions of tsetse populations in some places, and appearances in places hitherto too cold for them.

zoology↗

bioRxiv2018

Improved estimates for extinction probabilities and times to extinction for populations of tsetse (Glossina spp)

A published study used a stochastic branching process to derive equations for the mean and variance of the probability of, and time to, extinction in population of tsetse flies (Glossina spp) as a function of adult and pupal mortality, and the probabilities that a female is inseminated by a fertile male. The original derivation was partially heuristic and provided no proofs for inductive results. We provide these proofs, together with a more compact way of reaching the same results. We also show that, while the published equations hold good for the case where tsetse produce male and female offspring in equal proportion, a different solution is required for the more general case where the probability ({beta}) that an offspring is female lies anywhere in the interval (0, 1). We confirm previous results obtained for the special case where {beta} = 0.5 and show that extinction probability is at a minimum for {beta} > 0.5 by an amount that increases with increasing adult female mortality. Sensitivity analysis showed that the extinction probability was affected most by changes in adult female mortality, followed by the rate of production of pupae. Because females only produce a single offspring approximately every 10 days, imposing a death rate of greater then about 3.5% per day will ensure the eradication of any tsetse population. These mortality levels can be achieved for some species using insecticide-treated targets or cattle - providing thereby a simple, effective and cost-effective method of controlling and eradicating tsetse, and also human and animal trypanosomiasis. Our results are of further interest in the modern situation where increases in temperature are seeing the real possibility that tsetse will go extinct in some areas, without the need for intervention, but have an increased chance of surviving in other areas where they were previously unsustainable due to low temperatures.\n\nAuthor summaryWe derive equations for the mean and variance of the probability of, and time to, extinction in population of tsetse flies (Glossina spp), the vectors of trypanosomiasis in sub-Saharan Africa. In so doing we provide the complete proofs for all results, which were not provided in a previously published study. We also generalise the derivation to allow the probability that an offspring is female to lie anywhere in the interval (0, 1). The probability of extinction was most sensitive to changes in adult female mortality. The unusual tsetse life cycle, with very low reproductive rates means that populations can be eradicated as long as adult female mortality is raised to levels greater than about 3.5% per day. Simple bait methods of tsetse control, such as insecticide-treated targets and cattle, can therefore provide simple, affordable and effective means of eradicating tsetse populations. The results are of further interest in the modern situation where increases in temperature are seeing the real possibility that tsetse will go extinct in some areas, but have an increased chance of surviving in others where they were previously unsustainable due to low temperatures.

zoology↗