Regulation of seasonal occurrence in the tick Rhipicephalus appendiculatus Neumann, 1901

Characterising African tick communities at a wild-domestic interface using repeated sampling protocols and models

The sharing of habitat by wild and domestic animals may result in pathogen transmission, notably via ectoparasite vectors such as ticks. Interfaces between protected and communal lands constitute sharp transitions between areas occupied by host communities that are extremely contrasted in terms of composition, diversity and density. Empirical characterizations of tick communities and of their vertebrate hosts are strongly relevant for understanding the mechanisms leading to disease transmission between wild and domestic animals. In the present study we aimed at depicting the pattern of spatial variation in the density of immature ticks at such an interface located in Zimbabwe. At the end of the 2011 rainy season, we applied a hierarchical repeated protocol to collect ticks. We used the drag-sampling method in the vegetation surrounding water pans used by ungulates in 3 distinct landscape compartments (i.e. national park, mixed compartment and communal lands) characterized by a differential use by wild and domestic hosts. We combined generalized linear mixed models with site occupancy models to (1) assess tick aggregation levels at different spatial scales, (2) identify and disentangle factors which influence the density and probability of tick detection, and (3) compare robust estimations of tick densities among the landscape compartments. Ticks belonging to the Amblyomma and Riphicephalus genuses were found to be the most abundant. At small scale, ticks were more often detected in the afternoon and were more abundant close to water pans for Amblyomma and Riphicephalus genuses. Riphicephalus spp. density was also higher in grassland and bushland vegetation types as compared to woodland vegetation type. At large scale, for the three detected genuses, density was much higher near water pans located in the communal lands as compared to the national park and mixed compartment. Given that host community's diversity is much lower in the communal areas than in the two other landscape compartments, these results are compatible with a dilution effect but not sufficient to demonstrate this effect without additional studies. Up to date, it is the first utilization of these rigorous sampling and statistical modelling methodologies to estimate tick density in African ecosystem simultaneously at large and small scales.

Identification of hard ticks (Acari: Ixodidae) and seroprevalence to Theileria parva in cattle raised in North Kivu Province, Democratic Republic of Congo

This study aimed to identify tick species and to determine their relationship with the Theileria parva seroprevalence in cattle raised under an extensive farming system in North Kivu Province, Democratic Republic of Congo in two agro-ecological zones namely medium (1,000-1,850 m) and high (>1,850 m) altitude. Among the 3,215 ticks collected on 482 animals, from February to April 2009, Rhipicephalus appendiculatus (64.26 %), the main vector of T. parva, was the most abundant species followed by Rhipicephalus decoloratus (35.49 %) and Amblyomma variegatum (0.25 %). The mean burden of R. appendiculatus tick per infested animal appeared significantly higher at medium (6.5 ± 0.22 ticks) than at high (0.07 ± 0.3 ticks) altitude (P < 0.05). However, an indirect fluorescent antibody test carried out on 450 blood samples revealed a global T. parva seroprevalence of 43 % (95 % CI: 38-47) which was not significantly (P > 0.05) different between medium (48.4 %; 95 % CI: 38-49) and high (41.9 %; 95 % CI: 35-49) altitude. These relatively low seroprevalences suggest that there is a state of endemicity to T. parva infection in the study area. The presence of the tick vector on animals was associated with an increased risk of being seropositive to T. parva infection (odds ratio = 2.04; 95 % CI: 1.8-2.3; P < 0.001). The results suggest the need for a longitudinal study to investigate the seasonal dynamics of tick species and T. parva infection. The rate of tick infection should also be evaluated in order to determine the intensity of T. parva transmission to cattle.

An investigation into the distribution, host-preference and population density of ixodid ticks affecting domestic animals in Bangladesh

To study the distribution, host-preference and population density of ixodid ticks in Bangladesh, an attempt was made to collect adult ticks from various host animals in three distinct topographic zones, viz. flood plains, hills and steppe 'Barind'. Five species of ixodid ticks were recorded, namely, Boophilus microplus (56.3%), Haemaphysalis bispinosa (11.3%), Rhipicephalus sanguineus (14.7%), Hyalomma anatolicum anatolicum (15.0%) and Amblyomma testudinarium (2.8%). The data showed that B. microplus occurred predominantly on cattle (42.4%). The other hosts involved were buffaloes (12.5%), goats (25.5%) and pigs (8.2%). H. bispinosa mostly parasitized goats (31.5%) rather than cattle (12.0%) and buffaloes (10.8%). R. sanguineus was principally a dog tick (27.4%) but also parasitized cattle (10.8%) and goats (6.8%). H. a. anatolicum was restricted to cattle (19.2%) and A. testudinarium was found on both cattle (4.4%) and pigs (2.3%). These results indicate that ixodid ticks are not strictly host-specific except for H. a. anatolicum. The population density of these ticks was significantly (p < 0.01) influenced by the changing of seasons. B. microplus, H. bispinosa and R. sanguineus were by far the most widely distributed species; the distribution of H. a. anatolicum was restricted to the steppe 'Barind tract' and A. testudinarium was found in the hilly regions only.

Survival of unfed Rhipicephalus appendiculatus and Rhipicephalus zambeziensis adults

Two ixodid tick species, Rhipicephalus appendiculatus (Neumann) and Rhipicephalus zambeziensis (Walker et al.) (Acari: Ixodidae), both originating from the southern province of Zambia, were used to study the survival time of adults at a range of different humidities and temperatures. In general, the experiment clearly demonstrates the different survival times of the two species in relation to the climatic conditions tested: survival of R. zambeziensis was better under more extreme conditions of temperature and humidity. These findings offer an explanation for the different distribution ranges of the two tick species. Rhipicephalus appendiculatus is more confined to cooler and wetter conditions, whereas R. zambeziensis is adapted to hotter and drier areas.

Immunisation against Theileria parva in eastern Zambia: influence of maternal antibodies and demonstration of the carrier status

Immunisation of calves by the infection and treatment method (I & T) has been extensively used in the eastern province of Zambia to control East Coast fever (ECF), a protozoan tick-borne disease. This paper presents the results of a field longitudinal study, which included a total of 148 Angoni calves. After immunisation against ECF, they were monitored for a full rainy season, coinciding with the main peak of activity of the vector of Theileria parva, the tick Rhipicephalus appendiculatus. Dysimmunisation (acute reaction generated by I & T immunisation), seroconversion and mortality are among the parameters recorded. The effect of maternal antibodies on these parameters was analysed and also studied in experimental conditions on two calves. Before immunisation, young calves had a higher seroprevalence than older animals (maternal antibodies) but their post-immunisation seroprevalence was lower. There was no evidence that their immunoprotection was weaker but this indicates that the post-immunisation seroconversion is probably not a reliable tool to monitor the efficacy of calf immunisation. The carrier state of cattle after immunisation was investigated in experimental conditions on three bovines whereas in the field, the infection prevalence in the ticks was estimated using the relation between the tick burden and the T. parva contacts with the calves. The ability of larval and nymphal R. appendiculatus ticks to pick-up T. parva from carriers and to transmit it to naïve animals after moulting was assessed. It was found that both instars are able to transmit clinical and lethal ECF but that the prevalence of T. parva infection in nymphs is much lower than in adults, confirming the primary role of adults in the transmission of ECF in endemic conditions. Similar results were obtained from the field whereby the ECF peak corresponds with the peak of adult R. appendiculatus activity. The infection prevalence in the ticks was however much lower in the field than in experimental conditions indicating that an important proportion of them feed on alternative hosts. Old ticks seemed to have lost part of their infectivity.

Assessing infestation risk by vectors. Spatial and temporal distribution of African ticks at the scale of a landscape

Control of major livestock diseases in the tropics, such as theileriosis or trypanosomosis, is still largely based on the control of their vectors. Understanding the distribution of vectors, such as ticks and tsetse flies, is needed in order to improve the efficiency and economical viability of control operations. Technical improvements such as remote sensing and global information systems have allowed valuable improvements for the prediction of large-scale vector distribution (continental to national), but trying to make these predictions at the scale of a landscape is facing other challenges. At this scale, an analysis of host/vector interactions with an evolutionary point of view is useful. A study was undertaken on a mixed game/cattle ranch in Zimbabwe during which we monitored variations in the abundance and spatial distribution of the immature free stages of Rhipicephalus appendiculatus/R. zambeziensis and R. e. evertsi, two major groups of tick species in Southern Africa. We found two contrasting distributions in relation to contact between tick larvae. The ungulate-host R. e. evertsi appeared to be unpredictable, whereas R. appendiculatus/R. zambeziensis were predictable in time and space, but associated with key-resources for ungulates (water and forage resources). The consequences of such distributions are discussed in terms of vector control.

Ticks and tick-borne diseases: a vector-host interaction model for the brown ear tick (Rhipicephalus appendiculatus)

An analytical model is derived for the interaction of the brown ear tick (Rhipicephalus appendiculatus) with its hosts. Such models are rare due to the complexity and lack of information on the entire stages of ticks life cycles. Most models are simulations rather than analytical. The vector is categorized into a discrete number of compartments according to its life cycle. The starting model in this article consists of a system of differential equations with constant coefficients. A general model on a stage structured population with unlimited host density is developed. From the characteristic polynomial of the system a sensitivity analysis for the population parameters is carried out in detail. The model is then improved by incorporating host abundance and availability. This is done on the basis of a demand-driven and ratio-dependent functional response model. The improved model adequately represents the dynamics of a stage-structured vector population under conditions of varying host density. The model allows the qualitative evaluation of several management strategies and is expected to guide future research work.

A study of the diapausing behaviour of Rhipicephalus appendiculatus and R. zambeziensis under quasi-natural conditions in Zambia

The diapausing behaviour of Rhipicephalus appendiculatus was studied under quasi-natural conditions in the Eastern Province of Zambia. Newly-moulted adults of ticks indigenous to the study area entered a behavioural diapause when exposed to daylengths below a critical photoperiod, provisionally a daylength between 11 h 20 min and 11 h 45 min. In the Eastern Province of Zambia the diapause was apparently not terminated by a long-day signal, but by a weakening of the photoperiodic maintenance of the diapause because of increasing age of the ticks. Adults of a reference stock indigenous to Kenya also entered a diapause when exposed to daylengths below the same threshold and maintained this diapause for the same length of time. Adults of a reference stock of Rhipicephalus zambeziensis indigenous to Zimbabwe did not enter a diapause when exposed to the same daylengths. The relevance of the findings is discussed in relation to the distributions of the two species.

The effect of rainfall on tick challenge at Kyle Recreational Park, Zimbabwe

The effect of rainfall pattern on tick challenge was investigated at Kyle Recreational Park, Zimbabwe, from 1991 to 1992 using drag and removal plot methods to sample environmental tick density. The abundance of adults and nymphs of the brown ear-tick Rhipicephalus appendiculatus and larvae of the bont tick Amblyomma hebraeum was positively correlated with monthly rainfall, whereas no relationship with rainfall was revealed for larval R. appendiculatus, adults of the red-legged tick R. evertsi, or larvae of the blue tick Boophilus decoloratus. A comparison between 1991 (490 mm rainfall) and the drought year of 1992 (161 mm) revealed significant differences in the abundance of R. appendiculatus, A. hebraeum, and B. decoloratus. During the wet season, R. appendiculatus adults were 2-3 times more numerous in the environment during the higher rainfall year of 1991. A. hebraeum larval abundance exhibited a similar pattern to that of R. appendiculatus adults, but B. decoloratus larvae were more abundant in the drought year of 1992 during both the wet and dry seasons. Comparable tick abundance data collected at Kyle during the above-average rainfall years of 1975-1977 (mean = 1029 mm) were compared with tick challenge during the below-average rainfall years of 1991-1992 (mean = 326 mm). In grassland sand habitat and all habitats combined R. appendiculatus adults, nymphs, and larvae were much more abundant during the high rainfall years. In contrast, larvae of B. decoloratus were more numerous during the drier years. A hebraeum larvae were also more abundant during the drier years. The strong positive correlation of adult R. appendiculatus abundance with rainfall and the coincidence of increased adult tick challenge with increased rainfall indicate that adult R. appendiculatus tick burden on hosts would be heaviest during the wet season and high rainfall years.

Climate, satellite imagery and the seasonal abundance of the tick Rhipicephalus appendiculatus in southern Africa: a new perspective

Recent predictive models for the distribution of the African tick Rhipicephalus appendiculatus Neumann, based on the computer packages CLIMEX and BIOCLIM and data derived from meteorological satellites, and for the seasonal dynamics of the same tick using the computer simulation models ECFXPERT and T3HOST, all have their limitations. Statistical analysis of the relationships between the seasonal abundance of all three life stages of this tick and climatic and satellite-derived data from five sites in southern Africa, taken from the literature, supports a new perspective that it is the timing of the questing activity of the desiccation-vulnerable larvae that determines the pattern of the tick's seasonal dynamics. The timing of the activity of nymphs and adults is determined by temperature-dependent development rates plus the delaying phenomenon of photoperiod-sensitive diapause, the timing and duration of which have evolved to achieve maximum generation survival by ensuring the occurrence of eggs and larvae during periods of optimal climatic conditions. The most important environmental factor appears to be night-time minimum temperature, determining condensation and saturation deficit and thus the tick's ability to replenish moisture lost during the daytime and so to survive while questing for hosts. It is the larvae whose numbers are correlated most closely with these factors, consistent with earlier experimental results showing larvae to be most susceptible to desiccating conditions. There is a statistical linkage between larval tick numbers and satellite imagery, arising from the correlation between larval numbers and minimum temperature and saturation deficit conditions, and in turn the relationship between these climatic conditions and the subsequent vegetational changes monitored by the satellites. Moisture availability to larvae is likely to be the critical factor throughout the geographical range of R. appendiculatus, but the precise combination of climatic conditions that optimize moisture availability and questing tick survival can be expected to vary geographically. The relationships between ticks, temperatures and satellite data in parts of equatorial Africa have yet to be established. These correlative patterns highlight both the critical life stage and environmental factors when trying to understand temporal, and ultimately spatial, variations in tick abundance.

Theileria parva: influence of vector, parasite and host relationships on the epidemiology of theileriosis in southern Africa

The protozoan parasite Theileria parva, transmitted by the ixodid tick Rhipicephalus appendiculatus, is the cause of East Coast fever (ECF) and the related syndromes of Corridor disease and January disease in cattle of eastern, central and southern Africa. It is likely that buffalo (syncerus caffer) are the natural host of T. parva. In eastern and southern Africa, there exists both buffalo-adapted and cattle-adapted T. parva. Disease caused by buffalo-adapted parasites is called Corridor disease, and that caused by cattle-adapted parasites is termed East Coast fever. In eastern Africa, it has been shown experimentally that buffalo-adapted T. parva can, after serial passage in cattle, become adapted to cattle, in which it can then be maintained and cause ECF. This adaptation has been termed transformation. The transformation of buffalo-adapted T. parva to a cattle-adapted parasite has not been reported in southern Africa, and ECF, eradicated from South Africa, Swaziland and southern Mozambique by 1960, has not reappeared in the subcontinent. This paper discusses the possible reasons for this, and hypothesizes that vector population dynamics and the susceptibility of the vector population to infection with T. parva are among the most important factors which influence the expression of ECF as a disease entity, and the likelihood of transformation occurring. It also considers the possibility that disappearance of ECF from southern Africa resulted from the extinction, as a result of vigorous control measures and unfavourable climatic conditions, of non-diapausing populations of R. appendiculatus that may have been introduced from eastern Africa with cattle imported in 1901.

Retrospective observations on the transmission of East Coast fever in Zimbabwe

Records of 283 outbreaks of East Coast fever in Zimbabwe in the period 1914 to 1946 reveal that transmission of infection occurred throughout the year with peaks in January to March and May to July. The high level of transmission in January to March coincides with the known seasonal occurrence of adult Rhipicephalus appendiculatus. It is suggested that transmission at other times of the year may have been effected by a population of R. appendiculatus which did not exhibit a strictly seasonal life-cycle and has since been eradicated.

Immunization of cattle against Theileria parva bovis and their exposure to natural challenge

Theileria parva bovis isolates were tested for their immunizing capacity under natural field challenge on Willsbridge Farm in the highveld of Zimbabwe. Fifteen susceptible Sussex yearlings were immunized with the Boleni stock and 15 with a mixture of three isolates from the farm, using tick-derived sporozoite stabilates. No chemoprophylaxis was used. A dose of 0.1 ml of stabilate appeared to be safe in preliminary laboratory experiments, but the reactions were severe in the Sussex cattle and one died despite treatment. Twenty-nine immunized animals and 10 controls first experienced a mild infection, starting about 15 days after their arrival at the farm. Ten of the immunized animals and four controls had schizonts in peripheral lymph nodes for variable periods; one third of those had pyrexia. Nymphal Rhipicephalus appendiculatus ticks applied to three of the reacting immunized calves transmitted Theileria taurotragi to two animals and T. parva to a third. A second Theileria infection, due to T. parva bovis, was detected shortly after the first one. Schizonts were detected in seven out of 10 controls. Pyrexia was more severe and prolonged. Two of the controls died of theileriosis. At the same time schizonts were seen in three immune animals and eight of them had short periods of pyrexia. Intercurrent infections with Babesia bigemina, Borrelia theileri and Eperythrozoon were detected and may have contributed to the fever. Tick infestations were low during the exposure. In the second year of exposure, four out of eight new control animals had severe reactions, and one died. None of the immunized animals became ill, but one animal from the first year control group, which had not reacted previously, had clinical theileriosis. It is concluded that immunization provided an effective protection against field challenge.

A climate-based model for the development of the ixodid tick, Rhipicephalus appendiculatus in East Coast fever zones

East Coast fever (Theileria parva infection) is an important parasitic disease of cattle in East and Central Africa. Past experimental studies have provided a great deal of information on the dynamics of the life cycle of the tick vector, Rhipicephalus appendiculatus. This paper describes a mathematical model based on field observations to explain the close relationship between the tick life cycle and climate. The model provides a basis for the future study of different ECF control programmes using computer experiments.