Seasonal incidence of Karoo paralysis in relation to the infestation density of female Ixodes rubicundus

Tick Diversity and Distribution of Hard (Ixodidae) Cattle Ticks in South Africa

Ticks are amongst the important ectoparasites where livestock are concerned, as they adversely affect the animals through bloodsucking. In tropical and subtropical countries, they transmit pathogens such as babesiosis, theileriosis, ehrlichiosis, and anaplasmosis in cattle, causing a reduction in production rate and significant concomitant economic losses. Ticks affect 80% of the cattle population across the world, with an estimated economic loss of USD 20–30 billion per year. In South Africa, economic losses in the livestock industry caused by ticks and tick-borne diseases are estimated to exceed USD 33 million per year (ZAR 500 million). There are seven major genera of ixodid ticks in Southern Africa (i.e., Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes and Rhipicephalus). The environment in which a tick lives is made up of all the various biological and abiotic factors that are either necessary or unnecessary for its life. The areas where various ticks have been found have been documented in many publications. Using these data, maps of possible species’ habitats can be made. Historical records on tick distribution may be incorrect due to identification mistakes or a change in the tick’s name. All the sources used to generate the maps for this review were unpublished and came from a wide range of sources. To identify tick species and the accompanying microbial ecosystems, researchers are increasingly adopting tick identification methods including 16S and 18S rDNA gene sequencing. Indeed, little is known about the genetic alterations that give important traits, including the predilection for tick hosts, transmission, and acaricide resistance. Opportunities for exploring these changes in tick populations and subpopulations are provided by advancements in omics technologies. The literature on the variety of ixodid ticks, their direct and indirect effects, and control methods in South Africa is compiled in this review.

Data-driven predictions and novel hypotheses about zoonotic tick vectors from the genus Ixodes

Background

With the resurgence of tick-borne diseases such as Lyme disease and the emergence of new tick-borne pathogens such as Powassan virus, understanding what distinguishes vectors from non-vectors, and predicting undiscovered tick vectors is a crucial step towards mitigating disease risk in humans. We aimed to identify intrinsic traits that predict which Ixodes tick species are confirmed or strongly suspected to be vectors of zoonotic pathogens.

Methods

We focused on the well-studied tick genus Ixodes from which many species are known to transmit zoonotic diseases to humans. We apply generalized boosted regression to interrogate over 90 features for over 240 species of Ixodes ticks to learn what intrinsic features distinguish zoonotic vectors from non-vector species. In addition to better understanding the biological underpinnings of tick vectorial capacity, the model generates a per species probability of being a zoonotic vector on the basis of intrinsic biological similarity with known Ixodes vector species.

Results

Our model predicted vector status with over 91% accuracy, and identified 14 Ixodes species with high probabilities (80%) of transmitting infections from animal hosts to humans on the basis of their traits. Distinguishing characteristics of zoonotic tick vectors of Ixodes tick species include several anatomical structures that influence host seeking behavior and blood-feeding efficiency from a greater diversity of host species compared to non-vectors.

Conclusions

Overall, these results suggest that zoonotic tick vectors are most likely to be those species where adult females hold a fecundity advantage by producing more eggs per clutch, which develop into larvae that feed on a greater diversity of host species compared to non-vector species. These larvae develop into nymphs whose anatomy are well suited for more efficient and longer feeding times on soft-bodied hosts compared to non-vectors, leading to larger adult females with greater fecundity. In addition to identifying novel, testable hypotheses about intrinsic features driving vectorial capacity across Ixodes tick species, our model identifies particular Ixodes species with the highest probability of carrying zoonotic diseases, offering specific targets for increased zoonotic investigation and surveillance.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0163-2) contains supplementary material, which is available to authorized users.

Genetic parameters for tick count and udder health in commercial and indigenous ewes in South Africa

The genetics of tick infestation in sheep need study, as host resistance often forms part of integrated pest control programs. Repeated udder health scores, site-specific tick count, mating weight and reproduction records (N=879-1204) were recorded annually from 2010 to 2015 on ewes of the indigenous Namaqua Afrikaner (NA) fat-tailed breed, as well as the commercial Dorper and SA Mutton Merino (SAMM) breeds. Udders were scored subjectively on a 1-5 scale (1 - udder intact and 5 - udder damaged severely) and ticks were counted on three locations. The body sites counted were the head and thoracic limb (HTLTC), udder-pelvic limb (UPLTC) and perineum-breech-tail (PBTTC). These counts were also totaled for a total tick count (TTC). Reproduction traits were number of lambs weaned per ewe lambed and total weight of lamb weaned per ewe lambed. Udder health scores of NA ewes were lower than those of Dorpers, which in turn had lower scores than SAMM ewes. NA ewes had lower values for HTLTC, UPLTC and TTC than the commercial breeds, but higher values for PBTTC than Dorpers. Heritability estimates amounted to 0.26±0.04 for HTLTC, 0.53±0.04 for UPLTC, 0.07±0.06 for PBTTC, 0.44±0.06 for TTC and 0.61±0.03 for udder health score. Animal permanent environment also affected PBTTC (0.14±0.07). Significant genetic correlations were found between the HTLTC and UPLTC (0.47±0.10), UPLTC and udder health score (0.52±0.07), HTLTC and UPLTC (0.24±0.11) as well as UPLTC and PBTTC (-0.44±0.11). Heavier ewes had higher UPLTC (0.38±0.09), TTC (0.33±0.09) and impaired udder health (0.21±0.08). Udder health scores and tick counts at all sites were not related to reproduction traits. The indigenous NA breed outperformed the commercial breeds with lower values for HTLTC, UPLTC, TTC and a better udder health score. Mechanisms contributing to the better performance of the NA breed under pastoral conditions and the scope for selection for tick tolerance within breeds should be studied further.

Status of Dorper sheep as hosts of ectoparasites

Seventeen ixodid tick species or subspecies have been collected from Dorper sheep in South Africa. The diseases transmitted or the conditions caused by these ticks in sheep are discussed. Although Dorper sheep may be infested with the scab mite, Psoroptes ovis, this does not seem to cause production losses. These sheep may, however, serve as a source of mite infestation for woolled sheep. The majority of Dorper sheep are infested with the larvae of the nasal bot fly, Oestrus ovis, they may also be subject to myiasis caused by the larvae of two calliphorid flies. In certain regions of the country Dorper sheep are prone to serious fly worry by female Simulium chutteri. Although Dorper sheep may be infested by various louse species these do not seem to reach large numbers or cause production losses.

Interbreed differences in the efficacy of 1% deltamethrin pour-on to protect small livestock against infestation with Ixodes rubicundus (Acari:Ixodidae)

Synthetic pyrethroid pour-on compounds have been developed for use on cattle but many are now also registered for the control of ectoparasites on small ruminants. The breed of small livestock has been known to influence the efficacy of various pour-ons and we hypothesized that differences in the duration of protection afforded by a synthetic pyrethroid pour-on against ticks on small livestock, would be greatest between hairy and woolly breeds. Angora goats with mohair, Dorper sheep with hairy wool, and Merino sheep with wool were treated with 1% m/v deltamethrin pour-on with an oil base at a dosage rate of 1 ml 5 kg-1 body mass. The duration of effective protection against Ixodes rubicundus, the Karoo paralysis tick, was determined under field conditions. Angora goats proved to be the highest risk group, but were afforded the least protection. Efficacy never reached 100% in this group and within 3 weeks the goats were already at risk of paralysis. Dorper sheep were afforded above 90% protection for 3 weeks and 85% after 4 weeks. In the case of the Merino sheep efficacy of protection remained above 90% up to 7 weeks post-treatment and was still 87% during Week 8. Differences in the amount of sebum and suint in the epidermis of the different breeds may be responsible for differences in the rate of spread and the rate of dilution of the active compound in the epidermis.

Control of Karoo paralysis ticks through vegetation management

Karoo paralysis, caused by feeding Ixodes rubicundus females, is a major disease of small stock in South Africa. Control methods currently practised are almost exclusively chemical based. To limit overdependance on chemicals, vegetation management was investigated as a possible method for control, to be incorporated in an integrated tick management system. Laboratory and field experiments were conducted to determine, firstly, the extent of vertical migration and survival of ticks on long and short copper rods which simulated grasses as questing substrates; secondly, the infestation burdens of sheep exposed to similar tick challenges in pens with long and short grass; and thirdly, the effect of trimming the lower crown line of wild olive trees, simulating the browsing effect of goats, on tick density in the immediate environment of the trees. When ticks were exposed to optimal ( > 45 cm) and sub-optimal ( < 10 cm) length rods on which to quest, the extent of vertical migration over extended periods of time (up to 87 days) was significantly higher (P < 0.001) for the ticks exposed to long rods. Also, almost 3 times as many ticks exposed to long rods survived compared to those exposed to short rods. Sheep exposed to long grass were infested by twice as many ticks compared to those exposed to short grass. Tick density at modified wild olive trees (Olea europaea africana) (0.027 ticks/m2) differed significantly (P < 0.05) from that at control trees (0.088 ticks/m2). It is recommended that coarse grazers such as cattle and horses should be used to graze down long grasses before sheep are introduced into camps known to be infested with I. rubicundus. Similarly, domestic goats can, through their browsing effect on shrubs and trees, modify the vegetation and as such play an important role in an integrated tick management system.

An evaluation of strategic and threshold control measures against the Karoo paralysis tick, Ixodes rubicundus (Acari: Ixodidae) in South Africa

Paralysis caused by feeding female Ixodes rubicundus ticks is a major problem in large areas of South Africa. As the life cycle of the tick extends over a period of 2 years, it was hypothesized that strategic treatment of sheep with an acaricide over a 2 year period, timed to kill most engorging females, should markedly lower the biotic potential of the tick. Two flocks of sheep grazing in separate paddocks known to be infested with I. rubicundus were treated either strategically or on a threshold basis (i.e. only when tick challenge exceeded a predetermined critical level in terms of paralysis) for a 2 year period. The tick burdens of untreated control sheep running with the two flocks were monitored over a 4 year period and their seasonal dynamics determined. The times at which peak infestations occurred were similar for both flocks of sheep, but significant differences in mean tick burdens between the two flocks were recorded. Tick numbers on sheep in the strategically treated flock did not decrease during the third and fourth years of the trial as was expected. Possible reasons for this were low stocking densities, especially during times of peak abundance of adults and the presence of wild hosts which maintained tick populations.

Improvement of field sampling methods for adult Karoo paralysis ticks, Ixodes rubicundus (Acari: Ixodidae), through addition of host odour

The responsiveness of Ixodes rubicundus ticks on questing substrates and the success of their attachment to non-living substrates were investigated. The purpose of this study was to relate responsiveness to conditions of temperature and humidity, to compare the efficacy of flagging and dragging methods to estimate the size of populations of adult I. rubicundus in the field, and to determine the possible influence of host odour on the efficacy of these methods. Responsiveness was tested under varying conditions of temperature and humidity under field conditions, and the same ticks were used to determine the duration of attachment to a flannel cloth, either impregnated with host odour from sheep wool or without it. Flagging and dragging methods were compared under laboratory conditions with cloths either treated with host odour or not. Within the range of ambient temperatures recorded during this study (7-25 degrees C) most of the ticks (86%) were responsive. No obvious relationship between temperature, relative humidity and responsiveness of ticks was evident. Ticks remained on average 32.9 seconds (n = 64) on cloth treated with host odour compared to 9.1 seconds (n = 54) on untreated cloth. Flagging was 1.5-1.7 times as effective as dragging and treatment of the cloths with host odour increased the efficacy 2.4 (dragging) to 2.8 (flagging) times. The reactions of ticks to external stimuli should, therefore, be taken into account to maximize field sampling success.

The effect of feeding immature Karoo paralysis ticks Ixodes rubicundus (Acari: Ixodidae) on the metabolic rate of the rock elephant shrew

The effect of feeding immature Karoo paralysis ticks (Ixodes rubicundus) on the resting metabolic rate (RMR) of their principal natural host, the rock elephant shrew (Elephantulus myurus), was investigated under laboratory conditions. The elephant shrews were artificially infested with numbers of ticks simulating natural burdens. The RMR of the elephant shrews was quantified by measuring the oxygen consumption in an open through-flow system. The RMR of hosts infested only with larvae did not differ from that of the control group. Those infested with nymphs, or nymphs and larvae, had significantly lower RMR's compared to the control animals. There were no signs of paralysis in any of the infested hosts.

Vertical migration of adult Ixodes rubicundus, the Karoo paralysis tick (Acari: Ixodidae)

Mortality due to Karoo paralysis amongst livestock is of considerable economic importance in South Africa. Indications are that the extent of vertical migration of adult ticks, to reach questing positions on grasses, is influenced by sudden climatic changes. Such influences as well as selection of questing substrates were investigated under controlled conditions. Vertical migration of adult Ixodes rubicundus on copper rods was determined in a square metal arena within a closed glass container in a room in which temperature and light could be controlled. Significantly more female and male ticks migrated vertically on 1.5 mm diameter rods than on 4.0 mm rods and on 450 mm long rods than on 100 mm rods. More ticks of both sexes migrated vertically at RH > 80% than at RH < 50% and at 12 degrees C than at 21 degrees C. More female than male and more older than younger ticks migrated vertically in all experiments. The results of this study contribute towards an understanding of the epidemiology of Karoo paralysis.

The appetence behaviour of two South African paralysis-inducing ixodid ticks

The appetence responses of the ixodid ticks Ixodes rubicundus and Rhipicephalus punctatus to various stimuli were quantified under laboratory conditions. A greater percentage of I. rubicundus generally responded to the different stimuli at lower than at higher temperatures while the converse was usually true for R. punctatus. Radiation heat and shadowing evoked a response in the greatest percentage of I. rubicundus, whereas CO2 and host odour were responsible for a similar reaction in R. punctatus. A single perturbation of the substrate caused more than 50% of I. rubicundus to respond while constant perturbation resulted in a progressive decrease in the proportion of ticks reacting. Significantly more R. punctatus than I. rubicundus were attracted to a CO2 trap and the R. punctatus were also attracted to the trap over a greater distance. Significantly more older than younger ticks of both species responded to the various stimuli.

Seasonal occurrence of Karoo paralysis in Angora goats in relation to the infestation density of female Ixodes rubicundus

The seasonal occurrence of Karoo paralysis in relation to the infestation density of female Ixodes rubicundus was investigated in an enzootic area of the southwestern Orange Free State, South Africa. Young and adult Angora goats were exposed to infestation during two paralysis tick seasons. Only the young goats became paralysed and their infestation densities (female ticks kg-1 host body weight) were significantly higher than those of healthy animals. The infestation densities on paralysed goats varied between 0.64 and 3.56 ticks kg-1. During the 1988 season, 30% of the naive young goats became paralysed compared with 67% the following season. Twenty-nine percent of previously paralysed goats were paralysed again during the same season. Paralysis occurred during periods of peak tick activity (April-June).

Sites of attachment and intraspecific infestation densities of the brown paralysis tick (Rhipicephalus punctatus) on Angora goats

Significant differences in the distribution of brown paralysis ticks on various age classes of Angora goats were recorded. In kids, most (greater than 98%) of the ticks attached to the head and ears, whereas in older groups, in addition to the ears, a high proportion (greater than 20%) of ticks also attached to the ventral side of the neck. There were significant differences in the mean infestation densities of both male and female R. punctatus in kids and older animals. These differences were, however, only significant for the first two sample dates involving kids, and are probably related to behavioural attributes of the kids which enhance tick/host contact. Newly born Angora goat kids are considered a high-risk group with regard to paralysis caused by the brown paralysis tick. Methods of avoiding mortality amongst kids are suggested.

The dispersion of the Karoo paralysis tick, Ixodes rubicundus, within a naturally infested population of sheep in South Africa

The ratio of variance to mean (s2/means) of numbers of Ixodes rubicundus (males and females) per sheep was used as a measure of the dispersion pattern of this tick within a naturally infested population of sheep. Sheep were divided into two groups, one with previous contact with I. rubicundus and the other with no previous contact. Samples of 10 sheep from each group were searched for ticks on a weekly basis for 19 weeks during the winter of 1986. In each group, both male and female ticks had an s2/means ratio greater than 1 for the majority of observation periods indicating an aggregated dispersion pattern. The degree of aggregation was greater with increasing density of the ticks. The implications of these results are discussed both in relation to sampling procedures and potential control strategies.

The effect of the pattern of attachment weights of female Ixodes rubicundus on Karoo paralysis in a naturally infested flock of sheep

The weights of feeding female Ixodes rubicundus attaching to sheep was studied in a natural focus of Karoo paralysis. The studies showed fewer females in the middle weight ranges and more in the heavier and lighter categories than would be expected if the distribution was normal. Weights were also strongly skewed towards the lighter females between 0 and 400 mg. No differences in the pattern of attachment weights occurred over the four experimental groups used, suggesting that previous contact of sheep with I. rubicundus had no effect on the ability of the tick to engorge.