R. M. Newson

Host resistance in cattle tick control

Cattle ticks are an important constraint on the livestock industry, particularly in tropical and subtropical areas, mainly because of the diseases they transmit and the costs of control. Conventional control is by means of acaricides; although there are still serious drawbacks, these can be minimized by a strategic approach. In this review Julio de Castro and Robin Newson look at alternatives. One is to make use of the hosts innate abilities. In the simplest situation, where no control measures are applied, the animals develop their own protective immunity against ticks and tick-borne diseases (TBDs). Alternatively, breeds of cattle with naturally high resistance to ticks can be used, enhanced by selection. Methods of immunizing cattle against ticks are also under development. Ideally, ticks should be managed at an economically acceptable level by a combination of techniques, supported where necessary by vaccination against TBDs.

Predators, parasitoids and pathogens of ticks: A review

Research conducted on natural enemies of ticks, namely predators, parasitoids and pathogens, is reviewed. Predators include ants, birds, rodents, lizards, shrews, spiders and domestic chickens. All the known parasitoids are hymenopteran wasps belonging to the genera Ixodiphagus and Hunterellus. The work so far done on bacteria, fungi, viruses and rickettsiae which are pathogenic to ticks is reviewed, both with regard to experimental infections and natural infections. The possibilities of using natural enemies as biological control agents of ticks are discussed.

Predation of free-living engorged female Rhipicephalus appendiculatus.

In experiments done over a period of 1 1/2 years using engorged femaleRhipicephalus appendiculatus tethered in a grass plot, 42% predation was observed in long grass (40–60 cm), and 36% in short grass (6–10 cm). Deaths due to environmental factors were 4.8% and 6.8% in long and short grass, respectively.Six groups of animals were confirmed to be predators of the ticks, namely: ants, spiders, rodents, birds, lizards and shrews.The implications of these results in making tick population models, and the possibility of using predators in integrated tick-control packages are discussed.

Resistance in cattle againstRhipicephalus appendiculatus with an assessment of cross-resistance toR. pulchellus (Acari: Ixodidae)

A comparison of the yield of engorged nymphs ofRhipicephalus appendiculatus andR. pulchellus was carried out between cattle highly resistant toR. appendiculatus but susceptible toR. pulchellus, and cattle and rabbits susceptible to both species.Resistant cattle yielded fewer and smallerR. appendiculatus nymphs than either susceptible cattle or rabbits. More nymphs of both tick species dropped from susceptible cattle but no differences were detected in the mean weight of engorgedR. pulchellus nymphs fed onR. appendiculatus-resistant and susceptible cattle, although resistant animals yielded more nymphs ofR. pulchellus than ofR. appendiculatus.An acute inflammatory reaction, accompanied by intense pruritus and grooming, was observed in the ears of the resistant cattle infested withR. appendiculatus, but no reaction was detected in the opposite ears whereR. pulchellus was feeding.The low cross-resistance between these two closely related tick species suggests that different antigens may be involved in the development of resistance in cattle to different tick species.

Feeding Performance of Amblyomma variegatum (Acarina: Ixodidae) Fed Repeatedly on Rabbits

Rabbits infested with different stages ofAmblyomma variegatum Fabricius, 1794 became resistant to subsequent infestations by the same tick. Resistance was manifested by a reduction of 73.6% in the mean engorged weight of female ticks by the 3rd infestation. By the 5th infestation, only 70% of the nymphs engorged and their mean engorged weight was 57.7% of that of nymphs fed on tick-naive control rabbits. In the case of larvae, there was a 22.8% reduction in their mean engorged weight by the 6th infestation. Based on increases in body length while feeding on tick-naive rabbits, the nymphs could be divided into four feeding categories, i.e. Nl (unfed), N2, N3 and N4 (increasing states of engorgement). While 92–96% of ticks which dropped from susceptible rabbits fitted into feeding category N4, only 28% of those that dropped from tick-resistant rabbits fitted into that category. The remainder of the ticks fitted into categories N3 (60%) and N2 (12%). The majority of those ticks with reduced weight developed into adults which had an atypical scutal ornamentation pattern, in that an additional pair of lateral spots was regularly observed on the males. The females of such ticks deposited small numbers of eggs from which no larvae hatched.

Development of resistance in calves to nymphs ofRhipicephalus appendiculatus (Acarina: Ixodidae) during test feeds

A test of host resistance to infestation by the tickRhipicephalus appendiculatus was repeated 5 times, at weekly intervals, on 2 initially susceptible calves. One hundred nymphs (NN) were used each time. Pairs of tick-susceptible rabbits served as controls. The engorged NN were weighed, then allowed to moult individually, and the sex of each was noted after emergence of the adult. During the course of the experiment there was a significant reduction in mean engorged weight (especially in the pre-male NN) and also in the length of the feeding period, and a downward trend in the percentage of NN which engorged. There was a significant increase in the percentage of engorged NN that moulted. Pre-male NN were significantly heavier than pre-female NN after feeding on susceptible hosts, but though mean feeding times were the same, they took 1–2 days longer to moult than pre-females at 28°C. It was concluded that the test itself could induce resistance if applied to fully susceptible calves.

Drop-off patterns for engorged adult females, nymphs and larvae of Rhipicephalus appendiculatus

A drop off rhythm of engorged Rhipicephalus appendiculatus Neumann was demonstrated for females and engorged nymphs, but not for larvae. Seventy per cent of 1279 engorged R. appendiculatus females dropped off the animals between 0800 and 1000 hr, while 66% of 2736 engorged nymphs dropped between 1600 and 1800 hr.Females which were put on the animals at night (2000 hr), started to drop-off 24 hr later, than those put on in the day (0900 hr). Ticks which were fed on tick-sensitized animals also started to drop off 24 hr later, than those on tick-naive animals.The implications of these results on tick control management are discussed.RésuméRythme de détachement de femelles adultes, k]de nymphes et larves gorgées de Rhipicephalus appendiculatus. Un rythme de détachement de leurs hôtes de femelles et nymphes Rhipicephalus appendiculatus Neumann gorgées a été déjà décrit mais rien n’a été fait en ce qui concerne les larves. Soixante dix pourcent des 1279 femelles gorgées se détachent des animaux entre 0800 heures et 1000 heures tandis que 66% des 2376 nymphes gorgées tombent entre 1600 heurs et 1800 heures.Les femelles présentées aux animaux la nuit (20 heures) commencent á se détacher 24 heures après celles qui ont été mises sur les animaux le jour (9 heures). Les tiques nourries sur des animaux déjà sensibilisés commencent a se détacher 24 heures aprés celles nourries sur des animaux n’ayant jamais eu contact avec des tiques. Les implications que peuvent avoir ces résultats en matière de contrôle de tiques ont été discutées.

Diurnal and seasonal activity of unfed adults of Rhipicephalus appendiculatus (Acarina: Ixodidae) in relation to some intrinsic and extrinsic factors—III: Daily changes in water content

Groups of Rhipicephalus appendiculatus adults were prepared at three levels of hydration (100, 85 and 65%) and placed in small field enclosures. Some of the ticks were broadcast over the vegetation, while others were confined in nylon gauze bags and placed at soil level. Weight changes in the bagged ticks were monitored for 3 consecutive days by twice-daily weighing. Observations were made in the hot dry season, rainy season and the cool dry season. During the hot dry season there was a progressive loss in weight over the observation period, as during the night the ticks could not fully replace the water lost by day. In the rainy season there was an initial rapid uptake of water at night by the dehydrated groups and any weight losses during the day were easily replaced at night. In the cool dry season water losses were slight, and though ticks regained water at night, neither of the dehydrated groups became 100% hydrated during the period of observation.

Diurnal and seasonal activity of unfed adult Rhipicephalus appendiculatus (Acarina: Ixodidae) in relation to some intrinsic and extrinsic factors—II. Factors affecting vertical distribution of ticks in the habitat

When marked adults of the tick Rhipicephalus appendiculatus were released into 25 ° 25 cm plots in the field, their vertical distribution in the habitat was assessed after recovery, by comparing the number of ticks on the vegetation with those on the soil surface. In the hot dry season, 73% of the ticks were recovered. Of these a third were on the vegetation and two thirds at the soil surface. Out of the 67% recovered in the wet season, only 15% were found at the soil surface and the rest on the vegetation. In the cool dry season, 75% were recovered with roughly equal proportions on the vegetation and the soil surface. This suggests that only a third of the ticks would be available to be picked up by the hosts during the hot dry season. Two thirds would be available during the wet season and only half were available during the cool dry season.