Resistance of restrained Bos taurus dairy bull calves to the bont tick Amblyomma hebraeum (Acarina: Ixodidae)

Three infestations of restrained Jersey and Friesland bull calves with adult Amblyomma hebraeum did not affect the rate of engorgement of female ticks and resulted in an insignificant decline in the mean engorgement weight of female ticks recovered from the Friesland group. Repeated infestations significantly affected the egg-laying capacity of engorged female ticks recovered from both groups of hosts. The Friesland calves manifested an enhanced resistance to the larvae and nymphs of Amblyomma hebraeum resulting in prolonged attachment, and a significant decline in engorgement rate of these instars compared with the Jersey group. Skin reaction tests using crude tick extracts yielded antigen-specific hypersensitivity reactions which were not related to resistance to A. hebraeum. Histological changes at the attachment site associated with tick rejection are discussed in relation to host resistance mechanisms.     

The effect of the brown ear-tick Rhipicephalus appendiculatus on the growth of Sanga and European breed cattle

The effects of larvae, nymphs and adults of the brown ear-tick Rhipicephalus appendiculatus on the growth of Bos indicus X B. taurus (Sanga) steers and on B. taurus steers and heifers was measured by exposing groups, maintained in the same pastures, to different levels of tick challenge. Larvae and nymphs had no significant effect on liveweight gains, but adults had a large effect. Each engorging female tick counted was associated with a loss of 4 g. Their effect on the Sanga cattle was minimised by host resistance which severely limited the number of ticks feeding on them. The B. taurus animals were severely affected and suffered extensive ear damage. Both breeds were subjected to screw-worm fly (Chrysomya bezziana) strike in the lesions caused by heavy tick infestations, but the B. taurus animals were much more prone to attack. The results provide a basis for calculating losses in production caused by this species of tick which is widespread in Africa.     

Studies on naturally acquired immunity to African ticks. II. Observations on cattle exposed to Rhipicephalus appendiculatus under varying periods of repeated infestations.

Groups of Friesian cattle were infested repeatedly with Rhipicephalus appendiculatus at weekly intervals (eight infestations) and at intervals of 3 weeks (seven infestations) and 6 weeks (seven infestations). The engorgement weight of adults, nymphs and larvae decreased significantly initially, from the second infestation, but rose significantly at later infestations. This rise was more dramatic with larval instars than with any other. A new method for quantifying the resistance status of animals was introduced; this method showed that the resistance acquired by cattle during 6-weekly reinfestations was more stable than that acquired by cattle during weekly and 3-weekly reinfestations. The criteria for assessing natural resistance to ticks was reappraised. These are classified into those serving long- and short-term purposes, and those which have direct and indirect effects. Criteria which could serve long- and short-term purposes include a reduced number of engorged ticks, an increased number of dead male and female ticks, a decreased number of ticks which fed above the critical engorgement weight and an increased number which fed within the pre-mating weight and decreased the number of copulations. The criterion of reduced engorgement weight can serve the short-term purpose only. A new manifestation of resistance to ticks, i.e. decreased moulting weights of nymphs and adults, together with the criterion of decreased egg hatchability, were classified as indirect effects because they were manifested after the ticks had dropped off the resistant cattle.     

Parasites of domestic and wild animals in South Africa. XXX. Ectoparasites of kudus in the eastern Transvaal Lowveld and the eastern Cape Province.

Sets of four kudus were shot and examined for arthropod parasites at approximately monthly intervals from April 1981 to March 1983 in the southern part of the Kruger National Park, eastern Transvaal Lowveld. These animals harboured 10 ixodid tick species of which Boophilus decoloratus followed by Amblyomma hebraeum were the most abundant. The seasonal abundances of these ticks and of Amblyomma marmoreum, Rhipicephalus appendiculatus, Rhipicephalus evertsi evertsi and Rhipicephalus zambeziensis were determined. The kudus were also infested with 3 lice and 1 louse fly species, as well as the nymphs of a pentastomid. Sixteen kudus were shot in the Andries Vosloo Kudu Reserve, eastern Cape Province and 9 on an adjacent farm. These animals were infested with 12 tick species. A. hebraeum followed by Rhipicephalus glabroscutatum were the most abundant on kudus in the reserve and R. glabroscutatum followed by Haemaphysalis silacea on the animals on the farm. The seasonal abundances of A. hebraeum, A. marmoreum, H. silacea, R. appendiculatus, R. glabroscutatum and a Rhipicephalus sp. (near R. oculatus) were determined on the kudus in the reserve. The kudus were also infested with 3 lice and 1 louse fly species. Two kudus examined in the Addo Elephant National Park were infested with 6 tick, 1 louse and 1 louse fly species.     

Adult Amblyomma hebraeum burdens and heartwater endemic stability in cattle.

Several conclusions of importance to studies on the epidemiology of heartwater were drawn from an investigation in which the numbers of adult Amblyomma hebraeum ticks, to which a closed herd of Hereford cattle were exposed over a period of 6 1/2 years, were manipulated. With a tick Cowdria ruminantium infection rate of 3-5%, an endemically stable situation was created by dipping the herd only when an average of 10 adult male and female A. hebraeum ticks were counted on 10 animals. When the average was increased to 15 during the calving period, 97% of calves acquired a tick-mediated immunity at the age of 6 months. Because only adult ticks confined to the hindquarters are counted, this procedure is recommended as a feasible and practical guideline to stock owners wishing to determine a dipping programme that would ensure endemic stability. The indirect fluorescent antibody test gave a true reflection of the infection rate through ticks in calves 3-6 months old, but not in older animals that had been re-infected more than once. This is because on one hand antibody may persist for 2 years after withdrawal from tick exposure and on the other the artificial re-infection of cattle with a tick acquired immunity is not always followed by a rise in antibody titres and may even result in seronegativity. Four cows infected and re-infected through ticks, remained immune to challenge for 2 years after withdrawal from tick exposure. Within the confines of one farm 3 isolates of C. ruminantium that differed in pathogenicity and immunogenicity were recovered from ticks. One of these isolates was almost non-pathogenic to cattle.     

Exposure of cattle immunized with different stocks of Theileria parva to buffalo-associated Theileria challenge on two game parks in Zimbabwe.

Eight cattle immunized with cattle-derived Theileria parva Boleni stabilate together with six susceptible controls were released in Dombawera Game Park on the Highveld of Zimbabwe. This coincided with Rhipicephalus appendiculatus nymphal activity. The cattle grazed together with African buffaloes (Syncerus caffer) and were not treated against tick infestation. The nymphal tick infestation was high, and seven of the eight immunized cattle and three of the controls had severe and fatal reactions. Subsequently, two stocks of Theileria parva to be tested for their immunizing abilities were prepared-one from adult ticks which were fed as nymphs on one of the sick control animals (Dom 268) and the other from adult ticks collected from pastures grazed by buffaloes (Bv-1). Two groups of cattle were immunized with either the Dom 268-derived strain (eight animals) or the Bv-1-derived strain (four animals). These together with three non-immunized controls, were released in Bally Vaughaun Game Park in the Highveld, where buffaloes are present, during the season of nymphal tick activity. A third group of five cattle, immunized with stabilate Bv-1, and three non-immunized controls were released at the same site during the season of adult tick activity. The nymphal and adult tick infestations of the cattle were large and more than 2000 nymphs and 1000 adult ticks were counted per animal. Cattle were treated with a pyrethroid pour-on preparation to control the tick infestation and screw-worm strike. The immunized cattle in the three groups survived the theileriosis challenge for a period of 18 months, but the non-immunized control cattle suffered a severe and fatal theileriosis 19-23 days after being placed on the pasture.     

Efficacy of ivermectin delivered from an intraruminal sustained-release bolus against natural infestations of five African tick species on cattle

The efficacy of ivermectin delivered by an orally administered prototype sustained-release bolus over approximately 90 days was evaluated against natural infestations of five African tick species. Twenty cattle, allocated by restricted randomization based on counts of standard Boophilus decoloratus, were allocated to two groups and were either given an ivermectin bolus or designated as non-medicated controls. All cattle grazed a single pasture of native grasses for 20-40 days before treatment and until trial termination. Starting on Days 27, 40, 68 and 82 after bolus administration, four replicates were confined to individual tick-collection stanchions for 4 to 5-day periods. Ticks recovered from these cattle were counted by species, sex, and stage and degree of repletion; engorged females were weighed and incubated to determine the number which oviposited. For the other replicates, half-body counts of adult ticks (classified by species, sex and degree of repletion by females) were made at 1- and 2-week intervals through Day 90. Among replicates confined to stanchions periodically, fewer (P less than 0.05) engorged adult female B. decoloratus, Hyalomma spp., Rhipicephalus appendiculatus and Rhipicephalus evertsi evertsi were collected from bolus-treated cattle than from controls. Numbers of engorged adult female Amblyomma hebraeum were reduced, but differences were not statistically significant (P greater than 0.10). Among cattle maintained continuously on pasture, tick numbers were reduced on the ivermectin-treated groups. A significant (P less than 0.05) treatment by linear time effect was seen for all adult ticks counted except R. appendiculatus. A significant (P less than 0.05) treatment by quadratic time effect was seen for A. hebraeum, B. decoloratus and R. evertsi evertsi, and overall treatment differences were significantly different (P less than 0.05) for these species. The differences tended to increase with time. Except for Boophilus, reductions in tick numbers on treated animals relative to controls were not readily apparent. There were no adverse reactions attributable to ivermectin treatment or the presence of the bolus. Each treated animal retained its bolus throughout the trial, based on metal detection.     

Life cycle of the tick Amblyomma parvum Aragão, 1908 (Acari: Ixodidae) and suitability of domestic hosts under laboratory conditions

Amblyomma parvum is a widespread Neotropical tick found on several domestic animals and is known to harbor a Rickettsia species of yet unknown pathogenicity. However its life cycle on, and suitability of, several of these hosts has not been described. In this work the biology of A. parvum is presented when fed on seven domestic hosts (chicken, dog, rabbit, horse, guinea pig, cattle and goat). The complete life cycle of the tick varied from 96.8 to 102 days. Highest engorgement weight of larvae was from ticks fed on horses and that of nymphs from guinea pigs. Highest larval yield was from guinea pigs and that of nymphs from horses. Engorged female and egg mass weights, yield and conversion of female weight to eggs rates were the highest in dog ticks and the lowest in goat ticks. The highest egg hatching rate was seen in ticks from dogs and the lowest in ticks from cattle. Overall it was seen that dogs were the best host for adult A. parvum ticks, and guinea pigs for immatures. Horses were also shown to be a good host for all tick stages. It can thus be affirmed that A. parvum is a host generalist tick, and its distribution is probably determined by environmental requirements rather than by hosts.     

Problems with the interpretation of epidemiological data in heartwater: a study on 23 farms

In an epidemiological study undertaken on 23 farms where heartwater occurs endemically, it was found that on an overall average, antibodies to Cowdria ruminantium were detected in 64.3% of the cattle, 6 adult Amblyomma hebraeum ticks were counted per animal and 7.0% of ticks were infected with the heartwater agent. It was found that the seropositivity of the animals was determined largely by the tick loads to which they were subjected and that the influence of the tick C. ruminantium infection rate was less evident. There was no parallel between the prevalence of heartwater on the farms and the immune status of the animals. In general, higher tick counts were recorded in herds where strategic tick control is practised than on farms with a total tick control programme. The method of tick control did not, however, appear to influence the immune status of the cattle, the tick infection rate, or the prevalence of heartwater.     

Feeding predilection of Gulf Coast tick, Amblyomma maculatum Koch, nymphs on cattle

Gulf Coast tick nymphs successfully attached and fed on cattle after being freely released. Six Hereford heifers were each infested with approximately 2000 Gulf Coast tick nymphs, three with a strain originating from Refugio Co., TX, and three with ticks from Osage Co., KS by free release on the head and legs to simulate field acquisition of questing nymphs. Two re-infestations were conducted, the first at 7 days and the second at 28 days. Nymph dispersal was estimated by daily inspection of 22 body areas and removal of engorging ticks from the third to the fifth days post-infestation. Total recovery of engorging Texas nymphs was 3.0, 10.2, and 0% and Kansas nymphs was 21.5, 3.3, and 0% for infestations one, two and three, respectively. Immunological resistance to tick infestation expressed as cellular hypersensitivity was evident against Kansas nymphs in the second infestation and against both tick strains in the third infestation. Ticks removed from the withers, midline, and tail-head areas accounted for 68% of the total nymphs recovered in the first two infestations. Within these areas, nymphs were observed to aggregate in small spots where the hair was less dense or naturally parted and the remainder were found scattered in dense hair.     

Histology of bovine skin reactions to Ixodes holocyclus Neumann.

The histology of skin responses to infestations with female Ixodes holocyclus ticks was investigated in naive and previously infested cattle. In the latter hosts, infiltration of the skin site with basophil leucocytes was an early event. Concurrent spongiosis and vesiculation occurred in the epidermis adjacent to the tick mouthparts. This was followed by infiltration of the upper dermis by eosinophils and other leukocytes. Such reactions were evident one hour after attachment of ticks in hosts which had received two prior infestations. Pronounced epidermal vesiculation and bulla formation with eventual serous exudation followed in previously infested cattle, leading to entrapment of ticks.     

Goats as alternative hosts of cattle ticks

The objective of this study was to compare the presence on goats and cattle of adult ticks that usually infest cattle. To this end ticks collected from sets of five goats were compared with those collected from sets of five cattle at 72 communal dip-tanks in the eastern region of the Eastern Cape Province. Amblyomma hebraeum was present on goats at 25 and on cattle at 39 dip-tanks, and a total of 61 goats and 138 cattle were infested. Adult Rhipicephalus (Boophilus) microplus was present on goats at 48 and on cattle at 69 dip-tanks, and a total of 113 goats and 242 cattle were infested. The lengths of 84 of 148 female R. (Boophilus) microplus collected from the goats exceeded 5 mm or more, indicating that they could successfully engorge on these animals. The differences between the proportions of dip-tanks at which A. hebraeum or R. (Boophilus) microplus was present on goats and cattle and also between the proportions of goats and cattle that were infested were significant (Chi-square test, P < 0.01). Adult Rhipicephalus appendiculatus was present on goats at 70 and on cattle at 67 dip-tanks, and a total of 296 goats and 271 cattle were infested. The proportion of dip-tanks at which cattle were infested did not differ significantly from the proportion of tanks at which goats were infested (Fischer's exact probability test, P = 0.44), but the proportion of infested cattle was significantly lower than the proportion of infested goats (Chi-square test, P < 0.05). Adult Rhipicephalus evertsi evertsi was present on goats and cattle at all 72 sampling localities, and a total of 334 goats and 316 cattle were infested. The proportion of infested cattle was significantly lower than the proportion of infested goats (Chi-square test, P < 0.05). These results underscore the necessity of including goats in any tick control programme designed for cattle at the same locality.     

Relationship between the resistance of crossbred cattle to ticks, Boophilus microplus (Canestrini, 1887) and Hyalomma anatolicum anatolicum (Koch, 1844)

Studies were undertaken to determine the effect of repeated pure infestations with Boophilus microplus on susceptibility to subsequent pure infestations with Hyalomma anatolicum anatolicum, and the effects of pure infestations with both species of tick on susceptibility to a series of mixed infestations. Crossbred (Bos indicus X Bos taurus) calves were infested with Boophilus microplus (17 times), H. a. anatolicum (four times), followed by five mixed infestations of B. microplus and H. a. anatolicum. The decline in B. microplus engorgement from a mean yield of 274.4 +/- 60.3 ticks per host after the first exposure, to a mean yield of 9 +/- 4.6 per animal after the seventeenth exposure, was observed in animals exposed to only B. microplus. This might be due to acquired resistance. However, these animals were found to be as susceptible to H. a. anatolicum as animals which had never been exposed to ticks of either species. A decline in the yield of H. a. anatolicum from a mean yield of 92.1 +/- 10.7 after the first exposure to 54.7 +/- 11.3 after the fourth exposure, indicated that the cattle could also acquire resistance to repeated pure infestations with this species. After repeated pure infestations with both tick species, cattle reacted to five mixed infestations showing a high degree of resistance to B. microplus and low resistance to H. a. anatolicum (mean yield for B. microplus was only 10 +/- 8.1 ticks per host after the first mixed exposure and declined to 1.3 +/- 1.7 after the fifth, whereas the mean yield for H. a. anatolicum was 71.4 +/- 11.3 ticks per host following the first exposure and declined to 37.3 +/- 7.8 after the fifth). Host responses elicited to one species do not provide cross-resistance to the second species used in this study.     

Some avian and mammalian hosts of Amblyomma hebraeum and Amblyomma marmoreum (Acari: Ixodidae)

Large numbers of birds, wild mammals and domestic stock from a variety of localities within the Republic of South Africa were examined for infestation with the ixodid ticks Amblyomma hebraeum and Amblyomma marmoreum. Every warthog (Phacochoerus aethiopicus), Burchell's zebra (Equus burchelli), impala (Aepyceros melampus) and kudu (Tragelaphus strepsiceros) from the Kruger National Park in the north-eastern Transvaal Lowveld was infested with A. hebraeum. In the eastern Cape Province every helmeted guinea fowl (Numida meleagris), scrub hare (Lepus saxatilis) and kudu from the Andries Vosloo Kudu Reserve; all but 1 of the 22 domestic cattle examined on the farm "Bucklands"; and all Angora goats plus nearly all Boer goats examined on the farm "Brakhill" were infested with this tick. Most animals examined appeared to be good hosts of the immature stages, and the larger the host species the greater the chances of it harbouring large numbers of adult ticks. The largest animals examined, such as eland, buffalo, giraffe and rhinoceros, harboured very large numbers of adult A. hebraeum. No adult A. marmoreum was recovered from any host. However, 50% or more of helmeted guinea fowl and kudu from the Andries Vosloo Kudu Reserve; helmeted guinea fowl, scrub hares and eland (Taurotragus oryx) from the Mountain Zebra National Park; helmeted guinea fowl, kudu, domestic sheep, goats and cattle on the farm "Bucklands", and caracal (Felis caracal) from the Cradock and Southwell areas of the eastern Cape Province were infested with immature A. marmoreum. In the Bontebok National Park in the south-western Cape Province more than 35% of scrub hares, vaal ribbok (Pelea capreolus) and bontebok (Damaliscus dorcas dorcas) were infested with immature ticks.     

Hosts, seasonality and geographic distribution of the South African tortoise tick, Amblyomma marmoreum

The tortoise tick Amblyomma marmoreum was collected from large numbers of reptiles and other animals during the course of numerous surveys conducted in South Africa. A total of 1229 ticks, of which 550 were adults, were recovered from 309 reptiles belonging to 13 species, with leopard tortoises, Geochelone pardalis being the most heavily infested. The 269 birds sampled harboured 4901 larvae, 217 nymphs and no adult ticks, and the prevalence of infestation was greatest on helmeted guineafowls, Numida meleagris. Only two larvae were recovered from 610 rodents, including 31 spring hares, Pedetes capensis, whereas 1144 other small mammals yielded 1835 immature ticks, of which 1655 were collected from 623 scrub hares, Lepus saxatilis. The 213 carnivores examined harboured 2459 ticks of which none were adult. A single adult tick and 6684 larvae and 62 nymphs were recovered from 656 large herbivores, and a total of 4081 immature ticks and three adults were collected from 1543 domestic animals and 194 humans. Adult male and female A. marmoreum were most numerous on reptiles during January and February, and larvae during March. The largest numbers of larvae were present on domestic cattle and helmeted guineafowls in the Eastern Cape Province during March or April respectively, whereas larvae were most numerous on helmeted guineafowls, scrub hares and the vegetation in north-eastern Mpumalanga Province during May. In both provinces nymphs were most numerous between October and December. Amblyomma marmoreum appears to be most prevalent in the western regions of the Western and Eastern Cape and Free State provinces, and the north-eastern regions of the Northern Cape, KwaZulu-Natal, Mpumulanga and Limpopo provinces.     

Development of a novel self-medicating applicator for control of internal and external parasites of wild and domestic animals

Four trials, three in the United States and one in South Africa, were conducted to evaluate the potential value of a novel self-medicating applicator in the passive control of gastrointestinal nematodes in cattle and deer, and of files and ticks on cattle using oil-based treatments. The results of the trials demonstrated that this applicator is an effective and practical device for the passive treatment of both deer and cattle for trichostrongyle infections using the endectocide, moxidectin (Cydectin. Fort Dodge Animal Health, USA), of cattle for hom fly (Haemotobia irritans) infestations using the insecticide, cyfluthrin (CyLence, Bayer AG, Germany) and of cattle for tick infestations (in particular Amblyomma hebraeum and Rhipicephalus appendiculatus) using the acaricides deltamethrin and amitraz (Delete All, Intervet, South Africa).     

Ticks on crested francolins, Francolinus sephaena, and on the vegetation on a farm in Limpopo Province, South Africa

Ticks were collected at approximately bi-monthly intervals between June 1996 and June 1997 from crested francolins, Francolinus sephaena, and from the vegetation on a mixed cattle and wildlife farm in Limpopo Province, South Africa. The birds were infested with the immature stages of 13 tick species, of which Amblyomma hebraeum, Amblyomma marmoreum and Hyalomma marginatum rufipes were the most numerous and prevalent. Ten ixodid tick species were collected from the vegetation, of which the immature stages of Rhipicephalus appendiculatus, Rhipicephalus (Boophilus) decoloratus and Rhipicephalus evertsi evertsi were the most numerous. No adult ticks were collected from the birds and only two from the vegetation. The restricted home range of crested francolins implies that they could serve as a source of tick infestation only for other animals within the same habitat as the birds.     

Cross-reactivity between antigens from Amblyomma cajennense and A. hebraeum (Acari: Ixodidae)

Laboratory animals exposed to feeding ticks develop resistance which is reflected by a decline in tick engorgement weight, egg-laying by adults and reduced egg viability. Serum antibodies from these hosts and their reaction with tick antigens have been detected by different methods, including precipitation techniques, immunofluorescent techniques, ELISA and Western blots. However, little is known about the effects of antibodies on ticks that engorge on resistant hosts, or which tissues of the tick body are possibly immunogenic. Some researchers, using immunohistochemistry, have detected host antibodies in the gut, salivary glands and haemolymph of ticks engorged on resistant animals. The same technique has helped considerably in determining antigenic sites or antibody targets in other arthropods. Consequently, immunohistochemistry techniques were used in this study to detect cross-reactivity between sera raised against Amblyomma cajennense (Fabricius, 1787) with Amblyomma hebraeum (Koch, 1844), and vice versa. The results show the existence of shared antigens between the 2 tick species. In general, our results point more to a 1-way cross-reactivity of A. hebraeum with A. cajennense than a reciprocal cross-reactivity, suggesting that A. hebraeum is more immunogenic than A. cajennense.     

Efficacy of flumethrin 1% pour-on against ticks on cattle under field conditions in Ethiopia

The efficacy of a flumethrin 1% pour-on (Bayticol, Bayer AH) was evaluated against natural infestations of ticks on cattle on a dairy farm in Ethiopia during 1997/98. The cattle, (n = 92), which were Friesian/ Zebu crosses, were heavily infested with Boophilus decoloratus. Dry cows (n = 8) were randomly selected and allocated either into a treatment or a control group. Flumethrin 1 % pour-on was applied to the treatment group according to the manufacturer's recommendation, i.e. along the dorsal mid-line from the head to the base of the tail. Ticks were counted and identified in situ on treated and control animals. When the total tick counts of the treated group were similar to those on the control, the trial was discontinued. Excellent results were achieved with flumethrin against heavy tick infestations. There was a rapid kill after 24 h post-treatment, and from day 4 onwards 100% control was achieved and maintained for a further 29 days. The results obtained indicate that there was a significant difference between the mean tick counts of the control group compared with those of the treatment group (P < 0,05). The evidence presented here suggests that the flumethrin 1 % pour-on may be applied at 45 day intervals and should provide effective protection against the economically important ticks if it is used at the recommended dosage rate and applied correctly.     

Effect of host resistance on the feeding and reproductive performance of Haemaphysalis punctata and Ixodes ricinus ticks

A study was made to assess the influence of host response on Haemaphysalis punctata and Ixodes ricinus biology by comparing the effects of feeding ticks on previously uninfested hosts with those fed on previously sensitised hosts. Adult H punctata were fed on sheep; larvae and nymphs of H punctata and I ricinus were fed on rabbits. The engorgement weights of all instars studied that fed on sensitised hosts fell very gradually with successive infestations. These weight reductions were greater at high than at low levels of infestation. In previously sensitised hosts, the engorgement period of all instars was prolonged: the percentage of instars which completed engorgement and engorgement weights were reduced. The host's clinical and immune response markedly affected the fecundity of adult female ticks and the viability and hatchability of the laid eggs. Eighty per cent of female H punctata successfully laid eggs following the first infestation and the hatchability of eggs was 85 per cent. The numbers successfully laying eggs decreased to 60 per cent during the fifth and sixth infestations and was less than 20 per cent following the seventh infestation; the hatchability decreased similarly. There was no marked effect on the development of immature instars when fed on sensitised rabbits.