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.     

Seasonal dynamics and hosts of Amblyomma triste (Acari: Ixodidae) in Argentina

The seasonal dynamics and host usage of Amblyomma triste in Argentina were analyzed. Adults of A. triste were present from early winter to mid-summer, with the peak of abundance from late winter to mid-spring (August to October). Larvae and nymphs were found from December to June, with the peak of abundance in summer. There were no differences among the biological parameters (pre-moult period of larvae and nymphs, pre-oviposition period of females, and minimum incubation period of eggs) of engorged ticks exposed to different photoperiod regimens at the laboratory, but the periods for each biological parameter obtained from ticks exposed in the field were significantly longer than those from the laboratory. Field results fit better with the data of seasonal distribution of each stage. Morphogenetic diapause was not detected, but complementary studies should test the presence of behavioral diapause. Rodents of the subfamily Sigmodontinae (Akodon azarae, Oligoryzomys flavescens, Oligoryzomys nigripes, Oxymycterus rufus and Scapteromys aquaticus) are the principal hosts for immature stages of A. triste, the caviid Cavia aperea could be another potential host for these stages, and birds are exceptional hosts for larvae and nymphs. Regarding hosts of adults in Argentina, domestic and wild large-sized mammals belonging to different orders (cattle, dog, horse, Blastocerus dichotomus and Hydrochoerus hydrochaeris) serve as hosts for adults of this tick species. In conclusion, A. triste has a life cycle of 1 year with adults feeding on large endemic and introduced mammals and immature stages using sigmodontine and caviid rodents as hosts.     

The natural hosts of larvae and nymphs of Amblyomma tigrinum Koch, 1844 (Acari: Ixodidae)

The hosts of larvae and nymphs of Amblyomma tigrinum, a tick whose adults feed on wild and domestic Canidae in South America, are uncertain. A 17 months survey was carried out trapping wild vertebrates in north-western Córdoba, Argentina, to evaluate their parasitism with A. tigrinum subadults. Larvae and nymphs of this tick species were identified conventionally and by comparison of 16S rDNA sequences with GenBank deposited sequences. A total of 207 small and medium-sized rodents and 182 birds were captured and examined for ticks. Most ticks on birds were from ground forest feeding birds (BB) with a minimal contribution of birds feeding in open pastures. All ticks from rodents were obtained from representatives of the families Cricetidae (SR) and Caviidae (MR). Percent of larvae infestation was higher (P<0.01, Chi-square distribution) in BB (55.2%) and SR (46.4%) than in MR (17.4%) and the same trend was found for number of larvae on these hosts (test of Kruskal-Wallis). Caviidae (only representative Galea musteloides) rodents were extremely prone to be infested with nymphs of A. tigrinum (94.2%) followed by BB (50.6%) and SR (3.6%) (P<0.01) and the same tendency was found for number of nymphs (P<0.01). The index of aggregation for nymphs on MR was the lowest (0.409) followed by nymphs on BB (0.706) which may be a consequence of higher and homogenous exposure of G. musteloides to host-seeking nymphs. Several BB are food source for both larvae and nymphs of A. tigrinum while for rodents larvae were common only on SR (mainly on the Sigmodontinae Akodon dolores and Graomys sp.) and nymphs feed almost exclusively on MR. Therefore, both birds and rodents are of importance for the survival strategy of A. tigrinum subadults. The plasticity of A. tigrinum to colonize areas with different climates plus the capacity of their subadults to feed on hosts widely distributed indicates that this tick has the potential to become a widespread parasite but this does not seem to be the actual situation. Several proposals are presented to further understand its ecology.     

Amblyomma aureolatum (Pallas, 1772) and Amblyomma ovale Koch, 1844 (Acari: Ixodidae): hosts,distribution and 16S rDNA sequences

DNA sequences of Amblyomma aureolatum (Pallas, 1772) and Amblyomma ovale Koch, 1844 were obtained to determine genetic differences between these tick species. Collections of these species are discussed in relation to distribution and hosts. Seven ticks collections (four from Brazil, one from Argentina, one from Uruguay and one from USA) house a total of 1272 A. aureolatum (224 males, 251 females, 223 nymphs and 574 larvae) and 1164 A. ovale (535 males, 556 females, 66 nymphs and 7 larvae). The length of the sequenced mitochondrial 16S rRNA gene fragment for A. aureolatum was 370bp and for A. ovale was 373bp. The DNA sequence analysis showed a 13.1% difference between the two species. Apart from one male A. ovale found on a toad, all adult ticks were found on mammals. The majority of adult specimens of both tick species were removed from Carnivora (96.1 and 84.3% of A. aureolatum and A. ovale, respectively), especially from dogs (53.1% of A. aureolatum, and 46.4% of A. ovale). Collections on wild Canidae were higher for A. aureolatum (23.3%) than for A. ovale (7.1%). On the other hand, collections of A. ovale adults on wild Felidae were higher (18.3%) than findings of A. aureolatum (9.2%). The contribution of other mammalian orders as hosts for adults of A. aureolatum and A. ovale was irrelevant, with the exception of Perissodactyla because Tapiridae contributed with 13.0% of the total number of A. ovale adults. Adults of both tick species have been found occasionally on domestic hosts (apart of the dog) and humans. Most immature stages of A. aureolatum were found on Passeriformes birds, while rodents and carnivores were the most common hosts for nymphs and larvae of A. ovale. A. aureolatum has been found restricted to the Neotropical region, covering the eastern area of South America from Uruguay to Surinam, including northeastern Argentina, eastern Paraguay, southeastern Brazil and French Guiana. A. ovale showed a distribution that covers the Neotropical region from central-northern Argentina throughout the Neotropics into the Nearctic region of Mexico with a few records from the USA, also with collection sites in Paraguay, Bolivia, most Brazilian states, Peru, Ecuador, French Guiana, Surinam, Guyana, Trinidad & Tobago, Venezuela, Colombia, Panama, Costa Rica, Nicaragua, Belize, Guatemala and several states of Mexico.     

The developmental success of Amblyomma hebraeum and Amblyomma marmoreum on the leopard tortoise, Geochelone pardalis

The success of natural infestations of various life history stages of Amblyomma hebraeum and Amblyomma marmoreum on the leopard tortoise, Geochelone pardalis, was compared. Success was measured by the time taken for ticks to detach, as well as the percentage of ticks engorging and subsequently either moulting to the next life history stage or laying viable eggs. Larvae of A. hebraeum were the only developmental stage not recovered. Nymphae and female A. hebraeum were less successful in moulting or laying eggs than the corresponding stages of A. marmoreum. Nevertheless, 48,7% of A. hebraeum nymphae moulted, while 1 of 6 females laid viable eggs.     

Acquisition of resistance to the bont tick Amblyomma hebraeum (Acarina: Ixodidae) by goats

Goats acquired resistance to larvae of Amblyomma hebraeum following three repeated infestations. Resistance was associated with immediate Type I hypersensitivity reactions following intradermal skin tests using crude egg extracts. In a separate experiment, the inoculation of tick-naive goats with extracts of the larvae of A. hebraeum failed to induce observable effects on feeding larvae despite the development of anti-larval antibodies.     

Amblyomma triste Koch, 1844 (Acari: Ixodidae): hosts and seasonality of the vector of Rickettsia parkeri in Uruguay

Amblyomma triste Koch, 1844, the tick species most frequently involved in human bites in Uruguay, has been implicated as a vector of human rickettsiosis. Seasonal abundance of adult A. triste was examined by standard flagging at three sites where human tick bites and cases of the disease have been reported. Adult tick activity occurred from August to February (end of winter to mid summer in the southern hemisphere) with a peak in spring. Activity declined in step with decreasing temperatures and photoperiod during winter. This period of activity coincides with seasonal outbreaks of human rickettsiosis in the region. In a small mammal survey, the Sigmodontinae rodents Scapteromys tumidus (Waterhouse, 1837) and Oxymycterus nasutus (Waterhouse, 1837) and the small marsupial Monodelphis dimidiata (Wagner, 1847) were the main hosts for immature A. triste. Immature ticks were observed on hosts in November, well within the period of peak adult abundance. In stored collections, immature ticks were most often collected from January to March. These data suggest that one generation might be completed in 1 year. The main animal host for adult A. triste at our study sites was the domestic dog. Humans were afflicted by the tick in rural and suburban settlements where other host animals are scarce or extinct and where dogs are common.     

Climate changes and suitability for the ticks Amblyomma hebraeum and Amblyomma variegatum (Ixodidae) in Zimbabwe (1974-1999)

The spread of Amblyomma hebraeum has been reported in Zimbabwe. At the same time there was little or no spread in the distribution of Amblyomma variegatum. This paper examines the climatic cycles and their trends in the period 1974-1999 with a view to explaining the abiotic causes of this spread, and of forecasting the likely tendency in climate suitability for both tick species. An annual data-set of rainfall and air temperature was used as a source for climate, together with a habitat-modeling algorithm to estimate climate suitability for both ticks. Long-term suitable habitat for A. hebraeum exists mainly in the south and southeast of the country. Areas of adequate habitat for A. variegatum exist across the country, between approximately 17 degrees S and 18.5 degrees S, and are most suitable in regions of the northwest. The climate niches of the two species differ, and account for their almost allopatric distributions, as observed in the duration and intensity of the dry period and in total annual rainfall. Cyclic changes in both temperature and rainfall drive the periodic modifications in the distributions of the ticks. More intense periods of drought in the highveld, drive the expansion of A. hebraeum in this region. Temperature does not have any effect on the tendency in this area. Areas in south and southeast show a trend towards an increase in climatic suitability because of an increase in temperature. Zones in which habitat suitability is increasing for A. variegatum are restricted to the northwestern parts of the country, because warmer temperatures and a slight decrease in the intensity of the dry season. The progressive increase in temperatures seems to be forcing the dispersion of A. variegatum towards areas outside of zones that have a prolonged dry period. On the other hand A. hebraeum is compelled to spread northwards, following areas with adequate rainfall patterns, but halted by temperature limits and perhaps competition with A. variegatum. Without adequate control measures, invasive waves of A. hebraeum may occur over a background swell of northward expansion.     

Ability of adults of Amblyomma hebraeum (Acarina: Ixodidae) to feed repeatedly on sheep and cattle

Adult Amblyomma hebraeum ticks were exposed at 14-day intervals on five sheep on 46 occasions and on three cattle on 15 occasions to determine the effects of repeated feeding. The engorged weight and percentage of ticks which fed successfully on sheep declined significantly at first but subsequently they increased. On cattle there was no progressive decline in engorged weight, although the percentage which fed successfully declined significantly and was lower than that for sheep. This decline was attributed to increased grooming by cattle and was the only apparent mechanism by which resistance was expressed.     

Acaricidal activity of thymol on larvae and nymphs of Amblyomma cajennense (Acari: Ixodidae)

The aim of the present study was to evaluate the acaricidal efficacy of thymol on unengorged and engorged larvae and engorged nymphs of Amblyomma cajennense. To perform the test for unengorged larvae, the larval packet technique was employed and mortality was evaluated 24h after the test, while for engorged larvae and engorged nymphs the immersion technique was employed and mortality was evaluated after 15 days. In all the experiments, the following concentrations of thymol were tested: 2.5, 5.0, 10.0, 15.0 and 20.0mg the thymol/ml. The control group was exposed to water and 1% DMSO and there were 10 repetitions for each treatment. The values found for mortality of unengorged larvae were 18.2%, 51.8%, 97.6%, 93.5% and 94.5%, and for engorged nymphs the values were 26.0%, 92.2%, 100.0%, 100.0%, 100.0%, in concentrations of 2.5, 5.0, 10.0, 15.0 and 20.0mg of thymol/ml, respectively. In the test with engorged larvae, mortality was 100.0% in all treatments. Based on the results, it is possible to conclude that thymol has acaricidal activity against immature stages of A. cajennense.     

Some wild hosts of the Karoo paralysis tick, Ixodes rubicundus Neumann, 1904 (Acari: Ixodidae)

A variety of mammal and bird species in the Mountain Zebra National Park, eastern Cape Province, were examined for Ixodes rubicundus. In addition, caracal (Felis caracal) from the park and its vicinity as well as from the Graaff-Reinet region of the Karoo were examined. Amongst the animals collected, the red rock rabbit (Pronolagus rupestris) was a preferred host of the immature stages of the tick. Caracal were moderately good hosts of these stages and, with mountain reedbuck (Redunca fulvorufula) and eland (Taurotragus oryx), are the preferred hosts of adult ticks.     

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.     

Cutaneous hypersensitivity induced in rabbits by extracts of the tick Amblyomma cajennense (Acari: Ixodidae)

The cutaneous hypersensitivity test was used to correlate host resistance to ticks and type of reactions elicited by Aomblyomma cajennense (Fabricius, 1787) tick extract in rabbits. Rabbits were divided into 3 groups of 2 animals each: naive, pre-infested and control. Cutaneous hypersensitivity was induced by intradermal inoculation of 25 microg extract in 0.03 ml of phosphate buffered saline (PBS) in rabbit ears. Control rabbits were inoculated with PBS only. The ear thickness was measured with a Mitutoyo device before and 10 min, 1, 2, 4, 18, 24, 48, 72 and 96 h post-inoculation (PI). Pre-infested rabbits showed an immediate type reaction within the 1st 10 min PI (60% increase in ear thickness) and a delayed reaction (18 h) (85% increase), whereas the naive rabbits showed only the immediate reaction within the 1st 4 h (60% increase). PBS induced only mild reactions. These results point out the crucial role of the cellular immune response of rabbits in the expression of resistance to A. cajennense.     

Morphological characterization of the ovary and vitellogenesis dynamics in the tick Amblyomma cajennense (Acari: Ixodidae)

In this work we describe the internal morphology of the female reproductive system of the cayenne tick Amblyomma cajennense. This system is represented by a panoistic ovary, which lacks nurse cells in the germarium. This ovary consists of a single tube, in which a large number of oocytes develop asynchronously, thus accompanying the processes of yolk deposition in the oocytes. The oocytes were classified into stages that varied from I to V, according to: cytoplasm appearance, presence of the germ vesicle, presence of yolk granules, and presence of chorion. The study of vitellogenesis dynamics suggest that the yolk elements are deposited in the oocyte following a preferencial sequence, in which the lipids are the first to appear, followed by proteins an finally by the carbohydrates. In this way the yolk of A. cajennense ticks have these three elements that may be free in the cytoplasm or chemically bounded forming glycoprotein or lipoprotein complexes.     

Vitellogenesis in the tick Amblyomma triste (Koch, 1844) (Acari: Ixodidae) Role for pedicel cells

This study presents new information on the vitellogenesis of the tick Amblyomma triste. In this species, the ovary consists of a layer of epithelial cells, which form the ovarian wall, oogonia and developing oocytes; and the pedicel, a cellular structure that synthesizes and provides yolk precursors for developing oocytes. The pedicel also attaches oocytes to the external surface of the epithelial wall. In this study, evidence is provided in support of pedicel cells in providing compounds for oocytes during vitellogenesis.     

Horse resistance to natural infestations of Anocentor nitens and Amblyomma cajennense (Acari: Ixodidae).

The objective of this study was to investigate some aspects of horse resistance to natural infestations of Anocentor nitens and Amblyomma cajennense over a 2-year period. Free-living stages of A. nitens were used to evaluate the influence of season of the year on horse resistance. Every 2 weeks, 10 selected engorged females, weighing 150-329 mg, were removed from naturally infested horses and were individually placed into glass tubes closed with a cotton stopper, and maintained in an incubator. The biological parameters evaluated varied according to the season of the year, i.e., reproductive efficiency, maximum larval survival (MLS) and egg-hatching rates were significantly lower in spring/summer than in fall/winter. At 14-day intervals, females of A. nitens and A. cajennense were counted on each side of the body of five mares. A comparison between the animal with the lowest infestation and the animal with the highest infestation showed greater differences in animals infested with A. nitens than those infested with A. cajennense. Ten animals were found to carry 64.9% of the total parasite burden of A. cajennense and 88.6% of the highest infestations of A. nitens females. A seasonal variation on intensity of horse resistance to A. nitens was observed during the study. However, this was not observed for A. cajennense. Cross-resistance was observed in animals with the highest total number of A. cajennense and the highest number of A. cajennense adults (r=0.817), larvae (r=0.756), and A. nitens females (r=0.799). These results led to the conclusion that changes in horse resistance to A. nitens occur seasonally. As far as the total population of A. cajennense and the female stage of A. nitens are concerned, horses develop cross-resistance.     

Acaricidal activity of hydroethanolic formulations of thymol against Rhipicephalus sanguineus (Acari: Ixodidae) and Dermacentor nitens (Acari: Ixodidae) larvae

The aim of this study was to assess the acaricidal activity of hydroethanolic formulations of thymol at varying concentrations on Rhipicephalus sanguineus and Dermacentor nitens larvae. The larval packet test was used and the thymol concentrations tested were 2.5, 5.0, 10.0, 15.0 and 20.0 mg/ml. The control group was exposed only to water and ethanol (50/50%) and there were 10 repetitions for each treatment. The mortality was evaluated after 24 h. For the R. sanguineus larvae, the mortality rates were 47.5, 50.2, 96.7, 95.9 and 98.1%, while for D. nitens the rates were 14.1, 75.0, 90.2, 90.3 and 99.5%, at respective thymol concentrations of 2.5, 5.0, 10.0, 15.0 and 20.0 mg/ml. The results indicate that the hydroethanolic formulations of thymol tested have acaricidal activity on R. sanguineus and D. nitens larvae exposed topically, causing mortality greater than 90% 24 h post-treatment starting at the concentration of 10 mg/ml.     

The sex ratio of Amblyomma cajennense (Acari: Ixodidae) with notes on the male feeding period in the laboratory

To evaluate the sex ratio of field collected nymphal Amblyomma cajennense ticks, we collected 5326 engorged nymphs from naturally infested horses in Pirassununga county and allowed them to molt to adults in the laboratory. They yielded a sex ratio of 1:1.83 (M:F). Three and two engorged females were collected from horses pastured at Pirassununga county and from tapirs pastured in Sorocaba county, respectively. These females were allowed to oviposit and their progeny were reared until the adult stage in the laboratory. Engorged females collected from Pirassununga yielded a sex ratio of 1:1.57 (M:F) and a sex ratio of 1.14:1 (M:F) were obtained for those ticks collected from tapirs. In addition, unfed tick larvae were collected from Pedreira county and reared in the laboratory until the adult stage. This collection yielded a sex ratio of 1.11:1 (M:F). These results showed significantly different (P<0.05) sex ratio constitutions among different tick populations. Laboratory rabbits were infested once with A. cajennense male ticks, which showed feeding periods varying from 7 to 86 days. During this period, the rabbits were re-infested regularly with A. cajennense female ticks. A total of 179 engorged females were collected from the rabbits and their engorged weight, feeding, preovioposition and egg incubation periods, weight of deposited eggs, percent of hatched eggs and egg production efficiency were compared to the male feeding period and to the number of live males present on the host. None of the female variables were affected by the male feeding period. Male ticks remained fertile for the whole feeding period. Percent of hatched eggs was the only female variable that significantly decreased as the number of live males decreased on the host. The results showed that although some A. cajennense populations are composed of more females than males after molting, this female predominance is compensated by a long male feeding period and maintenance of its reproductive performance.     

Larval Gulf Coast ticks (Amblyomma maculatum) [Acari: Ixodidae] as host for Hepatozoon americanum [Apicomplexa: Adeleorina].

Laboratory-reared larval Gulf Coast ticks (GCTs) (Amblyomma maculatum) were exposed experimentally and found to acquire Hepatozoon americanum infection while feeding on parasitemic dogs. These ticks supported gamogonic and sporogonic development of the apicomplexan, and oocysts from newly molted nymphs were infectious for a dog. Other nymphs from this cohort that were allowed to feed on a blood-parasite naive sheep molted normally; the resulting adult ticks contained oocysts that were infectious for another dog. Merogonic development of H. americanum in the dogs and the resulting lesions/disease appeared similar, irrespective of whether infectious oocysts were derived from nymphal or adult ticks that acquired infection as larvae. In the system previously known, nymphal ticks acquire infection and adults harbor infective oocysts, which vertebrate hosts ingest. Given that larval A. maculatum can acquire infection and nymphs can harbor viable oocysts as demonstrated by this study, the potential variety of vertebrate hosts that can alternate with GCTs in maintaining an endemic cycle is considerably expanded.