The effects of the bont tick, Amblyomma hebraeum, on milk production of Sanga and Sanga x Brahman cattle.

The effects of adults on the bont tick, Amblyomma hebraeum on the milk production of Sanga and Sanga x zebu (Brahman) cattle were measured over a period of 11 weeks in the low veld of Zimbabwe in the summer of 1986. Four groups of lactating cows, consisting of two breeds, each divided into a high and low tick treatment, were exposed to very low or high challenges of ticks and their milk production measured by weighing their calves before and after suckling. The liveweight gains (LWG) of the calves were also measured. Tick burdens on the infested groups averaged around fourteen engorging females of A.hebraeum per day, which amounted to infestations of about 150 adult ticks. This is greater than most observed field infestations. This caused no significant reduction in milk yield or calf growth over the whole period, provided the teats of the dams had not been damaged by ticks. Mismothering occurred when teats were damaged. No breed differences were observed so all data was pooled for further analysis. Average calf LWGs of the high tick groups were reduced by 2.2 kg (P < 0.01) during one 4-week period but overall the 3.9 kg difference in LWG of the tick treatment groups was not quite significant (P < 0.10). Although there was a poor relationship between tick numbers and reduced milk yield or calf LWG, the effects were always in the direction expected. The effects averaged 6 +/- 10 g reduction of milk and 2.6 +/- 1.8 g loss of LWG of calves for every female tick that engorged. it was concluded that milk production is not an important consideration when estimating the losses in production caused by A.hebraeum on Brahman x Sanga or Sanga breeds of cattle. Losses due to teat or udder damage could be much more important and need to be quantified.

The effects of the brown ear-tick, Rhipicephalus appendiculatus, on milk production in dairy cattle.

The effect is reported of artificially controlled levels of infestation with adults of Rhipicephalus appendiculatus on the milk yield of twenty commercial Bos taurus dairy cattle on a high plane of nutrition and eighteen crossbred B.taurus x Sanga cattle on a lower plane of nutrition on the highveld of Zimbabwe. The results showed no significant effect on milk yield of infestations averaging twenty engorging ticks per animal per day, despite severe ear damage in some animals. They indicate that milk production of dairy cattle under commercial management is not sensitive to infestation with R.appendiculatus. The results have important implications for management of ticks in Africa, but need to be interpreted within the context of the control of tick-borne diseases.

The effects of the brown ear-tick, Rhipicephalus appendiculatus, on milk production of Sanga cattle.

Lactating Sanga cows of the Mashona breed from Zimbabwe, receiving either a low or high level of nutritional supplement, were exposed to two levels of infestation of adults of the brown ear-tick, Rhipicephalus appendiculatus in the highveld of Zimbabwe. The effect of the ticks on the milk yield was measured over an 11-week period during the rainy season from January to April 1986. A technique in which calves were weighed before and after suckling was used to estimate milk yield. There were significant treatment effects of ticks (P < 0.05) on milk production but no significant differences in liveweight gain between calves from tick-free and tick-infested dams. The loss in milk production was poorly related to the number of female ticks that engorged, being 9 g (SEM 4) per tick. A Friesian x Hereford (Bos taurus) reference group of cattle carried 50% more ticks than the Mashona cows, illustrating a difference in resistance between the breeds. Thirteen screw-worm (Chrysomya bezziana) strikes were recorded amongst the thirty-two Mashona cows compared with twenty-one amongst the ten Friesian x Hereford animals between January and the end of March.

Monitoring Babesia bovis infections in cattle by using PCR-based tests.

The sensitivity and specificity of PCR tests based on the small-subunit rRNA gene sequence of Babesia bovis were compared in a blind study of experimentally infected cattle with the corresponding parameters of the complement fixation (CF) test currently used in the United States to screen for bovine babesiosis. Cattle were experimentally infected with a single inoculum of a cloned laboratory strain of B. bovis. Blood samples were collected and tested over a period covering from the day of infection to 10 months postinfection. The level of parasitemia (percent infected erythrocytes) present in each sample was estimated from test results and was plotted as a function of time postinfection. These data are the first describing the course of infection by methods capable of detecting parasitemias in the range of 10(-7)%, which frequently occur in the carrier state. Parasitemias in the samples tested strongly influenced the sensitivity and negative predictive value of the PCR-based tests which varied with time postinfection. The average sensitivities of the three PCR-based tests for B. bovis ranged from 58 to 70% for a single determination, while the sensitivity of the CF test was only 6%. Both PCR-based and CF tests for B. bovis had high specificity values ranging from 96 to 100%.

Infestations of the bont tick Amblyomma hebraeum (Acari: Ixodidae) on different breeds of cattle in Zimbabwe.

Infestations of adults and nymphs of Amblyomma hebraeum were counted on Brahman (Br), Brahman x Simmental (BS), Sanga (Sa) and Hereford (He) steers exposed to infested pastures at Mbizi in southern Zimbabwe in 1986-1987. Herefords were always the most heavily infested, while the Sanga tended to carry the fewest ticks with the Brahman and Brahman x Simmental groups being in between. The ratios of the engorged females on the four breeds were 2.3:1.4:1.4:1.0 for He:Br:BS:Sa. The ratios of the standard nymphs were 2.2:1.4:1.7:1.0 for He:Br:BS:Sa. The results confirm earlier observations in Africa and support the view that there are genetic differences between breeds in the expression of resistance to this tick species.

Factors influencing infections in Rhipicephalus appendiculatus ticks fed on cattle infected with Theileria parva.

A large database on the transmission of a stabilate of the Theileria parva Muguga stock from one breed of cattle using two stocks of Rhipicephalus appendiculatus, Muguga and Ol Pejeta was developed and analysed. Factors associated with the ticks and cattle, and the infections developing in cattle were studied in relation to the infection variables in the tick batches harvested daily from cattle. Generalized Linear Interactive Modelling (GLIM) was used to determine the importance of factors and interactions in influencing the levels of tick infection variables using Type I and Type III sums of squares analyses. Analysis of the 6 variables, prevalence (percentage of ticks infected), abundance (mean number of infected salivary gland acini per tick examined) and intensity (mean number of infected salivary gland acini per infected tick) in batches of 30 male and 30 female ticks showed that 24 covariates, factors or interactions had a significant effect (P < 0.05). Certain covariates and factors were particularly important for all 6 tick infection variables; parasitaemia of animal on the day of tick harvest, stabilate dilution administered to animal, month in which tick batch was harvested, minimum packed cell volume of animal over the sampling period, age of animal, and the minimum leukocyte count of the animal over the sampling period. The GLIM analyses were found to be a useful tool in identifying factors that influence infection levels and in devising methods of producing tick batches with more predictable infections.

A self-medicating applicator for control of ticks on deer.

A self-medicating applicator for killing ticks on deer and domestic ungulates by passive transfer of acaricides during feeding is described. The applicator consists of a barrel divided into a food bin (above) and a sealed self-contained acaricide reservoir (below) with a vertical, centrally-located ceramic column that extends from the reservoir into the food bin. Acaricide is drawn up the column from the sealed reservoir by absorption. Animals attracted to the food in the device acquire the acaricide during feeding when they contact the uppermost portion of the column. White-tailed deer (Odocoileus virginianus) readily utilize the applicator in penned and natural conditions. In a field study, examination of hunter-killed deer demonstrated that animals from a treated site were infested with many fewer Ixodes scapularis (3.4 +/- 1.1) than those from a control site (10.8 +/- 3.0). Chromatographic analysis of hair samples revealed traces of permethrin on three of the four animals examined, suggesting use of applicators by these deer. In a study using penned goats (Capra hircus) infested with Amblyomma americanum ticks, treatment efficacy reached 86.4% within 4 days of exposure to the applicators. Visual observations confirmed that all animals used the applicators. Gas chromatographic analysis of goat hair samples indicated that permethrin was detectable on all of the treated animals exposed to the device.

Efficacy of pheromone-acaricide-impregnated tail-tag decoys for controlling the bont tick, Amblyomma hebraeum (Acari: Ixodidae), on cattle in Zimbabwe.

A large-scale field test using pheromone-acaricide-impregnated plastic tail-tag decoys demonstrated excellent efficacy of these devices for control of the bont tick, Amblyomma hebraeum, on cattle in Zimbabwe. The tail tags were impregnated with a mixture containing o-nitrophenol, methyl salicylate, 2,6-dichlorophenol and phenylacetaldehyde and one of three different acaricides (cyfluthrin, flumethrin or alphacypermethrin). o-Nitrophenol and methyl salicylate are components of the A. hebraeum attraction-aggregation-attachment pheromone, while 2,6-dichlorophenol and phenylacetaldehyde are proven attractants for this tick. Both o-nitrophenol and methyl salicylate were lost gradually from the tags over 12 and 14 week periods, respectively. In field trials, tick counts were compared between cattle that received tail tags either impregnated with pheromone mixture alone, cyfluthrin and pheromone mixture, flumethrin and pheromone mixture, alphacypermethrin and pheromone mixture or were left untreated. During the first 3 month trial period, control of adult bont ticks was 94.9% with cyfluthrin tail tags and 87.5% with flumethrin tail tags. In general, there was no significant difference in bont tick numbers on cattle without tags and those with tail tags containing pheromone only. When the trial was repeated for another 3 month period, control of bont ticks with tail tags containing cyfluthrin and flumethrin was 99.3 and 95.1%, respectively. However, control of bont ticks using alphacypermethrin was only 79.2%. Overall, retention of tail tags was excellent although some loss was encountered during the rainy season. In addition to controlling bont ticks, the tail tags provided moderate control of other tick species (Rhipicephalus evertsi evertsi, Rhipicephalus zambeziensis and Hyalomma spp.) simultaneously infesting cattle in the trials.

Demonstration of vertical transmission of Cowdria ruminantium, the causative agent of heartwater, from cows to their calves.

One of the most important questions about the epidemiology of heartwater in the field is how Cowdria ruminatium is transmitted within vertebrate host populations. In this study vertical transmission of C. ruminantium from cows to their calves was demonstrated. Twelve mixed-breed calves, born to dams living in a heartwater-endemic area of Zimbabwe, were tested post-natally for the presence of C. ruminantium. Vertical transmission was demonstrated to occur under natural field conditions using tests in which uninfected laboratory-reared Amblyomma ticks were fed on neonatal calves and subsequently either fed on, or inoculated into, susceptible small ruminants or tested by a polymerase chain reaction (PCR) assay. Prior to natural Amblyomma tick infestation, C. ruminantium infection in 5 of the 12 calves was confirmed by tick transmission to small ruminants and 11 of the 12 calves tested positive based on PCR analysis of ticks fed on them. The role of colostral cells, as one mode of infection, was demonstrated by the transmission of C. ruminantium to three out of five goats inoculated intravenously with viable colostral cells collected from dams living in a heartwater-endemic area. The significance of vertical transmission is presented in relation to the epidemiology and control of heartwater.

Colostrum from dams living in a heartwater-endemic area influences calfhood immunity to Cowdria ruminantium.

Two studies were carried out to determine whether colostrum from dams living in a heartwater-endemic area has an influence on calfhood immunity to Cowdria ruminantium infection. The initial study was conducted using Friesian calves originating from a heartwater-free herd. Experimental groups consisted of calves receiving colostrum from dams living in a heartwater-endemic area and known to be exposed to C. ruminantium (as determined by the indirect fluorescent antibody test) and calves receiving colostrum from dams located in a heartwater-free area. All calves were challenged at 3 days of age with a homologous C. ruminantium blood stabilate originating from the same area as the endemic colostrum. A significant difference in both the intensity of clinical reactions and percentage of fatal heartwater cases was noted between the two groups of calves. Three of the five calves receiving the colostrum free of C. ruminantium-specific antibodies succumbed to challenge, whereas none of the five calves receiving the colostrum from the heartwater-endemic area showed any clinical reactions. A second similar study was carried out using 12 mixed breed calves born in a heartwater-endemic area of Zimbabwe. The results from this study supported the findings of the first. The significant role of colostrum in calfhood immunity to C. ruminantium infection is discussed in relation to the epidemiology and control of heartwater.

Laboratory reared Amblyomma hebraeum and Amblyomma variegatum ticks differ in their susceptibility to infection with Cowdria ruminantium.

The susceptibility of laboratory reared Zimbabwean Amblyomma hebraeum and A. variegatum ticks to infection with geographically distinct Cowdria ruminantium strains was investigated by feeding both species simultaneously on individual sheep infected with one of the four strains (Crystal Springs [Zimbabwe], Ball 3 [South Africa], Gardel [Guadeloupe] and Nigeria [Nigeria]). A. hebraeum ticks demonstrated a high susceptibility to infection with all four C. ruminantium strains. In comparison, A. variegatum were less susceptible to infection with the Crystal Springs and Ball 3 strains (P < 0.001), but showed a similar susceptibility to the Gardel and Nigeria strains. The differences in susceptibility of A. variegatum to infection with the four strains of C. ruminantium correlated with the origin of these strains. The consistently higher susceptibility of A. hebraeum ticks to infection with geographically different C. ruminantium strains may be one explanation for the observation that heartwater is a more serious problem where A. hebraeum is the vector of the disease.

Effects of tick infestation and tick-borne disease infections (heartwater, anaplasmosis and babesiosis) on the lactation and weight gain of Mashona cattle in south-eastern Zimbabwe.

The effects of ticks and tick-borne disease infections on the lactation and weight gain of Mashona cattle were studied at Mbizi Quarantine Station in the south-eastern lowveld of Zimbabwe. Twenty-nine Mashona cows were allocated to 2 balanced groups and kept in separate paddocks at a stocking rate of one animal per 8 ha. One group received regular acaricide treatment to control bont (Amblyomma hebraeum) and other ticks. The other group was left untreated. The cows were artificially inseminated. The acaricide-treated cows and calves were essentially tick free throughout the experiment, while the untreated cows and calves were continuously tick infested. There was a drought-related decline in tick infestations in the second year of the experiment. Antibodies to Cowdria ruminantium, Babesia bigemina and Anaplasma marginale were detected in cows and calves from both groups, though the untreated group had significantly higher titres to C. ruminantium (P < 0.001). The total, measured amount of milk suckled by untreated calves was significantly more than treated calves (273 kg vs. 241 kg, P < or = 0.05). By interpolating between the twice weekly measurements, it was calculated that over the entire lactation untreated calves suckled an average of 935 kg/hd vs. 837 kg/hd for the treated group. There were no statistical differences in the weights of the 2 groups of calves at birth, weaning, 180 and 210 days post partum (P < 0.05). For cows, there were no statistically significant differences in gestation periods (288 vs. 279 days), reconception rates or weight patterns over time (P < 0.05). The results show that intensive acaricide treatment in areas of Zimbabwe where heartwater is enzootically stable is uneconomical. The maintenance of enzootic stability for tick-borne diseases through minimal tick control is clearly a more economic and practical control option.

Generation and characterization of cloned Theileria parva parasites.

A 3-step procedure for cloning Theileria parva parasites was developed. The first step involved the in vitro infection of a fixed number of bovine lymphocytes with titrated sporozoites. The cell lines obtained from infections initiated using sporozoite/lymphocyte ratios below 1:100 were then selected for cloning as these contained schizont-infected cells, each of which was derived from infection with a single sporozoite. In the second step, these cell lines were cloned by limiting dilution. As sporozoites infect lymphocytes and transform to induce clonal multiplication, this step produced infected cell lines containing both cloned parasites and cloned lymphocytes. In the third step, the cloned cell lines were used to infect cattle and isolation of the parasite in ticks was made during piroplasm parasitaemia. Finally, sporozoites were harvested from infected ticks and used for further characterization. Sporozoites derived from cloned cell lines of T. parva Muguga, Marikebuni, Boleni, Uganda and buffalo-derived 7014 were characterized using monoclonal antibody profiles, DNA restriction fragment length polymorphism detected using repetitive and telomeric probes, in vivo infectivity and, in one case, cross-immunity studies. Additionally, several distinct schizont-infected lymphocyte clones were isolated from the Muguga, Mariakani and buffalo-derived 7014 stocks. The combined results of the characterization revealed that the cloning procedure selected clones of T. parva from the parental stocks which were known to contain a mixture of genetically different parasite populations. The cloning method and the clones generated will be of value in studies of the biology of the parasite and in elucidating the strain specificity of immune responses in cattle.

The relationship between tick (Amblyomma hebraeum) infestation and immunity to heartwater (Cowdria ruminantium infection) in calves in Zimbabwe.

The occurrence of endemic stability for heartwater (Cowdria ruminantium infection) is a controversial issue, because the means by which young cattle and other neonatal ruminants become infected and acquire immunity to the disease have never been adequately explained. We conducted a study in a heartwater-endemic area in southeastern Zimbabwe to investigate the relationships between calf immunity to C. ruminatium, infestations of the tick vector Amblyomma hebraeum and dam, colostral and calf antibody titres to C. ruminantium. Two groups of cows (tick-infested and acaricide-treated) were artificially inseminated and then monitored through pregnancy and lactation by means of tick counts and serum antibody titres. The calves born to the cows in each treatment group were similarly monitored from birth until after weaning, when they were challenged with a heartwater stabilate (Ball-3 vaccine). Colostrum was collected from the cows on the days that the calves were born. Serum and colostrum samples were screened for Cowdria-specific antibodies using an indirect fluorescent antibody test. The cows and calves in the acaricide-treated group remained essentially tick-free for the duration of the study. The cows in the tick-infested group were continuously infested with A. hebraeum, but their calves only became infested between 12 and 31 weeks after birth. Cowdria-specific antibodies were detected in the sera of the cows and calves and in the colostrum of both treatment groups, but the titres were consistently higher in the tick-infested group than in the acaricide-treated group. Antibodies, probably of maternal origin since the precolostral serum titres were negative, were present in the calves of both groups for at least 8 weeks after birth. Between Weeks 24 and 52 postpartum, the antibody titres of the tick-infested group of calves were significantly correlated with the numbers of A. hebraeum present on the animals. The antibody titres of the calves in the acaricide-treated group increased considerably following challenge with heartwater stabilate, 60 weeks after birth. No clinical heartwater was detected in either group of calves following inoculation of stabilate, and it was concluded that the calves in both groups were immune to the disease. In the acaricide-treated group, immunity did not correlate with exposure to tick-transmitted infections. Hence, we concluded that the calves in both groups had probably been infected by vertical transmission around the time of birth. Such early infections are likely to have been controlled by maternal antibodies or by an unidentified immune mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Development and evaluation of PCR assay for detection of low levels of Cowdria ruminantium infection in Amblyomma ticks not detected by DNA probe.

The sensitivities of a PCR assay and a DNA probe assay were compared for the detection of Cowdria ruminantium in Amblyomma ticks that were fed on C. ruminantium-infected, clinically reacting, and recovered carrier animals. The PCR assay and DNA probe detected infection in 86.0 and 37.0%, respectively, of 100 ticks fed on a febrile animal. In 75 ticks fed on carrier animals, PCR and the DNA probe detected infection in 28.0 and 1.33% of ticks, respectively. This demonstrates that the DNA probe has poor sensitivity for the detection of low levels of infection in ticks and that PCR is necessary for this purpose. The PCR assay had a detection limit of between 1 and 10 C. ruminantium organisms and did not amplify DNA from Ehrlichia canis, which is phylogenetically closely related to C. ruminantium, Theileria parva, or uninfected Amblyomma hebraeum or A. variegatum. PCR detected infection in A. hebraeum and A. variegatum adult ticks infected with one of six geographically different C. ruminantium strains. Amplification was also possible from desiccated ticks and ticks fixed in 70% ethanol, 10% buffered formalin, or 2% glutaraldehyde. The PCR assay supersedes the DNA probe and older detection methods for the detection of C. ruminantium in ticks, particularly those fed on carrier animals, and is suitable for both prospective and retrospective studies which require accurate detection of C. ruminantium in individual ticks. Application of the PCR assay should significantly improve the understanding of heartwater epidemiology, particularly through the determination of field tick infection rates.

Factors affecting the distributions of the ticks Amblyomma hebraeum and A. variegatum in Zimbabwe: implications of reduced acaricide usage.

The ticks Amblyomma hebraeum and A. variegatum are the main vectors of heartwater, a disease of ruminants caused by Cowdria ruminantium, in the agricultural areas of Zimbabwe. At present, A. hebraeum is widely distributed in the dry southern lowveld, and occurs in at least seven foci in the higher rainfall highveld. Amblyomma variegatum occurs in the Zambezi valley and surrounding dry lowveld areas in the northwest. The distribution of A. hebraeum has changed considerably over the past 70 years, while that of A. variegatum appears to have remained fairly static. The distribution patterns of both species in Zimbabwe display anomalous features; the ticks occur in areas of lowest predicted climatic suitability for survival and development and in areas where the densities of cattle, the most important domestic host, are lowest. The only factor favouring the survival of the species in the lowveld habitats in which they occur is the presence of alternative wildlife hosts for the adult stage. Their absence from more climatically favourable highveld habitats appears to have been the result of intensive acaricide treatment of cattle over a long period and a historic absence of significant numbers of wildlife hosts. Eradication of A. hebraeum and A. variegatum by intensive acaricide treatment of cattle can be achieved in the absence of significant numbers of alternative hosts, because of the long attachment and feeding periods of the adults of these tick species. However, eradication becomes impossible when alternative hosts for the adult stage are present, because a pheromone emitted by attached males attracts the unfed nymphal and adult stages to infested hosts. The unfed ticks are not attracted to uninfested hosts, such as acaricide-treated cattle. Regular acaricide treatment of cattle is expensive and so, for economic reasons, the Government of Zimbabwe is no longer enforcing a policy of strict tick control. It is likely that reduced tick control will result in the spread of Amblyomma ticks to previously uninfested areas. Added to this, recent introductions of various wildlife species to highveld commercial farming areas have created conditions in which the ticks could become established in higher rainfall areas. Amblyomma hebraeum is more likely to spread than A. variegatum, because its adults parasitize a wider range of wildlife hosts (warthogs, medium to large-sized antelope, giraffe, buffalo and rhinoceros), whereas adults of A. variegatum appear to be largely restricted to one wildlife species (buffalo) in Zimbabwe, the distribution of which is now confined to very limited areas of the country, as part of foot and mouth disease control measures.(ABSTRACT TRUNCATED AT 400 WORDS)

Pheromonal composition of two species of African Amblyomma ticks: similarities, differences and possible species specific components.

Two species of bont ticks, Amblyomma hebraeum and Amblyomma variegatum, common to Africa were studied to compare types and quantities of compounds known or believed to serve as components of the attraction-aggregation-attachment pheromone (AAAP). A complex of attraction, aggregation and attachment stimulating pheromone components are used by these ticks to detect hosts, mates and, perhaps, minimize interspecific breeding. Solvent extraction of pheromone emitting ticks followed by gas chromatography and mass spectrometry revealed little qualitative difference in the composition of the AAAP in these two species. However, subtle differences in the relative proportions of the pheromonal blend are noted suggesting that such differences in concentration may facilitate species-specific discrimination during aggregation and attachment. Differences in the relative abundance of benzaldehyde and methyl salicylate in the males of the two species were especially noteworthy. Possible means by which such differences in phenolic compound composition may affect the behavior of these ticks are discussed.

Correlation between antibodies to Cowdria ruminantium (rickettsiales) in cattle and the distribution of Amblyomma vector ticks in Zimbabwe.

Cowdriosis, caused by Cowdria ruminantium, is transmitted by Amblyomma ticks, which are widely distributed in Zimbabwe. To assess the distribution of this disease in Zimbabwe, cattle either exposed to Amblyomma ticks or maintained in areas free from these ticks were tested for antibodies to Cowdria. A total of 324 sera were tested using competitive ELISA and the indirect fluorescent antibody test (IFAT). At diptanks in Amblyomma-infested areas 52% (n = 95) and 26% (n = 47) of sera were positive by cELISA and IFAT, respectively. At diptanks in Amblyomma-free areas 11% (n = 125) and 10% (n = 134) of sera were positive by cELISA and IFAT, respectively. The results were significantly different between Amblyomma-infested and tick-free areas (chi 2 = 24.73, P < or = 0.005 for IFAT and chi 2 = 57.53, P < or = 0.005 for cELISA). High background readings in field sera, possibly due to cross-reactive antibodies to Ehrlichia spp., complicated the determination of a realistic cut-off point, especially in cELISA. On the basis of the distribution of Amblyomma ticks, currently a large part of Zimbabwe can be considered endemic for the disease.