Amblyomma variegatum (Acari: Ixodidae): mechanism and control of arbovirus secretion in tick saliva

Saliva is considered to be the conduit by which pathogens are transmitted from blood-sucking arthropod vectors to their vertebrate hosts, but supporting evidence for this is fragmentary. To determine if Thogoto (THO) virus, a tick-borne member of the influenza virus family, is transmitted via tick saliva, and whether virus replication is a prerequisite for such transmission, two experimental conditions were compared: (1) "biological transmission" and (2) "mechanical transmission." In (1), THO virus was allowed to infect and replicate in a natural vector, Amblyomma variegatum: virus was detected in saliva collected from 3/22 (14%) ticks. In (2), virus was inoculated directly into the hemocoel with the drug used to induce salivation and saliva was collected immediately to preclude the possibility of virus replication: virus was detected in saliva collected from 31/170 (18%) ticks. The results demonstrate that THO virus is secreted in tick saliva and that virus can pass from the hemolymph to the salivary glands independently of viral replication within the tick. The comparatively low numbers of ticks that yielded virus-positive saliva samples together with the results from assays of serial saliva samples suggested that virus secretion may not be a continuous process during salivation. Ticks in which THO virus had established an infection showed an impaired secretory response compared with uninfected ticks and ticks used for mechanical transmission.     

Impact of natural infestation of Amblyomma variegatum on the liveweight gain of male Gudali cattle in Adamawa (Cameroon)

The effect of natural tick infestation on the liveweight gain (LWG) of male Gudali zebu cattle was studied throughout a year by comparing the performances of two herds, one of which was submitted to weekly acaricidal treatment and the other was left untreated against ticks. Six species of ticks were identified on the untreated animals: Amblyomma variegatum, Boophilus decoloratus, Rhipicephalus lunulatus, Rhipicephalus turanicus, Hyalomma nitidum and Hyalomma marginatum rufipes. Most of the losses observed in the untreated herd during the rainy season were due to A. variegatum, and the loss in LWG was estimated to be 55-76 g per engorged female A. variegatum. The infestation also leads to wounds and to lesions of dermatophilosis. There was an interval between the peak infestation by A. variegatum and the appearance of weight loss owing to them. The control of ticks on the Gudali zebu in Adamawa, during the months of high infestation by A. variegatum adults, is economically profitable. On the other hand, the performances of the two herds during the dry season were similar, showing that infestation by larvae and nymphs of A. variegatum has no impact on the zebu LWG, and that tick control during that period is not profitable.     

Acquisition of resistance to the tick Amblyomma variegatum in Boran cattle, Bos indicus and the effects of Trypanosoma congolense and Babesia bigemina on host resistance

Resistance was induced in cattle to the tick Amblyomma variegatum by five consecutive infestations with nymphs and adults. Using the principal component analysis (PCA), it was found that percentage of adults engorged, percentage of adults which died, percentage of nymphs which engorged, percentage of nymphs which moulted and percentage of nymphs which died, were the main indicators of resistance against A. variegatum. The percentages of nymphs which engorged or moulted after the third infestation were significantly (P < 0.01) reduced while the percentage of nymphs which died increased significantly (P < 0.01) after the third infestation. Percentages of adults which engorged or died started to decrease significantly (P < 0.01) from the fourth infestation after an initial increase during this period. The acquisition of resistance by cattle to the adult ticks was slower than to the nymphs. Infection of cattle with Trypanosoma congolense and Babesia bigemina after the fifth infestation enhanced the acquired immunity as revealed by the significantly (P < 0.01) increased feeding period of the adult ticks and changes in other parameters.     

Kinetics of male pheromone production by Amblyomma variegatum (Acari: Ixodidae)

Males of Amblyomma variegatum (F.), when attached on their host, produce a pheromonal blend composed of methyl salicylate, ortho-nitrophenol, and nonanoic acid, which acts as an aggregation-attachment pheromone. Using olfactometry assays, assays on hosts, and quantification of the compounds by capillary gas chromatography, the kinetics of pheromone production has been studied. Males engorged for at least 10 d elicit attachment from most of the tested females. Attractiveness of males for females is significant only between days 14 and 23 of engorgement. Before 10 d of attachment, the amounts of pheromones detected in hexanic male body extracts are very low (< 10 ng); they then increase to a maximum of 2 micrograms for the major compound per male. Large amounts can be detected until 80 d of engorgement. The inter-individual variation is very large throughout the engorgement period. The interest and the limitations of the different techniques used are discussed.     

Effects of cattle tick (Boophilus microplus) infestation on the bovine immune system

The immunosuppressive effect of experimental Boophilus microplus infestation on bovine peripheral blood lymphocytes (PBL) and on host antibody production to a protein antigen (ovalbumin) was examined. Boophilus microplus infestation caused a marginal decrease in the percentage of T lymphocytes in PBL, which was observed in both lightly (5000 larvae) and heavily (40,000 larvae) infested cattle, and began at the second infestation and continued until the end of the fourth infestation. The percentage of B lymphocytes in heavily tick-infested cattle was less than that in non-infested control cattle after the fourth infestation. The response of PBL from tick-infested cattle to phytohemagglutinin (PHA) was always less than that of tick-free cattle after the second infestation. No noteworthy differences were detected between the three stages of tick infestation, that is, 1 week before the peak of adult engorgement, the middle of the peak and 1 week after all ticks had dropped. Boophilus microplus saliva (100 microliters ml-1) suppressed 47% of the response of bovine PBL to PHA in vitro. This suppressive effect of saliva may contribute to the lower responsiveness of PBL from tick-infested cattle. Antibody production by tick-infested cattle was examined during the third and fourth heavy tick infestation. Tick-infested cattle showed a diminished response against ovalbumin after the second immunization. The immunosuppressive effects of tick infestation may play an important role in tick survival or in the transmission of tick-borne diseases in the field.     

A comparison of the effect of two regimens of infestation on the development of resistance by cattle to the cattle tick, Boophilus microplus (Can.)

The development of resistance to Boophilus microplus by cattle was studied using sets of cattle twins, in stalls. One twin from each set received 3 infestations of 40,000 larvae and the other a continuous infestation of 1,000 larvae a day over the same period. Sets of twins were then challenged with 1,000 larvae a day for 40 days and 2 field infestations of 20,000 larvae. Correlations of ranking for resistance made at 40-day periods during daily infestations of 1,000 larval ticks or for corresponding intermittent infestations of 40,000 larvae were low until animals had received 120,000 larvae. Thereafter irrespective of method of infestation correlations of ranking were relatively high (r = 0.61-0.96) between periods of infestations or method of infestation including 2 field infestations. Fewer adult female ticks matured on the daily infested animals than on the intermittently infested animals during the treatment period, but animals developed a similar resistance level whether infested with either technique.     

Attitudes and practices of Queensland dairy farmers to the control of the cattle tick, Boophilus microplus

OBJECTIVE: To determine practices for the control of cattle ticks on dairy farms in Queensland, the attitudes of farmers to tick infestations and to identify opportunities for and barriers against the introduction of non-chemical methods of tick control. DESIGN: A survey of 199 dairy farmers from tick-infested parts of Queensland was undertaken by 20 dairy advisers and stock inspectors from October 1996 to June 1997. The sample was a proportional, random selection of dairy farms from four regions. A personal interview was conducted with each farmer and answers to 134 questions were obtained. RESULTS AND CONCLUSIONS: Most farmers were not concerned by cattle ticks on their own farms, although they believed that ticks are important to the dairy industry. They were most concerned about the development of chemical resistance by cattle ticks. Inadequate facilities and lack of motivation appeared to be the factors most limiting to improving the methods of control. Most farmers claimed to have only small numbers of ticks at worst. Although a control program recommended by the Queensland Dairyfarmers' Organisation was well regarded by farmers, few had adopted it. Many farmers saw no need to implement a strategic control program.     

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)     

The utilisation of creatine and its analogues by cytosolic and mitochondrial creatine kinase

We have investigated the utilisation of four analogues of creatine by cytosolic Creatine Kinase (CK), using 31P-NMR in the porcine carotid artery, and by mitochondrial CK (Mt-CK), using oxygen consumption studies in isolated heart mitochondria and skinned fibers. Porcine carotid arteries were superfused for 12 h with Krebs-Henseleit buffer at 22 degrees C, containing 11 mM glucose as substrate, and supplemented with either 20 mM beta-guanidinopropionic acid (beta-GPA), methyl-guanidinopropionic acid (m-GPA), guanidinoacetic acid (GA) or cyclocreatine (cCr). All four analogues entered the tissue and became phosphorylated by CK as seen by 31 P-NMR, Inhibition of oxidative metabolism by 1 mM cyanide after accumulation of the phosphorylated analogue resulted in the utilisation of PCr, beta-GPA-P, GA-P and GA-P over a similar time course (approximately 2 h), despite very different kinetic properties of these analogues in vitro. cCr-P was utilised at a significantly slower rate, but was rapidly dephosphorylated in the presence of both 1 mM iodoacetate and cyanide (to inhibit both glycolysis and oxidative metabolism respectively). The technique of creatine stimulated respiration was used to investigate the phosphorylation of the analogues by Mt-CK, Isolated mitochondria were subjected to increasing [ATP], whereas skinned fibres received a similar protocol with increasing [ADP]. There was a significant stimulation of respiration by creatine and cCr in isolated mitochondria (decreased K(m) and increased Vmax vs control), but none by GA, mGPA or beta-GPA (also in skinned fibres), indicating that these latter analogues were not utilised by Mt-CK. These results demonstrate differences in the phosphorylation and dephosphorylation of creatine and its analogues by cytosolic CK and Mt-CK in vivo and in vitro.     

Methicillin-resistant Staphylococcus aureus and its relationship to antimicrobial use: possible implications for control

The control of methicillin-resistant Staphylococcus aureus (MRSA) is still an unresolved issue in numerous healthcare institutions worldwide. Guidelines for the control of MRSA in hospitals focus on measures to control cross-transmission and prevent colonization, but rarely specifically mention the control of antimicrobial use. We reviewed the different types of evidence for a causal relationship between MRSA and antimicrobial use by classifying them in four categories: consistent associations, dose-effect relationships, concomitant variations, and arguments to support a plausible biological model to explain this relationship. Although the relative participation of cross-transmission and antimicrobial selection pressure in the level of MRSA observed in a healthcare setting remains to be determined, we found lines of evidence to support the existence of a relationship between MRSA and antimicrobial use in each of the four categories. This review points out the relative lack of studies specifically designed to investigate this aspect of MRSA epidemiology and the need to implement such studies quickly. In the meantime, the results presented here should encourage the implementation of antimicrobial-use improvement programs in hospitals in addition to existing infection control measures, which are still a priority in countries with high MRSA prevalence.     

Trials for control of ixodid ticks using pheromone acaricide tick decoys

Experimental application of Hyalomma dromedarii pheromone-impregnated in cyfluthrin (pyrethroid) decoys on experimentally infested camels revealed efficacy rate of 85.33%, while the control group showed normal pattern of tick engorgement. This study proved the rapid movement of males toward decoys and some males took the mounting position with such decoys. On the other side, the same decoys (H. dromedarii) when used on naturally infested cattle with Boophilus annulatus did not even disturb the Boophilus ticks indicating that B. annulatus contain specific pheromones which should be furtherly identified.     

Field studies and cost-effectiveness analysis of vaccination with Gavac against the cattle tick Boophilus microplus

This study presents findings from an updated retrospective cohort mortality study of male police officers from January 1, 1950 to December 31, 1990 (n = 2,593; 58,474 person-years; 98% follow-up). Significantly higher than expected mortality rates were found for all cause mortality (Standardized mortality ratio [SMR] = 110; 95% confidence interval [95% CI] = 1.04-1.17), all malignant neoplasms (SMR = 125; 95% CI = 1.10-1.41), cancer of the esophagus (SMR = 213; 95% CI = 1.01-3.91), cancer of the colon (SMR = 187; 95% CI = 1.29-2.59), cancer of the kidney (SMR = 2.08, 95% CI = 100-3.82), Hodgkin's disease (SMR = 313; 95% CI = 1.01-7.29), cirrhosis of the liver (SMR = 150; 95% CI = 1.00-2.16), and suicide (SMR = 153; 95% CI = 1.00-2.24). All accidents were significantly lower (SMR = 53; 95% CI = 0.34-0.79). Mortality by years of police service showed higher than expected rates for (1) all malignant neoplasms in the 1- to 9-years-of-service group; (2) all causes, bladder cancer, leukemia, and arteriosclerotic heart disease in the 10 to 19-year group; and (3) colon cancer and cirrhosis of the liver in the over 30 years of service group. Hypotheses for findings are discussed.     

The use of vaccines and genetically resistant animals in tick control

The control of ticks and diseases transmitted by ticks is extremely difficult. Application of acaricides is the most common prophylactic and therapeutic control measure against these ectoparasites. The selection of tick strains which are resistant to these products, the appearance of chemical residues in milk and meat, and environmental pollution resulting from the use of acaricides pose real problems. This article deals with aspects of current work on the alternative control of ticks and places special emphasis on the development of vaccines and the utilisation of genetically resistant animals.     

Risk of contamination by Borrelia burgdorferi S. lato in a forest environment. Survey during 13 months of abundance of the tick Ixodes ricinus and its level of infestation by the Lyme borreliosis agent in Brittany

The authors, in a forest in Brittany previously studied for several years, caught by flagging, each month from April 1992 to May 1993, nymphs of I. ricinus tick, and looked by indirect immunofluorescence, for B. burgdorferi infestation. An amount of 1,506 ticks was thus studied. Infestation frequency was varying from 0 per cent in January and February to 14.4 per cent in August. Standarding of tick collecting method allowed to establish, for each month, a tick, "availability" index, and, according to the spirochete infestation frequency, to do estimation of the risk level, for human visiting the concerned forest, of being infected by B. burgdorferi. Obtained results show that this risk is the highest in August, and quite non-existent in January and February.     

The distribution of Schistosoma haematobium in the Isoka district, Zambia; and a possible strategy for its control

The distribution of schools prevalent for Schistosoma haematobium in the Isoka district, Zambia was estimated by examining haematuria in the urine of the pupils found in Grades Three, Four or Five using reagent sticks. Thirty three (57 pc) schools had prevalence rates of 25 pc or more. The distribution of S. haematobium was patchy with significant differences in prevalence rates between some areas only short distances apart. A sociological study in the same schools showed that 68 (97 pc) head/senior teachers associated the disease with blood in urine and agreed to perform a reagent stick test on their pupils' urine. Thirty five (50 pc) of these respondents considered S. haematobium infection as a major problem and 66 (94 pc) of them were ready to administer a diagnostic questionnaire to their pupils in a study to identify high risk schools for S. haematobium. We conclude that the identification of high risk schools in the Isoka district, Zambia, using a diagnostic questionnaire and reagent stick testing by teachers, should proceed as a step to controlling S. haematobium infection in the district.     

Variability of choline acetyltransferase in ocular tissues of rabbits, cats, cattle and humans

It has been proved in three rodent species that in the insoluble collagen fraction which accumulates in skin collagen with age of the two categories of collagen present (collagen type I and collagen type III), their proportion alters in favour of collagen type I with the advancing age. Since it has also been shown that collagen type I is less resistant towards proteolytic cleavage than is collagen type III its accumulation can be explained either by rapidly advancing cross-linking of this collagen type or more likely by different proteosynthesis. The second alternative is preferred since a step-wise polymerization of collagen type III was also observed. No information revealing to what extent the lysine derived cross-links can combine both collagen types is at present available. On the basis of this information the rapid decrease in insoluble collagen in very early ontogeny (rats below 8 weeks of age) is explained.