Biochemical investigation into the toxic salivary secretion of the tick, Ornithodoros savignyi Audouin (1827)

Abstract

Ticks, including the Sand tampan Ornithodoros savignyi, pre= sent important economic problems over a large area in the Republic of South Africa (S 1), In all known diseases of which ticks are the vectors, the causal agents (protozoa, rickettsias, Ana plasms, viruses, spirochetes and bacteria) have been identified with the exception of tick toxicoses in which pathogenic agents are evidently not involved (S 2) - Thus, the causal agents associated with tick toxicoses caused by the Sand tampan have not been identified. These causal agents appear to develop only in the invertebrate host (the ticks) and their timely removal from the vertebrate hosts, is followed by a subsidence of the symptoms. Recovered animals develop a durable immunity (S 3) The observations regarding tick paralysis and sweating sickness are most likely also applicable to tick toxicosis caused by the Sand tampan. Claims of toxins present in the salivary secretions of ticks have been made (S 1). Evidence supporting these claims arc presented in this thesis. Information is provided concerning the chemical composition of the salivary secretion and ~n isolation procedure by means of which a toxic component present in the secretion may be obtained in a pure form. In addition, some chemical components and activities in the oral secretion of the bontbosluis, l\. nblyomrna hebraeurn, were investigated and the results reported. Sand tampan ticks used for the investigations were collected in the North West Cape Kalahari with traps baited with solidified carbon dioxide (S 5), Oral secretions of the ticks were obtained by parasympathetic stimulation as described by Howell (S 6), Some properties of the secretion are shown in Table S l, the enzymatic activities undoubtedly enhance the entry of the toxin into the tissues of the host. Isolation of the toxic activity was achieved by means of gel and ion exchange chromatography and isoelectric focusing. The toxin was found to be homogeneous according to ultra-centrifugal and gel-electrophoretic techniques and isoelectric focusing. Some characteristics of the toxic component are presented in Table S 2, Removal of the carbohydrate moiety or s-carboxymethylation of the toxin results in a complete loss of toxic activity. A non-toxic glycopeptide which showed most of the above characteristics was found in the salivary secretion. It differed slightly from the toxin with respect to the isoelectric point (0,3 units lower) and amino acid sequence (Table S 3). A detailed structural comparison of these two glycopeptides could aid in the elucidation of the structural requirements essential for toxicity. The syndromes produced by the salivary secretion (S 25) indicate that the toxin may exert its effect via increased capillary permeability or fragility. Since a method has been developed for the purification of the toxin and its chemical nature has been partially described, the biochemical lesion(s} can now be investigated. The total free amino acids present in the salivary secretion of the bontbosluis, Amblyomma hebraeum was determined. A total of 1553 μg of amino acids per ml secretion was found. This value is ·approximately 3 times higher than the value for the Sand tampan and approximately ·20 times higher than for human saliva. The salivary secretion of this tick was fractionated by gel chromatography. One of the fractions showed weak hyaluronidase activity and was found to be homogeneous according to sedimentation equilibrium centrifugation. The molecular mass, determined by the latter method was found to be 9680. The amino acid composition of the fraction revealed, with a few exceptions, similarity with that of hyaluronidase isolated from bovine testicular tissue.