In vitro culture of Oestrus ovis (Linné 1761) first instar larvae: its application to antiparasitic drug screening

Nasal bots...a fascinating world!

Larvae causing obligatory myiasis are numerous and they may affect cutaneous and subcutaneous tissues, wounds, nasopharyngeal cavities (nasal bots), internal organs and the digestive tract (bots) of domestic and wild animals and humans as well. Nasal bots belong to the Family Oestridae, Subfamily Oestrinae, which includes several important genera: Oestrus, Kirkioestrus, and Gedoelstia infecting Artiodactyla (except Cervidae) in Africa and Eurasia, Cephenemyia and Pharyngomyia infecting Cervidae, Rhinoestrus infecting horses, Cephalopina infecting camels, Pharyngobolus infecting African elephants, and Tracheomyia infecting Australian kangaroos. Nasal bots are widespread in Mediterranean and tropical areas and in affected animals they induce sneezing and nasal discharge which may become caked with dust making breathing very difficult. The aforementioned species of larvae are host-specific but sometimes the may be deposited in human eyes inducing a painful opthalmomyiasis of short duration. The first fascinating trait of these parasites is the very efficient morphological and biological adaptations to parasitism they show either as larvae or as adults, in order to facilitate their survival and search for a suitable host. Nasal bots have reached different degrees of complexity in their life cycles. Indeed, while for some species (e.g., Oestrus ovis, Rhinoestrus usbekistanicus) larvae are injected by flies directly into nostrils and develop in the sinuses before being ejected for external pupation, some other species migrate from eyes to blood before returning to nasal cavities either through the ethmoid bone (Gedoelstia hässleri) or via lungs and bronchi (Gedoelstia cristata). Moreover, larvae are very well-adapted to their environment being able to undergo through hypobiosis either inside or outside the host, according to the climatic environmental conditions and seasonality. The second fascinating trait of nasal bots is related to host behavioural and immune responses against the infection. Host behaviour may in fact prevent larviposition and inflammatory/immune reactions limit larval development. The main pathophysiological mechanisms involve mast cells and eosinophils which destroy the larvae in sensitized animals. The intense eosinophilic reaction has side effects both locally (i.e. on the nasal mucosa) and also generally, with possible interactions with gastrointestinal strongyles (e.g., both worm burdens and fecundity decreased in lambs infected by O. ovis). Infected animals (e.g., sheep, goat, camel, and donkey) firstly suffer from fly strike, when adult flies inject first stage larvae on nostrils: sheep may try to avoid fly swarms but eventually Rangifer tarandus can only manage a terror-stricken look! Secondly, hosts will suffer from myiasis with typical nasal discharge and sneezing related to sinusitis. Clinical manifestations may vary: for example O. ovis induces severe clinical signs in sheep whilst produces few effects in goats! These parasites are diffused in many Mediterranean and tropical countries. Unfortunately, it is commonly believed that bacterial infections induced by nasal bots are of greater clinical importance: this view is not substantiated and the control of this condition depends on treatment with macrocyclic lactones, closantel and nitroxynil. Reinfections are common, and controlling nasal bots is not so simple.

Proteolytic activity in salivary gland products of sheep bot fly (Oestrus ovis) larvae

This study identified and characterized hydrolytic enzymes in salivary gland products of Oestrus ovis larvae. Third instars were collected from the heads of slaughtered goats. Salivary glands were extracted, their products obtained by centrifugation and the enzymatic profile determined. Optimum pH, temperature of maximum proteolytic activity, thermal stability, and resistance of salivary gland products were determined on collagen and subclasses of proteases were identified using protease inhibitors. Zymograms were used to determine the molecular weight of proteases. Antigenic protein bands were revealed by immunoblotting using sera obtained from experimentally infested goats. Seven positive enzymatic activities were detected in salivary gland products: acid phosphatase, naphthol-AS-BI-phosphohydrolase, esterase (C4), esterase lipase (C8), leucine arylamidase, alpha-glucosidase and N-acetyl-beta-glucosaminidase. Optimum pH for proteolytic activity was 8.0; proteolytic activity increased with temperature (10-50 degrees C) then drastically decreased at 60 degrees C. Proteases in O. ovis salivary gland products belong to the serine subclass. In Zymograms, bands of proteolytic activity were detected in the 20-63 kDa range; the immunoblot showed three antigenic bands, one of them related to a protease band (63 kDa). Serine proteases in O. ovis salivary gland products are most likely involved in larval nutrition and host immuno-modulation.

Examination of the migration of first instar larvæ of the parasite Œstrus ovis (Linne 1761) [Diptera: Œstridæ] in the upper respiratory tract of artificially infected lambs and daily measurements of the kinetics of blood eosinophilia and mucosal inflammatory response associated with repeated infection

Twelve lambs were divided into two groups: Group C control, non-infected, and Group O infected once a week for 5 weeks with OEstrus ovis L1 through the same nostril. The first objective of this experiment was to check whether larvae moving through a given nostril remain in the same side nasal cavity or might to spread in both nasal cavities. It has been observed that larvae invade and spread through the entire nasal cavities. The only possible passage way between both sides is via the choanae and velum palatinum. The second objective was to follow the kinetics of blood eosinophilia. A primary peak in eosinophil numbers was noted 4 days following infection, with a higher peak following the second infection. After that, no major changes were seen. Nevertheless, the numbers of eosinophils were always higher than in control animals until the end of the follow-up. The third objective of the study was an enumeration of reactive cells (mast cells, globule leucocytes, and eosinophils) in the mucosae of the upper and lower respiratory tract after necropsy of the animals of the two groups. As observed in previous experiments, there was a large accumulation of these cells in mucosae of the upper respiratory tract. It was also worth noting a significant accumulation of eosinophils in the tissues of the trachea, bronchae and lungs even though OE. ovis was not present there. This "distant" eosinophilic reaction may have important consequences on patho-physiology of other parasites living in these locations: eosinophils have the potential to kill them even though these cells are not activated by their specific antigens.

Genetic structure of Oestrus ovis populations in sheep and goats

A genetic analysis using RAPD markers was performed on 12 natural populations of Oestrus ovis (Linné, 1761). Three-hundred and six O. ovis larvae (first, second and third instars) were randomly recovered in nasal cavities of sheep and goats naturally infected in Algeria, Ethiopia, France, Mauritania, Rumania and Tunisia and were analysed by 56 RAPD fragments. The results showed a high diversity within all samples. A significant genetic divergence was showed by discriminant analyses among the 12 populations sampled (p<0.0001). Moreover, discriminant analyses showed significant differentiation (p<0.0001) between O. ovis larva populations of sheep and goats and also among samples collected in the same region.

Oestrus ovis (Diptera: Oestridae): effects of larval excretory/secretory products on nitric oxide production by murine RAW 264.7 macrophages

Larvae of Oestrus ovis (Insecta: Diptera: Oestridae) are common parasites of nasal and sinus cavities of sheep and goats. Previous studies revealed that crude extracts of larvae modify NO synthesis by ovine monocyte derived macrophages. The aim of this study was to investigate the larval excretory/secretory products effects on nitric oxide production by murine tumour macrophages RAW 264.7. Stimulation of RAW macrophages by excretory/secretory products of the three instars larvae (25 microg/ml) significantly increased nitrite concentrations in culture supernatants compared to negative and positive Escherichia coli lipopolysaccharide control. This effect was time and dose dependent. Nitrite production in culture supernatants was due to induction of isoform NOS-2 because both NG monomethyl L-arginine (100 microM) and dexamethasone (20 microM) inhibited, by 60 and 50%, respectively, nitrite accumulation in culture supernatants. First steps of purification, by ion exchange chromatography, indicated that one protein of 29 kDa was able to induce NO synthesis by macrophages. Further studies are needed for a better characterization of these molecule and to investigate their immunogenicity for a vaccine approach.

Pathophysiology of Oestrus ovis infection in sheep and goats: a review

Oestrus ovis is a very common parasite of sheep and goats in many countries. Its pathological effects are often underestimated because owners and veterinarians are used to seeing the infection. The study of natural and experimental infections has provided information about the evolution of the disease and its pathophysiology. Hypersensitivity is involved; the numbers of mast cells and eosinophils increase but changes in IgE have only recently been examined. Little is known about the development of immunity but it is possible that some animals are immunodeficient.