Ostertagia ostertagi: establishment of patent infections in calves by intravenous inoculation

A stochastic model to investigate the effects of control strategies on calves exposed to Ostertagia ostertagi

Predicting the effectiveness of parasite control strategies requires accounting for the responses of individual hosts and the epidemiology of parasite supra- and infra-populations. The first objective was to develop a stochastic model that predicted the parasitological interactions within a group of first season grazing calves challenged by Ostertagia ostertagi, by considering phenotypic variation amongst the calves and variation in parasite infra-population. Model behaviour was assessed using variations in parasite supra-population and calf stocking rate. The model showed the initial pasture infection level to have little impact on parasitological output traits, such as worm burdens and FEC, or overall performance of calves, whereas increasing stocking rate had a disproportionately large effect on both parasitological and performance traits. Model predictions were compared with published data taken from experiments on common control strategies, such as reducing stocking rates, the ‘dose and move’ strategy and strategic treatment with anthelmintic at specific times. Model predictions showed in most cases reasonable agreement with observations, supporting model robustness. The stochastic model developed is flexible, with the potential to predict the consequences of other nematode control strategies, such as targeted selective treatments on groups of grazing calves.

The effect of truncated infections with Ostertagia ostertagi on the development of acquired resistance in calves

The relative contribution of the third (L3), fourth (L4) and adult stages of Ostertagia ostertagi to the development of immunity was assessed in calves which were either continuously infected during 21 weeks or subjected to infections truncated by anthelmintic treatment at the L3 or L4 stage. A fourth group remained uninfected (control group). Faecal samples and blood samples were collected weekly for faecal egg counts and determination of pepsinogen and antibody levels. Only the continuously infected animals showed positive egg counts, which fell towards the end of the primary infection period. Pepsinogen and antibody levels remained high in the continuously infected group until the end of the primary infection period. At that time, they were significantly higher compared to the control calves, with intermediate values in the truncated infection groups. After the 21 weeks primary infection period all animals were dewormed. To evaluate the protection provided by the different immunisation protocols, all animals were challenged 1 week later with 156000 Ostertagia L3, spread over 12 consecutive days. The marked reduction in egg counts following challenge infection indicated a certain degree of immunity in the continuously infected calves, which was confirmed at necropsy by the reduced worm burdens, the high percentage of inhibited early L4 larvae, the reduced size of the adult worms and the higher numbers of mucosal mast cells in this group. Numbers of globule leucocytes and eosinophils were not significantly different from the control group. Infections truncated by anthelmintic treatment elicited poor development of immunity as shown by the egg output after the challenge infection and the percentages of arrested larvae and the lengths of adult worms which were intermediate to those of the continuously infected calves and control animals.

Isolation, characterization and immunolocalization of a globin-like antigen from Ostertagia ostertagi adults

Western blot analysis using an anti-globin rabbit serum Rb94 revealed a major band of 17 kDa in extracts of Ostertagia ostertagi adults and 4th-stage larvae. The adult stage globin-like antigen (OoAdGlb) was purified from total worm extracts by liquid chromatography. The protein has an estimated molecular mass of 36 kDa under non-reducing conditions, suggesting a dimeric structure containing 2 non-covalently linked 17 kDa monomers. Tryptic peptides were sequenced and showed strong similarities with the globins of free-living and parasitic nematodes. Immunolocalization revealed the presence of this globin-like antigen in the body wall musculature and/or the cuticle of O. ostertagi adults. An enzyme-linked immunosorbent assay based on the purified OoAdGlb showed no differences in response between calves infected by O. ostertagi and/or Cooperia oncophora and the negative controls.

Regulation of immunity to Ostertagia ostertagi

Knowledge of bovine immune response to ostertagiasis is important to understanding the mechanisms of innate and acquired immunity to this economically important helminth parasite that infects cattle worldwide. Infection causes both antibody and cellular immune responses. Evidence shows that Ostertagia possesses excretory-secretory (ES) molecules that may regulate immune cell responses that affect acquired immunity and pathophysiological changes to infection. Ostertagia can down-regulate antibody and cellular immune responses. One of these ES regulatory molecules is a lectin that causes eosinophil chemotaxis. In addition to its antigenicity, this regulatory molecule serves as a means of communication between the parasite and cells of the host immune system. It is suggested that, lacking this type of communication, Ostertagia infection may not be readily recognized by the host immune cells. A hypothesis is proposed for the mechanisms of acquired immunity to Type I ostertagiasis. Regulatory molecules of Ostertagia ES are suggested as suitable vaccine candidates.

Attempts to immunize cattle against Ostertagia ostertagi infections employing 'normal' and 'chilled' (hypobiosis-prone) third stage larvae

Immunity to Ostertagia ostertagi infections in calves develops slowly and only becomes manifest towards the end of a grazing season in which they have been exposed to the parasite. In an attempt to hasten the onset of immune reactions, three immunization protocols were set up. Twenty four heifers were allocated into four groups. Beginning in January, animals in two of the groups were inoculated with four 1-monthly increasing dosages of either 'normal' or 'chilled' (hypobiosis-prone) larvae, those in the third group received a single large infection with 'chilled' larvae and those in the fourth group served as non-infected controls. All animals were turned out on a common pasture in late April. Development of immunity was evaluated through determinations of faecal egg counts, live weight gains, serum pepsinogen levels and specific serum antibody responses of three isotypes (IgG1, IgG2 and IgA). Significantly reduced egg excretions in the immunized groups were apparent early in the season, indicating that the immunizations had, in this respect, been efficacious. The 'chilled' and 'normal' larvae seemed equally efficient given as multiple and single infections. A single large dosage of 'chilled' larvae seemed to have adverse effects. Only moderate antibody responses were elicited probably because of low challenge infection level on pasture. Considerable variation in responses existed between and within the four groups, for which reason conclusions regarding correlations between antibody isotype responses and immune effects on parasites could not be made.

Immunity to Ostertagia ostertagi

Control of ostertagiasis is a major economic problem in the temperate areas of the world. An immunological control scheme for cattle is an important aspect of integrated control and its achievement is dependent on our understanding of how immunity is acquired. Also, host mechanisms regulated by Ostertagia need to be understood in order to address immunological questions. Little is known about acquired immunity to Ostertagia infection in cattle. The degree of immunity is incomplete and its development slow. Evidence is accumulating for impairment of antibody and cellular immune responses by Ostertagia which is responsible for the survival of the parasite in the young host. Other experimental work suggests that local factors, such as eosinophil accumulation, mucosal mast cell kinetics and other cellular and humoral immune responses, may play a role in pathogenesis and immunity. At the present time these areas are also totally unexplored in the study of Ostertagia infection.

Observations on the routes of infection of Oswaldocruzia filiformis (Nematoda: Trichostrongylidae) in amphibia

The oral, percutaneous and subcutaneous routes of infection of Oswaldocruzia filiformis were investigated in amphibia. Tadpoles of Bufo bufo and Rana temporaria can be infected with O. filiformis when kept temporarily in a suspension of infective larvae in water. Larval stages and subadults were found in tadpoles. All stages of the parasite, including egg-producing females, were found after metamorphosis of the host. However, under natural circumstances infection of tadpoles seems unlikely. Oral infections in metamorphosed hosts of both species were successful in 97.5% of the host animals used. The first eggs appeared 29 days after infection in the faeces. The oral route seems to be normal for O. filiformis in amphibia. Experiments on percutaneous infections did not reveal actual penetration of larvae in or through the skin nor a subsequent migration through host tissues. Sometimes a few larvae were found in the stomach and intestine, but in these particular cases the experimental conditions did not totally exclude the possibility of oral infections. Consequently, the percutaneous route of infection is not plausible for O. filiformis. Subcutaneous inoculation of infective larvae seems to be a possible way of establishing experimental infections. Erratic localisation of the parasite in the enlarged gall bladder of the host was observed.