Experimental Eperythrozoon ovis infection of sheep

Abstract

Extract

Eperythrozoon ovis is an epi-erythrocytic sheep parasite of uncertain classification with a world-wide distribution (Sutton, 1970 Sutton, R. H. 1970. Eperythrozoon ovis — a blood parasite of sheep. N.Z vet. J., 18: 156–164. [Taylor &; Francis Online] , [Google Scholar]), but whether eperythrozoonosis is a disease of economic importance remains unresolved. In Scotland, Foggie and Nisbet (1964 Foggie, A. and Nisbet, D. I. 1964. Studies on Eperythrozoon infection in sheep. J. comp. Path., 74: 45–61.  [Google Scholar]), found that experimentally E. ovis caused a clinical anaemia, occasionally with jaundice, but there was no significant difference in weight gain between infected and control sheep. Similar observations in naturally infected sheep were recorded in Norway by Overas (1969 Overas, I. 1969. Studies on Eperythrozoon ovis infection in sheep. Acta vet. Scand., Suppl. 28 [Google Scholar]), and in Australia by Harbutt (1969b Harbutt, R. Patricia. 1969b. The effect of Eperythrozoon ovis infection on body weight gain and haematology of lambs in Victoria. Aust. vet. J., 45: 500–504.  [Google Scholar]), Other Australian authors have found infection by this parasite; associated with outbreaks of ill-thrift in lambs (Sheriff et al., 1966 Sheriff, D., Clapp, K. H. and Reid, A. Marjorie. 1966. Eperythrozoon ovis infection in South Australia. Aust. vet. J., 42: 169–176.  [Google Scholar]) and severe anaemia with some mortality (Littlejohns, 1960 Littlejohns, I. R. 1960. Eperythrozoon ovis in sheep. Aust. vet. f., 36: 260–265.  [Google Scholar]; Maxwell, 1969 Maxwell, J. A. L. 1969. Eperythrozoon ovis infection in Western Australia. Ausf. vet. J., 45: 436–436.  [Google Scholar]; Campbell et al., 1971 Campbell, R. W., Sloan, C. A. and Harbutt, R. Patricia. 1971. Observations on mortality in lambs in Victoria associated with Eperythrozoon ovis. Aust. vet. J., 47: 538–541.  [Google Scholar]). Although the presence of strains of different pathogenicity has been suggested (Foggie and Nisbet, 1964 Foggie, A. and Nisbet, D. I. 1964. Studies on Eperythrozoon infection in sheep. J. comp. Path., 74: 45–61.  [Google Scholar], 1966 Foggie, A. and Nisbet, D. I. 1966. Eperyihrozoon ovis. Vet. Rec., 79: 297–298.  [Google Scholar]; Harbutt, 1969a Harbutt, R. Patricia. 1969a. The incidence and clinical significance of Eperythrozoon ovis infections of sheep in Victoria. Ausf vet. J., 45: 493–499.  [Google Scholar]), other factors influencing the disease should also be considered (Littlejohns, 1960 Littlejohns, I. R. 1960. Eperythrozoon ovis in sheep. Aust. vet. f., 36: 260–265.  [Google Scholar]).     

Humoral immune response of sheep to infection with Eperythrozoon ovis

Circulating antibody was detected by an indirect fluorescent antibody test (IFAT) in the serum of sheep infected experimentally with Eperythrozoon ovis. Antibodies were first detected 15 to 32 days after infection with E ovis and titres peaked at 41 days. This antibody may be associated, at least in part, with protection against infection with E ovis since the initial increase in antibody titre coincided with a fall in the primary parasitaemia. A role for antibody is suggested further by the fact that the prepatent period of infection was prolonged by one day and the parasitaemia initially remained at low levels in infected sheep protected by passively transferred hyperimmune serum. Moreover, following primary infection, acquired immunity was manifest by a lack of parasitaemia following challenge infections while increased IFA titres were observed. No evidence of opsonic activity was observed in an in vitro erythrophagocytosis test in that neither mouse macrophages nor sheep monocytes phagocytosed E ovis infected or uninfected erythrocytes sensitised with hyperimmune serum.     

The effects of Eperythrozoon ovis in sheep

Infection of adult sheep with a single strain of Eperythrozoon ovis led to three different situations. First, the animal resisted the organism and no haematological changes occurred. Second, the host developed a controllable parasitaemia in which erythrocyte values fell shortly after peak parasitaemia and then returned to normal. Third, the host failed to control the parasitaemia and chronic low grade anaemia developed. Dexamethasone sometimes caused a resurgence of parasitaemia in sheep.     

Vaccination against Brucella ovis infection in sheep

Extract

Dairy cows in New Zealand subsist chiefly on pasture or pasture products such as hay and silage, whilst concentrates are fed only in exceptional cases. Town supply herds are concerned with the production of winter milk and payment by the dairy factory is based on their milk production assessment during the months of June, July, and August. It is therefore necessary to have maximum milk production during these winter months when pastures are at their minimum rate of growth. As a food supplement, therefore, many town supply herds augment their feeding with crops and wet brewers' grains. The grains are fed from hoppers in the bail and are also spread out in the paddocks or placed in large wooden troughs. Most grains are purchased on a contract basis from the brewers, more being available during the summer months, when more beer is consumed. Friday's load of grains is usually stored and the consumption spread out over the week-end until the first fresh load is brought in on Monday morning.     

Effect of Eperythrozoon ovis on the lysis of sheep erythrocytes in the complement fixation test

When erythrocytes from sheep experimentally infected with Eperythrozoon ovis were used in the titration of reagents for a standardised complement fixation test, increased amounts of both haemolysin and complement were required for erythrocyte lysis compared with preinfection titrations. The haemolysin requirement increased by up to 125% at 55 days post-infection and complement requirement increased by up to 40% at 40 days post-infection. These changes appeared to correlate with the development of a macrocytic anaemia in affected sheep rather than E. ovis parasitaemia. The results emphasise the need to carefully monitor the haematological parameters of sheep used as sources of erythrocytes for the complement fixation test.     

Seroprevalence of Oestrus ovis infection in sheep in southwestern Germany

The aim of the survey was to determine the seroprevalence of Oestrus ovis infection in flocks in southwestern Germany. Serum samples collected from 1497 sheep (>6 months of age) of 110 flocks in 1997 and 1998 were examined for antibodies to crude somatic antigens of O. ovis second-stage larvae using an ELISA test. Data on the farm management were obtained by a questionnaire. Overall, 76% of the flocks had at least one seropositive animal, and the seroprevalence of anti-Oestrus antibodies was 50% in sheep. Flock size was the only risk factor significantly associated with the detection of antibodies. Larger flocks (>50 ewes) were more likely to be seropositive than smaller ones. These results show that Oestrus infections are widespread in sheep in southwestern Germany. Further investigations are required to estimate the economic importance of oestrosis and the efficiency of control measures.     

Chemoprophylaxis of Anaplasma ovis infection in sheep with a long-acting oxytetracycline

The chemoprophylactic efficacy of long-acting oxytetracycline was determined in 13 susceptible ewes infected with Anaplasma ovis. The drug was administered intramuscularly at a dose rate of 20 mg kg-1 body weight during the prepatent period. When the ewes were exposed to an equivalent homologous challenge on Day 45 post-infection, each showed a mild or inapparent reaction. The host reactions, i.e., body temperature, parasitaemia, packed-cell volume (PCV), haemoglobin, humoral and cell mediated immunity (CMI) were also studied. The results indicate that the use of oxytetracycline during the incubation period would minimise clinical signs of an Anaplasma infection; this may be due to an increasing CMI response.     

Epidemiology of Oestrus ovis infection of sheep in the highveld of Zimbabwe

During a period of 13 months, 507 heads of sheep, obtained from an abattoir near Harare, were examined for infection with Oestrus ovis larvae. The prevalence of infection varied from 6 to 52%, the highest being in November and the lowest in April. The mean annual larval burden was 1.12. The maximum number of larvae recovered from a single head was 57 in the month of November. Two larval peaks were observed, the first and highest in November and the second in August/September. Some flies are present throughout the year, except in May. There are at least 3 generations of flies per year. The wet summer period from January to May seems to be unfavourable, as very few flies are present. There is no overwintering of first instar larvae in the heads of sheep.     

Antibody response and duration of latent infection in sheep following experimental infection with Babesia ovis

Babesia ovis isolated in Extremadura (Spain) was the subject of a serological study in experimentally inoculated sheep. The first antibody titres, determined by the indirect fluorescent antibody (IFA) test, were observed 7-8 days post infection (p.i.) in all animals except the splenectomized group, in which the only animal that survived showed antibody response 10 days p.i. A faster response following challenge was observed in sheep which were seropositive before inoculation, which suggests the existence of an antigen memory. The highest titres were reached 16-25 days p.i., and subsequently began to fall, reaching minima at the end of the experimental period (330 days p.i.). The chronic carrier state in experimental B. ovis infection had a duration of at least 2 years. Passive transmission of antibodies from experimentally infected mothers to newborn lambs was also detected. Antibody levels were observed for a period not longer than 2 months after birth.     

A survey of farmers'' attitudes to services provided by consulting veterinarians to the Western Australian sheep industry

A postal survey was conducted of 80 sheep farmers in the Kojonup and Esperance districts of Western Australia to establish what they wanted from a veterinary service. Twenty five of the farmers surveyed used a sheep consultant, 25 did not, and 30 were interested in employing one. Farmers were asked questions about themselves and their attitudes to private veterinarians who provide specialist services to sheep farmers. Data reported here showed that farmers wanted a veterinarian who lived in the district, was well trained in sheep management and production, was enthusiastic and had good communication skills. The service provided should be whole-farm and available to members of the consultant's group only. Regular newsletters and field days were necessary, but the provision of contract services, such as mulesing, lamb-marking, drenching, pregnancy testing and sheep classing, and 'fire-brigade' services for sick animals, were not rated as important. Most farmers were unwilling or unable to give a dollar value for the likely benefits of a consultancy service. Non-financial benefits included keeping farmers up to date with new technical developments and information. The survey also showed that a veterinarian specialising in services to sheep farmers could be confident of employment.     

A computer simulation study to evaluate resistance development with a derquantel-abamectin combination on UK sheep farms

UK guidelines for the sustainable control of parasites in sheep (SCOPS) aim to delay further development of anthelmintic drug resistance. This study describes a computer model evaluation of resistance development with a novel oral formulation of derquantel-abamectin, to inform recommendations for use. Two different farm management scenarios, based on UK field data, were modelled to simulate low refugia (non SCOPS) or high refugia (SCOPS) worm populations. The effect on resistance allele frequencies and field efficacy of several treatment scenarios using the novel active derquantel (DQL), a spiroindole (SI), as either a single or multiple active formulation with abamectin (ABA), a macrocyclic lactone (ML), under the two farm management systems was evaluated. The initial resistance allele frequency for DQL was set at 0.0001, assuming that resistance in the UK is low, and for ML at 0.165 or 0.8, assuming that resistant nematode populations exist in the UK. DQL resistance reached a level at which a reduction in field efficacy might be detected (resistance allele frequency 0.25) by year 16 when used sequentially, and by year 31 when used in annual rotation (ABA) with SCOPS management inputs, and by year 5 (sequential) and by year 10 (annual rotation) with non SCOPS management inputs. ML resistance reached a level at which a reduction in ABA field efficacy might be detected (resistance allele frequency 0.25) by year 4 when used sequentially, and by year 8 when used in annual rotation with DQL and SCOPS management inputs, and by year 1 (sequential) and by year 2 (annual rotation) with non SCOPS management inputs. No detectable reduction in field efficacy was observed for DQL-ABA after 40 years of use with SCOPS management inputs for simulations using an initial ML resistance allele frequency of 0.165 and 0.8. A detectable reduction in field efficacy was observed for DQL-ABA by year 32 (initial ML resistance allele frequency=0.165) and by year 6 (initial ML resistance allele frequency=0.8) with non SCOPS management inputs. In summary, the results suggest that formulating DQL in combination with ABA confers a substantial advantage in delaying the development of both DQL and ML resistance, and the provision of adequate refugia further extends this advantage.     

The acquisition of immunity to Histophilus ovis by sheep in nature

It was demonstrated that skin wound infection with Histophilus ovis elicits an immune response which can protect a ram against a challenge injection of the same organism into its epididymis.     

Comparison of infection rates of Oestrus ovis between sheep and goats kept in mixed flocks

Oestrosis is a nasal myiasis of sheep and goats caused by larvae of the fly Oestrus ovis and can lead to severe clinical signs, which together with the disturbance caused by the adult fly may result into serious economic losses. Infection rates and larval burdens are always higher in sheep than in goats after either natural or artificial infestation. The aim of this study was to compare the host preference of the adult fly O. ovis between sheep and goats in mixed flocks, where they are kept together under the same husbandry conditions and hence, are very similarly exposed to the fly preference. Blood sera samples were collected from a total of 397 sheep and 335 goats, from 43 mixed flocks located at different regions of Greece. Antibodies specific to O. ovis IgG were measured by ELISA. A flock was considered positive when at least one individual was positive, i.e. showed a seropositivity of >or=20% in relation to positive control sera. A total of 193 (48.6%) sheep and 58 (17.9%) goats were found to be seropositive against O. ovis. Thirty-eight (88.4%) out of 43 flocks had at least one seropositive animal. The mean seroconversion against O. ovis in animals from the different flocks was 38.6% and 13.6% for sheep and goats, respectively, whereas the variance of infection within each flock was 0-100%. The mean seropositivity between sheep that were found to be positive or negative was 60.6% and 5.4%, respectively, whereas the corresponding values between goats were 35.2% and 5.2%, respectively. No significant difference in the seroconversion values was noted between flocks from the different areas (P=0.817), whereas a very significant difference was observed between animal species (P=0.001). However, there was no significant difference when seroconversion comparisons were made within samples of the same animals species, sheep or goats from different flocks of all the regions included in the study (P=0.695). The results of this study clearly demonstrate that O. ovis has a widespread distribution in Greece, and the seroprevalence is significantly higher in sheep than goats (P=0.001).