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.     

Observations on the pathology of Eperythrozoon ovis infection in sheep

The main pathological features of experimental Eperythrozoon ovis infection were an increase of spleen weight by up to 250% at the peak of parasitaemia and an increase in liver weight by 36%. Haemosiderin was present in the kidneys, livers and spleens of all infected sheep at the peak and late stages of parasitaemia. Control sheep had haemosiderin in the spleen only. On the basis of these findings, intravascular haemolysis appears to be the predominant mode of red cell removal. Although not observed histologically, some erythrophagocytosis by the spleen and liver probably occurs in the course of the infection.     

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.     

The prevalence of Eperythrozoon ovis infection in weaner and adult sheep in north eastern Victoria

A survey of weaner (6 to 12 months) and adult sheep for the presence of Eperythrozoon ovis antibodies using an immunofluorescent antibody assay was carried out. In 22 shires in north eastern Victoria over 2 years infection was demonstrated in 10% and 51% of weaner and adult sheep respectively.     

A modified indirect immunofluorescent assay for the detection of antibody to Eperythrozoon ovis in sheep

Experimental ovine eperythrozoonosis was studied using Giemsa staining of blood films and a modified indirect immunofluorescent antibody assay (IFAA). The serums of 21 Border Leicester Merino cross lambs between 12 weeks and 7 months-of-age were analysed before and after infection with Eperythrozoon ovis (E. ovis) using the IFAA test. No rise in the IFAA titre was seen until day 7 and this coincided with the first detection of E. ovis organisms in blood smears stained with Giemsa. The percentage of E. ovis infected red blood cells peaked on day 14, but the IFAA titre did not peak until day 35. Titres to E. ovis, on average, had begun to drop by day 63. There was considerable individual variation in response to E. ovis infection as measured by the IFAA. Titres as high as 6,400 were observed in individual sheep at the peak of E. ovis parasitaemia of red cells. One sheep had a titre of 51,200 nineteen days after infection, and titres of 3,000 were maintained for several months in a few sheep. The assay proved reliable, and up to 100 samples per day could be tested. The antigenicity of the slide preparations was found to be satisfactory after storage for 6 months at -20 degrees C and 4 degrees C and for 28 months at -70 degrees C. Temperature fluctuations during storage rendered slides unsuitable for the IFAA after these times. A method of storing E. ovis infected blood in liquid nitrogen is described.     

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.