Serological Investigation of Granulocytic EhrlichiaInfection in Sheep in Norway
© The Author(s) 2002
Received: 27 November 1999
Accepted: 14 March 2001
Published: 30 September 2001
Serum samples of 749 sheep from 75 sheep flocks in Norway, i.e. 361 lambs (6 to 7 months old) and 388 adults (>1.5 year), were analysed for antibodies to Ehrlichia equi. Ten animals from each flock were examined. Seropositive animals were found along the coast of southern Norway from Vestfold to Sør-Trøndelag (as far north as 63°38'N). Seropositive sheep were not found in southeast, east or northern Norway. Thirty-two flocks were seropositive, although tick-borne fever had only been diagnosed earlier in half of these. In 78% of the seropositive flocks, more than 80% of the sheep were seropositive. A total of 35.7 % and 36.3 % of lambs and adults were found seropositive, respectively. However, the overall seroprevalence among animals that had been grazing on Ixodes pastures were 0.80 for the lambs and 0.84 for the adults. Mean antibody titres (± SD) (log10) in seropositive lambs and adults were 2.59 (± 0.449) and 2.70 (± 0.481), respectively. No significant differences in either seroprevalence or mean antibody titre between sheep of different ages were obtained in this study. Based on antibodies 94% of sheep flocks on Ixodes pastures were infected with a granulocytic Ehrlichia infection. The association between seropositive flocks and Ixodes infested pasture shows a very high degree of agreement (p < 0.00001). The present study indicates that granulocytic Ehrlichia infection in sheep is underdiagnosed in Norway.
KeywordsEhrlichia phagocytophila antibodies lambs seroprevalence.
Serologisk undersøkelse med hensyn på granulocyttær Ehrlichia infeksjon hos sau i Norge.
Serologisk undersøkelse med hensyn på antistoffer mot Ehrlichia equi ble foretatt på 749 sauer, fordelt på 75 flokker fra hele landet. Totalt ble 361 lam (6-7 måneder gamle) og 388 voksne (>1,5 år) undersøkt. Seropositive dyr ble funnet fra kysten av Sør-Norge fra Vestfold til Sør-Trøndelag (så langt nord som 63°38´N). Trettito flokker var seropositive, men granulocyttær ehrlichiose (sjodogg) hadde bare vært diagnostisert i halvparten av disse. I 78% av de seropositive flokkene var mer enn 80% av sauene seropositive. Totalt var 35,7% og 36,3% av henholdsvis lam og voksne seropositive. Av de sauene som hadde gått på Ixodes-infisert beite var imidlertid 79,6% av lammene og 83,9% av de voksne dyra seropositive. I middel var titret (log10 ± SA) hos seropositive lam og voksne henholdsvis 2,59 (± 0,449) og 2,70 (± 0,481). Det var ingen signifikant forskjell i seroprevalens og titer mellom sau av ulik alder. Nittifire prosent av flokkene på Ixodes-beite var infisert med granulocyttær Ehrlichia. Det var sterk assosiasjon mellom seropositive flokker og forekomst av Ixodes (p<0,00001). Undersøkelsen tyder på at granulocyttær ehrlichiose hos sau er underdiagnostisert i Norge.
The most common tick-borne disease in domestic animals in Norway is tick-borne fever (TBF), caused by Ehrlichia phagocytophila, and transmitted by the tick Ixodes ricinus [35, 27]. TBF may cause abortion in ewes and temporary infertility in rams , but the main consequence of an E. phagocytophila infection in sheep is the ensuing immunosuppresion that leads to secondary infections, such as Staphylococcus aureus pyaemia and Pasteurella hemolytica (trehalosi) septicaemia [5, 26]. In the UK, it has been estimated that more than 300 000 lambs develop tick pyaemia annually .
TBF has for decades been considered as an important disease in lambs in certain areas along the coast of southern Norway . The purpose of the present study was to investigate the distribution of E. phagocytophila infection in sheep in different areas of Norway, especially in areas with a distribution of I. ricinus.
Materials and methods
Flocks from each county in Norway were included in this study, such that flocks in Ixodes areas along the coast and areas with a high number of winterfed sheep were preferred. However, representative flocks in each area were chosen and sampled by the local veterinarians.
Serum samples from sheep flocks were obtained in October/November. Samples from 10 sheep were randomly collected in each herd, around half of the samples were from lambs (6 to 7-months-old). A questionaire was filled out by the veterinarian during the visit of each flock, including questions about ectoparasitic treatment, Ixodes infested pastures, earlier treatment against TBF, and occurrence of tick-associated infections. Four sheep flocks were chosen from each of the 18 counties in Norway, except from the county of Sør-Trøndelag, where 8 flocks were selected. The reason for this was that the northernmost observation of tick-borne fever so far has been in the county of Sør-Trøndelag .
An indirect immunofluorescence antibody assay (IFA) was used to determine the antibody titre to Ehrlichia equi . Two-fold dilutions of sera were added to slides precoated with E. equi antigen (Protatek International and Organon Teknika). Bound antibodies were visualized by fluorescein-isothiocyanate (FITC)-conjugated rabbit-anti-sheep immunoglobulin (Cappel, Organon Teknika). Sera were screened for antibodies at dilution 1:40. If positive, the serum was further diluted and retested. A titre of 1.6 (log10 reciprocal of 1:40) or more was regarded as positive.
The statistical analysis was done according to . The overall seroprevalence and mean antibody titre were estimated and stratified by ectoparasitic treatment and age. Statistical calculations were done by using Statistix®, version 4.0 (Analytical software). Statistical analyses on seroprevalence were performed using a chi-square test and the antibody titres were compared using a Students t-test for independent samples. Significance was set at p < 0.05.
Of a total of 749 sheep from 75 flocks, 71 flocks in 1996 and 4 flocks in 1997, 270 sheep (36%) were found positive for antibodies to granulocytic Ehrlichia infection. Seropositive flocks were found in the coastal areas from Vestfold to Sør-Trøndelag. The northernmost seropositive flocks were found south of Trondheimsfjorden on the island of Hitra (63°38'N). The geographical distribution of the flocks is shown in Fig. 1. Thirty-two flocks were found positive, but only 16 of these had a history of previous TBF infection (Table 1). Symptoms of disease were not observed in any sampled animal.
Serological investigation of sheep sera for antibodies to Ehrlichia equi from different counties of Norway.
Number of positive flocks/total number of flocks
Number of flocks on tick pasture
Number of flocks treated against ticks
Number of flocks with a history of tick-borne fever (during the year of sampling)
Møre og Romsdal
Sogn og Fjordane
Distribution of E. equi antibodies in seropositive sheep flocks in Norway. Ten animals were investigated in each flock.
Percentage of seropositive animals
Reciprocal antibody titres against E. equi in 361 lambs and 388 adult sheep (>1.5 years) in Norway.
Number of lambs
Number of adults
Seroprevalence and mean antibody titres (log10 ± SD) to granulocytic Ehrlichia in sheep of different ages that had been grazing on Ixodes pastures.
Mean titre values*
2.59 ± 0.449
2.59 ± 0.418
2.68 ± 0.387
2.79 ± 0.527
Mean antibody titre (log10 ± SD) in seropositive lambs and adults were 2.59 ± 0.449 and 2.70 ± 0.481, respectively. However, no significant differences in mean antibody titres between different age groups of seropositive animals were observed (Table 4).
Comparison of E. equi serology and tick pasture in relation to the number of seropositive sheep flocks in Norway.
Strong serological cross-reactions between E. equi, E. phagocytophila and the agent causing human granulocytic ehrlichiosis (HGE) have been reported [8, 19, 23]. It is therefore possible to use any of the 3 closely related Ehrlichia antigens to get acceptable results in serosurveys. The titre to a heterologous strain of Ehrlichia is normally less than against the homologous strain, but the IgG titres may also differ noticeably depending on the source of the antigen [4, 33]. The sensitivity of the present test could perhaps have been increased by use of a more proper antigen, but unfortunately E. phagocytophila was not available for use as antigen in this study.
All blood samples were collected in October/November. Earlier investigations indicate that the antibody titres can be detected for at least 6 months in sheep after the primary infection , also when E. equi was used as antigen in the serological test . In humans, serological titres may last for at least 30 months after an acute HGE infection . In horses, serological investigations indicate that a positive antibody titre to E. equi could persist for more than 12 months in naturally infected horses . The persistence of Ehrlichia antibodies therefore indicates that animals infected during the grazing season would be found seropositive the following autumn and winter.
The present study shows that granulocytic Ehrlichia infected sheep are found on the coast of southern Norway from Vestfold to Sør-Trøndelag (as far north as 63°38'N). No antibodies to granulocytic Ehrlichia were found on the southeast, east or northern parts of Norway. The distribution of seropositive animals in this study is in accordance with the distribution of I. ricinus in Norway, although scattered populations of I. ricinus have been found as far north as Brønnøysund (65°30'N) .
The present results are also in accordance with earlier reports on the distribution of clinical cases of TBF in domestic animals . In addition, in June 1997, cattle was found infected with E. phagocytophila for the first time in Stadsbygd (north of Trondheimsfjorden-63°32'N), in an area where Babesia divergens in cattle is common (Schei, personal communication). The present study indicates that the area around Trondheimsfjorden is so far the northernmost limit of Ehrlichia infections in domestic animals in Norway.
In comparison, babesiosis in cattle in Norway has been observed as far north as in Nordland county (65°47'N) . This difference in northern distribution between babesiosis in cattle and ehrlichiosis in sheep, may be due to differences in the maintenance of the respective infections in hosts or vectors. Sheep, wild deer and small rodents have been proposed as reservoir hosts for granulocytic Ehrlichia infection in Europe [20, 6], while B. divergens is regarded to be rather host specific . Both B. divergens and E. phagocytophila may cause persistent infection in cattle and sheep, respectively [13, 9, 28], so both infections could be brought from endemic areas by both ticks or hosts. Both microorganisms are transmitted by I. ricinus, the only tick in Norway known to transmit infections to animals . E. phagocytophila is transmitted transstadially in I. ricinus, and ovarial transmission has not yet been observed [15, 21]. In contrast, B. divergens infection could persist in I. ricinus for at least 2 generations even in the abscence of cattle [7, 10]. These observations might indicate a greater chance for maintenance of a B. divergens infection than a granulocytic Ehrlichia infection in I. ricinus populations in areas where competent hosts are sparsely scattered, as along the coast of northern Norway.
In the present study, 32 out of 34 flocks that grazed on tick infested pastures were infected with granulocytic Ehrlichia. The association between seropositive flocks and Ixodes infested pastures indicates a high degree of agreement. In 78% of the seropositive flocks, more than 80% of the sheep were seropositive. These results indicate a widespread Ehrlichia infection in areas where I. ricinus populations are present. Observations done in UK indicate a nearly 100% probability that a susceptible sheep will acquire granulocytic Ehrlichia infection on tick infested pasture . Earlier investigations indicate that the prevalence of granulocytic Ehrlichia infection in populations of I. ricinus varies between different countries . However, no information on the prevalence of Ehrlichia infection in I. ricinus populations in Norway is available.
No significant differences in antibody titres between different age groups of sheep were observed in this study. The titre values are in accordance with E. equi titres, found in experimentally E. phagocytophila infected lambs, 2 months after the initial infection .
No effect of acaricide treatment was observed on the prevalence of infection or the titre values in Ehrlichia infected sheep. Most lambs/sheep were treated only once with acaricides on tick pastures. Earlier observations indicate that synthetical pyrethroids only give 2 to 3 weeks of full protection against ticks [18, 12]. In addition, lambs grazing on tick pastures may seroconvert to E. phagocytophila after 3 weeks of tick exposure, although synthetical pyrethroids have been applied .
Only half of the seropositive flocks had a known history of TBF, indicating that granulocytic Ehrlichia infection is underdiagnosed in sheep flocks on tick infested pastures in Norway. This statement is supported by the fact that only 20 of 32 seropositive flocks (62.5%) had been treated prophylatically against TBF. Disease problems associated with tick infested pasture were only recorded in 6 flocks during the year of sampling; 4 of these had been treated with synthetical pyrethroids. These results indicate that some strains of granulocytic Ehrlichia may have low virulence in sheep, as observed earlier by [9, 32, 25, 28].  and  also observed that isolates of E. phagocytophila from cattle and sheep in different geographic areas of infection may vary considerably with regard to their ability to cross-protect. Antigenic diversity has also been observed in isolates of the HGE agent .
Mild or subclinical E. phagocytophila infection may also be due to breed variations in susceptibility to a TBF infection, as has earlier been reported in sheep . However, to the authors knowledge, no such breed differences have been observed in Norwegian sheep breeds. Few recorded disease problems may also indicate a recent introduction of TBF in the flock, since most primary infections of TBF in the field are not observed due to unobtrusive clinical signs . The main disease problems associated with TBF are seen in lambs, and in sheep purchased from tick-free areas and put onto tick infested pastures.
In conclusion, the present results indicate that granulocytic Ehrlichia infection is abundant on tick infested pastures in Norway. The total sheep population in Norway during summer time is around 2.4 million, and the average flock size is approximately 100 sheep . In 1996, more than 5100 flocks were treated prophylatically against TBF with tick repellents/insecticides (Norwegian Animal Disease Report 1996). However, in the present investigation only around 60% of the seropositive flocks had been prophylatically treated against tick infestation; all animals were treated in 75% of these flocks. These results indicate that more than 850 000 sheep in Norway are grazing on I. ricinus infested pastures and may be exposed to infection with E. phagocytophila. It is therefore probable that TBF infection in sheep may have a wider distribution in Norway than earlier believed.
The authors wish to thank all local veterinarians participating in this study by collecting blood samples, and Pfizer AS and the Norwegian Research Council for economic support of the study. We also want to thank Ulla-Britt Wikstrøm for excellent technical assistance.
- Artursson K, Gunnarsson A, Wikström U-B, Olsson Engvall E: A serological and clinical follow-up in horses with confirmed equine granulocytic ehrlichiosis. Equine Vet J. 1999, 31: 473-477.View ArticlePubMedGoogle Scholar
- Asanovich KM, Bakken JS, Madigan JE, Aguero-Rosenfeld M, Wormser GP, Dumler JS: Antigenic diversity of granulocytic Ehrlichia isolates from humans in Wisconsin and New York and a horse in California. J infect Dis. 1997, 176: 1029-1034.View ArticlePubMedGoogle Scholar
- Bakken JS, Krueth J, Tilden RL, Asanovich MN, Asanovich K, Walls J, Dumler JS: Duration of IFA serologic response in human infected with the agent of human granulocytic ehrlichiosis (HGE). Abstract of the IDSA 35th Annual meeting. Reviews of infectious diseases. 1997, 25: abst 73Google Scholar
- Bjoersdorff A, Brouqui P, Eliasson I, Massung RF, Wittesjö B, Berglund J: Serological evidence of Ehrlichia infection in Swedish Lyme borreliosis patients. Scand J infect Dis. 1999, 31: 51-55. 10.1080/00365549950161880.View ArticlePubMedGoogle Scholar
- Brodie TA, Holmes PH, Urquhart GM: Some aspects of tick-borne diseases of British sheep. Vet Rec. 1986, 118: 415-18.View ArticlePubMedGoogle Scholar
- Brouqui P: Ehrlichiosis in Europe. Rickettsiae and rickettsial diseases at the turn of the third millenium, Elsevier, Paris. Edited by: Raoult D, Brouqui P. 1999, 220-232.Google Scholar
- Donnelly J, Pierce MA: Experimental transmission of Babesia divergens to cattle by the tick Ixodes ricinus. Int J Parasitol. 1975, 5: 363-367. 10.1016/0020-7519(75)90085-5.View ArticlePubMedGoogle Scholar
- Dumler JS, Asanovich KM, Bakken JS, Richter P, Kimsey R, Madigan JE: Serologic cross-reactions among Ehrlichia equi, Ehrlichia phagocytophila, and human granulocytic ehrlichia. J clin Microbiol. 1995, 1098-1103.Google Scholar
- Foggie A: Studies on the infectious agent of tick-borne fever in sheep. J Path Bact. 1951, 63: 1-15. 10.1002/path.1700630103.View ArticlePubMedGoogle Scholar
- Gray JS, Murphy TM: Bovine babesiosis in Ireland. Irish vet News. 1985, 9-14.Google Scholar
- Hardeng F, Baalsrud KJ, Øvernes G: Controlling tick infestations and diseases in sheep by pour-on formulations of synthetic pyrethroids. A field study Vet Res Comm. 1992, 16: 429-436. 10.1007/BF01839020.View ArticleGoogle Scholar
- Henderson D, Stevens DP: Cypermethrin pour-on for the control of ticks (Ixodes ricinus) on sheep. Vet Rec. 1987, 121: 317-19.View ArticlePubMedGoogle Scholar
- Joyner LP, Davies SFM: Acquired resistance to Babesia divergens in experimental calves. J Protozol. 1967, 14: 260-262.View ArticleGoogle Scholar
- Martin SW, Meek AH, Willeberg P: Veterinary epidemiology. Principles and methods. 1987, Iowa State University Press, AmesGoogle Scholar
- MacLeod J, Gordon WS: Studies in tick-borne fever of sheep. I. Transmission by the tick Ixodes ricinus and the description of the disease produced. Parasitology. 1933, 25: 273-283.View ArticleGoogle Scholar
- Mehl R: The distribution and host relations of Norwegian ticks (Acari, Ixodides). Fauna Norv Ser B. 1983, 30: 46-51.Google Scholar
- Mehl R, Sandven P, Braathen LR: Skogflåtten Ixodes ricinus. (The tick Ixodes ricinus). Tidsskr Nor Lægefor. 1987, 107: 1642-1644.Google Scholar
- Mitchell GBB, Webster KA, Wright CL: Use of deltamethrin 'pour on' for control of the sheep tick Ixodes ricinus. Vet Rec. 1986, 119: 156-57.View ArticlePubMedGoogle Scholar
- Nicholson WL, Comer JA, Sumner JW, Gingrich-Baker C, Coughlin RT, Magnarelli LA, Olson JG, Childs JE: An indirect immunofluorescence assay using a cell culture-derived antigen for detection of antibodies to the agent of human granulocytic ehrlichiosis. J clin Microbiol. 1997, 35: 1510-1516.PubMed CentralPubMedGoogle Scholar
- Ogden NH, Woldehiwet Z, Hart CA: Granulocytic ehrlichiosis: an emerging or rediscovered tick-borne disease?. J med Microbiol. 1998, 47: 475-82.View ArticlePubMedGoogle Scholar
- Ogden NH, Bown K, Horrocks BK, Woldehiwet Z, Bennett M: Granulocytic Ehrlichia infection in Ixodid ticks and mammals in woodlands and uplands of the UK. Med vet Entomol. 1998, 12: 423-429. 10.1046/j.1365-2915.1998.00133.x.View ArticlePubMedGoogle Scholar
- Paxton EA, Scott GR: Detection of antibodies of the agent of tick-borne fever by indirect immunofluorescence. Vet Microbiol. 1989, 21: 133-38. 10.1016/0378-1135(89)90025-4.View ArticlePubMedGoogle Scholar
- Pusterla N, Wolfensberger C, Gerber-Bretscher R, Lutz H: Comparison of indirect immunofluorescence for Ehrlichia phagocytophila and Ehrlichia equi in horses. Equine Vet J. 1997, 29: 490-492.View ArticlePubMedGoogle Scholar
- Scott GR: Tick-associated infections. Diseases of sheep. Edited by: Martin WR. 1983, Blackwell Scientific Publications, Oxford, 209-213. 1Google Scholar
- Stannard AA, Gribble DH, Smith RS: Equine ehrlichiosis: A disease with similarities to tick-borne fever and bovine petechial fever. Vet Rec. 1969, 84: 149-150.View ArticlePubMedGoogle Scholar
- Stuen S: Tick-borne fever (TBF) and secondary infections in sheep. Rickettsiae and rickettsial diseases. Edited by: Kazár J, Toman R. 1996, Veda, Bratislava, 347-349.Google Scholar
- Stuen S: Utbredelsen av sjodogg (tick-borne fever) i Norge. (The distribution of tick-borne fever (TBF) in Norway). Norsk Vet Tidsskr. 1997, 109: 83-87.Google Scholar
- Stuen S: Sjodogg (tick-borne fever) – et historisk tilbakeblikk. (Sjodogg (tick-borne fever) – a historical review). Norsk Vet Tidsskr. 1998, 110: 703-706.Google Scholar
- Stuen S, Olsson Engvall E, Artursson K: Persistence of Ehrlichia phagocytophila infection in lambs in relation to clinical parameters and antibody responses. Vet Rec. 1998, 143: 553-55.View ArticlePubMedGoogle Scholar
- Stuen S, Artursson K, Olsson Engvall E: Experimental infection of lambs with an equine granulocytic Ehrlichia species resembling the agent that causes human granulocytic ehrlichiosis (HGE). Acta vet scand. 1998, 39: 491-497.PubMedGoogle Scholar
- Trodahl S: Sauen som husdyr (The sheep as a domestic animal). 1998, Saueboka, A/S Land-bruksforlaget, Oslo, 11-27. 2Google Scholar
- Tuomi J: Experimental studies on bovine tick-borne fever (3) Immunological strain differences. Acta pathol microbiol scand. 1967, 71: 89-100.View ArticleGoogle Scholar
- Walls JJ, Aguero-Rosenfeld M, Bakken JS, Goodman JL, Hossain D, Johnson RC, Dumler JS: Inter- and intralaboratory comparison of Ehrlichia equi and human granulocytic ehrlichiosis (HGE) agent strains for serodiagnosis of HGE by the immunofluorescent-antibody test. J clin Microbiol. 1999, 37: 2968-2973.PubMed CentralPubMedGoogle Scholar
- Woldehiwet Z, Scott GR: Tick-borne (pasture) fever. Rickettsial and chlamydial diseases of domestic animals. Edited by: Woldehiwet Z, Ristic M. 1993, Pergamon Press, Oxford, 233-254.Google Scholar
- Øverås J: Sjukdom hos sau på Ixodes ricinus infisert beite. (Diseases of sheep on Ixodes ricinus infested pasture). Norsk Vet Tidsskr. 1972, 83: 561-67.Google Scholar