Article — Artikel

The prevalence of serum antibodies to ruminantium infection in ranch in Tanzania: a cross-sectional study

E S Swaia*, P F Mtuia A K Chang’ab and G E Machangea

across farming systems in the country. ABSTRACT This investigation was designed to provide Serum samples collected in a cross-sectional survey of grazing cattle on Manyara Ranch, information on the risk of heartwater and Monduli district, Tanzania, were tested by indirect major antigenic protein 1 fragment B to assist in the development of appropri- (MAP 1-B) ELISA to determine the seroprevalence of Ehrlichia ruminantium and to assess ate control strategies. The prevalence of ranch-level risk factors for heartwater. Heartwater-exposed cattle were widespread on the E. ruminantium was assessed, and factors ranch and overall seroprevalence was 50.3 % (95 % CI, 44.9 –55.6), enough to indicate an associated with infection in extensively endemically unstable situation. Multivariate logistic regression modelling was used to raised indigenous cattle on Manyara identify risk factors associated with seropositivity. Two factors appeared to increase the Ranch, Tanzania, were explored. herd’s risk for contracting heartwater. Seroprevalence increased significantly with age (β = 0.19 per year of age, P < 0.001) and animals carrying ticks of any species were associated MATERIALS AND METHODS with an increased risk of infection with E. ruminantium (Odds ratio, OR = 3.3, P < 0.001). The force of infection based on the age seroprevalence profile was estimated at 18 per 100 Study site cattle year-risk. The current control measures on the ranch were associated with a decreased risk of infection with E.ruminantium (OR = 0.25 for no dipping and OR = 0.31 for Manyara Ranch (03°34’04’’S, 36°05’ low dipping, P < 0.001). Six tick species were identified; in order of frequency these were: 01”E) occupies 17 871 hectares (~45 000 Ambylomma variegatum 59.9 %, Rhipicephalus evertsi evertsi 13.9 %, Rhipicephalus pulchellus acres) of land and is located 92 km 12.5 %, Hyalomma truncatum 7.03 % and Rhipicephalus appendiculatus 6.07 %. The least southwest of Arusha city in the Maasai encountered tick was Rhipicephalus simus, which accounted for 0.38 %. The cattle seemed Steppe Heartland26. The ranch is lo- well adapted to their environment and capable of resisting the tick burden under this cated between 2 major national parks: extensive wildlife/livestock grazing and interaction system. Tarangire to the south and Lake Manyara Key words: E. ruminantium, indigenous cattle, risk factors, seroprevalence, Tanzania. to the north. Wildlife and livestock inter- act freely when grazing. The ranch is Swai E S, Mtui P F, Chang’a A K, Machange G E The prevalence of serum antibodies to Ehrlichia ruminantium infection in ranch cattle in Tanzania:a cross-sectional study. Jour- semi-arid with annual average rainfall of nal of the South African Veterinary Association (2008) 79(2): 71–75 (En.). Veterinary Investiga- 600–700 mm and is at an elevation of tion Centre, PO Box 1068, Arusha, Tanzania. 1200 m above sea level. The vegetation is mainly Acacia–Commiphora bushland, wooded savanna, with short grass. The rains are usually concentrated in 2 INTRODUCTION results in considerable economic losses seasons: end of March to May, and end of Heartwater (cowdriosis), caused by the due to loss of production, treatment costs October to December.The mean tempera- rickettsial organism Ehrlichia ruminantium and reduced initiatives for the upgrading ture ranges from 15 to 30 °C. The study (formely Cowdria ruminantium), is an in- of local breeds of livestock with more sus- was carried out in October and Novem- fectious disease of ruminants. The organ- ceptible exotic breeds 16. ber 2006. ism is transmitted by 3 host of the Heartwater can be unequivocally diag- genus Amblyomma33. Heartwater is one of nosed by the identification of E. rumi- Cattle husbandry system the most important tick-borne diseases nantium by demonstration of rickettsial The cattle are maintained in a pastoral (TBD) affecting both wild and domesti- inclusion bodies in the endothelial cells of open grazing system from approximately cated grazing ruminants in Tanzania18, brain crush smears24. This method is, 07:30 to 16:30 daily. No supplementary and it has been estimated that the direct however, only applicable to clinical cases. feeding is provided. The animals are economic loss due to the disease is about Serological tests specifically targeting housed at night in bomas or kraals con- US$20.5–24.5 million10. The mortality rate bovines have been applied but are of structed from thorny tree branches to due to heartwater is around 15 % with a limited use under field conditions owing protect them from theft and predators. morbidity rate of 5 %, although the sever- to cross-reactions with other Ehrlichia spe- The breeding system used on the ranch ity of the disease varies widely between cies.6,9. The MAP1-B ELISA based on the is natural mating, with service bulls run- cattle types, agro-ecological zones, truncated major antigenic protein 1 of ning freely with females year-round. socio-economic conditions and cattle pro- E. ruminantium has been shown to be of Calves are weaned at 8 months of age. duction systems10. In tropical and sub- value under field and experimental Young calves are isolated and penned tropical areas, the disease is endemic and conditions, especially in the regions until the return of their dams from graz- a where the distribution of cowdriosis is ing, and then allowed to suckle. Cattle are Veterinary Investigation Centre, PO Box 1068, Arusha, 14,15,17,33 Tanzania. unknown . vaccinated yearly against anthrax, haem- bHerd Health Division, Manyara Ranch, PO Box 2658, Although the disease is known to be orrhagic septicaemia, lumpy skin disease, Arusha, Tanzania. endemic in various parts of Tanzania, and female calves <10 months of age with *Author for correspondence. E-mail: [email protected]. there is very little information on the Brucella Strain 19. Control of ectoparasites, Received: September 2007. Accepted: April 2008. epidemiology of the disease between and mainly ticks, is carried out by using

0038-2809 Jl S.Afr.vet.Ass. (2008) 79(2): 71–75 71 Decatix® (2.5 % m/v, Deltamethrin, Statistical analysis Tick infestation Coopers, Zimbabwe). The frequency of Descriptive statistics for the animal and A total of 2600 ticks (mature and imma- dipping is coded as follows: none: no ranch-level explanatory variables (inclusive ture) were counted during sampling. Of dipping, low: once every 4 weeks; moder- of tick infestation) examined in the study the 360 cattle examined and sampled, 320 ate: twice every 4 weeks. Anthelmintic were developed using Epi-Info version were found to carry ticks, giving an infes- treatment using Albendazole (10 %, Kela 6.04d7. Cross-correlations7 were performed tation rate of 88.9 % and the overall mean N.V., Phenix, Belgium) is generally lim- on all explanatory variables to investigate (mean± SE) tick density of 8.12 ± 0.44 ited to calves. the potential for confounding amongst tick/cattle. Tick infestation rate was signif- χ2 them (defined as those with a coefficient icantly ( = 31.7, df = 4, P = 0.001) higher in low, intensive and non-dipped and Study animals and sampling of 0.7 or greater). The relationship between least in moderately dipped cattle. The A systematic sampling procedure was explanatory variables and outcome re- mean tick-specific densities per animal adopted and every 4th animal was selected sponse (seroconversion to E. ruminan- are detailed in Table 2. as the cattle passed through a crush2.A tium) were investigated in 2 steps by logis- 4 Six tick species were identified, with total of 360 cattle (all ages and sexes), out tic regression (using Egret for Windows A. variegatum being the most abundant of the total ranch herd size of 1481, were version 2.0). Explanatory variables inves- (59.9 %), followed by R. evertsi evertsi used in the study. The cattle types reared tigated were ‘presence of a tick of each (13.9 %), R. pulchellus (12.5 %), H. trun- on the ranch are Boran (75 %) and Tanza- species’ investigated as a binary variable catum (7.03 %) and R. appendiculatus nia short horn zebu (TSHZ) (20 %), and (i.e. yes or no); body score (coded as fat, (6.07 %). The least encountered tick was their crosses (5 %). Each selected animal medium, thin); dipping (yes or no), R. simus at 0.38 %. was subjected to a thorough, whole-body acaricide application frequency (coded as inspection for the presence of ticks. Other none, low, moderate); sex (female or Age–antibody prevalence profiles information recorded included: sex, age male) and age. An increasing seropositivity was re- (estimated by ear notch, examination of In the 1st step, the relationships between corded with respect to increasing age. The teeth and written records). The age of each explanatory and outcome variable estimated force of infection was at 0.18 each animal was transformed age centred were individually investigated. In the animals per cattle year-risk. The graphical to normalise the data. Tick infestation, 2nd step, any variables that were signifi- age seropositivity relationship and forces with both mature and immature stages, cantly associated at P <0.25 were in- of infection are shown in Fig. 1. was coded as low: ≤20; moderate: >20 cluded in multivariate logistic regression4 and ≤40; high: >40. Tick infestation was models, producing, by forwards and Factor influencing antibody response moderate, therefore whole body count- backwards substitution and elimination, to E. ruminantium ing was adopted8. Ticks on the study ani- the most parsimonious models in which The factors that were significantly asso- mals were identified to genera and all explanatory variables remained signif- ciated with E. ruminantium antibody preva- species and counted using standard pro- icant at the P < 0.05 level. The criteria for lence in the multivariate regression model cedures8. The body condition score of inclusion and exclusion were a change in are given in Table 3. Cattle treated with each sampled animal was determined deviance significant at the 5 % level acaricide at an interval of once per month using the 9-point system developed at according to the maximum likelihood and not dipped were associated with the International Livestock Centre for Af- ratio test-chi square distribution. significantly lower antibody prevalence rica20. Scores were categorised as ≥1–3: Forces of infection were estimated from than those that were treated at an interval thin; ≥4–6 medium; and ≥7–9: fat. The age seroprevalence profiles using maxi- of twice per month (OR = 0.31 for once responses to many of these questions mum likelihood methods (MLM) in Excel per month, OR = 0.25 for not dipped, P < were investigated as explanatory vari- (Microsoft, USA) with the Solver add-in29. 0.001). Animals that carried ticks (of any ables in the analyses of seroconversion to Assuming a stable population size and species) at the time of survey were E. ruminantium. age structure and a constant force of associated with significantly higher sero- infection across all age groups, the log positivity than those that did not carry Collection of sera and laboratory analysis likelihood (LL) was derived using the ticks (OR = 3.30, P < 0.001). Multivariate Blood samples for sera were collected following equation: analysis of risk factors showed that age aseptically by jugular venipuncture into a was the risk factor for the occurrence of a β plain vacutainer tube (Becto-Dickson, =+−−ll−−λλi i seropositivity to heartwater in cattle ( = LLRneNRne∑ i ()()ii 1 , UK) from selected animals. The blood i = 1 0.19 per year of age, P < 0.001). samples were labelled and transported in refrigerated cool boxes to the Veterinary where Ri = number of seropositive in DISCUSSION Investigation Centre (VIC) laboratory group i, Ni = number tested in age group i Based on a serological survey, the pres- where they remained overnight. Serum and λ = the force of infection. ent investigation revealed that bovine was then separated by centrifugation and cowdriosis due to E. ruminantium infec- stored at –20 °C until used. The sera were RESULTS tion exists on Manyara Ranch, supporting analysed for antibodies against E. rumi- the presence of heartwater (cowdriosis) nantium using indirect MAP1-B ELISA15,34. Descriptive analysis of antibody (A Chang’a, pers. obs., 2006) and in other The results were expressed as per cent response to E. ruminantium parts of Tanzania10,13. The detected preva- positive (PP) calculated as a percentage of Of the animals sampled (n = 360), 249 lence of infection in the cattle was inter- the optical density (OD) value of the ref- (69.2 %) were females and 111 (30.8 %) mediate in comparison to that observed erence positive control. The cut-off point were males. The mean serum antibody in other studies in Africa; from 31 %11 in was obtained by plotting a graph of the prevalences of the 360 animals studied by the Ivory Coast, 41 % in Malawi25, and up OD readings of heartwater-negative sera. variable categories are shown in Table 1. to 61 % in Ghana12, but generally higher Seroconversion status was considered The overall mean antibody prevalence than observed in studies in Caribbean, positive if the PP cut-off was ≥20 %33 was 50.3 % (95 % CI, 44.9 –55.5). e.g. from 7.3 % reported in indigenous

72 0038-2809 Tydskr.S.Afr.vet.Ver. (2008) 79(2): 71–75 cattle on 19 islands19 to 30 % in cattle in Table 1: List of variables, proportion and seroprevalence of each category investigated (n = Guadeloupe3. The estimated seropre- 360) (CI = confidence interval). valence was considerably less than 70 %, Variable Category Number of animals Seroprevalence % suggesting that this pathogen exists in a (%) (± 95 % CI) state of ‘endemic instability’ as defined5,21. However, these levels are sufficiently Score Fat 298 (82.8) 48.6 (42.8–54.4) high to ensure that clinical disease would Medium 49 (13.6) 61.2(46.2–74.8) be a risk. Thin 13 (3.6) 46.2 (19.2–74.8) The most commonly detected tick species Sex Female 249 (69.2) 58.2 (51.8–64.4) was A. variegatum, which was consistent Male 111 (30.8) 32.4 (23.8–41.9) with serological responses to E. rumi- Age (in years) ≥0.5 to ≤1.5 101 (28.05) 30.6 (21.8–40.6) nantium (transmitted by A. variegatum), >1.5 to ≤3.0 102 (28.3) 41.1 (31.5–51.3) having the highest prevalence amongst >3.0 to ≤4.5 80 (22.2) 67.5 (56.1–77.5) the cattle13,25. R. evertsi evertsi constituted >4.5 to ≤6.0 56 (15.5) 66.07 (52.1–78.1) 13.9 % and was the 2nd-most abundant >6.0 to 9.0 21 (5.8) 80.9 (58.0–94.5) tick. However, this result is not consistent Dipping Yes 328 (91.1) 53.3 (47.4–58.5) with findings in Ethiopia, where it No 32 (8.9) 21.8 (9.2–39.9) formed only 6.4 % of the total ticks Acaricide None 32 (8.2) 21.8 (9.2–39.9) 28 counted in a survey . The ability of this Low 96 (26.7) 29.1 (20.3–39.3) tick to survive in open grassland, compa- Moderate 232 (64.4) 62.9 (56.3–69.1) rable to Manyara Ranch, and its perennial Carrying tick (of any sp.) Yes 320 (88.9) 51.8 (46.2–57.4) breeding habit, have both been advanced No 40 (11.8) 37.5 (22.7–54.1) as the reasons for high incidences. Tick Tick spp. (n = 320) burdens were low (mean 8.2, range 4–64), A. variegatum Yes 278 (86.8) 50.3 (44.3–56.3) but with no adult ticks of any species No 42 (13.1) 49.3 (38.7–61.2) being observed on cattle in 11 % of obser- R. evertsi evertsi Yes 133 (41.6) 44.3 (35.7–53.2) vations. Such low tick burdens could No 187 (58.4) 53.7 (47.0 –60.3) reflect the poor off-host survival of ticks in H. truncatum Yes 93 (29.06) 49.4 (38.9–60.03) cattle ranchland. No 227 (70.9) 50.6 (44.4–56.7) The present study indicated that ani- R. pulchellus Yes 146 (45.6) 45.9 (37.7–54.3) mals that were not dipped, and those No 174 (54.3) 53.3 (46.3–60.1) dipped at an interval of once every 4 R. appendiculatus Yes 50 (15.6) 42.0 (28.1–56.7) weeks, were all associated with lowered No 270 (84.4) 51.6 (45.8–57.2) seropositivity. It is not clear, however, why the detected rate of infection was low, particularly for undipped cattle, con- sidering the number and level of tick infestation recorded during the survey. One possible explanation could be that the current acaricide (at least at ranch level) regimen (once every 4 weeks, and twice every 4 weeks) is more frequent than necessary, although it might not necessarily be true because of other tick species that are potential vectors of other tick-borne diseases. It is not clear, there- fore, to what extent these cattle contrib- ute to the maintenance of ticks and the E. ruminantium pathogen on Manyara Ranch, and this warrants further investi- gation. Presence of vectors (ticks) was another Fig. 1: Age seroprevalence and maximum likelihood force of infection estimate (λ = 0.18) for factor associated with cattle seropositive Ehrlichia ruminantium on Manyara Ranch.

Table 2: Number of infested cattle and infestation rate and density of 7 tick species.

Tick species No. of cattle infested Infestation rate Tick density Total ticks (%) (mean ± SE)

A. variegatum 278 77.2 5.60 ± 0.344 1558 R. evertsi evertsi 133 36.9 2.72 ± 0.195 362 R. pulchellus 148 41.1 2.20 ± 0.128 326 H. truncatum 93 25.8 1.96 ± 0.152 183 R. appendiculatus 50 13.9 3.16 ± 0.478 158 R. simus 7 1.9 1.42 ± 0.202 10 Overall 320 88.9 8.12 ± 0.44 2600

0038-2809 Jl S.Afr.vet.Ass. (2008) 79(2): 71–75 73 Table 3: Significant factors associated with seropositivity to Ehrlichia ruminantium of cattle REFERENCES β on Manyara Ranch using multivariable logistic models ( = coefficient of regression 1. Bell-Sakyi L, Koney E B, Dogbey O, Walker (or parameter estimate), SE = standard error of coefficient, LRS = likelihood ratio statistic, LRP = A R 2004 Ehrlichia ruminantium seroprev- likelihood ratio P-value) alence in domestic ruminants in Ghana; I. Longitudinal survey in the Greater Accra Variable $ (SE) Wald P LRS LRP Odds Region. Veterinary Microbiology 100: 175–188 ratio 2. Cameron A 1999 A practical manual and soft- (±95% CI) ware package for active surveillance of livestock diseases in developing countries. ACIAR Constant –0.61 (0.33) Monograph No. 4, Canberra, Australia Application frequency: 3. Camus E, Martinez D, Beauperthuy L, Benderdouche A, Coisne S, Corbette C, Low vs moderate –1.17 (0.32) 0.001 61.5 0.001 0.31 (0.16–0.58) Denormandie N, Garris G, Harris D, King T None vs moderate –1.35 (0.53) 0.001 0.25 (0.09–0.73) 1993 Heartwater in Guadeloupe and in the Carrying tick: yes vs no 1.19 (0.36) 0.001 0.001 3.30 (1.61–6.76) Caribbean. Revue d’Élevage et de Médecine Vétérinaire des Pays Tropicaux 46: 109–14 Age (centred) in years 0.19 (0.079) 0.001 0.001 1.21 (1.03–1.42) 4. Cytel Software Corporation 1999 Statistics and Epidemiology Research Corporation, version 2.0. Seattle, USA 14 to E. ruminantium. Animals that carry ticks study , the findings of this study did 5. Deem S L, Perry B D, Katende J M, (of any species) had higher seroprevalence provide evidence of previous exposure to McDermott J J, Mahan S M, Maloo S H, since they were at a higher risk of expo- the pathogen and suggest that there is a Morzaria S P,Musoke A J, Rowlands G J 1993 sure than those that had no ticks. Other need for strict tick-control measures. The Variations in prevalence rates of tick-borne diseases in zebu cattle by agro ecological factors investigated such as body score serological evidence of cattle exposure to zone – implications for East-coast were not associated with E.ruminantium E. ruminantium infection highlights the immunization. Preventive Veterinary Medi- infection. need for taking cowdriosis into consider- cine 16: 171–187 Age-related differences in prevalence ation in the envisaged future crossbreed- 6. Du Plessis J L, Bezuidenhout J D, Brett M S, and force of infection were significant. A ing scheme on Manyara Ranch, where Camus E, Jongejan F,Mahn S M, Martinez D 1993 The sero-diagnosis of heartwater: a strong positive correlation was observed over 70 % of the ranch herd is of the Boran comparison of five tests. Revue d’Élevage et with increasing age and reactor rates, re- breed. This breed has been shown to have de Médecine Vétérinaire des Pays Tropicaux 46: flecting more innate resistance to primary a higher growth rate and reproductive 123–129 infection and a high level of exposure to performance, and better adaptability to 7. Epi-info 1996 Centres for Disease Control, infective ticks. This is in agreement with stressful nutritional, parasitic and disease version 6.04d, Atlanta, USA, and Geneva, 32 Switzerland the findings of other authors . High anti- conditions, than the exotic (Bos taurus) 30 8. Hoogstraal H 1956 African Ixodoidea Volume body response to the E. ruminantium para- breed . These potentials have made 1. Ticks of Sudan. Department of the Navy, site in animals above 6 months of age (the Boran cattle a standard breed in cross- Bureau of Medicine and Surgery, Washing- youngest animals sampled) demon- breeding for both beef and milk produc- ton, DC strated a high level of and early exposure tion in low-input pasture-based systems31 9. Jongejan F, Thielemans M J 1989 Identifica- to infective ticks consistent with a high tion of an immunodominant antigenically conserved 32-kilodalton protein from Cow- CONCLUSIONS degree of infection. Studies have shown dria ruminantium. Infection and Immunity 57: that maternally derived antibodies de- The use of serum antibody profiles in 3243–3246 1 cline to zero within 2–4 months .Inthe the detection of exposure of cattle to the 10. Kivaria F M 2006 Estimated direct economic light of our findings, the detected anti- E. ruminantium parasite has an important costs associated with tick borne diseases on body titres could be due to field exposure diagnostic role. Despite the fact that cattle in Tanzania. Tropical Animal Health Production 38: 291–299 to infective ticks rather than maternally animals seem well adapted to ranch envi- 11. Knopf L, Komoin-Oka C, Betschart B, derived antibodies. As far as is known, im- ronments, the use of antibody profiles Jongejan F,Gottstein B, Zinsstag J 2002 Sea- munisation against E. ruminantium has alone is not adequate to explain disease sonal epidemiology of ticks and aspects of never been carried out in Tanzania, and status, as a number of animals devel- cowdriosis in N’dama cattle in the central any seropositive cattle are most likely to oped antibodies without obvious clinical Guinea Savannah of Côte d’Ivoire. Preven- have been naturally infected. Nonethe- disease. Consistent with serology, other tive Veterinary Medicine 53: 21–30 12. Koney E B, Dogbey O, Walker A R, less, this is the first structured study to prospective morbidity and mortalitity Bell-Sakyi L 2004. Ehrlichia ruminantium have identified and quantified the deter- studies should be carried out to arrive at a seroprevalence in domestic ruminants in minants of heartwater in an extensively better-informed indication of the disease Ghana. II. Point prevalence survey. Veteri- grazed local herd in rural Tanzania. status in this complex livestock/wildlife nary Microbiology 103: 183–193 Although MAP-1B ELISA is rated as interaction system. 13. Lynen G, Bakuname C, Sanka P 1999 Tick more specific and sensitive to ovine and and tick borne survey in northern regions ACKNOWLEDGEMENTS of Tanzania. Proceedings of the Tanzanian Vet- caprine than bovine sera compared to erinary Association (TVA)Scientific Conference polyclonal competitive ELISA (PC- The authors gratefully acknowledge held at AICC Arusha, 30 November – 2 ELISA)27, our seroconversion estimates FAO country office in Tanzania for financ- December 1999: 24–31 are likely to be lower or higher than the ing the E. ruminantium serology kit. Weare 14. Mahan S M, Semu S M, Peter T F,Jongejan F true prevalence in the cattle. A wider use also grateful to the manager of Manyara 1998 Evaluation of the MAP-1B ELISA for cowdriosis with field sera from livestock in of MAP-1B-ELISA in cattle under field Ranch, who kindly allowed us to use his Zimbabwe. Annals of the New York Academy conditions is hampered by the down- reg- ranch and facilities. My sincere thanks of Sciences 849: 259–261 ulation of antibody responses to E. rumi- go to Drs A. Nijhof and B. Faburay for 15. Mboloi M M, Bekker C P, Kritwagen C, nantium following 1st exposure to the reviewing an earlier draft of this paper Grener M, Jongejan F 1999 Validationof the organism, and also by cross-reactions and for providing useful suggestions. indirect MAP 1B enzyme-linked immuno- sorbent assay for diagnosis of experimental with other Ehrlichia secies such as E. canis This paper is published with the permis- 22,23 Cowdria ruminantium infection in small and E. phagocytophilum . However, con- sion of the Director of the Veterinary ruminants. Clinical and Diagnostic Labora- sistent with the results of Zimbabwean Services in Tanzania. tory Immunology 6: 66–72

74 0038-2809 Tydskr.S.Afr.vet.Ver. (2008) 79(2): 71–75 16. Meltzer M I, Norval R A 1993 Evaluating the Ehrlichia ruminantium infection (heart- 28. Tafesse B 1996 Survey on the distribution economic damage threshold for bont tick water) in wild animals. Trends in Parasitol- of ticks of domestic animals in eastern ( hebraeum) control in Zimbabwe. ogy 18: 214–218 Ethiopia. Tropical Animal Health Production Experimental and Applied Acarology 17: 171– 23. Semu S M, Peter T F,Mukwedeya D, Barbet 28: 145–146 185 A F, Jongejan F, Mahn S M 2001 Antibody 29. Thrusfield M 2000 Veterinary epidemiology. 17. Mondry R, Martinez D, Camus E, Liebisch responses to MAP 1B and Cowdria rumi- Blackwell Science, London A, Katz J B, Dewald R, Van Vliet A H, nantium antigen are down regulated in cattle 30. Trail J C M, Gregory K E, Marples H J, Jongejan F 1998 Validation and comparison with tick transmitted heart water. Clinical Kakonge J 1982 Heterosis, additive maternal of three enzyme-linked immunosorbent and Diagnostic Laboratory Immunology 8: and additive direct effects of the Red Poll assays for detection of antibodies to Cowdria 388–396 and Boran breeds of cattle. Journal of Animal ruminatium. Annals of the New York Academy 24. Smith G E, Anderson E C, Burridge M J, Science 54: 517–523 of Sciences 849: 262–272 Peter T F, Mahan S M 1998 Growth of 31.TrailJCM,Gregory K E, Stanford J, Durkin 18. Mtei B M, Msami H 1998 Tick and tick- Cowdria ruminantium in tissue culture endo- J 1984 Crossbreeding cattle in beef produc- borne disease control. Strategy paper of thelial cell lines from wild African mam- tion programmes in Kenya. I. Comparison the Livestock Department, Ministry of mals. Journal of Wildlife Diseases 34: 297–304 of purebred Boran and Boran crossed with Agriculture. Vol.1, Dar es Salaam, Tanzania 25. Soldan A W,Norman T L, Masaka S, Paxton the Chorolais, Aryshire and Santa Gertrudis 19. Muller K A, Martinez D, Camus E, Jongejan E A, Edelsten R M, Sumption K J 1993 breeds. Tropical Animal Health Production 16: F 1992 Distribution of heart water in the Sero-conversion to Cowdria ruminantium of 181–186 Caribbean determined on the basis of de- Malawi zebu calves, reared under different 32. Trueman K F,Blight G W 1978 The effects of tection of antibodies to the conserved tick control strategies. Revue d’Élévage et de age on the resistance of cattle to Babesia 32-kilodalton protein of Cowdria ruminan- Médecine Vétérinaire des Pays Tropicaux 46: bovis. Australian Veterinary Journal 54: 301– tium. Journal of Clinical Microbiology 30: 171–177 305 1870–1873 26. Sumba D, Bergin P,Jones C 2005 Land con- 33. Uilenberg G 1997 The epidemiology of 20. Nicholson M J, Butterworth M H 1986 A servation trusts: a case study of Manyara heartwater: establishment and maintenance guide to condition scoring of zebu cattle. Inter- ranch. AWF working paper. Online at: of endemic stability – two comments. Para- national Livestock Centre for Africa, Addis http://www.awf.org (accessed July 2007) sitology Today 13: 243 Ababa, Ethiopia 27. Sumption K J, Paxton E A, Bell-Sakyi L 2003 34. Van Vliet A H, van der Zeijst B A, Camus E, 21. Norval R A I, Lawrence A J, Young A S, Development of a polyclonal competitive Mahan S M, Martinez D, Jongejan F 1995 Perry B D, Dolan T T,Mukhebi W A, Bishop, enzyme-linked immunosorbent assay for Use of a specific immunogenic region on R, McKeever D 1992 The epidemiology of detection of antibodies to Ehrlichia rumi- the Cowdria ruminantium MAP1 protein in a theileriosis in Africa. Academic Press, London nantium. Clinical and Diagnostic Laboratory serological assay. Journal of Clinical Microbi- 22. Peter T F, Mahan S M, Burridge M J 2002 Immunology 10: 910–916 ology 33: 2405–2410

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