Evaluation of the MAP-1B ELISA for cowdriosis with field sera from livestock in Zimbabwe

Tick-borne diseases in the Union of the Comoros are a hindrance to livestock development: Circulation and associated risk factors

Tick-borne diseases (TBD) occur in many temperate countries and are economically important in most tropical and subtropical areas, affecting dairy and beef cattle, as well as small ruminants. Four major tick-borne diseases have been detected in eastern and southern Africa: East Coast fever (ECF) caused by Theileria parva, Theiler 1904, anaplasmosis caused by either Anaplasma marginale, Theiler 1910, Anaplasma centrale, Theiler 1911, or Anaplasma ovis, Bevan 1912, babesiosis caused by Babesia bovis, Babes 1988 and Babesia bigemina, Smith & Kilborne 1893, and heartwater caused by Ehrlichia ruminantium Cowdry 1925. A cross-sectional survey was undertaken to determine the antibody prevalence of these TBDs and to identify the risk factors for TBD infections in the Union of the Comoros. In 2016 and 2017, 903 individual animal serum samples were collected from 429 separate farms, where the farmers answered individual questionnaires. The antibody prevalence of anaplasmosis, babesiosis (B. bigemina) and heartwater was determined by enzyme-linked immunosorbent assays (ELISA) and the antibody prevalence of ECF was assessed using an immunofluorescence antibody test (IFAT). The relationship between TBD seropositivity and livestock-related variables was assessed by multivariate analyses with standard logistic regression models. The results showed that these four TBDs were present in the Union of the Comoros with a global antibody prevalence of 15% (95% CI [12.7%; 17.3%]) for anaplasmosis, 9.2% (95% CI [6.5%, 11.9%]) for B. bigemina babesiosis, 5.3% (95% CI [3.2%, 7.4%]) for ECF and 4.6% (95% CI [3.2%, 6%]) for heartwater. We compared these findings with the abundance and distribution of several tick species known to be TBD vectors and we found a significant correlation between Rhipicephalus appendiculatus and ECF, and between Amblyomma variegatum and heartwater. We also found that two major variables were significantly correlated with B. bigemina antibody prevalence ("island" and "breeding area"), four variables were significantly correlated with anaplasmosis antibody seroprevalence ("island", "number of cattle per farmer", "number of farmers per village" and "breeding area"), two were significantly correlated with ECF antibody prevalence ("number of farmers in village" and "presence of ticks"), and three were significantly correlated with heartwater ("island", "number of cattle per farmer" and "number of farmers in the village"). Our findings confirmed livestock exposure to the four targeted TBDs of major concern for livestock development. Consequently, raising farmers' awareness and setting up a period of quarantine should be considered a priority.

Seroprevalence of Ehrlichia ruminantium antibodies and its associated risk factors in indigenous goats of South Africa

The present study investigated the seroprevalence of antibodies to Ehrlichia ruminantium and the associated risk factors in goats from five different farming provinces of South Africa. Sera collected from 686 goats of the commercial meat type (n=179), mohair type (n=9), non-descript indigenous goats from Eastern Cape (n=56), KwaZulu-Natal (n=209), Limpopo (n=111), North West (n=61) and Northern Cape (n=11) provinces and a feral Tankwa goat (n=50) were tested for the presence of immunoglobulin G (IgG) antibodies to antigens of E. ruminantium using the indirect fluorescent-antibody test (IFAT). Fifty two percent of these goats had ticks. The overall seroprevalence of antibodies to E. ruminantium was 64.87% (445/686) with the highest seroprevalence reported for Limpopo (95.50%) and lowest for Northern Cape (20.29%). Highest seroprevalence for antibodies to E. ruminantium was observed in goats from endemic regions (76.09%), and from smallholder production systems (89.54%). High seroprevalence was also observed in non-descript indigenous goats (85.04%), adult goat (69.62%), in does (67.46%) and goats infested with ticks (85.79%). The logistic model showed a gradient of increasing risk for commercial meat type Savanna (OR=3.681; CI=1.335-10.149) and non-descript indigenous (OR=3.466; CI=1.57-7.645) compared to Boer goats and for goats from the smallholder production system (OR=2.582; CI=1.182-5.639) and those with ticks (OR=3.587; CI=2.105-6.112). Results from this study showed that E. ruminantium infections were prevalent but were widely and unevenly distributed throughout South Africa. Findings from the study facilitate identification and mapping of risk areas for heartwater and its endeminicity in South Africa and should be taken into consideration for future disease control strategies and local goat improvement programs.

Diversity of Ehrlichia ruminantium major antigenic protein 1-2 in field isolates and infected sheep

Proteins expressed from the map1 multigene family of Ehrlichia ruminantium are strongly recognized by immune T and B cells from infected animals or from animals that were infected and have recovered from heartwater disease (although still remaining infected carriers). Analogous multigene clusters also encode the immunodominant outer membrane proteins (OMPs) in other ehrlichial species. Recombinant protein analogs of the expressed genes and DNA vaccines based on the multigene clusters have been shown to induce protective immunity, although this was less effective in heterologous challenge situations, where the challenge strain major antigenic protein 1 (MAP1) sequence differed from the vaccine strain MAP1. Recent data for several ehrlichial species show differential expression of the OMPs in mammalian versus tick cell cultures and dominant expression of individual family members in each type of culture system. However, many genes in the clusters appear to be complete and functional and to generate mRNA transcripts. Recent data also suggest that there may be a low level of protein expression from many members of the multigene family, despite primary high-level expression from an individual member. A continuing puzzle, therefore, is the biological roles of the different members of these OMP multigene families. Complete genome sequences are now available for two geographically divergent strains of E. ruminantium (Caribbean and South Africa strains). Comparison of these sequences revealed amino acid sequence diversity in MAP1 (89% identity), which is known to confer protection in a mouse model and to be the multigene family member primarily expressed in mammalian cells. Surprisingly, however, the greatest sequence diversity (79% identity) was in the less-studied map1-2 gene. We investigated here whether this map1-2 diversity was a general feature of E. ruminantium in different cultured African strains and in organisms from infected sheep. Comparison of MAP1-2s revealed amino acid identities of 75 to 100% (mean of 86%), compared to 84 to 100% (mean of 89%) for MAP1s. Interestingly, MAP1-2s varied independently of MAP1s such that E. ruminantium strains with similar MAP1s had diverse MAP1-2s and vice versa. Different MAP1-2s were found in individual infected sheep. Different regions of a protein may be subjected to different evolutionary forces because of recombination and/or selection, which results in those regions not agreeing with a phylogeny deduced from the whole molecule. This appears to be true for both MAP1 and MAP1-2, where statistical likelihood methods detect heterogeneous evolutionary rates for segments of both molecules. Sera from infected cattle recognized a MAP1-2 variable-region peptide in enzyme-linked immunosorbent assay, but less strongly and consistently than a MAP1 peptide (MAP1B). Heterologous protective immunity may depend on recognition of a complex set of varying OMP epitopes.

Molecular characterization of Ehrlichia interactions with tick cells and macrophages

Several tick-transmitted Anaplasmataceae family rickettsiales of the genera Ehrlichia and Anaplasma have been discovered in recent years. Some species are classified as pathogens causing emerging diseases with growing health concern for people. They include human monocytic ehrlichiosis, human granulocytic ewingii ehrlichiosis and human granulocytic anaplasmosis which are caused by Ehrlichia chaffeensis, E. ewingii and Anaplasma phagocytophilum, respectively. Despite the complex cellular environments and defense systems of arthropod and vertebrate hosts, rickettsials have evolved strategies to evade host clearance and persist in both vertebrate and tick host environments. For example, E. chaffeensis growing in vertebrate macrophages has distinct patterns of global host cell-specific protein expression and differs considerably in morphology compared with its growth in tick cells. Immunological studies suggest that host cell-specific differences in Ehrlichia gene expression aid the pathogen, extending its survival. Bacteria from tick cells persist longer when injected into mice compared with mammalian macrophage-grown bacteria, and the host response is also significantly different. This review presents the current understanding of tick-Ehrlichia interactions and implications for future.

Longitudinal monitoring of Ehrlichia ruminantium infection in Gambian lambs and kids by pCS20 PCR and MAP1-B ELISA

BACKGROUND

The epidemiology of E. ruminantium infection in extensively managed young animals is not adequately understood. Thus in this study, we monitored the onset (age at first infection) and kinetics of E. ruminantium infection and antibody response in extensively managed newborn lambs and kids at three sites in The Gambia.

METHODS

We used a nested pCS20 PCR and MAP1-B ELISA in a longitudinal study to monitor the onset (age at first infection) and kinetics of E. ruminantium infection and antibody response respectively, in 77 newborn lambs and kids under a traditional husbandry system at three sites (Kerr Seringe, Keneba, Bansang) in The Gambia where heartwater is known to occur. The animals were monitored for field tick infestation and the comparative performance of the two assays in detecting E. ruminantium infection was also assessed.

RESULTS

The infection rate detected by pCS20 PCR varied between 8.6% and 54.8% over the 162-day study period. Nineteen per cent of the animals in week 1 post-partum tested positive by pCS20 PCR with half of these infections (7/14) detected in the first 3 days after birth, suggesting that transmission other than by tick feeding had played a role. The earliest detectable A. variegatum infestation in the animals occurred in week 16 after birth. Antibodies detected by MAP1-B ELISA also varied, between 11.5% and 90%. Although there is considerable evidence that this assay can detect false positives and due to this and other reasons serology is not a reliable predictor of infection at least for heartwater. In contrast to the pCS20 PCR, the serological assay detected the highest proportion of positive animals in week 1 with a gradual decline in seropositivity with increasing age. The pCS20 PCR detected higher E. ruminantium prevalence in the animals with increasing age and both the Spearman's rank test (rs = -0.1512; P = 0.003) and kappa statistic (-0.091 to 0.223) showed a low degree of agreement between the two assays.

CONCLUSION

The use of pCS20 PCR supported by transmission studies and clinical data could provide more accurate information on heartwater epidemiology in endemic areas and single-occasion testing of an animal may not reveal its true infection status. The view is supported because both the vector and vertical transmission may play a vital role in the epidemiology of heartwater in young small ruminants; the age range of 4 and 12 weeks corresponds to the period of increased susceptibility to heartwater in traditionally managed small ruminants.

Point seroprevalence survey of Ehrlichia ruminantium infection in small ruminants in The Gambia

Using the MAP1-B enzyme-linked immunosorbent assay, we tested 1,318 serum samples collected from sheep and goats at 28 sites in the five divisions of The Gambia to determine the Ehrlichia ruminantium seroprevalence rates and to assess the risk for heartwater. About half (51.6%) of 639 sheep were positive, with seroprevalence rates per site varying between 6.9% and 100%. The highest seroprevalence was detected in the western part of the country (88.1% in the Western Division and 62.1% in the Lower River Division). Sheep in the two easterly divisions (Central River and Upper River divisions) showed the lowest seroprevalence of 29.3% and 32.4%, respectively, while those in the North Bank Division showed an intermediate prevalence of 40.6%. In goats, less than one-third (30.3%) of 679 animals tested were positive. The highest seroprevalence was detected in goats in the North Bank Division (59%) and Western Division (44.1%). Goats in the Lower River Division showed an intermediate level of 21.9%, whereas the lowest rates were found in the eastern part of the country (4.8% in the Central River Division and 2.3% in the Upper River Division). At nearly all sites, seroprevalence rates were higher in sheep than in goats. The results show a gradient of increasing heartwater risk for susceptible small ruminants from the east to the west of The Gambia. These findings need to be taken into consideration when future livestock-upgrading programs are implemented.

Detection by two enzyme-linked immunosorbent assays of antibodies to Ehrlichia ruminantium in field sera collected from sheep and cattle in Ghana

Two serological tests for detection of antibodies to Ehrlichia (previously Cowdria) ruminantium, the causative agent of heartwater, were compared by using field sera collected from sheep and cattle as part of serosurveys in Ghana. Sera selected as either negative or positive by a new polyclonal competitive enzyme-linked immunosorbent assay (PC-ELISA) were tested by the indirect MAP1-B ELISA. Cutoff values of 14 percent positivity (14 PP) for both ruminant species were obtained for the MAP1-B ELISA by using preseroconversion Ghanaian sera and were compared with previously recommended cutoff values of 29 PP for sheep and 38 PP for cattle. With the 14-PP cutoff, of 151 sheep sera which tested negative by PC-ELISA, 89% were also negative by MAP1-B ELISA, while of 419 sheep sera positive by PC-ELISA, 98% were also positive by MAP1-B ELISA. Of 261 bovine sera negative by PC-ELISA, 82% were also negative by MAP1-B ELISA. Of 511 bovine sera positive by PC-ELISA, only 47% were positive by MAP1-B ELISA; these included 168 sera collected from cattle following first seroconversion as detected by both tests, with 125 of these sera positive by PC-ELISA but only 59 and 5 positive by MAP1-B ELISA with the 14- and 38-PP cutoff levels, respectively. These results indicate that both assays are highly sensitive and specific for detection of E. ruminantium exposure in sheep but that the MAP1-B ELISA lacks sensitivity for postseroconversion bovine sera in comparison to the PC-ELISA. Both tests confirm E. ruminantium seroprevalence of at least 70% in Ghanaian sheep; levels of exposure among Amblyomma variegatum-infested Ghanaian cattle are likely to be higher than the seroprevalence value of 66% obtained with the PC-ELISA.

Development of a polyclonal competitive enzyme-linked immunosorbent assay for detection of antibodies to Ehrlichia ruminantium

A polyclonal competitive enzyme-linked immunosorbent assay (PC-ELISA) is described for detection of antibodies to Ehrlichia (Cowdria) ruminantium by using a soluble extract of endothelial cell culture-derived E. ruminantium as the antigen and biotin-labeled polyclonal goat immunoglobulins as the competitor. For goats, the diagnostic sensitivity and specificity were both 100% with a cutoff of 80% inhibition (80 PI), with detection of antibodies for 550 days postinfection. For cattle, diagnostic sensitivity and specificity were 86 and 100%, respectively, with a cutoff of 50 PI and 79 and 100% with a cutoff of 70 PI. Cross-reactions with high-titer experimental or field antisera to other Ehrlichia and Anaplasma species were observed at up to 68 PI in cattle and up to 85 PI in sheep, and therefore to exclude these cross-reactions, cutoffs of 70 PI for bovine serology and 85 PI for small-ruminant serology were selected. Application of the PC-ELISA to bovine field sera from South Africa gave a higher proportion of positive results than application of the murine macrophage immunofluorescent antibody test or indirect ELISA, suggesting a better sensitivity for detection of recovered cattle, and results with bovine field sera from Malawi were consistent with the observed endemic state of heartwater and the level of tick control practiced at the sample sites. Reproducibility was high, with average standard deviations intraplate of 1.2 PI and interplate of 0.6 PI. The test format is simple, and the test is economical to perform and has a level of sensitivity for detection of low-titer positive bovine sera that may prove to be of value in epidemiological studies on heartwater.

Antibody responses to MAP 1B and other Cowdria ruminantium antigens are down regulated in cattle challenged with tick-transmitted heartwater

Serological diagnosis of heartwater or Cowdria ruminantium infection has been hampered by severe cross-reactions with antibody responses to related ehrlichial agents. A MAP 1B indirect enzyme-linked immunosorbent assay that has an improved specificity and sensitivity for detection of immunoglobulin G (IgG) antibodies has been developed to overcome this constraint (A. H. M. van Vliet, B. A. M. Van der Zeijst, E. Camus, S. M. Mahan, D. Martinez, and F. Jongejan, J. Clin. Microbiol. 33:2405-2410, 1995). When sera were tested from cattle in areas of endemic heartwater infection in Zimbabwe, only 33% of the samples tested positive in this assay despite a high infection pressure (S. M. Mahan, S. M. Samu, T. F. Peter, and F. Jongejan, Ann. N.Y. Acad. Sci 849:85-87, 1998). To determine underlying causes for this observation, the kinetics of MAP 1B-specific IgG antibodies in cattle after tick-transmitted C. ruminantium infection and following recovery were investigated. Sera collected weekly over a period of 52 weeks from 37 cattle, which were naturally or experimentally infected with C. ruminantium via Amblyomma hebraeum ticks, were analyzed. MAP 1B-specific IgG antibody responses developed with similar kinetics in both field- and laboratory-infected cattle. IgG levels peaked at 4 to 9 weeks after tick infestation and declined to baseline levels between 14 and 33 weeks, despite repeated exposure to infected ticks and the establishment of a carrier state as demonstrated by PCR and xenodiagnosis. Some of the serum samples from laboratory, and field-infected cattle were also analyzed by immunoblotting and an indirect fluorescent-antibody test (IFAT) to determine whether this observed seroreversion was specific to the MAP 1B antigen. Reciprocal IFAT and immunoblot MAP 1-specific antibody titres peaked at 5 to 9 weeks after tick infestation but also declined between 30 and 45 weeks. This suggests that MAP 1B-specific IgG antibody responses and antibody responses to other C. ruminantium antigens are down regulated in cattle despite repeated exposure to C. ruminantium via ticks. Significantly, serological responses to the MAP 1B antigen may not be a reliable indicator of C. ruminantium exposure in cattle in areas of endemic heartwater infection.

Detection of the agent of heartwater, Cowdria ruminantium, in Amblyomma ticks by PCR: validation and application of the assay to field ticks

We have previously reported that the pCS20 PCR detection assay for Cowdria ruminantium, the causative agent of heartwater disease of ruminants, is more sensitive than xenodiagnosis and the pCS20 DNA probe for the detection of infection in the vector Amblyomma ticks. Here, we further assessed the reliability of the PCR assay and applied it to field ticks. The assay detected DNA of 37 isolates of C. ruminantium originating from sites throughout the distribution of heartwater and had a specificity of 98% when infected ticks were processed concurrently with uninfected ticks. The assay did not detect DNA of Ehrlichia chaffeensis, which is closely related to C. ruminantium. PCR sensitivity varied with tick infection intensity and was high (97 to 88%) with ticks bearing 10(7) to 10(4) organisms but dropped to 61 and 28%, respectively, with ticks bearing 10(3) and 10(2) organisms. The assay also detected C. ruminantium in collections of Amblyomma hebraeum and Amblyomma variegatum field ticks from 17 heartwater-endemic sites in four southern African countries. Attempts at tick transmission of infection to small ruminants failed with four of these collections. The pCS20 PCR assay is presently the most characterized and reliable test for C. ruminantium in ticks and thus is highly useful for field and laboratory epidemiological investigations of heartwater.