The transmission of Cowdria ruminantium by Amblyomma tholloni

Transmission of the rickettsial organisms Cowdria ruminantium (cause of heartwater) by the elephant tick Amblyomma tholloni has been demonstrated. Larvae fed on infected sheep successfully transmitted the disease causing organisms in both the nymphal and adult stages. Nymphs similarly infected transmitted the organisms as adults. Transovarian transmission did not occur. The results are discussed in relation to the epidemiology and control of heartwater in areas infested with A. tholloni.     

Natural history of Ehrlichia ruminantium

Ehrlichia ruminantium is an obligately intracellular proteobacterium which causes a disease known as heartwater or cowdriosis in some wild, and all domestic, ruminants. The organism is transmitted by ticks of the genus Amblyomma, and it is of serious economic importance wherever the natural vectors occur, an area which includes all of sub-Saharan Africa, and several islands in the Caribbean. The disease was first recognized in South Africa in the 19th century, where its tick-borne nature was determined in 1900, but the organism itself was not demonstrated until 1925, when it was recognized to be a rickettsia, initially named Rickettsia ruminantium. It was thus the first species of what are now known as Ehrlichia to be discovered, and most of the early work to elucidate the nature of the organisms, and its reservoirs and vectors, was performed in South Africa. The next milestone was the development, in 1945, of an infection and treatment regimen to immunize livestock, and this is still the only commercially available “vaccine” against the disease. Then in 1985, after fruitless attempts over many years, the organism was propagated reliably in tissue culture, opening the way for the first application of the newly developed techniques of molecular genetics. From 1990 onwards the pace of heartwater research accelerated rapidly, with notable advances in phylogeny, diagnosis, epidemiology, immunology, and vaccine development. The complete genome sequence was published in 2005, and during the last two years a new understanding has arisen of the remarkable genetic variability of the organism and new experimental vaccines have been developed. Despite all this the goal of producing an effective vaccine against the disease in the field still remains frustratingly just beyond reach. This article summarises our current understanding of the nature of E. ruminantium, at a time when the prospects for the development of an effective vaccine against the organism seem better than at any time since its discovery 83 years ago.     

Brain biopsy for the diagnosis of heartwater

Summary The technique of brain biopsy and its successful role in confirming a diagnosis of heartwater in the live animal are described.

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Biopsia Cerebral Para El Diagnóstico De Hidropericardio

Resumen Se describe la técnica de la biopsia cerebral para el diagnóstico de hidropericardio. Esta se ha utilizado exitosamente en Medicina Veterinaria.

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Biopsie Du Cerveau Pour Le Diagnostic De La Heartwater

Résumé La technique de la biopsie cérébrale et son rôle efficace dans la confirmation du diagnostic de la heartwater sur l'animal vivant sont décrits.

     

Mortality percentages related to heartwater and the economic impact of heartwater disease on large-scale commercial farms in Zimbabwe

A postal questionnaire asking for data concerning heartwater deaths was sent to all large-scale commercial farmers in Zimbabwe's lowveld, where heartwater is endemic. One hundred and fifty returns (24% return rate) provided usable data, with 85 farms (57% of sample) reporting heartwater deaths in 1990/91. The median heartwater mortality percentage for calves (0–12 months) was 2.3%. This was statistically significantly different (Wilcoxon signed rank test, P < 0.005) from the median mortality percentage of 0.6% for cattle 13 months and older. Heartwater accounted for 51% of all mortalities on farms reporting heartwater deaths. There were no meaningful correlations between farm or herd size and heartwater mortality percentages. A plot of the farms reporting heartwater deaths found that the heartwater endemic area in 1991 is essentially unchanged from 1979. Inspection of the geographic distribution of farms reporting heartwater showed no obvious patterns, and distances from communal land farms were not significantly correlated to heartwater mortality percentages. An economic model found that the annual financial impact of heartwater on large-scale commercial farms in Zimbabwe to be some Z$19.4 million (1991 prices). The upper bound, that allowed for inflation since 1991, was Z$33.4 million. The lower bound estimate was Z$8.1 million (Z$3 = US$1, 1991 rates). Eighty-nine percent of these costs can be attributed to the cost of dipping. Since it has been shown experimentally that the carrier state of heartwater may not affect the productivity of cattle, intensive dipping may not be the best means of controlling heartwater.     

The effect of prolonged oral administration of oxytetracycline on the course of heartwater (Cowdria Ruminantium) infection in sheep

Summary The effect of the prolonged oral administration of tetracycline on the course of artificially-induced heartwater in sheep was investigated. It was shown that the course of the disease could be greatly modified by this treatment, only one sheep in thirty showing typical heartwater during a 38-day period post inoculation. All the remaining sheep were immune when challenged with heartwater after 38 days. The practical implications of these findings and the possible development of drug-resistance are discussed. Attention is drawn to the absence of significant side effects following high-level feeding of oxytetracycline to sheep for a prolonged period.

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Sumario El efecto de la administración oral prolongada de la tetraciclina sobre el curso del corazón de agua artificialmente inducido en ovinos fue investigado. So demostró que el curso de la enfermedad podía ser grandemente modificado mediante este tratamiento, mostrando solamente un ovino de treinta, típico corazón de agua durante un periodo de 38 dias despues de la inoculación. Todo el remanente de los ovinos fueron inmunes cuando se les desafió con corazón de agua despues de 38 dias. Se discuten las implicaciones práticas de estos hallazgos y el posible desarrollo de resistencia a las drogas. Se hace notar la ausencia de efectos secundariosde significación despues de la alimentación prolongada de altos niveles de oxitetraciclina en ovinos.

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Résumé L’effet de l’administration orale de longue durée de tétracycline sur l’évolution de la heartwater expérimentale chez le mouton a été étudié. Il a été montré que l’évolution de la maladie pouvait être grandement modifiée par ce traitement, étant donné qu’un seul mouton sur trente a montré la heartwater typique pendant une période de 38 jours suivant l’inoculation. Tous les autres moutons étaient immunisés contre l’épreuve par la heartwater après 38 jours. La signification pratique de ces résultats et le développement possible d’une résistance au médicament sont discutés. L’attention est attirée sur l’absence d’effets toxiques notables de l’administration orale à haute dose d’oxytétracycline au mouton pendant une longue période.

     

Heartwater (Cowdria ruminantium infection) on São Tomé

Summary

The supernatant obtained after centrifugation of an emulsion of engorged female Amblyomma astrion ticks, collected from cattle on the West African island of São Tomé, was injected intravenously into a goat, which contracted heartwater. This confirmation of the existence of the disease on the island makes strict control measures necessary if present efforts at improving livestock production by imported exotic cattle are to be successful.     

Mining the genetic diversity of Ehrlichia ruminantium using map genes family

Understanding bacterial genetic diversity is crucial to comprehend pathogenesis. Ehrlichia ruminantium (E. ruminantium), a tick-transmitted intracellular bacterial pathogen, causes heartwater disease in ruminants. This model rickettsia, whose genome has been recently sequenced, is restricted to neutrophils and reticulo-endothelial cells of its mammalian host and to the midgut and salivary glands of its vector tick. E. ruminantium harbors a multigene family encoding for 16 outer membrane proteins including MAP1, a major antigenic protein. All the 16 map paralogs are expressed in bovine endothelial cells and some are specifically translated in the tick or in the mammalian host.In this study, we carried out phylogenetic analyses of E. ruminantium using sequences of 6 MAP proteins, MAP1, MAP1-2, MAP1-6, MAP1-5, MAP1+1 and MAP1-14, localized either in the center or at the borders of the map genes cluster.We show that (i) map1 gene is a good tool to characterize the genetic diversity among Africa, Caribbean islands and Madagascar strains including new emerging isolates of E. ruminantium; (ii) the different map paralogs define different genotypes showing divergent evolution; (iii) there is no correlation between all MAP genotypes and the geographic origins of the strains; (iv) The genetic diversity revealed by MAP proteins is conserved whatever is the scale of strains sampling (village, region, continent) and thus was not related to the different timing of strains introduction, i.e. continuous introduction of strains versus punctual introduction (Africa versus Caribbean islands).These results provide therefore a significant advance towards the management of E. ruminantium diversity. The differential evolution of these paralogs suggests specific roles of these proteins in host–vector–pathogen interactions that could be crucial for developing broad-spectrum vaccines.     

Problems encountered in the control of heartwater in Angora goats

This preliminary investigation confirmed that when Angora goats were immunized against heartwater, either when kids or young goats, they still had an immunity to heartwater a year later when they were challenged with a vaccine containing Cowdria ruminantium. An overwhelming majority of the uninoculated animals which were challenged at the same time were susceptible to clinical heartwater. Strategic immunization to obtain enzootic stability to heartwater is suggested. In these experiments the immunization of Angora goats was not accompanied by severe losses.     

Heartwater in Nigeria

A study was carried out to determine which materials from animals dying or dead of heartwater could initiate the disease in susceptible goats, using the intravenous and subcutaneous routes. C. ruminantium was consistently isolated by intravenous injection of the whole blood or of lung macrophages and by subcutaneous injection of brain homogenate. In animals dead of heartwater, it appeared that isolation of the organism was achieved only when extensive post-mortem autolysis had not supervened. Experiments with blood fractions showed that leucocytic and plasma fractions of infective blood transmitted heartwater; the erythrocytic fraction consistently failed to induce an infection.     

The control of heartwater on large-scale commercial and smallholder farms in Zimbabwe.

As part of a series of studies associated with the development of improved vaccines for heartwater (a tick-borne disease of ruminant livestock caused by Cowdria ruminantium), field surveys were carried out to assess losses associated with the disease and the costs associated with controlling it in the two main agro-ecological zones of Zimbabwe (lowveld and highveld) where heartwater is believed to be endemic and epidemic, respectively. In each zone, a cross-sectional study was performed in the main farming systems (smallholder (SH) and large-scale commercial (LSC) beef and dairy), followed by longitudinal studies in the same sectors to improve data accuracy for some parameters. Suspected heartwater-specific mortality in cattle was similar in all LSC sectors (p = 0.72) accounting for a median 1% mortality risk. Heartwater-specific mortality in SH areas was not assessed due to poor diagnostic ability of the farmers. Few LSC farms and SH households kept sheep; suspected heartwater-specific mortality in LSC sheep was 0.8% in the lowveld and 2.4% in the highveld. Goats were a major enterprise in SH areas but not on LSC farms. Suspected heartwater mortality in LSC goats was 0.8% at one site in the highveld and 17.5% on a farm in the lowveld. Application of acaricides was the major control method for heartwater and other tick-borne diseases on both SH and LSC farms. On LSC farms, plunge dipping was used most frequently and the number of acaricide applications ranged widely between 3 and 52 per year. The total cost of acaricides per head per annum was higher in highveld dairies than in highveld and lowveld beef enterprises (p = 0.03). In SH areas, cattle plunge dipping was conducted by the government with an average frequency of 8 +/- 2 (sd) immersions per annum in both the lowveld and highveld. The type of tick control on sheep and goats in all production systems was highly variable (ranging from none to hand removal or intensive acaricide treatment). Suspected heartwater cases on LSC farms were treated with tetracyclines; treatment was not reported in SH areas. Reported treatment costs were high (median Z$ 120) and highly variable (range Z$-833). Vaccination against heartwater with the live, blood-based vaccine was reported on only one LSC farm. LSC farms applying acaricide 30 or more times per year reported higher morbidity (p < 0.0001) and mortality (p < 0.0001) than farms applying acaricides less than 30 times a year. This finding supports the use of reduced tick control in the management of heartwater in Zimbabwe.     

The use of electro-encephalography and brain biopsy in the clinical diagnosis of heartwater

Both electro-encephalography and brain biopsy were examined as possible aids in the confirmation of a clinical diagnosis of heartwater in field cases. Results showed that electro-encephalographic changes associated with heartwater were non-specific and probably caused by the associated brain oedema. Brain biopsy on the other hand appears to be a reliable technique for the confirmation of heartwater, and the technique if properly performed holds little risk for the animal.     

Isolation and transmission ofCowdria ruminantium (causal agent of heartwater disease) in Blue Nile Province,Sudan

Heartwater was diagnosed in sheep at the Umm Banein Livestock Research Station, Blue Nile Province, Sudan.Cowdria ruminantium was isolated from sheep by subinoculation of blood into a goat. AdultAmblyomma lepidum ticks fed as nymphs on this goat transmitted heartwater to a Friesian (Bos taurus) calf. Brain stabilates from this calf, either snapfrozen into liquid nitrogen or frozen slowly in a deepfreezer at –70°C, with DMSD as cryoprotectant, were infective to cattle when inoculated subcutaneously.     

T cell-mediated immunity to Cowdria ruminantium in mice: the protective role of Lyt-2+ T cells

The inability of athymic nude mice to make a drug-aided recovery from infection with either the Kümm or the Welgevonden stocks of Cowdria ruminantium and their inability to mount an immune response, suggest that immunity in heartwater is cell-mediated. The adoptive transfer of immunity with the spleen cells of mice immune to the Welgevonden stock is supportive evidence. Immune spleen cells depleted of Lyt-2+ T cells are unable to confer resistance to challenge to recipient mice, whereas the depletion of L3T4+ T cells had no effect on the protection conferred by immune spleen cells. This is conclusive evidence that immunity in heartwater is largely cell-mediated. Immune serum, C. ruminantium and complement incubated in the presence of mouse peritoneal macrophages, inhibits the infectivity of the heartwater agent, but not in the absence of macrophages. The decreased resistance to challenge of immune mice treated with gloxazone adds further support to the concept that in heartwater persistence of C. ruminantium in the host is associated with immunity.     

The treatment of heartwater

This paper reviews the available literature on the treatment of heartwater and draws comparisons with results obtained from recent work on experimentally-induced heartwater in sheep, calves and mice.     

Models for heartwater epidemiology: practical implications and suggestions for future research

We present a simple model of the dynamics of heartwater that we use to explore and better understand various aspects of this disease. We adapted the Ross-Macdonald model for malaria epidemiology so that we could consider both host and vector populations, and evaluate the interactions between the two. We then use two more biologically detailed models to examine heartwater epidemiology. The first includes a carrier state and host mortality, and the second includes density dependence. The results from all three models indicate that a stable equilibrium with high disease levels is probably the standard situation for heartwater (R0 between 5.7 and 22.4). More than 80% of cattle become infected with heartwater if only 12% of infected tick bites produce an infection in cattle, if tick burdens are as low as only five ticks per host per day, or if tick lifespans are as short as 7 d. A host recovery rate of 30 d results in over 50% of the cattle becoming infected with heartwater. Our analyses indicate that it is quite difficult to prevent the establishment and maintenance of high levels of heartwater in a herd, thereby supporting previous suggestions that any attempts at controlling this disease through stringent tick control regimens are not warranted.     

Susceptibility to heartwater of calves born to non-immune cows

The resistance to artificial infection with Cowdria ruminantium of calves born to cows fully susceptible to heartwater is no different from that of calves bred in heartwater endemic areas where the tick challenge is negligible to considerable. The sub-inoculation into mice of blood collected 14-26 days after infection proved the presence of the heartwater agent in the blood of 8 out of 10 calves with no other clinical signs than mild to moderate fever. The combined use of a mouse model and the indirect fluorescent antibody test revealed considerable variation in the degrees to which calves become infected and react to artificial infection.     

Problems with the interpretation of epidemiological data in heartwater: a study on 23 farms

In an epidemiological study undertaken on 23 farms where heartwater occurs endemically, it was found that on an overall average, antibodies to Cowdria ruminantium were detected in 64.3% of the cattle, 6 adult Amblyomma hebraeum ticks were counted per animal and 7.0% of ticks were infected with the heartwater agent. It was found that the seropositivity of the animals was determined largely by the tick loads to which they were subjected and that the influence of the tick C. ruminantium infection rate was less evident. There was no parallel between the prevalence of heartwater on the farms and the immune status of the animals. In general, higher tick counts were recorded in herds where strategic tick control is practised than on farms with a total tick control programme. The method of tick control did not, however, appear to influence the immune status of the cattle, the tick infection rate, or the prevalence of heartwater.     

Transmission of Cowdria ruminantium by Amblyomma gemma from Infected African Buffalo (Syncerus caffer) and Eland (Taurotragus oryx) to sheep

Two African buffalo (Syncerus caffer), an eland (Taurotragus oryx) and a waterbuck (Kobus defassa) were intravenously inoculated with Cowdria ruminantium (Kiswani). Amblyomma gemma nymphs were fed on the animals at 3 weekly intervals. Jugular blood was also collected at 3 weekly intervals and inoculated into sheep. Nymphal ticks that fed on one buffalo on days 16 and 37 and on the other buffalo on day 58 after infection transmitted the disease as adults to sheep. Nymphs that were applied to the eland 16 days after infection also transmitted the disease to sheep. No nymphs that had fed on the waterbuck transmitted the disease. This is the first report of transmission of heartwater by Amblyomma gemma from infected wild ruminant species to a susceptible domestic ruminant species.     

Some observations on the sero-prevalence of heartwater and tick infestation in Zambian goats

A survey was carried out to define the distribution of heartwater in goats that originated from six districts in communal grazing semi-arid areas of Zambia. A total of 181 samples (40.1%) out of 451 serum samples from adult goats were positive for Ehrlichia ruminantium antibodies after screening using indirect MAP-1B antigen ELISA technique with statistically significant differences (P < 0.01) between the six districts. Out of 1 036 adult goats examined for tick infestation, 105 were infested by ticks, with Amblyomma species being the most dominant tick encountered. Amblyomma variegatum, which is the vector for heartwater transmission in Zambia constituted 42.4% of the tick species, identified. The overall tick infestation rate was 10% while the tick:goat ratio was 2.1:1. Amblyomma variegatum appears to be widespread throughout the study area, as are antibodies to E. ruminantium.     

The application of the indirect fluorescent antibody test in research on heartwater

The preparation of the antigen, details of the reagents, the titration of the antispecies conjugates and the execution of the indirect fluorescent antibody test are described. The sensitivity and specificity of the test and its applicability to the detection of antibodies to Cowdria ruminantium are recorded. The test is both highly specific and sensitive and can be applied to a wide range of studies on heartwater, including epidemiology, determination of the C. ruminantium infection rate of Amblyomma ticks and the evaluation of immunization against heartwater. The test can also be used to detect antibodies to the heartwater agent in the sera of game.