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.     

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.

     

Specific immunity in mice to heartwater

Mice develop a specific immune response following infection with the mice strains of heartwater. In the case of the Kümm strain the agent can persist in some tissues for up to 365 days. Transfer of spleen cells from immune mice confers protection against homologous challenge in recipient mice showing that cell mediated immunity is important. A comparison with immune mechanisms occurring in other Rickettsia is discussed.     

Molecular epidemiology of Mycobacterium avium subsp. paratuberculosis at a regional scale in Germany

Mycobacterium avium subsp. paratuberculosis (MAP) is the causal agent of Johne's disease in dairy cattle. Genotyping of MAP is useful to gain a better understanding of the origin of infection, to evaluate regional control programs, to improve diagnostics, and to develop vaccines. In this study 91 MAP isolates mainly from symptomatic dairy cattle in Rhineland-Palatinate (RP, Germany), its neighbor federal states, and Luxembourg were genotyped using Mycobacterial Interspersed Repetitive Units-Variable Number Tandem Repeat (MIRU-VNTR) and Multilocus Short Sequence Repeats (MLSSR). MIRU-VNTR and MLSSR produced 11 and 6 different genotypes among the 91 isolates, respectively. The combined analysis of both methods produced 25 genotypes with an index of discrimination (D) of 0.93 (95% CI: 0.91-0.95). The results revealed the genetic diversity of MAP and the dominance of two MAP genotypes commonly found in Europe, showed the usefulness of MAP genotyping in studies at a regional scale, and provided useful information for control initiatives in RP.     

Specific immunity in farm animals to heartwater

The duration of immunity to heartwater normally varies from 6 months to 4 years. In Angora goats it may be difficult to produce a specific immune response with vaccination. Humoral immunity does not appear to play a role in the development of immunity. It has been suggested that cellular immunity may play a role, but the actual mechanism involved remains obscure. The specific immune response developing in farm animals following infection with Cowdria ruminantium is reviewed.     

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 clinical pathology and pathophysiology of heartwater: a review

This paper reviews the available literature on the clinical pathology and pathophysiology of heartwater and makes comparisons with unpublished results obtained from a recent study in experimentally-induced heartwater in calves. The pathophysiological changes seem to center on an increased capillary permeability the result of which is reflected most noticeably in cardiac and lung function. There is a marked drop in cardiac output in severe cases and some workers have recorded a severe drop in diastolic blood pressure in the advanced stage of the disease. Changes in lung function are variable, depending on the stage of the disease, and may change from a respiratory alkalosis in the early febrile stage to a respiratory acidosis in more advanced cases. The basic cause for the increased capillary permeability is not known. The main clinical pathological changes measured include a progressive anaemia, fluctuations in total and differential white cell count, of which an eosinopenia and a lymphocytosis are the most marked, increases in total bilirubin which coincide with darkening of plasma colour, and a drop in total serum proteins mostly shown in the albumin levels.     

Epidemiology of heartwater in Guadeloupe and in the Caribbean

At present, heartwater in the Caribbean is known with certainty only on Guadeloupe, Marie Galante and Antigua; the first 2 islands are widely infected. The most important factors responsible for particular aspects of heartwater in Guadeloupe are: Cowdria ruminantium of high virulence. A very resistant cattle population (Creole), not normally clinically affected. A fairly susceptible goat population (Creole) (22% goats born in endemic areas die after experimental inoculation) which, fortunately, includes breeding lines with inherited resistance characteristics. Amblyomma variegatum which is present all over the island and all through the year, but with a low infection rate (1-2% of adult ticks are infected) because of the short period of rickettsemia in infected animals. The low rate of tick infection results in a low endemicity of the disease. For goats, the epidemiologic situation can be regarded as unstable because the low rate of infection in ticks does not allow a natural immunization of the majority of young kids when they still have a non-specific resistance. The possible evolution of heartwater in the Caribbean and in the United States in considered.     

The clinical pathology of heartwater. II. Studies on cardiac and pulmonary function in 4 calves with experimentally-induced heartwater

Studies to evaluate cardiac and pulmonary function were undertaken in 4 calves suffering from experimentally-induced heartwater. There was a marked variation in the course of the disease. Three of the calves recovered spontaneously after developing clinical signs. These included a rectal temperature in excess of 40 degrees C, anorexia and listlessness but no neurological signs. The remaining calf died 2 days after developing a fever and neurological signs. In the 3 calves that recovered, a mild hypoxemia developed during the acute stage of the disease. Arterial CO2 tension remained within normal limits, but there was a tendency towards an alkalosis. Increases in pulmonary dead space and fluctuations in venous admixture were observed. The calf that died showed similar mild changes in blood gas parameters, despite the presence of a marked reduction in minute volume, and a lung oedema was demonstrated on post-mortem examination. No marked changes in systolic and diastolic blood pressures and in right cardiac intraventricular pressures were observed. Terminally, however, there were marked decreases in stroke volume and cardiac output. These changes were associated with a sharp increase in heart rate. No primary cardiac pathology was observed on clinical and post-mortem examinations.     

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.     

Competitive enzyme-linked immunosorbent assay for heartwater using monoclonal antibodies to a Cowdria ruminantium-specific 32-kilodalton protein

Hybridomas producing monoclonal antibodies (mAb) to Cowdria ruminantium were raised. Four mAbs of the IgG isotype reacted in western blots with a 32-kilodalton Cowdria protein (Cr32), which had previously been shown to be conserved and immunodominant. A fifth mAb of the IgM isotype recognized a 40-kDa Cowdria protein. The latter mAb was negative in an indirect fluorescent antibody test (IFA), whereas the other four were positive. mAb No. 4F10B4 showed the strongest signal in western blots using three different stocks of Cowdria. Immuno-gold labeling of Cowdria organisms in vitro using 4F10B4 showed that Cr32 has surface-exposed antigenic determinants. Using mAb 4F10B4, a competitive ELISA was developed which detected specific Cowdria antibodies in goat, sheep and cattle sera. Antibodies in animal sera competed with binding of mAb 4F10B4 to a crude sonicated Cowdria antigen obtained from infected endothelial cell cultures. The competition ELISA (CELISA) detected antibodies in 55 out of 70 (79%) goats experimentally infected with one of eight different Cowdria stocks. Fourteen out of the 15 sera which were shown negative in the CELISA were also negative in the IFA. Nevertheless, all 15 sera recognized some epitopes of the immunodominant Cowdria-specific 32 kDa protein as judged from their reaction with this protein in western blots. Overall, there was 89% agreement between CELISA and IFA considering all 70 goat sera. Moreover, antibodies were detected in nine out of nine sheep infected with one of three different stocks of Cowdria and in sera from calves experimentally infected by two different strains of heartwater. There were no cross-reactions with Ehrlichia phagocytophila antibodies in goat sera, nor with Anaplasma marginale antibodies in bovine sera. Lack of cross-reactivity and detection of antibodies to eight geographically widely distributed stocks of Cowdria, makes the competition ELISA a promising test for use in heartwater endemic areas.     

Rearing and infection techniques for Amblyomma species to be used in heartwater transmission experiments

Techniques for the rearing of Amblyomma species to be used in heartwater (Cowdria ruminantium) transmission experiments are discussed. These involve the breeding and maintenance of infected and non-infected strains of ticks. They include the feeding of ticks on sheep, rabbits, mice, tortoises and guinea fowl.     

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.     

The control of heartwater in west Africa—Present and future

Tropical Animal Health and Production - Heartwater (HW) is the name given to the acute, febrile disease of ruminants caused by infection with the obligate intra-cellular rickettsiaCowdria...     

Comparison between 3 antigens for the serodiagnosis of heartwater disease by indirect immunofluorescence

A cell line of bovine endothelial cells (E5), infected with 3 different stocks of Cowdria ruminantium, was used as antigen in an indirect fluorescent antibody test for the serodiagnosis of heartwater. These antigens were compared to peritoneal macrophages from mice infected with the Kümm stock and to caprine neutrophils in primary cultures from goats infected with 4 different stocks of Cowdria. The use of endothelial cell cultures proved to be superior in all respects. The antigens can be produced in large quantities at a low cost, contrary to the other types. The reaction is easily and quickly read, compared to the laborious reading of neutrophil or macrophage antigens which often contain few and small colonies of Cowdria. Moreover, not all stocks are suitable for the preparation of neutrophil antigens, while macrophage antigen can only be obtained with the Kümm stock. Endothelial cell antigens also distinguish serotypes in C. ruminantium, but these differences seem to be less pronounced than those found with neutrophil antigens. Finally, the specificity of endothelial cell antigens appears to be better than that of Kümm antigen and comparable to that of neutrophil antigens. The use of Kümm antigen may have been responsible to a large extent for past unexplained positive serological results on certain Caribbean islands where it has not been possible to isolate Cowdria and where no clinical evidence of the disease has been found.     

The presence of endotoxin activity in cases of experimentally-induced heartwater in sheep

The presence of endotoxin was examined in 5 sheep with experimentally-induced heartwater. Two peaks in endotoxin levels were recorded in 4 out of the 5 sheep during the acute stage of the disease. The 1st peak coincided with or occurred shortly after the febrile reaction (over 40 degrees C). The 2nd peak occurred 3-5 days after the 1st, and in 2 sheep this 2nd elevation in endotoxin levels was associated with severe clinical signs (rapid and laboured breathing, cyanosis and recumbency), and 1 of the 2 sheep died on the day of the 2nd elevation. Both endotoxin peaks were of short duration and levels had decreased in the 24-h follow-up samples.     

Adult Amblyomma hebraeum burdens and heartwater endemic stability in cattle.

Several conclusions of importance to studies on the epidemiology of heartwater were drawn from an investigation in which the numbers of adult Amblyomma hebraeum ticks, to which a closed herd of Hereford cattle were exposed over a period of 6 1/2 years, were manipulated. With a tick Cowdria ruminantium infection rate of 3-5%, an endemically stable situation was created by dipping the herd only when an average of 10 adult male and female A. hebraeum ticks were counted on 10 animals. When the average was increased to 15 during the calving period, 97% of calves acquired a tick-mediated immunity at the age of 6 months. Because only adult ticks confined to the hindquarters are counted, this procedure is recommended as a feasible and practical guideline to stock owners wishing to determine a dipping programme that would ensure endemic stability. The indirect fluorescent antibody test gave a true reflection of the infection rate through ticks in calves 3-6 months old, but not in older animals that had been re-infected more than once. This is because on one hand antibody may persist for 2 years after withdrawal from tick exposure and on the other the artificial re-infection of cattle with a tick acquired immunity is not always followed by a rise in antibody titres and may even result in seronegativity. Four cows infected and re-infected through ticks, remained immune to challenge for 2 years after withdrawal from tick exposure. Within the confines of one farm 3 isolates of C. ruminantium that differed in pathogenicity and immunogenicity were recovered from ticks. One of these isolates was almost non-pathogenic to cattle.     

Ehrlichia ruminantium variants which do not cause heartwater found in South Africa

In 1994 a batch of apparently healthy goats was selected for intended export to the USA from a heartwater-free and vector tick-free region of South Africa. The animals were tested serologically for heartwater, using either or both an IFA and an ELISA test, and 52% were found to be serologically positive. A PCR assay based on Ehrlichia ruminantium 16S gene sequences gave positive results for 54% of the animals, suggesting that apparently non-pathogenic E. ruminantium variants existed in this heartwater-free area. To identify and characterise the agents responsible for the positive serological and PCR results, ticks and animal blood samples were collected from two of the three farms involved in the original survey during two successive seasons of expected peak tick activity. Ticks were kept alive for a minimum of 3 weeks to allow digestion of any blood meal before being processed. Over the two seasons, 28% of the livestock and 15% of the ticks sampled were found to be carrying E. ruminantium. E. ruminantium 16S and pCS20 sequences were detected in all of the four tick species collected from the livestock (Rhipicephalus evertsi evertsi, Rhipicephalus evertsi mimeticus, Hyalomma truncatum, Hyalomma marginatum rufipes), suggesting that some of the species may act as vectors. Animals generally carried multiple E. ruminantium 16S genotypes, whereas ticks rarely carried more than one. Infection levels in both animals and ticks were too low to generate a marked response when a blood stabilate was sub-passaged in a clean sheep, preventing the subsequent establishment of any of the organisms in culture.