Heartwater is an infectious, noncontagious, rickettsial disease of ruminants in areas infested by ticks of the genus Amblyomma. These include regions of Africa south of the Sahara and the islands of the Comores, Zanzibar, Madagascar, Sao Tomé, Réunion, and Mauritius. Heartwater was introduced to the Caribbean, and it and its vector there (A variegatum) are endemic on the islands of Guadeloupe and Antigua. A variegatum, but not the rickettsia, has since spread to several other islands despite attempts at eradication. Possible spread to the mainland threatens the livestock industry of regions from northern South America to Central America and the southern USA. Many ruminants are susceptible, including some antelope species. In endemic areas some animals may become subclinically infected and act as reservoirs. Indigenous African cattle breeds (Bos indicus) appear more resistant than B taurus breeds.
Etiology and Transmission
The causative organism is an obligate intracellular parasite, previously known as Cowdria ruminantium. Molecular evidence led to reclassification of several organisms in the order Rickettsiales, and it is now classified as Ehrlichia ruminantium. Under natural conditions, E ruminantium is transmitted by Amblyomma ticks. These 3-host ticks become infected during either the larval or nymphal stages and transmit the infection during one of the subsequent stages (transstadial transmission). The progeny of an infected female tick are most probably not infective (ie, there is no epidemiologically significant transovarial transmission). Therefore, the infection rate in tick populations tends to be low. Intrastadial transmission by male ticks may also occur, as well as some degree of vertical transmission from cow to calf (eg, via colostrum), in areas where the disease is endemic.
E ruminantium can be propagated experimentally by serial passage, either by inoculating infective blood into, or by feeding infected nymphal or adult stages of a vector tick on susceptible animals. The organism can also be propagated in tissue culture, most reliably in endothelial cells, but also in primary neutrophil cultures and macrophage cell lines. At room temperature, infective material loses its infectivity within a few hours, but the organism, together with suitable cryoprotectants, may be viably preserved in liquid nitrogen for years.
Immunity to heartwater appears to be chiefly, if not exclusively, cell mediated. There is no, or only partial, cross-protection between different strains (stocks) of E ruminantium. Most of these stocks are infective for, but cannot be serially passaged in, mice; however, a few are pathogenic to mice infected by the IV route. One of these, the Kümm stock, can even be passaged by the intraperitoneal route. Molecular analysis has established that the traditional Kümm stock was made up of organisms of 2 distinct genotypes.
Clinical Findings, Pathogenesis, and Lesions
The signs are dramatic in the peracute and acute forms. In peracute cases, animals develop fever, which is followed rapidly by hyperesthesia, lacrimation, and convulsions. In the acute form, animals show anorexia and nervous signs such as depression, a high-stepping stiff gait, exaggerated blinking, and chewing movements. Both forms terminate in prostration and convulsions. Diarrhea is occasionally seen. In subacute cases, the signs are less marked, and CNS involvement is inconsistent.
E ruminantium seems to initially reproduce in macrophages; it then invades and multiplies in the vascular endothelium. During the febrile stage, and for a short while thereafter, the blood of infected animals is infective to susceptible animals if subinoculated. Signs and lesions are associated with functional injury to the vascular endothelium, resulting in increased vascular permeability, without recognizable histopathologic or even ultrastructural pathology. The concomitant fluid effusion into tissues and body cavities precipitates a fall in arterial pressure and general circulatory failure. The lesions in peracute and acute cases are hydrothorax, hydropericardium, edema and congestion of the lungs and brain, splenomegaly, petechiae and ecchymoses on mucosal and serosal surfaces, and occasionally hemorrhage into the GI tract, particularly the abomasum. The typically straw-colored effusions are so high in large-molecular-weight proteins, including fibrinogen, that the fluid readily clots on exposure to air.
Clinical cases must be differentiated from a wide range of infectious and noninfectious diseases, especially plant poisonings, that manifest with CNS signs. In acute clinical cases in endemic areas, clinical signs alone may suggest the etiology, but demonstration of colonies of organisms in the cytoplasm of capillary endothelial cells is necessary for definitive diagnosis. Traditionally, this is done with “squash” smears of cerebral or cerebellar gray matter, stained with Romanovsky-type stains. Diff-Quick or CAM-Quick stains are adequate for experienced diagnosticians, but low concentration Giemsa stain developed for 30 min gives the best color differentiation and batch-to-batch consistency. Organisms in autolyzed material lose their stainability, and diagnosis then becomes difficult.
For the “brain squash smear,” a piece of gray matter (∼3 × 3 mm) is macerated between 2 microscope slides; the softened material is then spread like a blood smear with the material pushed rather than pulled along. A slight lifting of the spreader slide about every 5–10 mm creates several thick ridges across the slide, from which capillaries are arranged straight and parallel in the thin sections of the smear for easier examination. The endothelial cells of all the capillaries on a smear should be carefully scrutinized for the presence of the dark purple colonies of E ruminantium. The colonies must be identified on the strength of identifiable substructure to differentiate them from any other phagocytized matter; they are clusters made up of individual granules. The size of the granules can vary between patients, or smears from the same case, or even between colonies on the same smear, but is usually uniform within a particular colony. Small colonies generally have fewer, bigger granules, while big colonies are made up of many small organisms.
Using immunoperoxidase staining methods, a definitive diagnosis can be made on any formalin-fixed tissue samples, even from autolyzed carcasses. The contrasting color makes the search for and identification of the rickettsial colonies much quicker, although the substructure of the colonies should be identified before the diagnosis is confirmed. Due to the nature of the test, false-positive reactions may arise with some closely related organisms. On brain squash smears, Chlamydia pecorum can be confused with E ruminantium, but histopathology or the immunoperoxidase technique allow differentiation. Serodiagnosis of animals previously exposed to the disease, ie, recovered from subclinical or clinical infection, still poses problems. Several tests are currently in use, including several indirect fluorescent antibody and ELISA tests. All serologic tests, including an ELISA that uses recombinant antigen, are plagued by cross reactions with sera from animals infected with one of several Ehrlichia or Anaplasma organisms (false positive) and the fact that immune cattle on repeated exposure may become seronegative (false negative). DNA probes, available at research institutions, can be used together with PCR technology. A combination of a pCS20 probe and probes to 16S ribosomal RNA of several of the stocks are used routinely to examine samples from animals when permits for movement of animals from endemic to nonendemic areas are required. Another technique that has recently come into use is real-time PCR.
Treatment and Control
Although an effective and safe attenuated vaccine based on the Welgevonden stock was developed several years ago, it is not commercially available. For practical purposes, therefore, there is no widely effective and safe vaccine available to immunize against E ruminantium. Control of tick infestation is a useful preventive measure in some instances but may be difficult and expensive to maintain in others. Excessive reduction of tick numbers, however, interferes with the maintenance of adequate immunity through regular field challenge in endemic areas and may periodically result in heavy losses. For immunization, the “infection and treatment method” is still in use in southern Africa: infected sheep blood containing fully virulent organisms is used for infection, followed by monitoring of rectal temperature and antibiotic therapy after fever develops. In certain circumstances, the “controlled” infection is followed by preventive “block treatment” without temperature recording (cattle on day 14 [susceptible Bos taurus breeds] or day 16 [for the more resistant B indicus breeds]; sheep and goats on day 11). In South Africa, a doxycycline implant is available for SC deposition in the fat cushion at the caudal ear base at the time of IV infection. Young calves (<6–8 wk old) and lambs and kids (<1 wk old) are fairly resistant and may recover spontaneously from natural or induced infections. If immunized at that early age, block treatment can be avoided.
For treatment, oxytetracycline at 10 mg/kg or doxycycline at 2 mg/kg usually effect a cure, if administered early in the course of the disease. In sheep, goats, and susceptible cattle breeds, a higher dosage (10–20 mg/kg) of oxytetracycline may be required, particularly if treatment begins late during the febrile reaction or after other clinical signs appear. In such cases, the first treatment should preferably be given IV. A second and third dose may be necessary before the fever abates, or a second injection IM with a long-acting tetracycline formulation may be given. The withdrawal times for milk and meat after treatment with doxycycline or short- or long-acting oxytetracycline must be observed based on the regulations of the country. Corticosteroids have been used as supportive therapy (prednisolone, 1 mg/kg), although there is debate as to the effectiveness and the rationale for using potentially immunosuppressive drugs in an active infectious disease.
Last full review/revision March 2012 by Stephan W. Vogel, BVSc (Hons)