Heartwater: Introduction
(Cowdriosis) |  |
| 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 and its vector are also endemic on the islands of Guadeloupe and Antigua. Possible spread to the American mainland threatens the livestock industry of regions from northern South America to Central America and the southern USA. Many ruminants, including some antelope species, are susceptible. 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 larval or nymphal stages and transmit the infection during one of the subsequent stages (trans-stadial 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 macrophages. At room temperature, infective material loses its infectivity within a few hours, but the organisms, 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 recently established that the traditional Kümm stock actually 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, 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 of eyes, 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 and then invades and multiplies in the vascular endothelium. During the febrile stage, and for a short while thereafter, the blood is infective to susceptible animals if subinoculated. Signs and lesions are associated with functional injury to the vascular endothelium, resulting in increased vascular permeability. 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 the mucosal and serosal surfaces, and occasionally hemorrhage into the GI tract, particularly the abomasum. |
| |
| Diagnosis: |
| 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
Romanowsky-type stains, of which low concentration Giemsa affords the best color differentiation. Organisms in autolyzed material lose their stainability, and diagnosis then becomes difficult. For the “brian squash smear,” a piece of gray matter (~3 × 3 mm) is macerated between 2 glass slides; the softened material is then spread like a blood smear. 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 capillaries on a smear should be carefully scrutinized for the presence of the dark purple rickettsial colonies of
E
ruminantium
. Using an immunoperoxidase staining method, a definitive diagnosis can be made on any formalinized tissue samples, even from autolyzed carcasses. The contrasting color makes the search much faster, although the substructure of the rickettsial 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.
Chlamydophila
pecorum
on brian squash smears may 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 reaction 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). Several DNA probes, available at research institutions, can be used together with PCR technology. A combination of a pCS20 probe and probes to 16S rRNA of several of the stocks are routinely used to examine samples from animals when permits for importation into nonendemic areas are required. |
| |
| Treatment and Control: |
| There is as yet 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 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 conditions, the controlled infection is followed by preventive “block treatment” without temperature recording (cattle on day 14 [susceptible
Bos
taurus
breeds] or day 16 [resistant
B
indicus
breeds], and small stock on day 11). In South Africa, a doxycycline implant is available for SC deposition behind the ear at the time of 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. 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 treatment may be necessary before the fever abates, or a second injection IM with a long-acting tetracycline formulation may be given. In the USA, the
appropriate withdrawal times for milk and meat after treatment with doxycycline or short- or long-acting oxytetracyclines must be observed. Corticosteroids have been used as supportive therapy (prednisolone, 1 mg/kg), although there is debate as to their effectiveness and appropriateness of their use in an active infectious disease. Inactivated, attenuated and recombinant vaccines are currently being developed but are at an experimental stage. |
| |
