Heartwater, Ehrlichia Ruminatium, in Ruminants
Heartwater is a major disease in Africa and the Carribean. Caused by EHRLICHIA RUMINANTIUM, an obligate-intracellular gram-negative bacterium spread by AMBLYOMMA genus ticks.
This disease can present in a peracute, acute, subacute, or clinically inapparent form; in the common acute form mortality is high. Main lesions are hydropericardium and hydrothorax.
HW – Hydopericardium
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.
Peracute disease is usually seen in Africa in nonnative breeds of sheep, cattle, and goats. Heavily pregnantcows are particularly susceptible to this form. The clinicalsigns may include a fever, severe respiratory distress,hyperesthesia, lacrimation, terminal convulsions, andsudden death. Some breeds of cattle, including Jerseysand Guernseys, may develop severe diarrhea as well. Theperacute form of heartwater is relatively rare.The most common form of heartwater is acute disease.This syndrome is seen in both nonnative and indigenouscattle, sheep, and goats. The symptoms beginwith a sudden fever (up to 42C), anorexia, listlessness,and rapid respiration. Occasionally, animals also havediarrhea. These symptoms are followed by nervous signs,particularly chewing movements, protrusion of thetongue, twitching of the eyelids, and circling, often with ahighstepping gait. Affected animals sometimes standwith their heads lowered and legs apart. Some animalsmay become aggressive or anxious. As the disease progresses,the neurologic signs become more severe, and theanimal goes into convulsions. In the terminal stages, gallopingmovements, opisthotonos, hyperesthesia, nystagmusand frothing at the mouth are common. Animals withthe acute form of heartwater usually die within a weekafter the onset of the disease.On rare occasions, heartwater appears as a subacutedisease. In this form, the clinical signs include a prolongedfever, coughing, and mild incoordination. Theanimal either recovers or dies within 1 to 2 weeks.Mild or subclinical infections are seen in calves lessthan 3 weeks old, partially immune cattle or sheep, antelope,and some indigenous breeds of sheep and cattle. Theonly symptom is a transient fever. This form of the diseaseis known as ‘heartwater fever.’Cattle, sheep, goats, and wild buffalo are severely affected by heartwater, although in some indigenous African breeds of sheep and goats, the symptoms are mild.Blesbok, wildebeest, guinea fowl, leopard tortoises, and scrub hare are carriers. E.ruminantium can also infect eland, springbok, antelope, white-tailed deer, ferrets, thestriped mouse, the albino mouse, and the multimammate mouse.
Heartwater should be suspected in animals with the typical clinical signs including fever, respiratory distress, characteristic nervous symptoms, and sudden death. The presence of Amblyomma ticks and typical post-mortem lesions support the diagnosis of heartwater.
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.