Etiology
Heart water is caused by rickettsia, Cowdria ruminantium. It occurs in closed packed colonies in endothelial cells of blood vessels in various organs. Multiplication of the organism appears to be by binary fission. It has been suggested that cowdria may arise at first in the reticulo-endothelial cells of lymphnodes before invading endothelial cells of the blood vessels and the organism has been reported to occur in various other cell types including macrophages, monocytes and cells of renal tubules. Although animal invasion shows that the organism occurs in blood smears.
Distribution
Heart water has been reported from several countries in Africa, south of the Sahara but in most areas, little is known about the incidence of the infection. The potential and possibly natural distribution of heart water covers at least that of the tick vectors, Amblyoma such as Amblyoma herbraeum, Amblyoma variegatum, Amblyoma pomposum, Amblyoma gemma and Amblyoma lepidum. These ticks occupy most of Africa south of Sahara and Madagascar and other offline Irelands.
Susceptibility
Apart from cattle, other domestic ruminants like sheep, goat and avian, buffalo are susceptible to infection and the disease is in fact as important for sheep and goat as it is for cattle. Some species of African antelopes and European deer have been shown to be susceptible to experimental infection. Other susceptible animals are Eland and spring buck.
Epidemiology
There is no doubt that heart water occurs only in Africa. It is apparent that natural reservoirs of the disease are undoubtedly wild African ruminants. Domestic ruminants suffice to maintain the disease at high level as is the case in Madagascar where wild ruminants are not existed. Five species of Amblyoma have been shown experimentally to be able to transmit Cowdria rumunantium. Recently, it has been demonstrated in Zimbabwe that elephant tick called Amblyoma tholloni is an experimental vector. Laboratory transmission of heart water by American tick, Amblyoma maculatum has been reported. It is believed that dissemination is only transtadial so that infected larva is free of infection. Transovarian transmission has been shown in one conclusive experiment in South Africa. The later transmission is probably infrequent. A larva feeding on infective ruminant is able to transmit the disease, not only in the next instar, the nymph but it is still infective in the adult stage. On the other hand, as the larva commonly feeds on non-susceptible animal, it is often the nymph which first gets infection and in that case, only the adult transmit the disease. As is in the case with other tick borne diseases, infected ticks do not start to transmit the disease immediately upon attachment. They usually feed for some time before transmission occurs. In heart water, this is said to be about 24 hours while in theleriosis and babesiosis, it is a matter of days. Artificial transmission is possible by means of blood and tissues of infected animals.
It is unanimously agreed that intravenous infection is necessary for successful transmission. Subcutaneous route of infection was achieved in brain tissue material obtained from infected animal. Calves even of highly susceptible stocks are relatively resistant to the disease for 2-3 weeks after birth. This resistance is independent of the immune status of the dam. Some calves infected throughout this period recover but as majority are not infected until later after the age, resistance (immunity) is lost. A state of endemic stability without apparent disease does not exist. In endemic regions, cattle usually suffer little mortality. A considerable degree of innate resistance presumably has been acquired through long natural selection. Where the vector population is low by acaricidal or unfavorable ecological factors, sporadic cases of heart water may occur with long intervals of seeming absence of disease. It has been reported that infection can be maintained in fasting adults for 15 months. The disease may be carried over long distances and reintroduced into pastures by nymphs or birds. If infection was acquired in larval stage, heart water occurs throughout the year but its incidence may reduce during dry season in countries with pronounced seasonal climatic changes which influence seasonal activity of different stages of the vector.
Clinical signs
Clinical reactions may be peracute, acute or subacute. Peracute cases exhibit sudden high fever, collapse and death in convulsion. Acute cases are manifested by fever and signs of nervous disturbance. Acute cases start with sudden high fever, nervous symptoms like staggering, drunken gait, cycling movement, abnormal postures, twitching of eyelid, frequent sticking out of the tongue, haggard facial expression, tremors of individual muscles which can often be provoked by touching the animal or by noise. Pregnant animals may abort. Finally, the animal may collapse in convulsion and with paddling movement of the limbs and frequently nystagmus, opisthtonus and chewing movement of the mouth. There may be profuse fetid diarrhea. Finally, death follows. Before dying, the animal may pass bloody faeces. In subclinical cases, there is transient or mild fever and sometimes diarrhea.
Diagnosis
This is done by microscopic detection of the organism in brain cortex, capillaries obtained by biopsy or inoculating blood from affected animal into susceptible ruminant.
The organism can be demonstrated after death in brain smears stained with methylene blue.
Differential diagnosis
Nervous symptoms or disorders may stimulate diseases such as rabies, cerebral babesiosis, hypomagnesemic tetany, tetanus, strychnine poisoning .
Haemorrhagic diarrhea is found in coccidiosis, hay fever and sudden death in peracute case of anthrax, high fever in foot and mouth disease, rinderpest, trypanosomosis, babesiosis and anaplasmosis.
Prevention and control
Disease transmission can be prevented by vector control. Amblyoma species are three host ticks and each stages remains on host for relatively short period. These ticks are less susceptible to most acaricides than Boophilus.
Frequent acaricidal treatment is necessary to prevent disease transmission as transmission may start within a day after tick attachment.
Dipping or spraying would have to be carried out at least every 3 days. If the acaricide used has a residual activity of 2 days, effective weekly acaricidal treatment will often bring disease incidence to low level. There is no effective vaccine for heart water disease prevention. The method adopted in some countries is to infect and treat. This involves infection intravenously of infective blood into animal to be vaccinated. The temperatures of the infected animals are taken regularly. Animals are then treated with tetracycline as soon as fever occurs. Antibiotics of tetracycline group are active in curing heart water in early stage. Oxytetracycline and clortetracycline have been found effective. Two therapies on consecutive days give a better result than only one treatment.
Suspension formulations administered intramuscularly areknown to give better results in small ruminants than soluble ones. This is possible because their actions are more durable. Sulfonamides possess chemotherapeutic activity against heart water disease but even less than that of tetracycline so that they are now being abandoned. Because of the presence of satisfactory strain of immunization and treatment, introduction of susceptible exotic animals into heart water disease areas must be strongly advised against. The importation can only be done where there is strict efficient acaricidal control.
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