- Lets talk Heartwater (Cowdria ruminantium)
|Last update: November 23, 2010 03:43:58 PM|
Lets talk Heartwater
Dr P G Marais
Senior Specialist Scientist
Grootfontein Agricultural Development Institute, Middelburg, 5900
The first reference to what could have been heartwater was made by the Vootrekker pioneer, Louis Trichardt, on 8 March 1838. While trekking through the north-eastern Transvaal many of his sheep succumbed to a disease known as =a type of nintas=. The disease, characterized by nervous symptoms, was preceded three weeks after they had suffered massive tick infestation. The eastern Transvaal is characteristically inhabited by the main southern African vector of the disease, Amblyomma hebraeum, and the hot, wet season of mid-February is compatible with peak activity of the tick. It has not yet been possible to trace the origin of heartwater, and it is doubtful whether the disease is indigenous to South Africa. According to evidence given by John Webb before the Cattle Diseases Commission in 1877, heartwater was first observed near the coast along the borders of Kingwilliamstown and Peddie in 1858. Although the incidence and spread of heartwater were then definitely connected with the advent of the bont tick (Amblyomma hebraeum), it would appear that bont ticks had been observed in the Cape Colony more than twenty years before the first recorded outbreak of heartwater occurred, because in 1837 William Bowker had found bont ticks on cattle introduced from Zululand.
For most of this last century, heartwater has been considered one of the most important livestock diseases in Africa. The economic importance of heartwater as a disease of domestic ruminants in Africa is surpassed only by Theileriosis (East Coast fever) and Trypanosomiosis (Tsetse-borne). In the endemic areas in South Africa, mortalities due to heartwater are three times greater than the combined deaths caused by Babesiosis (Redwater) and Anaplasmosis (Gallsickness). North of the equator, heartwater is probably the third most important disease of cattle, after rinderpest and schistosomiasis. In Western and Southern Africa, it is at least one of the most important tick-borne diseases. Bigalke reported that losses from heartwater in South Africa during the period 1981 - 82, represented 30.3% of 1.54 million head lost to the three major tick - borne diseases, heartwater, gallsickness and redwater. The cost incurred from losses due to heartwater was estimated to lie between 21.2 and 54.5 million Rand. Another survey during 1994 on the incidence and importance of tick-borne disease in heartwater-endemic areas of South Africa showed that the losses in livestock from heartwater greatly outnumber those from redwater and gallsickness combined. Furthermore, mortality due to heartwater was three times higher in sheep and goats than in cattle.
Hosts and susceptibility
A large number and variety of African and Non-African ruminants and other animals are susceptible to infection with heartwater, giving rise to speculation that some, in heartwater- endemic areas, may serve as reservoirs of the disease. The controversy has been resolved with the finding that African buffalo (Synceros caffer), which serve as excellent natural hosts for the vectors are, after recovery from heartwater, chronically infected and intermittently infective for ticks for many months, and perhaps permanently. The range of susceptible vertebrates and potential carriers is not limited to ruminants, but includes birds and reptiles as well. Both guinea fowl and leopard tortoises can be infected; they respond subclinically and transfer the infection to Amblyomma ticks, which feed on these hosts in nature. The participation of wild animals in the cycle of heartwater, complicates efforts at control but may also serve to maintain a state of endemic stability.
Susceptibility and resistance
Animals exposed to heartwater respond in a variety of ways, depending on a number of factors, including species, breed, age, immune status and stock of Cowdria.
Heartwater causes severe disease in cattle, sheep, goats, and water buffalo; mild disease in some indigenous African breeds of sheep and goats; and inapparent disease in several species of antelope indigenous to Africa. The blesbok (Damaliscus albifrons), the black wildebeest (Connochaetes gnu), the eland (Taurotragus oryx oryx) and the springbok (Antidorcas marsupialis) have experimentally been shown to be susceptible to heartwater and although the natural disease in these animals is usually mild, deaths in springbok have been attributed to heartwater. The blesbok and wildebeest are known carriers of C. ruminantium and are believed to play a role in the maintenance of the disease in nature.
Exotic breeds of sheep and goats are of uniformly high susceptibility and mortalities of 50% or greater due to heartwater have been seen in sheep and cattle imported into sub-Saharan Africa. Merino sheep are highly susceptible. Goats, particularly Angoras, are the most receptive to heartwater of all domestic ruminants with mortalities surpassing 90 % in imported stock. In contrast, annually mortality in indigenous goats of an endemic area has been estimated at round 10%.
The innate resistance of indigenous breeds in endemic areas has not been fully examined and, in many instances, it is not possible to judge whether resistance is a characteristic of the breed, or if the resistant state has been acquired, through natural selection that favours survival of the least susceptible animals upon prolonged exposure. The resistance varied from flock to flock, depending on its history of exposure to heartwater. The rate declined progressively when animals had been reared in disease - free areas. Male goats were significantly more resistant than females and paternity influenced the resistant status of the kids. This resistance, apparently under control of a sex-linked recessive gene, suggests that selection for the trait is feasible.Of great potential significance for the southern African region is the recent establishment of genetic resistance in native goats to heartwater caused by C. ruminantium. A heritability estimate of 0.49 supported a hypothesis which proposed a clear role for sex-linked, recessive determinant in the resistance to heartwater
Factors relating to the tick vector, causative organism (C. ruminantium), and the vertebrate host are important in the epidemiology of heartwater. These include possible immunological strain differences of C. ruminantium, availability of wild animal reservoir hosts or vectors for the heartwater organisms, infection rates of ticks, age and a genetic resistance of domestic ruminant populations, seasonal changes influencing tick abundance and activity, and the intensity of tick control..
Heartwater occurs only where its vectors are present, and ten Amblyomma spp. capable of transmitting the organism occur in Africa. The major vectors are A. variegatum, the tick of the genus Amblyomma with the widest distribution in Africa and the only originally African species that has established itself successfully outside the continent, and A. hebraeum, which is the main vector of heartwater in southern Africa .
Vectors of lesser importance include A. pomposum, which is prevalent in Angola; A. lepidum in East Africa north of latitude 8E S; A. astrion, which primarily parasitizes African buffalo (Syncerus caffer) in Central Africa; and A. cohaerens and A. gemma, which occur in East Africa. An increased incidence of heartwater usually occurs when peak numbers of ticks are present. Good rains are often followed by a transient increase in the prevalence of the disease; its occurrence is not strictly seasonal, a fact which is particularly true in regions like the Caribbean and certain parts of Africa which have a temperate climate and a poorly-defined rainy season.
As little as 2.7-5.5 Fl of blood collected from a host during the febrile stage can infect Amblyomma larvae. Since C. ruminantium replicates in the tissues of the tick, the infection is amplified so that a single infected tick can transmit the disease to a large ruminant. Ticks may retain their infectivity for life so that small numbers of infected ticks could presumably maintain the infection in a particular herd or area.
It is generally assumed that the infection rates in tick populations are low, as is evidenced by the extended periods (even up to several years) during which susceptible domestic ruminants may escape infection in endemic regions. Although the infection rates of ticks may vary according to the season and locality in which they are collected, limited surveys in South Africa indicate that 1 to 7 per cent of A. hebraeum in endemic areas are infected at anyone time. Reasons for this low infection rate could be that not all ticks which feed on animals with a rickettsaemia become infected, and that not all the infected nymphs which fed as larvae on such animals retain the infection to the adult stage. Amblyomma spp. are also indiscriminate feeders, especially in the immature stages, and many ticks may therefore never acquire the infection, having fed during the larval or nymphal stages on a non-susceptible wild animal.
Through the years a great number of Cowdria isolates have been made, most being named after the farm or country of origin. Because these isolates have not as yet been sufficiently characterized, they are referred to as =stocks= , according to the terminology used for tryponsome populations. Cowdria stocks differ in their pathogenicity for ruminants. The existence of antigenically different stocks of C. ruminantium with varying virulence has recently been conclusively demonstrated despite numerous unsuccessful attempts in the past. Most of the older studies have indicated that, despite differences in the pathogenicity of different stocks of C. ruminantium, all stocks, with the exception of Kumm. However, more recent investigations in this regard have demonstrated a lack of or poor cross-protection between stocks. The introduction of animals which are immune to a particular stock into an endemic region where a different stock occurs may therefore result in cases of heartwater. Infections with different stocks may be necessary for the development of immunity to them.
Whether in apparent or overt disease will develop after the infection of a susceptible vertebrate host depends on its species, breed, age, degree of natural resistance, and immune status. Yong calves, lambs and goat kids possess a reverse age resistance which is independent of the immune status of the dam. Generally, this resistance is of short duration, lasting for only the first- four weeks of life in calves and the first week in lambs and kids, although it may persist for six to eight months in calves. This age resistance is not absolute as infection of some calves less than three weeks of age and of some lambs and kids less than one week old, may result in fatal disease.
The conformation of a diagnosis of heartwater requires the demonstration of C ruminantium in the cytoplasm of the endothelial cells of blood vessels, and the easiest, most efficient and quickest way of doing this is to visualize them in stained smears of the brain, although they may also be found in histological sections of organs such as the brain and kidneys. Although colonies of C. ruminantium are generally easy to detect in smears of untreated cases of heartwater, their numbers vary from one animal to another.
Various formulations of the tertracyclines are almost invariably used for treatment of heartwater especially when animals are treated early in the course of the disease. Tetracycline antibiotics administered before signs appear will suppress the disease entirely, but will allow immunity to develop. These are administered at the rate of 8 - 10 mg/kg or, in the case of doxycycline, at 2 mg/kg. Two injections on consecutive days, or on the first and third day after onset of fever are advisable. Intravenous, intramuscular and oral routes of administration are effective. Treatment of small stock showing clinical signs has resulted in a 48% recovery rate.
Control is usually achieved through tick control or vaccination. The decision as to which approach to use, requires that the question of endemic stability/instability first be resolved.
In endemically stable areas where tick control is not practised, a high level of immunity in livestock, particularly cattle, is typically seen. Here, if acaricidal programmes are instituted and rigidly enforced (intensive control), the level of herd immunity that results from exposure to ticks, will decline. Livestocks will then be highly susceptible and at risk, should control break down for any reason. Hence, intensive control may actually increase losses due to heartwater.
In unstable or transitional areas, where tick population are subject to climatological extremes, where intensive control has been practised, or where wild hosts are excluded, tick numbers may be insufficient to maintain immunity in livestock.
Four methods of control are generally applied to the control of heartwater - specific therapy of clinical cases, vaccination, tick control and chemoprophylaxis.
Several drugs have been used with varying success to treat animals suffering from heartwater. Tetracyclines, especially oxytetracycline, however, are most effective and are widely used for the specific treatment of the disease. Short-acting formulations of oxytetracycline are most commonly used at a dosage rate of 10-20 mg/kg body weight, either administered intramuscularly as a single dose, or half the calculated dose is given intravenously and the other half intramuscularly. This treatment is usually repeated 24 hours later. A long-acting oxytetracycline preparation has given similar results to the short- acting formulations of the drug in the treatment of clinical cases of heartwater.
Methods for the production of heartwater vaccines and the immunization of animals against the disease are far from ideal. Currently, there is only one method of vaccination of domestic animals against heartwater. This method involves infection of the animal intravenously with live virulent. This method of vaccination is both labourious and expensive and is accompanied by considerable risk and uncertainty. All heartwater vaccines contain live virulent organisms, which come from three sources:
♦ blood from reacting sheep or cattle (which is, at present, the only commercially available Cowdria vaccine)
♦ tick suspensions prepared from infected ticks
♦ suspensions of infected brain material
The major problems encountered with immunization against heartwater are that the vaccines are cumbersome to produce and contain organisms which are not only virulent (thus rendering the vaccines risky to use), but are very labile, which necessitates special deep-freezing facilities for their storage and transportation. With the exception of vaccines made from infective brain suspensions, which are usually effective when administered sub-cutaneouly, blood and tick suspension vaccines are usually only effective when administered intravenously.
The duration of immunity in goats following vaccination is poorly documented. In Angora goats the degree of immunity seems to depend largely on the time at which the animals are treated therapeutically during the reaction, subsequent to the vaccination.
When very young animals are vaccinated, advantage is taken of their age-related innate resistance to heartwater. Vaccination of calves under the age of one month, and of lambs and kids younger than seven days old, do not generally result in clinical disease, but they do develop an immunity. It is, however, the method with the lowest risk of losses due to fatal heartwater reactions after inoculation. This immunization procedure is usually followed in heartwater-endemic areas to establish a basic immunity in calves, lambs and kids, but the value of this method in these areas is not unequivocal.
The block method of immunization also known as 'systematic treatment' , is widely practised in South Africa, especially when immunizing large numbers of kids and lambs. It involves the intravenous inoculation of Cowdria-containing infective blood or tick suspensions into animals, and subsequent indiscriminate treatment on a specific day or days, irrespective of whether the animals show a temperature reaction or not. Although this procedure makes vaccination on a large scale possible, it has some serious disadvantages.
The heartwater organism is extremely fragile and cannot persist outside of a host for more than a few hours. The principal mode of bringing the disease into an area is thus through introduction of infected ticks or carrier animals. It is not known for how long wild or domestic ruminants can be a source of infection for ticks in nature. This prolonged carrier state needs to be considered when animals are moved from heartwaterâ€‘endemic to heartwaterâ€‘free areas. It is also not known for how long a tick can remain a carrier of the organism. Careful dipping and handâ€‘dressing followed by inspection to ensure the absence of ticks is recommended for animals in transit to heartwater free areas.
Vector control measures aimed at eradication of Amblyomma ticks by dipping of cattle have failed, principally because the vector is a multihost tick with a high rate of reproduction. The development of acaricide resistance has further complicated attempts at tick control. In endemic areas, tick levels are now allowed to remain at levels high enough to permit reinfection of immune animals to booster the immunity.
The use of indigenous, resistant breeds is recommended for areas lacking adequate tick control and veterinary care.
This is a procedure by which a series of oxytetracycline injections is used to protect susceptible animals against heartwater when they are introduced into an endemic area . The development of such a prophylactic regime holds great promise as a practical and reasonably safe method for the introduction of large groups of animals into endemic areas. However, its success is almost exclusively dependent on all the animals becoming naturally infected with heartwater during the time that they are protected by the oxytetracycline.