|The Courier N° 127 May - June 1991- Dossier 'New' ACP Export Products - Country Reports Cape Verde - Namibia (EC Courier, 1991, 104 p.)|
by Jos MORTELMANS D.V.M
CARDI (the Caribbean Agricultural Research and Development Institute) and the CTA (the Technical Centre for Agricultural and Rural Cooperation) ran a workshop in Antigua in November to look at two diseases posing major problems to traditional and modern rearers of small and large ruminants - heartwater disease, and dermatophilosis, also known as bovine streptothricosis.
While heartwater is, typically, an African diseases dermatophilosis, a serious handicap to herdsmen, particularly in tropical Africa, is found more or less all over the world. The appearance and persistence of both diseases are closely related to a tick of the genus Amblyomma and, in the specific case of the Caribbean, to Amblyomma variegatum, which comes from tropical Africa.
Heartwater disease, is caused by a Rickettsia, Cowdria ruminantium, which is transmitted by ticks of the genus Amblyomma. The multicoloured Amblyomma variegatum is the main vector in sub-Saharan Africa, another species, Amblyomma hebraeum, is the main vector in Southern Africa and there are a further eight African species of the same genus which are known to be potential vectors. Recent experiments have shown that two American species can also transmit the agent causing the disease.
Heartwater is one of the main diseases of domestic ruminants in some parts of Africa and some species of wild ruminants can, in turn, host Rickettsia, as can (albeit very rarely) some rodents and other vertebrates. Acute forms of the disease start with very high temperatures and extreme exhaustion, with nervous symptoms developing soon afterwards, often accompanied by serious cardiac and respiratory symptoms and diarrhoea, bringing rapid death. Animals with a high milk yield are very susceptible and, as immunisation is very difficult or entirely out of the question and courses of antibiotics are often started too late, any attempt at improving herds by crossbreeding or bringing in exotic, high yield species is obviously likely to be hampered or neutralised by heartwater disease. Then, of course, tick control is an expensive undertaking and the price of antibiotics and vaccines, which can be difficult and sometimes impossible to obtain, is high, so traditional herdsmen are left to their fate in the vast majority of cases.
Fortunately, some native species of domestic ruminants are fairly resistant to the infection and some Rickettsia strains are only minimally pathogenic, so heart-water disease is not considered to be serious in some places, although it is the major animal health constraint in others.
Dermatophilosis, a skin disease caused by Dermatophilus congolensis (an actinomycete), can infect a whole series of animals, particularly in the tropics. It has even been found, albeit rarely, in man. Once again, ruminants and cattle especially are the hardest hit. And once again, the high yield species are extremely prone to the disease, although this is something which may also vary in native strains. The germ was discovered by Renan Saceghem in Africa in 1914, in what is now Zaire (then the Belgian Congo) and some writers have no hesitation in labelling it one of the major cattle diseases of tropical Africa today. The symptoms are fairly small scabs, which may merge to form rough patches, sometimes with secondary infections. In extreme cases, almost all the body is covered, with death virtually inevitable as a result. Antibiotics often bring only illusory help and there are no efficient vaccines so far.
The question now is the role the ticks play. Amblyomma is both vector and cause in heartwater disease, with ticks which have themselves caught it from infected animals at the larva stage passing it on through blood-sucking. With dermatophilosis, the process of infection is different and the tick, whose role is in this case only incidental, repeatedly pierces the skin with its very long (2-4mm) buccal appendage, enabling the germ to penetrate, multiply and cause scabs. Minute wounds of a similar kind may also be caused by the thorny bushes which sometimes appear on grazing land and grow so thick that cattle have hundreds or even thousands of scratches for microbes to penetrate.
In the Caribbean, both diseases are a reality or a threat in an area from Trinidad & Tobago north to Cuba, the Dominican Republic, Puerto Rico and so on. The USA (Florida), Central America and the countries of north western South America (Guyana, Venezuela, Suriname etc) have a problem too.
Although scientists have only been investigating the problem for something like 10 years, it dates back to the last century (1828-30), in fact, when France took zebus - which ultimately became what are now known locally as Creole zebus - from Senegal to Guadeloupe. Little or no thought was given to cattle health measures at that time, as may easily be imagined. Acaricides had yet to be invented and it was unlikely that such plant-based remedies as did exist (tobacco leaf extracts, for example) would be applied to animals which were being shipped a long way off for good. Let there be no illusion about tobacco leaves either. There were none of the anti-smoking campaigns of today and no-one was going to waste good leaves on making animals comfortable on the voyage.. it seems certain that the ticks travelled over with the cows and more than likely that some of the ticks and some of the cows were Rickettsia-infected.
More cattle were moved to the Caribbean after that. In the last century, the tick was called the Senegal tick in the French possessions in that part of the world and the shiny, variegated and very beautiful Amblyomma variegatum became the great Antigua gold tick in Antigua. Infected ticks and animals were probably transported from Africa to the Caribbean on several occasions.
In 1932, a mad cattle disease (generating nervous symptoms, which could be very spectacular in small ruminants) was known in the Caribbean. Various etiologies were considered, but no causal agent was ever isolated or identified with any certainty. In 1966-67, heartwater was suspected for the first time by French veterinary doctors with a lot of experience of Africa, but it was not until 1980 that Rickettsia was found in goats on Guadeloupe and Cowdria ruminantium actually confirmed in the Caribbean.
The discovery caused an immediate sensation in scientific and veterinary circles all over the Caribbean. American veterinary departments were alerted as soon as it was known that two Amblyomma ticks which were very common on that continent (Amblyomma maculatum and Amblyomma cayennense) could be infected with Rickettsia and thus transmit the infection.
Everyone was convinced that if the tick had been able to cross the Atlantic Ocean, even by accident, it would have no problem in spreading over the dozens of islands in the region, sooner or later even getting as far as the American coast. Although there is a legal, controllable and properly controlled trade in cattle in the Caribbean, the region, with its centuries old reputation for piracy, of course has its black market too. The islands geographical situation and coastlines are an open invitation to it and the contraband cattle, sheep and goats take the pretty little Senegal tick with them - and with the sea trip often taking less than an hour there is not even time to succumb to Rickettsia.
Migratory birds are another, major, epidemiological factor and an uncontrollable one. Some of them live near the herds on the prairies, are known to carry tick larvae and chrysalises and find it easy to fly from one island to another and the epidemiological conclusion is obvious.
With the female Amblyomma able to lay as many as 20 000 eggs at a time, giving 10 000 more females in what can be a very short period if conditions are right, it is clear just what a disaster is looming.
What is the situation of heartwater disease and the tick which carries it in the Caribbean at the moment? Heartwater has been firmly diagnosed on Guadeloupe, Marie-Galante and Antigua. While natural selection has enabled the local species to develop a fairly good resistance over more than a century, exotic species are very prone to infection, which is a serious handicap when it comes to improving genetic potential with a view to, say, better milk production. Native cattle on the islands where Rickeffsia has not yet been found and cattle on the American continent are under serious threat because they have been unable to build up any resistance over the years and the improved cattle strains will obviously be the first to succumb if the disease arrives.
It is a very real danger. Amblyomma variegatum is already on 15 other Caribbean islands. The tick is well established on some islands not infected with Rickettsia so far (St Christopher and Nevis, for example), but has colonised only part of others (Martinique, Puerto Rico etc), while only the odd specimen and little spread have been found elsewhere. But the potential danger is there and the arrival of one Rickettsia-infected animal is all it would take to trigger an outbreak.
Dermatophilosis also goes back to the last century and the connection between the tick and a very severe skin disease was already known in the Caribbean in 1895. It is worth remembering that Amblyomma variegatum is not a vector of the germ which causes dermatophilosis, simply something which prepares the ground with the many tiny pits it makes in the skin when sucking blood. In many parts of the African tropics, thorns are usually behind the scratches through which the germ penetrates, but in the Caribbean, by contrast, it is ticks which are the main (but indirect) cause of dermatophilosis. The disease occurs where there are ticks, on Martinique, for example. Thorny bushes are also far more common in tropical Africa than in the Caribbean.
The high humidity of most of the islands is very conducive to the development of the germs and, as a result, dermatophilosis is sometimes considered to be the regions worst cattle disease threatening exotic and improved species above all and killing up to 50% of some herds.
Both political and veterinary authorities in the region are paying particular attention to Amblyomma variegatum, which is so closely associated with heartwater disease and dermatophilosis, both of which kill ruminants and trigger the physiological depressions which hold back production and reproduction. International organisations - the FAO, CARICOM (the Caribbean common market), the IICA (the inter-American agricultural cooperation institute) and many others - are keeping a close eye on the situation, offering advice, launching control campaigns and helping seek the requisite funds.
The CTA and CARDI were jointly responsible for organising the most recent contribution - the workshop in Antigua in November, which gave the regions scientists the opportunity to take stock of the situation, with the help of various international experts. The unanimous conclusion was that the Amblyomma variegatum tick has to be wiped out before it can create any more damage and certainly before it invades the mainland - something which would cause vast, uncontrollable and unforeseeable damage. Now is the time for action.