LUSH green pastures stretch for miles around David Kinyanjui Njuguna鈥檚 compound near Muguga, just west of Nairobi. But his two cows are confined to a small mud-floored pen behind his house. Like many farmers across Kenya, Njuguna is reluctant to let his cattle graze freely for fear that they will be bitten by a brown ear tick (Rhipicephalus appendiculatus) lurking in the grass.
Such bites are likely to be lethal. The tick carries a parasite called Theileria parva, which causes East Coast fever (ECF), a devastating disease that kills more than a million cattle a year across eastern and southern Africa, and cuts the milk yield of millions more. Economic losses total more than $165 million a year. And Kenya, because of its climate and farming practices, is the hardest hit.
Now its veterinary researchers are hitting back with a major campaign to protect livestock against ECF. They have opted for an unusual 鈥� and risky -form of immunisation. In selected parts of the country, vets are infecting cattle with live, virulent T. parva while at the same time injecting the animals with antibiotics to keep the deadly parasite at bay.
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Similar 鈥渋nfection and treatment鈥� campaigns have been waged against animal diseases in the past. Early this century, for example, vets would immunise cattle against rinderpest by exposing them to the virus and giving them serum from an animal that had recovered from the infection. But today, ECF is the only animal disease being tackled in such a dangerous fashion. It has some serious drawbacks. All immunised cattle become carriers of the disease for life. And unless further measures are taken to protect the treated animals, the procedure can kill 1 in 20 of them. 鈥淚t鈥檚 immensely crude and very risky if you don鈥檛 know what you鈥檙e doing,鈥� warns Duncan Brown of the Centre for Tropical Veterinary Medicine at the University of Edinburgh.
So why has Kenya turned to this form of immunisation? Is it, perhaps, because there is no other way to deal with the ECF parasite or the tick that carries it? Not so. Over the past 20 years, scientists in Kenya have helped to produce three effective drugs against T. parva, while Zimbabwe and South Africa have brought the tick to heel with measures such as chemical dipping. But in Kenya, as so often in Africa, the harsh realities of economics rule these approaches out. Many of the country鈥檚 cattle are owned by farmers like Njuguna who simply cannot afford to buy drugs or use chemical dips regularly. The infection and treatment approach may be risky, but Kenya鈥檚 veterinary researchers calculate that the risk is worth taking.
In Muguga, you can begin to see why. Farmers here have an average of two or three milk cows. Most are high-yielding European breeds, or crossbreeds with local animals. Unlike indigenous animals, such as zebu cattle, they have no immunity to ECF and are highly vulnerable. Njuguna has not yet lost a cow to ECF, and he dreads the possibility. 鈥淚t would interfere with my daily bread,鈥� he says. To buy a dairy cow costs about 40 000 Kenyan shillings (about 拢460), and it will produce milk worth up to KSh7000 a month. This is no mean sum. Njuguna鈥檚 job as an animal health assistant at the nearby National Veterinary Research Centre (NVRC) earns him around KSh3000 a month. Drugs to treat an infected animal would cost KSh4000.
If ECF is a disaster for individual farmers, it is also a huge drain on resources nationally, says Brown. One reason why it is a 鈥渂igger and more important problem鈥� than in other East African nations, he says, is because Kenya has some 3.2 million crossbred dairy cows 鈥� far more than neighbouring countries.
Further south, Zimbabwe and South Africa claim to have been free of ECF since the 1950s. They banished the disease with a strict system of dipping of cattle in acaricides, to kill the ticks, combined with slaughtering of infected animals. Before independence in 1963, Kenya also dipped cattle to control ECF and other tick-borne diseases. But today, says Njuguna, 鈥渢he dips are dying鈥�. Njuguna prefers to keep his cows under 鈥渮ero-grazing鈥� conditions and to inspect them for any ticks that he or his dog may bring into his compound. Besides, while dipping cattle once a week may be fine on big Zimbabwean ranches, for the small-scale farmers who keep most of Kenya鈥檚 dairy cows it means walking their animals for an hour or more to communal dips.
And there are good reasons for not relying too heavily on dipping, says Sam Mbogo, project manager for tick-borne diseases at the NVRC. Ticks develop resistance to a particular acaricide within a few years, and each new generation of acaricide is more expensive than the last. Also, cattle that have been dipped all their lives develop no immunity to T. parva which can be disastrous if the dips stop working. When Zimbabwe鈥檚 dipping system broke down in the chaotic run-up to its independence in 1980, for example, tick-borne diseases wiped out hundreds of thousands of animals.
Finding alternative ways to control ECF has been a central mission for researchers at Muguga since the British set up a veterinary research centre there in 1954. Parallels between ECF and malaria were clear from the beginning. Both are spread by arthropods 鈥� ticks in the case of T. parva and mosquitoes for Plasmodium, the malaria parasite 鈥� and both enter their mammalian hosts when the arthropods feed on blood. Plasmodium multiplies first in liver cells before breaking out and infecting red blood cells. T. parva hides in white blood cells before it, too, colonises red blood cells.
Grinding up ticks
There is a further similarity: vaccines against the two diseases have proved extremely difficult to design. In the mid-1970s, efforts to create a conventional ECF vaccine seemed to be going nowhere, and attention turned to the immunisation method now being tested in Kenya. It had been known since the 1950s that a cheap antibiotic could help ward off the disease if given very soon after a tick bite. Vets tried infecting cattle with T. parva, by applying ticks, and then injecting the antibiotic. But they could control neither the timing of the bite nor the dose of parasite transferred. The procedure killed some animals and left others unprotected.
A breakthrough came in the early 1970s, when Brown and his colleagues found that by grinding up ticks and centrifuging the remains they could produce consistent doses of parasite. This allowed the first trials to begin of infection and treatment procedure in which animals were injected with T. parva and injected with oxytetracycline at the same time. But only now, after two more decades of research, is Kenya ready to take the plunge.
One of the biggest delays was caused by the discovery that immunising animals with one strain of T. parva did not protect against all others, says John Wafula, assistant director of animal health at the Kenya Agricultural Research Institute (KARI), which runs the NVRC. 鈥淎 major search was mounted to find a 鈥榤aster鈥� immunising strain.鈥�
It was soon discovered that blood taken from animals harbours not just one strain of the parasite, but many. The International Livestock Research Institute (ILRI) in Nairobi now holds 60 of these 鈥渟tocks鈥�, taken from animals in different parts of the country, each one containing many strains. Unfortunately, no laboratory method has been found to test whether one stock will protect against others. This can only be checked by time-consuming trials in which researchers inoculate animals with one stock, and expose them to others.
In the mid-1970s, researchers found that a mixture of three stocks gave wide 鈥� but not complete 鈥� protection. The UN Food and Agriculture Organization has since tested this 鈥淢uguga cocktail鈥� in a number of countries. Demand is growing fast in Uganda and Tanzania, while Zambia has already used it to immunise around 10000 head of cattle. Kenya chose not to use it, partly on grounds of cost but also because one stock in the cocktail came from Tanzania. Kenyan officials were worried that introducing foreign parasites into the country would make matters worse.
What Kenya wanted was a local solution, and by 1980 it seemed to have found one. A stock named Marikebuni, after its area of origin in Coast Province, 鈥減erformed at least as well as the cocktail鈥�, says Wafula. But plans to test the stock hit a snag. Everyone had assumed that cows immunised by infection and treatment were left free of parasites, but in the 1980s scientists found otherwise. Oxytetracycline did not kill T. parva; it simply protected animals during the acute phase of infection. 鈥淚mmunised and recovered cows become chronic carriers of the infection,鈥� says Wafula. These animals, if bitten by ticks, provide a source of infection for other, unimmunised cattle. This finding forced the immunisation programme to shut down.
After about three years, pragmatism won out. 鈥淚t was finally decided that the risk was acceptable,鈥� says Wafula. In areas where ECF was endemic, immunised cattle posed no more threat than animals infected naturally.
Now came the problems of putting the programme into practice, and more than five years on, the difficulties remain. Vets need training and many rural practices do not have the necessary equipment, says Mbogo. Doses of parasites must be kept cold with liquid nitrogen, for example. 鈥淚f a district doesn鈥檛 have a liquid nitrogen container, you can鈥檛 give them the parasite,鈥� he says.
Danger of death
Also, the logistics of infection and treatment are complicated. As many as 5 per cent of cows that are treated stand to contract ECF and die. To guard against this, each cow has to be checked about 14 days after immunisation to ensure that it is not running a temperature. Often this means that the vet has to make a return visit.
The pilot programme for testing immunisation and treatment has taken place in Coast Province, where until recently more than 60 per cent of all deaths among dairy cows were caused by ECF. But that could soon change, says Mbogo. 鈥淲e鈥檙e at a stage now where in some places we can give it to the field vets to use.鈥�
The treatment costs farmers a hefty KSh800 per animal, but for those with high-yielding cows it is still worthwhile. 鈥淭he farmers at the coast think it鈥檚 great,鈥� Mbogo reports. 鈥淭hey are willing to pay for it.鈥� In a recent study in Coast Province, researchers from KARI and the ILRI weighed the costs of veterinary bills against the benefits of improved milk yields. Once immunisation and treatment are available across the province, the population of dairy cows is likely to grow by 8.6 per cent a year. That growth could be crucial to feeding Kenya鈥檚 mushrooming population: the country鈥檚 demand for meat and milk is expected at least to double by the year 2020.
One potential danger of infection and treatment is that cattle released from their mud-floored pens could succumb to other serious tick-borne diseases such as anaplasmosis, babesiosis and heartwater. But Wafula does not see this as a major problem. 鈥淒eliberate exposure of calves to these infections early in life almost invariably results in the establishment of immunity to all three without the risk of significant losses鈥�, he says. In any case, treatment for babesiosis is cheap: just KSh40.
Today鈥檚 infant immunisation programme is not the end of the ECF story either, as another advance in preventing the disease may be just around the corner. In the late 1980s, researchers came across a farmer in Lanet, near Nakuru, whose cattle were infested with ticks but did not suffer ECF. The stock of parasites from Lanet appeared to confer immunity to the disease. It was duly isolated, frozen 鈥� and then forgotten. All mild stocks that had been tested up to that time had offered little protection against lethal strains, so the researchers simply ignored it.
Mbogo has now thawed out the Lanet stock, injected it into five groups of animals, and exposed them to a range of pathogenic strains. He believes it gives better cross-protection than Marikebuni and, being milder, will be easier to use. Vets will need only one visit to each farm and the animals will not need oxytetracycline.
Meanwhile, researchers elsewhere are continuing to look for a conventional vaccine against ECF. Tony Musoke, Vish Nene and Subhash Morzaria at the ILRI have made more progress than most by focusing on a protein called p67, which sits on the surface of T. parva sporozoites, the form of the parasite that ticks inject into cattle. The researchers snipped out the DNA that codes for p67 and inserted it into Escherichia coli, a bacterium that they grow in culture to produce large amounts of the protein.
Trials of the vaccine show that it protects about 70 per cent of cattle, which Morzaria describes as an 鈥渦ncomfortable鈥� result. 鈥淲e don鈥檛 want to jump up and down and say we鈥檝e got this great vaccine. People will start demanding it,鈥� he says. The next step is to include another of the parasite鈥檚 proteins in the vaccine, in the hope that this will increase the proportion of animals protected.
For now, though, farmers will have to rely on Marikebuni stock and antibiotic. Some of the first tests of the method were carried out in cows belonging to Njuguna鈥檚 neighbours around Muguga. Farmers are starting to graze their cows on the pastures around the NVRC, says Njuguna, even though they are notorious for harbouring ticks infected with T. parva. 鈥淧eople used to be frightened to graze their cattle,鈥� he says. 鈥淣ow they鈥檙e happy to take the risk.鈥�