Deadly Tsetse Fly : East Africa: New Fight on Old Scourge

In Lucas Oyango’s village, they needed to refer only to “the disease.” For sleeping sickness was familiar to almost every family. Periodically, it turned the villagers into nomads, forcing them across the border into Tanzania.

“They were driven away because of the flies,” Oyango said.

Years later, one family returned with 10 head of cattle to hitch to their two plows. Before the crops could even be planted, the disease had killed all the animals.

The Lambwe Valley, in this remote southwestern corner of Kenya along the shores of Lake Victoria, is one of the last refuges of human and cattle sleeping sickness in East Africa. And that is a testament to the exceptional hardiness and adaptability of the disease’s carrier: the tsetse fly.

‘A Great Scourge’

“Sleeping sickness has been a great scourge here,” said Father Marinus Tielen, who has lived in this area for nearly 30 years. Today, he and Oyango, his assistant at the Roman Catholic mission overlooking Lake Victoria, look back over the decades of grief that the fly has brought, as well as the signs of improvement detectable in the last few years.

Said Oyango: “Now, where there are 50 homes, there were only four before. Many people have come back.”

Nevertheless, the tsetse fly has prevailed over 30 years of human effort to eradicate it from the valley, which is home to thousands of subsistence farmers and cattle herders as well as the wild denizens of one of Kenya’s smallest and most isolated game parks.

Problem of Rains

In that time, the valley has been among the most heavily sprayed districts anywhere in the world. The first insecticide campaign took place from 1955 through 1957. There was further heavy spraying of dieldrin, a potent pesticide, in 1961, but heavy rains apparently rendered that project ineffective even as they provided the flies with superb breeding conditions.

From 1968 through 1971, with the assistance of the World Health Organization and the U.N. Development Project, the government deployed a small air force against the flies--helicopters and airplanes spraying the valley every year. There was further spraying in 1981 and 1983 with the most advanced pesticides available at the time.

And yet when the spraying stopped, the flies soon recovered to pre-spray concentrations. Sleeping sickness remained endemic.

So today, the Lambwe Valley is the focus of another attempt to defuse the fly as a disease carrier. This effort is decidedly low-tech. Under the direction of the International Center for Insect Physiology & Ecology (ICIPE), which is headquartered in Nairobi, locally recruited hands are setting out newly developed traps in the hope of catching so many tsetse flies that the remaining population will be too sparse to cause significant outbreaks of sleeping sickness in humans or cattle.

No matter how effective they are, researchers say, the traps will have to be permanent fixtures of the land here.

“You can’t eradicate the flies,” remarked Robert Copeland, a Boston University-trained researcher supervising the field trapping, as he piloted a bouncing Land Rover across the valley’s rutted trails. “The farmers around here are going to have to set the traps out forever. But that’s good, because anything that has to be done forever becomes part of the culture.”

Hard Lessons Learned

The Lambwe Valley project also reflects some hard lessons learned about pest control in remote regions of Africa. One is that highly technological programs face heavy obstacles.

Spraying, for example, must conform to a tight schedule too often frayed by unpredictable conditions of weather and logistics. Too much rain dilutes the pesticide below effective concentration. It must be sprayed during a temperature inversion, lest it evaporate. A contrary wind can blow the cloud off course; if the droplet size is wrong, it can be deposited ineffectually in the treetops.

Chemical pesticides are also losing favor in much of the developed world on the reasoning that they pose too great a danger and are often too costly, subject to scarcity and difficult to use.

ICIPE’s marching orders from the international agencies that provide its funding are to develop more organic measures to fight crop pests, says K. N. Saxena, a New Delhi- and Munich-trained zoologist who heads its crop pest program. So the center is also searching for natural predators of tsetse flies and investigating whether releasing sterilized males in the wild--each female is only fertilized once, for life--will cut down breeding.

Dangerous Parasitic Disease

Although it first appeared in this region in the 1940s, sleeping sickness itself has long since become part of the local culture.

The World Health Organization today classifies the malady as one of the five most dangerous parasitic diseases in the developing world, with an average of 20,000 new cases a year and a population of 50 million persons at risk.

That is far below the threat of malaria, which strikes 97 million people a year and menaces 2.2 billion. But several types of trypanosomes--the microscopic parasites that enter a body via tsetse bite--also cause sleeping sickness in cattle. And that has an incalculable economic impact here, where cattle are prized more as convertible assets, draft animals and producers of fertilizer than as sources of food. So sleeping sickness ripples geometrically through the agriculture and the very fabric of village life.

Families dependent on selling their cattle to pay school fees cannot educate their children. The loss of cattle as potential dowries can make women unmarriageable or destabilize existing marriages, for which a parent’s cattle often serves as financial security.

Difficult Coexistence

For those reasons, humans often find it harder to coexist with tsetse flies than even with mosquitoes, which year-round cause a high level of endemic malaria in this region.

As a WHO manual for tsetse control workers notes, “Small farmers in tsetse areas cannot use draft oxen to cultivate fields and cannot fertilize their fields with cattle dung. They cannot move cattle through tsetse areas for fear of loss. . . . Villages may have to be abandoned and whole areas may become depopulated. Places that were once established communities may give way to bush once more.”

The human toll is frightful.

“I maintain you can always tell a man who has been affected by sleeping sickness,” said Father Tielen. “They get very slow mentally, and bodily. They hate the sun, perspire far more than ordinary people.”

Of the two varieties of human sleeping sickness caused by two different but related parasites, one, known as Gambian, is today more common in West Africa, although it appeared in Uganda and other areas around Lake Victoria in the 1950s. This is the sleeping sickness of legend, a slow killer that produces chronic lethargy and eventually death.

Unhinging Headaches

The variety dominant in East Africa is so-called Rhodesian sleeping sickness. This is more painful and more rapidly fatal. Victims first notice a chancre at the site of a fly bite. What follows are blinding, unhinging headaches.

“They get such severe headaches that they fall into a trance,” said Father Tielen. “Most come out of the trance just before they die. We always say when they come out of the trance they’re nearly dead.”

The onset of either illness occurs after a victim is bitten by a tsetse fly that has previously bitten a host--man, cattle or other animal--harboring the parasite in its blood. Like a malaria-carrying mosquito, the fly injects a fluid containing the parasite into the victim as it draws blood.

“The parasite then invades practically all tissues,” said Pierre Cattand, training director for the trypanosomiasis program of the World Health Organization in Geneva. Every physiological system reacts in defense against the invading protein, creating the first symptoms of illness. Because the parasite continually changes its “antigenic code”--the array of protein signals that provoke the host’s immune response--eventually “the body’s immune system simply gets exhausted,” Cattand says.

A Pseudo-Coma

It is the prostration afflicting victims in the last stages of both varieties that gives sleeping sickness its name. The patient falls into what is known as a pseudo-coma, so-called because the victim can be briefly awakened and thus appears to have only fallen into a deep sleep.

As for treatment, in the bush that is always a cultural and logistical nightmare. Farmers often cannot or will not pay for the costly medicines effective on their diseased cattle. Existing clinics for humans are sparsely scattered and often shunned by the Luo tribesmen of this area.

“If 10% of the victims go to the hospital when they’re sick, that’s a lot,” said Tielen. “People here have often feared to go to the health officer. They think it’s a very queer thing to go and sleep in a room--a ward--with 60 other people.”

The trypanosomes that cause human and cattle disease are of different varieties, but the human parasite can live in cattle blood, making the animals symptomatic reservoirs of the human disease, like bovine Typhoid Marys. When a tsetse takes a blood meal from such an infected cow, it becomes a threat during its months-long life to any human it subsequently bites.

Sheltered From Insecticides

Meanwhile, the tsetse has evolved in ways that confound man’s attempts at eradication. Tsetse larvae develop largely within the female’s body. When the larvae eventually drop to the ground, they instantly burrow deep under the soil to pupate. Consequently, during these two most vulnerable stages of their lives, the flies are sheltered from insecticides.

“Unlike mosquitoes,” said Copeland, supervisor of the trapping program, “you have to aim eradication measures at the adults.”

Over the years, sleeping sickness has been periodically suppressed by man-made changes in this ecosystem. The steady encroachment of agriculture cleared away the thorny thickets that were the flies’ preferred habitat. Then, in 1980, researchers were astonished to discover that the flies had again demonstrated their fearsome adaptivity by colonizing a plantation of pine trees. In this way, the flies regained acres of lost habitat. Other studies have found tsetse colonizing plantations of sugar cane and even citrus orchards and banana farms.

So, keeping in mind that the destruction of the flies’ natural habitat is unlikely to take place fast enough to eradicate sleeping sickness anytime soon, Copeland and his crew have been testing new ways to pare the fly population.

Family Members Lost

With three crew members in the back seat, Copeland guides his Land Rover through the cleared trails that transect his sample thicket. One, John Asiago, relates how he lost two family members to sleeping sickness in two separate epidemics: his father in 1966 and his mother in 1973. A brother contracted the disease as well, but reached a clinic in time for treatment.

At regular points along the trail Copeland stopped to check his tent-shaped traps. They are fashioned out of deceptively simple materials: a swath of blue cloth--blue is particularly alluring to tsetse flies--hung over a network of twigs, supporting a plastic bag with an opening at the bottom.

On the ground in front is the bait--a can of cattle urine and an open bottle of acetone. The whole array, known as a Nguruman Trap after the Kenya valley where it was developed, is the latest and so far most effective tsetse trap designed by ICIPE.

“Fabric, twigs, cattle urine--these materials are simple enough that any farmer can get his hands on them,” Copeland explained.

He fingered the cloudy plastic bag. Examining a mass of thousands of dead insects, he says approvingly, “This is almost pure tsetse fly.”