Tsetse fly

• 1. Introduction

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  The tsetse fly, a large biting insect of tropical Africa, can swallow a blood meal that weighs as much as its own body. Its abdomen changes dramatically in volume while it feeds. It is also remarkably tough. A housefly or a horsefly dies easily under a flyswatter, but crushing a tsetse takes a great deal of effort.

  Every species of tsetse belongs to a single genus, Glossina, and most scientists place that genus alone in its own family, Glossinidae. English speakers also call the insect the tik-tik fly, and the name is sometimes spelled tzetze. Researchers have studied it closely for two reasons. It spreads serious disease, and it can be raised in a laboratory.

  How does a fly raise its young on nothing but blood? What does it carry inside its body, and what does that cargo do to the animals it bites? And how could one insect have shaped African farming, African wildlife reserves, and even arguments about why some African states grew the way they did?

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• 2. One Egg at a Time

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  A female tsetse fertilizes a single egg at a time and keeps it inside her body while the young one develops. Scientists call this adaptation adenotrophic viviparity. During the first three larval stages, the offspring grows in the uterus, and the mother feeds it with a milky substance made by a modified gland.

  In the third larval stage, the young tsetse leaves the uterus and begins an independent life. The newborn crawls into the ground and grows a hard outer shell called the puparial case. Inside that case it completes its change into an adult fly. In this free-living larval form it looks like an ordinary maggot, yet it stays that way for at most a few hours, so almost no one sees it outside a laboratory.

  The whole larval period varies from four to ten days, and the young fly depends entirely on what its mother provides. An adult emerges from the puparial case after 35 days, and every bit of nourishment it needs along the way comes from her. The offspring can end up heavier than the mother herself. She must therefore find enough blood to keep herself alive and to supply a child that may outweigh her.

  This arrangement may explain why blood is such an important food for the species. Both males and females feed on it. Pupae, the hard-shelled stage in between, are small oblongs under 1 cm long, with two small dark lobes at the tail end, which is where they breathe.

  Whatever finally emerges from that shell has a body plan so distinctive that biologists can name it from a short checklist.

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• 3. A Distinct Fly

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  During World War II, a British de Havilland antisubmarine aircraft called the Tsetse Mosquito helped fix the insect's name in everyday English. In the Sotho languages the word is pronounced tseh-tseh, and it is easily rendered in other African languages. In science and development circles, people now often say simply tsetse, without the word fly.

  Adults measure between 0.5 and 1.5 cm. They have large heads, widely separated eyes and unusual antennae. The thorax is large, and the abdomen is wide and shorter than the wings. Four features together set a tsetse apart from other flies. The proboscis is long and thin, points straight forward, and joins the underside of the head through a distinct bulb. At rest the wings fold completely, one directly on top of the other. A cell in the middle of the wing has the shape of a hatchet or meat cleaver. And the hairs on the antennal arista are themselves branched.

  Twenty-three living species are known from the African continent and the Arabian Peninsula. They fall into three groups. The savannah flies belong to the Morsitans group, the forest flies to the Fusca group, and the riverine and lacustrine flies to the Palpalis group. As of 1990, flies had been reported from about 15 degrees north in the Niayes region of Senegal to 28.5 degrees south in KwaZulu-Natal, South Africa. Only two subspecies, G. f. fuscipes and G. m. submorsitans, live in the far southwest of Saudi Arabia. In 1903 a man named Carter found G. tachinoides near Aden in southern Yemen, and no one has confirmed it there since.

  The family is ancient. Fossil tsetse have been found in the Florissant Formation in North America and at Enspel in Germany, dating to the late Eocene and the late Oligocene respectively. In the first half of the 20th century, C. F. M. Swynnerton did much of the earliest ecological work on living flies in East Africa. In the early 1920s the entomologist E. E. Austen honored him by naming Glossina swynnertoni after him.

  Those scientists were not only cataloguing insects. They were trying to learn what each fly fed on.

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• 4. The Tsetse Menu

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  Warthogs supply most of the blood for Glossina swynnertoni, and for G. morsitans they are again the largest single source, with giraffe a smaller one for the first of those species. Across the genus, members of the pig family are the most important hosts. G. fuscipleuris takes about 65 percent of its meals from bushpigs and giant forest hogs, and up to 20 percent from hippopotamus.

  The diets of the forest and riverine flies are far more varied. G. palpalis draws roughly 10 percent of its blood from waterside birds, including cormorants. It also drinks from monitor lizards and crocodiles, and in some places as much as half of its meals come from crocodiles. G. longipennis depends unusually heavily on the rhinoceros, at about 60 percent of its meals. It takes up to 12 percent from elephants and up to 7 percent from ostriches.

  One animal is conspicuously left alone. The waterbuck gives off volatile chemicals that repel the flies, and researchers are testing those odors as a way to protect livestock.

  What the flies take from these animals is not only blood. Sometimes they also pick up a passenger.

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• 5. The Parasite Within

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  The trypanosomes that tsetse carry are single-celled protozoans of the genus Trypanosoma, and they are about the size of a red blood cell. Their effects vary widely. A species called T. theileri seems to cause no trouble except perhaps in animals that are already sick. Other strains are far more virulent, and they have evolved a life cycle that needs time in both a vertebrate host and a tsetse.

  The flies pass the parasites on in two ways. Mechanical transmission works like a syringe. A fly feeds on an infected animal, is interrupted, and soon feeds again, regurgitating some of the first blood meal into the second victim. Horseflies can spread trypanosomes this way too. Biological transmission is more intimate. The parasites must multiply through several generations inside the fly, changing form along the way and passing through a sexual phase. Because that phase happens only inside the tsetse, biological transmission is a required step in the life cycle.

  The fly is most likely to be infected during its first few blood meals, and once infected it is thought to stay that way for life. Infection is not passed from a pregnant fly to her young, so every newly emerged adult starts out clean. An infected fly injects parasites in its saliva or regurgitates them from an earlier meal. A successful inoculation needs at least 300 to 450 cells and may contain as many as 40,000.

  The parasite also seems to change how the fly behaves. Infected flies have altered saliva, which makes feeding less efficient and lengthens each meal. In one study of T. b. brucei in G. p. gambiensis, the proteins in the fly's head changed. The fly fed more often than it needed to, which gave the trypanosomes more chances to spread. The tsetse is not defenseless, however. It produces hydrogen peroxide, which damages DNA, and its defenses limit how many flies end up infected.

  The fly also hosts its own helpers. Tsetse have at least three kinds of symbiotic bacteria. The primary one is Wigglesworthia, which lives inside special cells. The others are Sodalis and some form of Wolbachia. The genome of Glossina morsitans was sequenced in 2014.

  In a bitten vertebrate, the trypanosomes enter muscle tissue and then move into the lymphatic system. From there they reach the bloodstream and finally the brain. The animal's lymph glands swell and its body wastes away until it dies. An uninfected tsetse that bites the dying animal picks up the parasite and closes the cycle.

  In humans, this illness is sleeping sickness. It starts with a bite that deposits parasites under the skin, and the swelling of the lymph glands that follows has a name, Winterbottom's sign. The parasites then cross into the central nervous system, causing extreme lethargy and eventually death. Left untreated the disease is invariably fatal, yet it can almost always be cured if it is caught early.

  The causative species, Trypanosoma brucei, was long split into three subspecies. T. b. brucei infects animals but not people. T. b. gambiense, found in West Africa, was thought to bring a slower, chronic illness. T. b. rhodesiense, found in East Africa, was thought to bring a faster, more virulent one. A molecular study using restriction fragment length polymorphism analysis suggests the subspecies are polyphyletic, so the real picture is more complicated.

  Farm animals suffer a disease with a different name. Nagana is the usual term for the illness in cattle and horses, and it saps growth, milk production and strength before it kills. Some cattle breeds are trypanotolerant, meaning they survive and grow even when infected, though less productively. Research by ILRI in Nigeria, the Democratic Republic of the Congo and Kenya found the N'Dama breed to be the most resistant. T. congolense and T. vivax are the two main threats to cattle in sub-Saharan Africa, while T. simiae causes a virulent disease in pigs.

  The damage to animals ripples far past the farm, and it began to rewrite the map of Africa in the 1880s.

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• 6. Rinderpest and the Empty Land

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  In 1887, an Italian expeditionary force brought livestock to Eritrea, and with them came the rinderpest virus, a cattle plague believed to have originated in Asia. According to an article in New Scientist, that accident helped create the empty, apparently primeval Africa seen in wildlife documentaries. The virus reached Ethiopia by 1888, the Atlantic coast by 1892 and South Africa by 1897. It killed over 90 percent of the cattle of pastoral peoples such as the Masai of east Africa. In South Africa about 5.5 million domestic cattle died.

  The pandemic coincided with a drought, and famine followed. Starving people fell to smallpox, cholera, typhoid and sleeping sickness. It is estimated that two-thirds of the Masai died in 1891. Germany and Britain were able to take over Tanzania and Kenya with little effort.

  With the cattle gone, grassland turned rapidly to bush. Within a few years the closely cropped grass gave way to woody grassland and thornbush, ideal tsetse habitat. Wild mammals multiplied, and the flies came with them. Highlands of east Africa that had been free of tsetse were colonized, and sleeping sickness arrived in places that had never known it. Millions of people died of it in the early 20th century.

  Conservationists misread what they saw. They called sleeping sickness "the best game warden in Africa" and assumed the land, empty of people and full of game, had always been that way. Julian Huxley of the World Wildlife Fund described the plains of east Africa as "a surviving sector of the rich natural world as it was before the rise of modern man". Large hunting reserves followed. In 1909 the newly retired president Theodore Roosevelt went on a safari that sent more than 10,000 animal carcasses to America. Much of the land later became nature reserves and national parks, including the Serengeti, Masai Mara, Kruger and Okavango Delta. Eastern and southern Africa now has a landscape of manmade ecosystems, with farmland largely free of bush and tsetse, and bush controlled by the fly.

  Colonial governments, worried for their own interests, brought transmission of sleeping sickness almost to a halt by the 1960s. Newly independent governments then relaxed surveillance and management in the same areas, and the disease surged back into a crisis in the 1990s.

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• 7. The Price of a Green Desert

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  The tsetse fly lives across nearly 10 million square kilometres of sub-Saharan Africa, much of it fertile land left uncultivated, a so-called green desert. Of the 38 countries with tsetse, 32 are low-income, food-deficit countries and 29 are among the least developed. Only 45 million of the 172 million cattle in sub-Saharan Africa graze in tsetse-infested areas. Those herds are often pushed into fragile highlands or the semiarid Sahel, which worsens overgrazing.

  Cattle deaths from trypanosomiasis are estimated at 3 million a year. That cuts the annual value of cattle production by between 600 million and 1.2 billion US dollars. The overall annual direct loss of potential livestock and crop production is put at 4.5 billion to 4.75 billion US dollars. Farmers in infested areas often work the soil with hoes, because nagana weakens and kills draught animals. Surviving cows give little milk, pregnant cows often abort, and without manure the worn-out soils go unfertilized. Keeping cattle at all usually requires constant preventive drugs, with their own troubles of resistance, counterfeit supplies and wrong dosages.

  People bear a heavy burden as well. An estimated 60-70 million people in 20 countries face some risk of sleeping sickness, yet only 3-4 million are covered by active surveillance. The yearly burden is estimated at 2 million disability-adjusted life years. Because the disease tends to strike economically active adults, a family with a patient loses about 25 percent of a year's income.

  Some economists have argued that the fly did more than impoverish farms. A 2012 study built a tsetse suitability index from insect population growth, climate and geospatial data. Higher scores were associated with a statistically significant weakening of agriculture, urbanization, institutions and subsistence strategies in the pre-colonial period. The authors proposed that the flies decimated livestock, forcing early states to rely on slave labor to clear land and denying farmers natural fertilizer. Low population density and weak cooperation between small communities may then have blocked the rise of stronger nations. They also noted that Great Zimbabwe stands in highlands where the fly does not occur, and it was the largest and most technically advanced pre-colonial structure in southern sub-Saharan Africa.

  Other authors are more skeptical. The conventional argument is that the fly made draught animals hard to use, so wheeled transport was never adopted. That holds true where the flies are dense, but similar cases exist outside tsetse country. The fly clearly influenced which technologies Africans adopted, yet critics contend it was not the single root cause.

  Whatever its historical weight, the fly can be beaten in particular places, and engineers have tried many ways to do it.

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• 8. Fighting the Fly

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  In the 1930s, the island of Principe off the west coast of Africa was cleared of every feral pig, and the fly disappeared with them. Tsetse reinvaded in the 1950s, but the new population carried no disease. That kind of mass slaughter of wild hosts was one of the earliest control methods.

  Another early method removed brush and woody vegetation, because tsetse rest on tree trunks. Until about 1959 the work was done by hand. Glover and colleagues described a faster technique that they called chain clearing, in which a chain is dragged between two heavy vehicles. Regrowth demanded continuous clearing, which was costly, and clearing came to be seen as an environmental problem, so the method was abandoned.

  Pesticides came next. Early in the 20th century, local efforts used inorganic metal-based compounds. After the Second World War, massive aerial and ground campaigns applied organochlorines such as DDT as ultra-low-volume aerosol sprays. Later techniques poured more advanced organic pesticides directly onto the backs of cattle.

  Traps are cheaper and simpler. Many use blue cloth, in sheets or in a biconical shape, because blue attracts the flies. The cloth either funnels them into a collection chamber or carries insecticide. Early traps imitated the shape of cattle, since tsetse are drawn to large dark forms like the hides of cows and buffaloes. One idea holds that zebra have stripes not for camouflage in long grass but because black and white bands confuse tsetse and discourage attack. Scientists have also tested scents that mimic animal breath, such as carbon dioxide, octenol and acetone. Placing cattle urine in a half gourd near the trap is a cheaper alternative, and for large operations extra traps usually cost less than artificial attractants. In Ethiopia, the BioFarm Consortium treats traps as an entry point to wider improvements in farming, health and marketing, a method in its final testing stage as of 2006.

  The sterile insect technique uses gamma rays or X-rays to sterilize mass-reared male flies. Released from the ground or the air, the males mate with wild females, which then produce no offspring. Insecticides are poor at removing the last remnants of a population, while sterile males are very good at finding the last remaining females. For that reason the technique usually comes at the end of an area-wide campaign. It has also been used against other pests, including the medfly.

  On the island of Unguja in Zanzibar, a project running from 1994 to 1997 first suppressed the flies with insecticides and then used sterile males to remove Glossina austeni entirely. The disappearance of the fly in 1997 was followed by the disappearance of nagana. Farmers could finally combine livestock with crops, using animals for transport and traction. Surveys in 1999, 2002-2014 and 2015 confirmed that neither tsetse nor nagana had returned.

  In the Niayes region of Senegal, near Dakar, a population of Glossina palpalis gambiensis lived in fragmented habitats and was genetically isolated from the main tsetse belt in the southeast. Feasibility studies ran from 2006 to 2010, and an area-wide campaign including sterile males began in 2011. By 2015 the region was almost free of tsetse. The target area covers 1,000 square kilometres in three blocks. In the northern block, intensive monitoring has found no wild tsetse since 2012. The prevalence of animal trypanosomiasis in the first two blocks fell from 40-50 percent to under 10 percent.

  A benefit-cost survey predicts that farmers there will swap local breeds for improved ones and raise their yearly income by 2.8 million euros. The number of cattle is expected to fall by 45 percent, which would reduce the environmental footprint of herding in the Niayes.

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