SEVEN NEGLECTED ENDEMIC ZOONOSES - some basic facts
Anthrax is primarily a disease of herbivores, although all warm-blooded species are susceptible to an extent. The causative agent is the spore–forming bacterium Bacillus anthracis. The ‘reservoir’ of the disease is soil contaminated by spores in the recent past or even several decades ago. Humans normally acquire anthrax either by direct or indirect contact with infected animals, or through occupational exposure to contaminated animal products. Of 60 countries reporting anthrax in 2004, nearly 60% were developing countries. In animals, the disease is almost always rapidly fatal. In people the disease takes three forms. Inhalation anthrax is an occupational disease reported only in industrialized countries and acquired by breathing in spores; gastro-intestinal anthrax is acquired from eating infected meat from an animal that died of the disease and the cutaneous form, which accounts for more than 95% of reported cases in developing countries, is acquired through skin lesions. In strong contrast to the fear of this disease in the West caused by its bio-terrorist potential, its role, year in, year out, in causing illness in poor livestock-keeping communities and sudden deaths in their herds and flocks, is largely ignored.
In humans, the vast majority of cases of tuberculosis are caused by Mycobacterium tuberculosis. However, TB can be caused by a number of other bacteria, of which Mycobacterium bovis, causing so-called ‘bovine tuberculosis’ is one of the more prevalent and has the widest host range of all TB bacteria. TB due to M. bovis often occupies sites other than the lungs (it is extra-pulmonary), but in many cases is clinically indistinguishable from M. tuberculosis infection. However, patients with M. bovis often do not respond to the drugs commonly used to treat TB, sometimes resulting in a fatal outcome. Other, far more expensive, drugs are often needed – placing an added burden on health services. Very little is known about the share of bovine TB in the global TB epidemic, but sporadic reports of cases are received from many African and Asian countries and recent work undertaken in the United Republic of Tanzania indicates that this may be a substantial fraction. Bovine TB appears to be increasing at a similar rate to the total number of cases of TB, and HIV is the greatest factor for progression of TB infection to active TB disease. In livestock, particularly cattle, the disease causes lowered productivity, but seldom death. Like brucellosis, bovine TB has been largely eradicated from herds in the developed world by a test-and-slaughter programme.
Brucellosis is one of the world’s most widespread zoonoses. Caused by various bacteria in the genus brucella which affect cattle, sheep, goats, pigs and some other animals, it leads to abortion, later permanently reduced fertility and chronically lowered milk yields in affected animals. It can be passed to people via direct contact with livestock or through drinking unpasteurized milk from an infected animal. In people, the main symptom is recurrent bouts of high temperature, hence its other name ‘undulant fever’ – and its tendency to be misdiagnosed as drug-resistant malaria in tropical countries. A chronic debilitating disease, it can cause a variety of other symptoms, including joint pain, fatigue and depression. It causes substantial losses to livestock producers in herds or regions where it is endemic. In most developed countries, testand-slaughter programmes, together with compensation for farmers and accreditation and financial incentives for disease-free herds have more or less eliminated brucellosis in livestock and few human cases occur.
Cysticercosis and neurocysticercosis (NCC)
Cysticercosis is emerging as a serious public health and agricultural problemin many poorer countries of Africa, Asia and Latin America. Humans acquire Taenia solium tapeworms when eating raw or undercooked pork meat contaminated with cysticerci, the larval form of the tapeworm which develop in the intestine of humans where they establish and become adult tapeworms which can grow to more than three metres long. These adult worms shed eggs in human faeces that can infect in turn, the same or other humans as well as pigs – by direct contact with tapeworm carriers or by indirect contamination of water or food. The disease is thus strongly associated with pig keeping in conditions of poor hygiene. Ingested eggs result in larval worms that migrate to different parts of the human and pig body and form cysts (cysticercosis). Pigs can harbour thousands of these cysts, making the pork from these animals unsafe to eat and often resulting in the total condemnation of the pig’s carcass. A principle site of migration in humans is the central nervous system. Human neurocysticercosis (NCC) occurs when the cysts develop in the brain. It is considered to be the most common parasitic infection of the human nervous system and the most frequent preventable cause of epilepsy in the developing world. WHO estimates that cysticercosis affects some 50 million people worldwide and, in endemic areas, causes some 50 000 deaths (map).
Cystic echinococcosis or hydatid disease
Cystic echinococcosis (CE) or hydatid disease is caused by the larval stage of the tapeworm Echinococcus granulosus. Its natural cycle is as a cyst in sheep and as a tapeworm in dogs. Dogs feed on infected sheep meat and in turn shed eggs in their faeces which are ingested by sheep. Humans become infected by ingesting food or water contaminated with faecal material containing tapeworm eggs passed from infected carnivores, or when they handle or pet infected dogs. Cysts, often sited in the abdomen, grow slowly over time and can become very large. The cure is usually surgery. Hydatid disease is found throughout the world in communities where sheep are reared together with dogs. It is highly prevalent in many developing countries, especially in poor communities. In humans, the incidence of surgical cases ranges from 0.1 to 45 cases per 100 000 and the real prevalence ranges between 0.22% to 24% in endemic areas. Control is through deworming of dogs and preventing dogs from eating undercooked sheep meat, especially offal, as well as abattoir control and health education. It causes serious human suffering and considerable losses in agricultural and human productivity. Transmission is facilitated by the general lack of awareness of transmission factors and prevention measures among the population at risk, abundance of stray dogs, poor meat inspection in abattoirs, improper disposal of offal and home slaughtering practices. The economic consequences are often not known, resulting in subsequent neglect during priority setting.
Rabies is probably the best known zoonotic illness. It is caused by a virus, which usually enters the body through a bite or skin lesion and makes its way to the brain. Its dramatic symptoms in animals and people and inevitably fatal outcome in untreated patients after a cruelly unpleasant illness have made it by far the most feared of the zoonoses. From the public health point of view, the single most effective measure for preventing it remains dog vaccination, despite the occasional risk to dogs and people from infected wild animals. People who have been bitten by a suspect animal should first of all wash the wound and then seek post-exposure treatment. Various types of post-exposure treatment exist, but are often unavailable in isolated rural areas or too expensive for governments or individuals to afford. From age-stratified incidence rates, on average between 30% and 50% of human cases of rabies (and therefore rabies deaths) occur in children under 15 years of age. In some areas significant losses to livestock, especially cattle, have been recorded. More than 99% of all human deaths from rabies occur in the developing world, with domestic dogs the source of the vast majority of human cases. Despite being one of the oldest diseases known to man and the existence of highly effective dog vaccine and post-exposure treatments for people, it is estimated that some 55 000 people a year suffer a dreadful death from this disease.
Zoonotic sleeping sickness or human African trypanosomiasis (HAT)
Unlike the other six diseases described, whose distribution is worldwide, sleeping sickness or human African trypanosomiasis (HAT) is limited to the continent of Africa where its insect vector, the tsetse fly, is found. There are two forms of sleeping sickness. The chronic gambiense form is found in Central and West Africa and although an animal can be found infected, the disease is maintained by transmission between the insect vector and humans. However, the animal reservoir is important in the acute rhodesiense form found in Eastern and Southern Africa. The causal agent, Trypanosoma brucei rhodesiense, infects humans, wild animals and domestic livestock, which maintain infection between epidemics, and coexists in animals with a complex of pathogenic trypanosomes (T. congolense, T. vivax and T. b. brucei) that present a major problem for livestock keepers in Africa. Untreated, the disease is always fatal in humans and devastating epidemics have occurred over the last century. Treatment is expensive, normally ranging from US$150 to US$800 per person, and in the later stages of the disease treatment itself involves some 5% mortality. Control is via the vector or the disease’s human and livestock reservoirs. For rhodesiense, the key to preventing the disease in people is now thought to be by treating the cattle reservoir, using drugs which are effective not only against the trypanosomes pathogenic to humans but also those which cause substantial losses to livestock production – backed up by appropriate vector control measures.