Getting the measure of tapeworms
The livestock revolution is stimulating some unpleasant side effects. Cysticercosis, a debilitating disease that threatens household food security and farming livelihoods, seems to be getting worse as farming practices and food supply patterns change. Endemic in much of the developing world - some 50 million people are thought to be infected - this parasitic zoonosis is caused by the larval stage of Taernia solium and T saginata, the tapeworms of pigs and cattle respectively. T. solium is by far the more dangerous. Not only is it more widespread but it can lead to the killer form of the disease, neurocysticercosis. In theory, cysticercosis is easily preventable. As with most zoonoses, the answer is better awareness and better hygiene. But again, as with most zoonoses, the question should be; how can this be achieved in practice and who should take responsibility for action?
It is unpleasant to have to accept that humans are the primary hosts of T. solium and T. saginata. The animal host becomes infected by ingesting tapeworm eggs passed in human faeces. Cysts of the intermediate parasite, Cysticercus cellulosae, form in the animal and, if its meat is eaten without adequate cooking, T. solium develops in the person's intestines where it can grow to several metres in length and result in malnutrition and general debility. A far more life-threatening cycle occurs where human faeces containing T. solium eggs infests waste water that is then used for human consumption either directly or through use in growing vegetables or other food products. The larval, cyst-forming stage of the lifecycle that normally takes place in the pig, now takes place in the human. Cysts can form in the muscle, the eye or, most seriously, in the brain. Symptoms are similar to brain tumours or epilepsy and many cases diagnosed as epilepsy are, in fact, due to neurocysticercosis. WHO estimates 50,000 deaths per annum and much suffering for those who survive but who live subsequently with recurrent seizures and the associated social stigma.
Farmer families most at risk
A high proportion of the 50 million people estimated by WHO to be carrying
T. solium are directly associated with agriculture because it is a
condition that affects those living in close proximity to animals. The greatest
risk is where pigs roam freely and can pick up human excrement. It is,
therefore, a problem that has traditionally been associated with rural
communities in poorer developing countries where there is no form of even
rudimentary sanitation. Here pigs are slaughtered in the backyard or on an open
village slaughter slab and there is, of course, no formal meat inspection. The
owner may get a lower price if his pig shows signs of infection but the meat
will nevertheless be eaten. Even vegetarians and non-pork eaters are at risk,
particularly in crowded urban areas where the risk of people consuming
contaminated waste water is also high. And global travel brings that risk even
to countries where sanitation is good and pigs are housed. Cysticercosis is now
considered an emerging disease in the USA. Another change in recent years, also
linked to both urbanisation and the livestock revolution, is the move away from
cattle production - as grazing land becomes more scarce - to monogastrics like
pigs. Farmers recognize the greater productivity of pigs and the opportunity
for marketing pork products to urban populations. But if meat is found to
contain cysts of T. solium it is likely to be rejected by meat
inspectors threatening the livelihood of the pig producer.
T. solium has only two hosts - pigs and humans. All that is required in order to control cysticercosis is to break the cycle between the two. The use of latrines to ensure pigs have no access to human excrement, washing hands before eating, and thoroughly cooking pig meat are relatively simple measures. Indeed the International Task Force for Disease Eradication considers cysticercosis to be one of six infectious diseases that could, in theory, be eradicated. A control programme that took place in Cameroon between 1987 to 2000 reduced cysticercosis in the pig population from about 24% to about 3% and, in the human population, from 2.4% to 0.5%. There are vaccines available that have been developed by Australian scientists and field tested in Africa and Latin America that provide 93 to 95% protection for pigs and cattle. Detection is also improving. In pigs, when cyst infestation is high, it is visible on the under side of the tongue but more sensitive ELISA and immunoblot serodiagnostic kits are now available. For humans, magnetic resonance imaging and CT scanning are the most reliable diagnostic tools but these are expensive and the equipment is often unavailable in developing countries. However serological tests are improving in sensitivity and specificity. Treatment is also improving for both pigs and humans. Research continues on all these fronts and also to gather baseline data but, when it comes down to control, is the disease a public health or a livestock problem? Is it an issue of trade, meat inspection or meat marketing, a matter for education or a social challenge? Everyone should be concerned about cysticercosis. No-one is immune.
