Alveolar echinococcosis (al-VEE-oh-ler ee-keye-ni-kah-KOH-sis) is an infection caused by the tapeworm Echinococcus multilocularis. The infection is rare in humans, although is serious when it occurs as, if not treated, the infection is nearly always lethal. Tumorlike formations— due to the growth of the larval form of the tapeworm— occur most commonly in the liver, but can also be present in the brain, lungs, and elsewhere in the body.
E. multilocularis has a life cycle that consists of an egg phase and a larval phase. The eggs are excreted in the feces of the infected animal. If the feces are eaten by another animal, the eggs can germinate to form the larva, which matures in the intestine. As part of the maturation process, eggs are produced, which are shed in the feces. The cycle can then repeat in another animal.
Humans acquire the infection by ingesting the eggs. This usually occurs in one of two ways. First, food that is contaminated with fox or coyote feces is eaten. This can happen when, for example, a person hiking in the woods eats herbs or berries collected along the route. Second and more commonly, eggs that have stuck to the fur of family pets as they have been shed by the animal (or picked up as the dog or cat has rubbed against vegetation) transfer to the hands of the owner when the animal is petted or groomed, and are accidentally ingested when the hands are put into the mouth.
As the larvae of the tapeworm grow, they aggregate (come together) to create tumorlike formations. These typically occur in the liver, but can spread elsewhere. The symptoms, which develop slowly over years, include abdominal pain, a feeling of weakness, and loss of weight. The symptoms can be mistaken for the slow growth of a liver tumor or the type of progressive liver damage that can result from the chronic over-consumption of alcohol.
Alveolar echinococcosis is widespread in animal populations in northern latitudes including Europe, China, Russia, Asia, Japan, and North America (primarily in the north-central area of the United States from Montana to Ohio, Alaska, and most of Canada). In these regions, the tapeworm is present in over 50% of the fox and coyote populations. Human cases have rarely been reported in North America; in the twentieth century only two cases are known to have occurred: one in the state of Minnesota and the other in the western Canadian province of Manitoba.
In addition to foxes and coyotes, E. mulitlocularis can be found in the intestinal tract of dogs and cats. The animals can also become infected when they eat rodents, voles, or field mice that are infected with the tapeworm larvae.
Because the infection occurs most often in wild foxes, people who are most at risk are those who spend much of their time outdoors. These include park rangers, trappers, and hunters. Urban and rural veterinarians also run a risk of contact because they handle animals that may carry the tapeworm eggs. There is no evidence of racial or gender association with the infection. The higher tendency of men to be infected could reflect the traditional dominance of men in occupations like logging and in pursuits like hunting.
The distribution of the infection may be spreading as the territory of wild foxes contracts and they come into closer contact with humans. This is partially due to the expansion of urban areas; the availability of food attracts foxes that had not previously inhabited these areas.
WORDS TO KNOW
LARVA: Immature form (wormlike in insects; fishlike in amphibians) of an organism capable of surviving on its own. A larva does not resemble the parent and must go through metamorphosis, or change, to reach its adult stage. The initial stage of a mosquito after it hatches from its egg.
PARASITE: An organism that lives in or on a host organism and that gets its nourishment from that host. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.
As of 2007, there is no cure for alveolar echinococcosis. The most common treatment involves surgery to remove the tumorlike larval mass, followed by drug therapy to attempt to prevent the germination of eggs that may still be present in the individual. Drug therapy, typically with the anti-fungal compound benzimidazole, can be long and expensive.
The best prevention is to lessen the chances of contacting E. multilocularis. Avoiding contact with living or dead wild animals unless protective gloves are worn is a sensible precaution. Keeping domestic pets close to home and away from contact with wild animals is another wise move. Washing hands after handling pets is another preventative step, but one that is difficult for most people to consistently follow.
The main impact on human health of alveolar echinococcosis is the high death rate of the infection if it is not treated. The odds of survival for five years if the infection is untreated is only 40% versus almost 90% if treatment is provided.
Even with treatment, persons treated for alveolar echinococcosis often face a diminished quality of life.
Furthermore, treatment comes with a high price tag; up to $300,000 per person.
The infection is an example of how political or economic decisions can influence a disease. The clearing of forests to provide more farmland or timber can cause rodent populations to become more concentrated in urban areas, increasing the chances for the spread of E. multilocularis.
IN CONTEXT: PERSONAL RESPONSIBILITY AND PREVENTION
The Centers for Disease Control and Prevention recommends that people who live in an area where E. multilocularis is often found in rodents and wild canines, take the following precautions to avoid infection:
- Don't touch a fox, coyote, or other wild canine, dead or alive, unless you are wearing gloves. Hunters and trappers should use plastic gloves to avoid exposure.
- Don't keep wild animals, especially wild canines, as pets or encourage them to come close to your home.
- Don't allow your cats and dogs to wander freely or to capture and eat rodents.
- If you think that your pet may have eaten rodents, consult your veterinarian about the possible need for preventive treatments.
- After handling pets, always wash your hands with soap and warm water.
- Fence in gardens to keep out wild animals.
- Do not collect or eat wild fruits or vegetables picked directly from the ground. All wild-picked foods should be washed carefully or cooked before eating.
SOURCE: Centers for Disease Control and Prevention (CDC)
Black, Jacquelyn G. Microbiology: Principles and Explorations. New York: John Wiley & Sons, 2004.
Pelton, Robert Young. Robert Young Pelton's The World's Most Dangerous Places. 5th ed. New York: Collins, 2003.
Wobeser, Gary A. Essentials of Disease in Wild Animals. Boston: Blackwell Publishing Professional, 2005.
Sréter, Tamás, Zoltáa Széll, Zsuzsa Egyed, István Varga.