Hydatidosis / Echinococcosis in India — A Neglected Zoonosis
Human echinococcosis is a parasitic disease caused by tapeworms of the genus Echinococcus.Echinococcosis is a zoonotic disease caused by tapeworms in the genus Echinococcus. The organisms live in the intestines of the definitive hosts as adults, and in the internal organs of intermediate hosts as cyst-like larvae. Some species of Echinococcus circulate in domestic animals; others occur in wildlife (sylvatic) cycles or have both domestic and sylvatic cycles. Adult tapeworms are usually carried asymptomatically in their definitive hosts, which can include dogs, cats and wild carnivores. Dogs are particularly important in zoonotic transmission due to their defecation habits and close relationships with humans. Many mammals and marsupials, including humans, livestock, pets and wildlife, can act as intermediate hosts. Intermediate hosts are usually asymptomatic in the initial stages, but the growing larvae, which often develop in the liver or lungs, may eventually cause illness and sometimes death. Echinococcosis is a major public health problem in some countries, and it may be emerging or re-emerging in some areas. Cystic echinococcosis, the most common form of the disease, is caused by the members of the Echinococcus granulosus sensu lato complex. Because the larvae of these organisms usually develop as discrete single cysts, it is the least severe and most treatable form. E. multilocularis (alveolar echinococcosis) and E. vogeli (polycystic echinococcosis) are more serious and difficult to treat. The latter organisms can proliferate in intermediate hosts, forming masses that can infiltrate entire organs and may disseminate to distant sites including the brain.
The two most important forms of the disease in humans are cystic echinococcosis (hydatidosis) and alveolar echinococcosis. Humans are infected through ingestion of parasite eggs in contaminated food, water or soil, or through direct contact with animal hosts.
Echinococcosis is often expensive and complicated to treat and may require extensive surgery and/or prolonged drug therapy. Prevention programs focus on the deworming of dogs and sheep, which are the definitive hosts. In the case of cystic echinococcosis, control measures also include improved food inspection, slaughterhouse hygiene, and public education campaigns. The vaccination of lambs is currently being evaluated as an additional intervention.
Humans become infected when they swallow the tapeworm eggs in contaminated food. The eggs then form cysts inside the body. A cyst is a closed pocket or pouch. The cysts keep growing, which leads to symptoms.
E granulosus is an infection caused by tapeworms found in dogs and livestock such as sheep, pigs, goats, and cattle. These tapeworms are around 2 to 7 mm long. The infection is called cystic echinococcosis (CE). It leads to growth of cysts mainly in the lungs and liver. Cysts can also be found in the heart, bones, and brain.
E multilocularis is the infection caused by tapeworms found in dogs, cats, rodents, and foxes. These tapeworms are around 1 to 4 mm long. The infection is called alveolar echinococcosis (AE). It is a life-threatening condition because tumor-like growths form in the liver. Other organs, such as the lungs and brain can be affected.
Children or young adults are more prone to get the infection.
Echinococcosis is common in:
- Africa
- Central Asia
- Southern South America
- The Mediterranean
- The Middle East
Risk factors include being exposed to:
- Cattle
- Deer
- Feces of dogs, foxes, wolves, or coyotes
- Pigs
- Sheep
- Camels
SUSCEPTIBLE HOSTS
It is becoming increasingly evident that human behavior especially in relation to dogs and cats, uncontrolled slaughter of food animals, indiscriminate disposal of offal and carcasses and eating habits of the people play an important role in epidemiology of disease. Human infection is acquired usually in childhood through contact with infected dog. The impact of Hydatidosis can be described only in terms of human suffering, cost of medical diagnosis, hospitalization and surgery, man days lost as well as in terms of temporary or permanent incapacity. The retarded growth of animals and reduction in the quality and yield of meat, milk, wool and condemnation of offal are also very important point. Hydatid disease is an occupational disease of certain groups i.e. Shepherds and their family in the endemic areas and shoe makers
What are the symptoms of Echinococcosis?
Cysts may produce no symptoms for 10 years or more.
As the disease advances and the cysts get larger, symptoms may include:
- Pain in the upper right part of the abdomen (liver cyst)
- Increase in size of the abdomen due to swelling (liver cyst)
- Bloody sputum (lung cyst)
- Chest pain (lung cyst)
- Cough (lung cyst)
- Severe allergic reaction (anaphylaxis) when cysts break open
Transmission and Life Cycle
Basically it is a “dog – sheep” cycle with man as an accidental intermediate host. The adult tapeworm lives in the small intestine of dog, which is definitive host for 2 – 4 years. The eggs are voided in the feces and contaminate the soil vegetation and drinking water. They are highly resistant and can survive for several months in pastures and household. Sheep, Cattle and other intermediate host become infected when they use vegetation which has been contaminated with feces from infected dog. Ingested eggs hatch in the intestine and larvae penetrate the intestinal lining and migrate to various organs as liver, lungs and brain and develop into hydatid cyst. The life cycle is completed when sheep or cattle viscera containing hydatid cyst are eaten by dogs. Infected dogs begin to pass of the parasite approximately 7 weeks after infection. Man does not harbor adult worm.
Echinococcus tapeworms have an indirect life cycle, which can only be completed with both an intermediate and a definitive host, typically a predator or scavenger and its prey. Adult tapeworms develop in the small intestine of the definitive host, and the larval metacestodes form bladder like hydatid cysts in the intermediate host, usually in the internal organs.
Treatment
Definitive hosts can be treated with various anthelminthic drugs. Praziquantel, the most commonly used agent, is effective against both immature and adult tapeworms. There is relatively little experience in treating animal intermediate hosts, especially those infected with E. multilocularis or E. vogeli. Surgery is often the treatment of choice for cysts that can be completely resected. Longterm treatment with benzimidazoles (e.g., albendazole or mebendazole) may suppress some cysts and/or prolong the animal’s life in non-surgical cases. Drugs are also used as an adjunct to surgery, especially for E. multilocularis. Non-curative debulking of an E. multilocularis mass is no longer recommended in humans infected with this organism, and there was no evidence that it was helpful in a few dogs.
- There is no specific treatment except surgical removal of cyst. 2. Mebendazole and Praziquantel may be used.
Prevention
Intestinal carriage of Echinococcus can be prevented in cats, dogs and captive carnivores by not feeding them raw or undercooked animal tissues (e.g., sheep entrails) and by not allowing them to hunt or roam. Potentially infected tissues can be cooked or frozen before feeding to destroy the parasite. Echinococcosis in the intermediate host is controlled by reducing its exposure to parasite eggs. Periodic deworming of farm dogs is the cornerstone of control or eradication programs for E. granulosus s.s. in sheep. Dogs that might be infected with Echinococcus should not be allowed onto pastures where livestock may graze. Stray dogs have been culled in control programs in some areas, particularly where echinococcosis is an issue in humans. In some countries, foxes are treated with praziquantel in bait to decrease the incidence of E. multilocularis. Foxes have also been culled, but these programs are controversial, the effects on the fox population can be complex, and their effectiveness is debated. One zoo with E. multilocularis issues in nonhuman primates set up a program to safeguard the animals’ food by washing and steaming vegetables, sourcing vegetables to be fed raw from areas where this organism is not thought to occur, and storing all food indoors. Wood chips used in the exhibit are heat treated. A recombinant vaccine (EG95) for E. granulosus in sheep has been licensed in a few countries. It can reduce the risk of infection but is not useful against existing cysts. This vaccine may also be also effective in goats and cattle. 1. Preventing dogs from gaining excess raw offal at slaughter houses and on farms to bed animals. This involves control of slaughter houses, proper meat inspection and destruction of infected viscera. 2. Control of dogs: This involves elimination of stray dog, drastic reduction of dog population and effective dog registration system, surveillance of dogs based on periodic stool examination after administration of Arecholine hydrochloride followed by isolation and treatment of praziquantel. A single oral dose of 5 mg/kg body wt. will remove all adult worms from the dog. 3. Health education of the public: Particularly Butchers, Dog owners, Animal breeders and Shepherds are the basis of effective prevention.
EDITED BY-DR B. MUKHOPADHYA, KOLKATA
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