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Principles of control on Cestodes

The principles of control of tapeworms are to prevent livestock from becoming infected, and when infection has occurred, to limit the pathogenic effect. The strategies available to achieve these differ according to whether the ruminants act as the final host for the tapeworm or the intermediate host for the larval stages.

1. ?Ruminants as final hosts

1.1 ?Intestinal tapeworms

As infections with Moniezia, Thysaniezia and Avitellina species are considered virtually non-pathogenic, specific treatment for these parasites are not recommended. Furthermore, young animals which are not treated develop an immunity to reinfection, whereas regular treatment may interfere with the development of immunity and result in repeated re-infection. In the case of severe infections, it is advisable to modify the treatment and control strategies selected for nematodes to include the use of a broad spectrum anthelminthic that controls both tapeworms and roundworms. A number of the benzimidazoles, such as albendazole, cambendazole and mebendazole, effectively control tapeworms.

Control may also be directed at the intermediate host, the oribatid mites. It has been shown that the habitat of the mites can be destroyed by ploughing and where this is feasible, newly sown pastures can be kept free of tapeworms for several years.

1.2 ?Hepatic tapeworms

The apparent lack of pathogenic effects and the absence of diagnostic techniques render treatment of hepatic tapeworms unjustified.

2. ?Ruminants as intermediate hosts

2.1 ?Cysticercosis

With the availability of sensitive and specific serological tests, it is now possible to diagnose cysticercosis in living ruminants. However, at present it is not considered feasible to treat animals due to the high cost and the possible public health significance of dead, calcified cysts in meat and organs. Drugs which have shown efficacy against larval cestodes include praziquantel, mebendazole and albendazole. Control of cysticercosis and the adult tapeworms is therefore based on the prophylaxis of the infections.

To prevent infection of cattle, sheep and goats, all final hosts, namely man (Taenia saginata) and dogs (T. ovis, T. hydatigena) should be treated according to need. In addition humans should be educated in the use of high standards of personal hygiene and latrines to prevent the spread of T. saginata eggs.

Standardized methods of meat inspection should be implemented to detect infections at slaughter, and infected meat and organs should be condemned, treated (by cooking until grey) frozen to prevent infected meat reaching humans and dogs.

Considerable progress has been made in the development of vaccines against cestode larvae in ruminants, and a commercial vaccine is now available in Australia against Cysticercus ovis. It is anticipated that vaccines against other larval cestodes may become available in the future.

2.2 ?Coenurosis

Clinical diagnosis of coenurosis is difficult and clinical signs can be confused with other disorders of the central nervous system, such as hypocalcaemia, listeriosis, cerebral abscess, tumours and Oestrus ovis infection (in sheep).

No specific treatment is available for this infection and slaughter of the animal is usually recommended.

Prevention of the disease includes the treatment of dogs with taenicidal drugs and the education of farmers and butchers so that offal and condemned material are not fed to dogs after slaughtering a parasitized animal.

2.3 ?Hydatidosis/echinococcosis

Immunodiagnostic methods, radiology and ultrasound scanning are used routinely in diagnosing hydatidosis in humans, but these methods are rarely if ever applied in veterinary medicine.

Surgery to remove cysts is still the only effective treatment in human cases. Various drugs, such as mebendazole and praziquantel, are presently being tested; it appears that high doses are required, administered over a long period of time. Animals are not treated for hydatidosis.

Control of hydatidosis and Echinococcus granulosus is therefore based on prophylaxis of the infection in man, livestock and dogs.

???Public education programmes should convey the message that dogs infected with E. granulosus present a danger to the human population and their livestock. Farmers and other dog owners must be aware that uncooked viscera containing hydatid cysts should not be given to dogs, that dogs should be dewormed regularly, and that the handling of infected dogs may increase the risk of becoming infected.

???Several anthelminthics are available for the treatment of dogs but a number of these cause segments to disintegrate, allowing the eggs to remain viable and thus spread after being passed. If possible, faeces passed by dogs after treatment should be buried or burned. However, experience gained during several control programmes has shown that these eggs are of minor importance in the epidemiology of hydatidosis. An alternative treatment is to use the purgative drug, arecoline hydrobromide; this is administered following 12 hours of fasting. The parasites are expelled during the subsequent 6 hours, during which time the animal should be confined. The faeces should be destroyed.

???All abattoirs, slaughterhouses and slaughter slabs should rigorously prevent dogs from gaining access to the premises; all offal and condemned material containing hydatid cysts should be destroyed.

2.4 ?Regional/national hydatidosis control programmes

Several control programmes in different parts of the world have reduced transmission to the stage where infection of humans is rare. The best known are on the islands of New Zealand, Tasmania, Cyprus and the Falkland Islands. A successful programme has also been undertaken in two regions of Chile. These control programmes have all been based on effective cooperation between veterinarians, technicians and dog owners and have had a strong educational component to each programme.

The objective of control is to reduce the transmission of echinococcosis/hydatidosis from animals to humans. The objective of eradication is to eliminate the transmission between animal hosts. It is very important to assess the potential for eradication/control and it is advisable to exercise extreme caution before decisions are made regarding the preference for eradication over control. Among other factors the following should be considered:

(a) Socioeconomic importance (prevalence, severity of disability, risk of mortality).

(b) Epidemiological features of the infection

(c) Annual losses due to infections in livestock

(c) Availability of adequate operational and financial resources

(d) Feasibility of control

Based on evaluations of effective control programmes the following four phases can be distinguished:

(a) Planning. The planning phase includes (1) appointment of the appropriate control authority supported by the necessary legislation. The majority of successful programmes has been implemented by the Ministry of Agriculture or its equivalent. (2) Collection of data from which a benefit-cost analysis can be made and appropriate control strategies identified. Data should be collected on the size of rural dog populations, the incidence of hydatidosis in humans by age group, the reinfection rate of rural dogs and the number of veterinarians and technicians needed to treat and test every 10,000 rural dogs. (3) Development of a computer-based surveillance programme from which progress in control can be determined and cost-effective modifications can be made. (4) Selection and training of staff. (5) Securing of appropriate funding for the programme before entering the next phase.

(b) Attack. The attack phase is labour-intensive and therefore very costly. It involves the use of arecoline surveillance and/or 6-weekly dog dosing. The duration of this phase depends on the strategy in use but according to experiences gained takes at least 10 years. To secure success it is important that funding is identified for the whole period before the programme is started. This phase requires constant monitoring to determine when a transfer can safely be made to the next phase.

(c) Consolidation. The consolidation phase transfers activities from nondiscriminatory dog dosing to quarantine of infected farms (or high risk farms). This transfer is often accompanied by the introduction of penalties for keeping infected dogs.

(d) Maintenance of eradication. During this phase all special activities cease and the normal resources of the meat inspection services of the Ministry of Agriculture are used to prevent reintroduction.

?Note: Source of this article: from www.ilri.org

 
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