Treatments for parasite control

A guide to what’s available to control parasites in livestock this season.Prof Mike Taylor, a veterinary parasitologist at the Central Science Laboratory, York, outlines the choices

ANTHELMINTICS (WORMERS) are used widely to treat and prevent worm infections – roundworms, tapeworms and flukes – in domestic livestock. Most products are related by their chemical structures and modes of action and fall into three main broad-spectrum groups – the benzimidazoles (white drenches) (Group 1-BZ), the imidazothiazoles and tetrahydropyrimidines (Group 2-LM) and the macrocyclic lactones (Group 3-AV) that include the avermectins and milbemycins.

Anthelmintics belonging to these groups are active against the major species of gut roundworms and lungworms. Some will also have activity against liver fluke and tapeworms. Macrocyclic lactone compounds (Group 3-AV) often referred to as endectocides, also have activity against some ectoparasites.

Other products are more specific in the parasites they kill and are referred to as narrow spectrum drugs. Most anthelmintics in this category are active against liver fluke. Some are also active against tapeworms or certain ectoparasite species.

Formulating a broad-spectrum anthelmintic with a narrow spectrum flukicide can further increase the range of activity of an anthelmintic product. Such combinations constitute the so-called fluke and worm drenches.


Broad-spectrum wormers have until recently been highly effective against adult and developing larval stages of the common gastrointestinal nematodes causing PGE and, in general, there was little to choose, in terms of efficacy, between any of the available worming products for treating sheep during the grazing season. The situation is changing with the advent of anthelmintic resistance.

Benzimidazole anthelmintics (BZ) are effective against BZ-sensitive nematodes and are ovicidal. Most are effective against tapeworms. The BZs possess high activity against the adult and immature larvae of Nematodirus battus. Some BZ anthelmintics at increased dose rates (albendazole, ricobendazole) are also active against liver fluke. Triclabendazole is narrow spectrum (liver fluke only) and differs from all the other BZs in many respects but is classed with them because of its chemical structure.

Levamisole and morantel are effective against gastrointestinal nematodes and Nematodirus battus, but have no activity against tapeworms or fluke.

Macrocylic lactones (ML) are available for sheep either as injectable formulations (ivermectin, moxidectin, doramectin), or oral drenches (ivermectin, moxidectin). All injectable MLs and oral moxidectin have persistent activity against some, but not all, worm species and can be used at extended treatment intervals in strategic dosing strategies, although unlike in cattle, the treatment intervals have not been clearly defined. Only one product has a licensed claim for persistent activity in sheep; moxidectin will prevent re-infection with ML-susceptible Teladorsagia spp and Haemonchus for five weeks. The period of protection from re-infection with other genera is much shorter. The MLs have variable activity against N battus although doramectin at increased dose rate is active against L4 larvae of this species. Moxidectin (oral or injectable) has no persistent activity against N battus. The MLs are also active against sucking lice (Linognathus spp), nasal bot flies (Oestrus) and mange mites (Psoroptes, Sarcoptes, Chorioptes). There is little or no activity against chewing lice (Bovicola ovis), ticks or keds.

Dormant (arrested) fourth-stage larvae of abomasal roundworms of sheep are susceptible to most of anthelmintics now available and can be used for treating and controlling winter scours in sheep. Anthelmintics that lack activity against arrested larvae may result in the need for repeated treatment.

Outbreaks of coughing associated with infection by the lungworm Dictyocaulus filaria may be seen during autumn. All available products have activity against the adults of this parasite.

Possible presence of resistant roundworms should always be considered when choosing an effective product. Wormer efficacy can be checked by use of a number of techniques and wherever possible appropriate advice should be sought. More details on anthelmintic resistance and measures to avoid its development are available in the form of advisory leaflets from SCOPS “Sustainable Control Of Parasites in Sheep” available from vets, product suppliers or by visiting

Tapeworm infections

Despite their size and the fact segments are often seen in infected animals’ faeces, there is little convincing evidence that tapeworms reduce performance. Benzimidazole wormers are generally effective in controlling tapeworm infections. Praziquantel has been recently introduced as a specific treatment for tapeworm infections.


Oxyclozanide, nitroxynil, closantel, triclabendazole and albendazole/ricobendazole – at increased dose rates, are all effective against adult flukes present in the bile ducts, making them suitable for treating chronic fascioliosis in late winter and spring. Disease seen during autumn and winter is commonly associated with mixed infections of adult and immature flukes and is best treated with triclabendazole, closantel or nitroxynil because of their higher activities against immature flukes. In cases of acute disease, due to large numbers of immature fluke, triclabendazole is the drug of choice. As with roundworms, possible presence of resistant fluke should also be considered when choosing an effective product.

Combined treatments

 A number of products are marketed for combined treatment of gastrointestinal roundworms and liver fluke, or roundworm and tapeworm infections in sheep. Combinations available include oxfendazole/oxyclozanide, levamisole/oxyclozanide, mebendazole/closantel, levamisole/triclabendazole for controlling worms and fluke; and levamisole/praziquantel for roundworm and tapeworm control. The rationale for using these combination products should be carefully assessed.


All available anthelmintics are highly effective against adult and developing larval stages of the common gastrointestinal roundworms. In general, there is little to choose, in terms of efficacy, between any of the anthelmintics currently available for treating parasitic gastroenteritis in calves during the summer. Injectable and pour-on endectocides have persistent anthelmintic activity and can be used in control strategies based on treatment at strategic intervals post turn-out.

When selecting an anthelmintic to treat animals on housing in autumn to prevent disease in late winter and early spring of the following year, it is advisable to use a product with high activity against arrested fourth stage larvae. Using an anthelmintic lacking such activity may result in the need for repeated treatment when arrested larvae resume development. Left untreated, these larvae can cause damage to the stomach wall, causing severe diarrhoea and even death. Evidence suggests that at manufacturers recommended dose rates, most products, with the exception of those containing levamisole, are active against arrested fourth stage larvae of Ostertagia ostertagi – the cause of bovine ostertagiosis.

With the exception of morantel, most anthelmintics are highly effective against developing fourth stage larvae and adult lungworms (Dictyocaulus viviparus). A live, attenuated, and highly effective oral vaccine for use in the prevention of lungworm in calves has been available since the 1960s.


Bolus devices are a popular labour-saving means of administering anthelmintics to cattle. A range of shapes and designs exist, each being administered by specially provided dosing guns.

Care is required when administering boluses and it is important to ensure the gun is inserted over the back of the tongue, depressing the plunger to eject the bolus as the animal begins to swallow. Devices fall into two categories either sustained-release where anthelmintic is released constantly over a period of time, or pulse-release where drug is released at intervals.

Sustained-release devices deliver anthelmintic over 90-140 days, depending on the product. Pulse-release devices consist of five or seven annular tablets of anthelmintic, mounted on a central metal core with a weighted end. The release of each annular tablet is determined by the corrosion rate of the central core. The first dose is released about three weeks after administration with the remaining doses released at regular intervals, of about 21 days, thereafter. This gives such boluses active lives of 105 or 147 days, depending on the number of tablets and release profile.

Boluses are designed for use primarily in first year grazing animals at turnout, but they can be used in cattle already at grass, or in animals in their second grazing season – see product labels or consult a vet for advice.


Oxyclozanide, nitroxynil, clorsulon, triclabendazole and albendazole/ricobendazole – at increased dose rates, all have activity against adult liver flukes and are, therefore, suitable for treating chronic fascioliosis in late winter and early spring. When treating cattle in late autumn and winter, a time when infections will consist of both adult and immature flukes, triclabendazole is the drug of choice – because it possesses activity against immature flukes.

Combined treatments

As with sheep, a number of products are marketed for combined treatment of gastrointestinal roundworms and liver fluke in cattle. Combinations available consist of oxfendazole/oxyclozanide, levamisole/oxyclozanide, ivermectin/clorsulon and levamisole/triclabendazole.

Where animals are treated in autumn and winter for both fluke and worms, the anthelmintics employed should ideally have activity against arrested fourth stage larvae and immature fluke, in addition to adult gastrointestinal roundworms and liver flukes. It will be seen that while some of these products may have activity against arrested fourth stage larvae and some have activity against immature flukes, none of them has both. Again, the rationale for using these products should be carefully assessed.

Milk Withdrawal Periods

On occasions, it may be necessary to treat dairy cattle with an anthelmintic – such as for fluke or lungworm. Wherever possible, treatments should be given during the dry period. When treatment has to be given during lactation it is important to observe milk withdrawal periods. Many of the macrocyclic lactones, with the exception of eprinomectin, and fluke products containing oxyclozanide or nitroxynil, should not be used in cows producing milk for human consumption and in some cases for a specified period prior to calving. It is important to read product labels or seek advice.


All available wormers are highly effective against adult and developing larvae of gastrointestinal roundworms parasitising pigs. All are active against adult and developing larvae of lungworm (Metastrongylus species), although fenbendazole should be given in divided doses at seven days apart.

Fenbendazole, flubendazole are generally more effective against whipworm infections (Truchuris). Ivermectin and doramectin are also active against lice and sarcoptic mange mites.


Anticoccidials are used to treat coccidiosis, a disease caused by protozoan parasites – “coccidia”. All ages of animals are susceptible to coccidial infection, but younger neonatal animals are more susceptible to disease, particularly when intensively reared.

Unlike anthelmintics, only a limited number of anticoccidial drugs to treat and prevent coccidia exist. Clinically affected animals can be treated with either a sulphonamide antibiotic, or diclazuril (Vecoxan). Severely affected animals may also require fluid therapy and it is advisable to consult your vet.

Only two products are currently licensed for the prevention of coccidiosis. Decoquinate (Deccox) is administered as a creep feed for a continuous 28-day period, usually from three to four weeks of age. Deccox is currently marketed by Forum Products and has a one-day meat withdrawal period for calves and lambs. Diclazuril (Vecoxan), an oral drench for lambs, is usually given as a single treatment at 4-6 weeks of age, but a second treatment may be given three weeks later if considered necessary. Vecoxan has no meat withdrawal period and is marketed by Janssen AH.

Cryptosporidiosis is a disease caused by the coccidial genus, Cryptosporidium and occurs in many animals, including cattle, sheep and pigs and has the potential to infect humans. Disease is usually seen in young animals or those with a reduced immunity. Halofuginone (Halocur) is now available from Intervet for the prevention of diarrhoea in newborn calves caused by Cryptosporidium parvum and also for reducing the severity of clinical disease. It has a 13-day meat withdrawal period.

Toxoplasmosis is an infection caused by Toxoplasma gondii with a wide host range including all domestic animals, birds and man. Infection in sheep can cause abortion, stillbirth or the birth of weak lambs. Decoquinate (Deccox) can be given to ewes in mid-pregnancy to prevent abortions and perinatal losses. A live vaccine is also available for immunising sheep against the disease.


As with anthelmintics, there is a wide choice of ectoparasiticides available with varying activity against the ectoparasites described. These can be divided into groups based on chemical structure and mode of action.

Ectoparasiticides act either systemically – carried in the bloodstream – or topically, by direct contact with the target organisms. Systemic acting products may be given by injection or applied topically to the skin as pour-on or spot-on formulations, from which chemical compound is absorbed through the skin and is taken up into the blood. Most existing products act on the parasites’ nervous system.

Some groups, such as the organophosphates and synthetic pyrethroids, have broad-spectrum activity against ectoparasites. These chemicals are administered as either as dips, sprays, pour-ons, spot-ons or in ear tags.

Macrocyclic lactone compounds have activity against both roundworms and ectoparasites – including warbles in cattle and mange mites in sheep – sheep scab. Drugs within this family are available as injectables, pour-ons, or drenches.


The organophosphate, diazinon, and the pyrethoids are all active against lice. Pyrethroids have variable activity against headflies and other biting and nuisance flies. Diazinon, the pyrethroids, cypermethrin, deltamethrin, and the insect growth regulators, cyromazine and dicyclanil, are active against blowfly larvae. Ivermectin, doramectin, and moxidectin and closantel are active against the nasal bot fly.

Diazinon, flumethrin, high cis-cypermethrin (all by dip) and macrocyclic lactones (by injection) are active against sheep scab mites. With injectables, the number of doses required for the treatment or prevention of sheep scab varies with product used, so when in doubt it is advisable to check individual product labels or consult a vet.

 Diazinon, cypermethrin, deltamethrin, flumethrin are active against the sheep tick, Ixodes ricinus.


Pyrethoids – (alpha) cypermethrin, deltamethrin and permethrin, together with the macrocyclic lactones and amitraz are all active against lice. Their activities against different types of chewing or sucking lice vary. Correct parasite identification and treatment are therefore important.

Macrocyclic lactone compounds are active against warble fly larvae (warbles) and have variable activity against mange mites. Pyrethroids are active against hornflies and other biting and nuisance flies.


The macrocyclic lactones (ivermectin and doramectin) are active against lice and mites found on pigs.