ABSTRACT: Parasiticides are pharmaceutical agents used to treat internal and external parasites. Each agent has its own indications and side effects. Anthelmintics must be selectively toxic to the parasite involved.

The suitably qualified persons (SQP) course, run by the Animal Medicines Training Regulatory Authority (AMTRA), refreshes those long-forgotten parasite life cycles and gives you the confidence to hand on advice to clients. There are three frequently asked questions.

Where do the worms come from?

Most infections are acquired by ingestion of microscopic eggs. This occurs when a pet licks areas where others have defecated, such as yards, parks or grass.

Many puppies are born with intestinal parasites (usually roundworms) that have been passed from the mother, where the parasite was in an encysted, quiet state.

Tapeworms are transmitted by an intermediate host when a cat or dog swallows a flea or eats a rabbit.

Are worms any risk to me and my children?

It should be emphasised that some parasites – particularly roundworms and hookworms – can also affect humans, especially children. For that reason, it is essential to prevent intestinal parasites in our pets and to treat any resultant infection.

People are infected by swallowing embryonated eggs from the environment (children playing in soil, for instance) and the larvae hatch out and attempt to migrate but cannot complete their life cycle. These migrating larvae may cause serious disease (visceral larva migrans) particularly in children, with symptoms including fever, pneumonia and, sometimes, encephalitis.

The larvae can also invade the eye (ocular larva migrans) causing potentially severe damage such as a detached retina and blindness. However, to put this in proportion, there are about 12 cases annually in the UK.

How serious is it if my pet has worms?

Parasitic diseases range from trivial to fatal. Parasites can cause severe disease in immature kittens and puppies, sick or debilitated pets, or in pets with a suppressed immune system. Younger pets often develop acute disease (vomiting, diarrhoea, dehydration and anaemia) whereas older pets show more chronic signs, such as intermittent diarrhoea.

Preventive approach

While it has long been recognised that transplacental and transmammary infection of ascarids and hookworms could be prevented through prophylactic treatment of pregnant dogs, few medicines are currently approved for this use. However, the effectiveness of this approach in dogs has been well documented.

Daily treatment of pregnant dogs with fenbendazole from the 40th day of gestation through the 14th day of lactation has been shown to inhibit Toxocara canis larvae in tissues, thereby preventing or greatly reducing the incidence of infection in puppies.

If the mother did not receive prophylactic treatment, puppies and kittens must be treated early and repeatedly in order to prevent patent infections. Nursing dogs and queens should be treated concurrently with their offspring because they often develop patent infections along with their young.

Because most puppies and kittens are not routinely brought to a veterinarian before six to eight weeks of age, they will already have patent infections and be actively contaminating the environment. For this reason, it is important to reach out to clients who have pregnant or newly born animals at home, and provide these animals with early prophylactic treatment for intestinal parasites.

There is a variety of anthelmintic drugs available that are safe and effective against ascarids, hookworms, and other intestinal helminths of dogs and cats. For animals that live in areas where heartworm (Dirofilaria immitis) infection is enzootic, many of the heartworm preventives are also effective against intestinal parasites.

Among the important gastrointestinal parasites of cats are roundworms (Toxocara species), hookworms (Ancylostoma tubaeforme, Ancylostoma braziliense and Uncinaria stenocephala), stomach worms (Physaloptera spp.), tapeworms (Dipylidium caninum, Taenia taeniaeformis) and microscopic parasites Coccidia, Giardia and Strongyloides species.

All the products that are authorised for use in the UK have been proven safe and efficacious when administered correctly (Figure 1). Other worming preparations are available from non-veterinary suppliers but they may be less safe and effective than the products we sell.

Figure 1: Selection of medications

Avermectins and milbemycin

Avermectins and milbemycins are active against both internal and external parasites and are therefore called endectocides.


Remember that doses of ivermectin necessary to kill parasites are toxic to certain genetic lines of collies. At high doses, toxicity shows as ataxia, tremors, mydriasis, depression, coma and death.

Dog breeds sensitive to ivermectin include collies, Australian Shepherd Dogs and both Old English and Shetland Sheepdogs. This sensitivity does not necessarily occur in every individual, but in about half of them owing to a recessive autosomal gene.

Mibemycin oxime (Milbemax, Program – Novartis)

This active ingredient may be used once weekly for three weeks to control sarcoptic mange, even in sensitive dogs. It is also effective against Cheyletiella, nasal mite infestation and generalised demodicosis. Side effects include transient vomiting and neurological signs.

Moxidectin (Advocate – Bayer)

This agent is effective against sarcoptic mange and daily administration is active in cases of generalised demodicosis.

Selamectin (Stronghold, Revolution – Pfizer)

Selamectin is effective against Cheyletiella (Figure 2) and notoedric mange. It is absorbed percutaneously within a few hours and then stays in the blood stream, protecting against heartworm disease. It then passes into the gastrointestinal tract where it can kill certain intestinal parasites, and also via the sebaceous glands on to the hair and skin, thereby providing protection against fleas and certain mites and ticks.

Figure 2: Cheyletiella mite. (Image courtesy of S. Kanagasundaram)

Toxicity is low because mammals are less sensitive than parasites to this agent. Over 98 per cent of fleas on the pet are killed within 36 hours of application and between 98 and 100 per cent of Toxocara infections are resolved.


Pyrethrins are one of the most widely used insecticides in today's flea and tick products and have been used as insecticides for over 100 years. Pyrethrins are natural extracts made from flowers of chrysanthemum plants. These plants grow naturally in the Middle East, Europe, Japan, and most importantly, Kenya. There are six different pyrethrins: pyrethrin I and II, cinevin I and II, and jasmolin I and II. All six are found in flea and tick products, but generally, the label only reads ‘Pyrethrin’, regardless of which of the six types is actually pr

Pyrethrins are used to control ticks, fleas, lice (Figure 3), Cheyletiella mites and mosquitoes. They are mainly found in products applied directly on the pet. Household insecticidal products generally contain either pyrethrins, a combination of pyrethrin and permethrin, or pyrethrins plus a synergist. 

Figure 3: Louse and egg from guinea pig. (Image courtesy of S. Kanagasundaram)

Synergists are chemicals that enhance the performance of other drugs. The synergist most widely used with pyrethrins is piperonyl butoxide.

 All pyrethrins are easily hydrolysed and degraded by stomach acids, so toxicity following ingestion by pets is very low. Toxicities, although rare, do occur. A cat or dog with pyrethrin toxicosis generally will salivate, tremor, vomit and may have a seizure. Generally, signs of toxicosis will be gone after 24 hours.


Pyrethroids – for example Permethrin, Phenothrin and Etofenprox – are synthetic pyrethrin compounds. That means they are made in a laboratory and are not natural plant extracts. These may be slower in action than the natural pyrethrins, but have a longer effect.

Because permethrins last longer than natural pyrethrins, they are commonly found in premise sprays and in products intended for slower, but sustained action. Permethrins are soluble in oils, but not in water. This is used to advantage in products that use an oil carrier to enhance distribution of the pyrethroid over the animal’s body and prolong its activity.

Permethrin should not be used on kittens or cats. A pyrethroid that is often used in the environment is fenvalerate. 

Pyrethroids are less easily broken down than pyrethrin, so this makes their toxic risk, though low, higher than that of pyrethrin. As with any pesticide, some animals may show a temporary sensitivity where the product is applied. A patient with pyrethroid toxicity will salivate, tremor, vomit, and may develop a seizure. The Veterinary Poisons Unit receives up to 50 enquiries a year regarding toxicity, 65 per cent of which are related to cats. The usual cause is over-zealous use of the product.

Organophosphates (and organocarbamates)

Organophosphates and organocarbamates are another class of medicines that have had wide usage as insecticides both in agricultural settings and for pet animals. Organophosphates should not be used on cats.

Organophosphates and organocarbamates are the class of insecticides most likely to cause toxic reactions in pets. If toxic levels are applied to the pet – or the pet ingests them – it will exhibit neurological signs, showing excessive salivation, tremors, staggering and maybe seizure. An antidote is available to counteract its effect.

Greyhounds and Whippets can be overly sensitive to organophosphates, so use a different product in these breeds.

Imidacloprid and moxidectin

The combination of imidacloprid and moxidectin (Advocate – Bayer) like most adulticides, kills fleas in 24 to 48 hours. It is also licensed to treat environmental life stages (Figure 4), Sarcoptes mites, Demodex mites (Figure 5), heartworms, Otodectes (ear) mites, ascarids (roundworms), hookworms (Ancylostoma and Uncinaria) and whipworms.

Figure 4: Flea larva

Figure 5: Demodex mite. (Image courtesy of Mark Craig)

It doesn’t treat tapeworms and is not a complete worming product.


Fenbendazole (Panacur – Intervet/Schering-Plough) is a popular wormer belonging to the benzimidazole family of safe, broad spectrum anthelminitics which also includes albendazole and mebendazole. The reason for its high safety factor is that it is insoluble in water which means it is poorly absorbed from the gut.

Peak plasma levels take six to 30 hours after dosing and are never greater than one per cent of the dose administered regardless of the formulation – paste, suspension, granules or bolus. Adverse effects are rare and attempts to cause poisoning in mice have been unsuccessful because they can tolerate the maximum quantities that can be physiologically given.

Febantel-pyrantel embonate-praziquantel

This combination is marketed as Drontal Plus by Bayer. Febantel is converted to fenbendazole in the gastrointestinal tract and its effects are seen in the larvae, in eggs, as well as in the different stages of the helminths.

Benzimidazoles possess a wide safety margin, pyrantel is not absorbed systemically to any extent and praziquantel has a wide safety margin, – equivalent to a plus-five times overdose. Trials on weaned puppies have found an efficacy of 83 per cent, rising to between 94 and 98 per cent, if repeated three times.


The pharmaceutical company, Novartis, formulates milbemycin in combination with praziquantel to produce Milbemax and with lufenuron to make Program Plus. The combination has been shown to significantly reduce Toxocara egg shedding and is well tolerated even by young animals. Results of one trial showed an efficacy of 97 per cent against Ancyclostoma spp.


Nitroscanate is highly effective in a single dose against common canine cestodes and nematodes. However, it is irritant and tablets should not be crushed, broken or divided. The product must not be given to sick or convalescing animals and it should not be used in cats. This agent is found in Johnson’s wormers.

Pyrantel (Drontal Cat, Bayer)

Pyrantel (Drontal Cat – Bayer) is a broad spectrum anthelmintic and is well absorbed after oral administration. The drug is free of adverse effects in all hosts at doses of up to seven times the therapeutic dose; but high level resistance has been found in the hookworm, Ancyclostoma, against which parasite the drug was only 25 per cent effective.


Very young pups (two weeks old) can be treated safely with piperazine and there is a wide margin of safety. Large oral doses produce vomiting, diarrhoea, inco   ordination and head pressing in dogs and cats. The drug is effective against ascarids (Toxocara canis, T. cati, and Toxascaris leonina) eliminating 82 per cent of the worms in one study of weaned pups. It is found in several over-the-counter (OTC) preparations, such as Sherley’s cat wormers and Johnson’s wormers.

Reducing resistance to nematocides

Antibiotic resistance is a topic with which you will be familiar, but have you considered the growing problem of resistance to parasiticides? Your choices and advice can make a difference.

   Resistance appears when drugs are used intensively against parasites which have a g
ood ability to survive in the host and where the host develops little acquired immunity

   Nematodes develop resistance to a mode of activity rather than a specific compound and so the entire family of drugs is at risk

   Once use of the anthelmintic class stops, reversion back to sensitivity is slow

   Drugs should be rotated and the frequency of treatment considered. Optimal impact from a minimum number of treatments is the aim, timing them to disrupt key events in the annual cycle of the parasite.

It should be clear to see that the most efficacious products are those which are restricted to sale by vets, pharmacists and SQPs. So next time you slip into automatic pilot when selling parasite control, stop and think. Is this the right product for this animal and what advice should you be giving to accompany the sale to ensure effective and safe use of the product?



Amanda graduated from Liverpool University in 1998 and has since worked as a veterinary surgeon in Cornwall, first in mixed practice and later in critical care and emergency work. She enjoys writing for the veterinary press and has contributed to several nursing textbooks.

Further reading

DUWEL, D., STRASSER, H. (1978) Birth of helminth- free canine pups through maternal fenbendazole therapy. Dtsch Tierarztl Wochenschr 85(6): 239-241. HSU, W. H. J. (1980) Toxicity and drug interactions of levamisole. Am Vet Med Assoc (1980) 1 76: 1 166-1 169. JACOBS, D. E. J, (1987) Control of Toxocara cams ir puppies. Vet Pharmacol Ther 10(1): 23-29.

PAYNE, P. A., RIDLEY, R. K. (1999) Strategic use of ivermectin during pregnancy to control Toxocara cams in greyhound puppies. Vet Parasitol 85(4):305- 312.

PAYNE-JOHNSON, M., MAITLAND, T. P., SHERINGTON, J., SHANKS, D. J., CEMENTS, P. J., MURPHY, M. G., MCLOUGHLIN, A., JERNIGAN, A. D. and ROWAN, T. G. (2000) Efficacy of selamectin administered topically to pregnant and lactatinc female dogs in the treatment and prevention of adult roundworm [Toxocara cams] infections and flea [Ctenocephahdes fels fels) infestations in the dams and their pups. Vet Parasitol 91(3-4): 347-358.

STOYE, M. (1992) Biology, pathogenicity, diagnosis and control of Ancylostoma camnum. Dtsch Tierarztl Wochenschr 99(8): 31 5-321.

REINEMEYER, C. R., FAULKNER, C. T., ASSADI-RAD, A. M., BURR, J. H. and PATTON, S. (1995) Comparison of the efficacies of three heartworm preventatives against experimentally induced infections with Anycylostoma camnum and Toxocara cams in pups. J Am Vet Med Assoc 206(1 1): 1 710-1715 ROCK, A. Veterinary pharmacology – A guide for the Veterinary Nurse.

SCHENKER, R., CODY, R., STREHLAU, G., ALEXANDER, D. and JUNQUERA, P. (2006) Comparative effects of Milbemycin oxime-based and febantel-pyrantel embonate- based anthelmintic tablets on Toxocara cams egg shedding in naturally infected pups. Vet Parasitology 137(3-4): 369-373.

• VOL 25 • No8 • August 2010 • Veterinary Nursing Journal