T. leoni -- there is no tracheal involvement in the life cycle of the worm.
T. canis -- placental transfer from bitch to pup may occur (in utero transmission).
T. cati -- the larvæ may be transferred from the queen's milk to kittens.

Strongyloidiasis (Strongyloides stercoralis, S. tunefaciens -- dogs & cats, respectively)
The parasitic worms of the strongyloid spp. are all female.  They are intestinal parasites in dogs, cats, foxes, and man.  They may infect by the paratenic cycle or through epidermal penetration (producing a focal dermatitis).  NOTE that in man, strongyloidiasis is a chronic, debilitating disease and infection may readily occur from dogs.  Effect drugs include ivermectin, thiabendazole, diethylcarbamazine, and dithiazanine.

Cestodiasis (Tapeworms) -- Tapeworm infection in pets is generally not a problem, since most foods are cooked.  However, three types of infection may occur from other sources.

Dipylidium caninum -- infection in dogs by the intermediate host, fleas.
Tænia pisiformis -- infection in dogs that eat wild rabbits (the natural host)
Tænia tæniæformis -- infection in cats that eat rats (host for the larval stage)

Ancylostomiasis (Ancylostoma caninum, A. braziliense, Uncinaria stenocephala), hookworms
Hookworm infection may occur via oral, dermal, transmammary, or intrauterine routes.  The infection may transmit to man, where it causes cutaneous larval migrans (creeping eruption).  Drugs effective against these nematodes include dichlorvos, pyrantel, and nitroscanate (oral); butamisole and disophenol (intravenous).  Other drugs that are effective include dithiazanine, febantel, fenbendazole, flubendazole, and mebendazole.

Lungworms (Ælurostrongylus abstrusus, Paragonimus kellicotti, Capillaria ærophilia)
The first stage larvæ of lungworms are viable for up to 5 ½ months in most soil.  They may be taken up by mollusks (i.e. snails), where they are viable for up to 5 months (in live mollusks, 3 weeks in dead mollusks).  The food chain then proceeds through crawfish and amphibians to birds and reptiles.  Infection may then spread to dogs and cats.  In small animals, the worm reproduces in the air passages.  Worms may be coughed up, swallowed, and passed through fæces.  Infection is common in the Great Lakes, Midwest, and South.  Effective agents include levamisole, ivermectin, and the benzimidazoles.

Spirocercosis (Spirocerca lupi)
This helminthic infection is most common in dogs, but may also occur in cats, foxes, and jackals.  Transmission is from coprophagic beetles to dogs, via ingestion.  Once in the stomach, the helminth enters the gastric artery and proceeds back along the arterial tree to the thoracic aorta.  Prophylaxis (prevention of the animal eating the beetle) is the best therapy.  If infection does occur, levamisole, disophenol, and albendazole are effective in treatment.

Dirofilariasis (Dirofilaria immitis), heartworms
Heartworms most often infect dogs.  Infection occurs via bites from infected mosquitoes (incidence is highest during the mosquito season -- summer to early fall).

Clinical signs and symptoms include weight loss, extreme fatigue, chronic cough, dyspnœa, radiographic evidence of enlarged heart or more commonly pulmonary arteries, ECG abnormalities, and characteristic signs of congestive heart failure.  Cats may also be infected (the frequency of feline infection is directly correlated to the canine population in the area).  Signs and symptoms in cats include acute collapse, dyspnœa, convulsions, vomiting, diarrhœa, tachycardia, and syncope.

Positive diagnosis is most often made with ELISA tests for antibodies to the heartworm.  NOTE, however, that worms must reach maturity prior to a positive test (approximately 6 ½ months post infection).  Therefore, testing a pup less than six months old will not yield conclusive results.  ALSO NOTE that ivermectin will cause the sterility of the helminth, producing a false negative test.  If using that drug as prophylactic therapy, confirmation of heartworm infection must include an occult heartworm test.

Prophylactic therapy may be effective with diethylcarbamazine, ivermectin, and milbemycin.  Therapeutic treatment (when infection has already occurred) generally uses either thiacetarsamide or melarsomine.

Gastric nematode of cats (Ollulanus tricuspis) and Stomach worm (Physaloptera)
These infections cause chronic gastritis with persistent vomiting.  Effective drugs include fenbendazole and pyrantel.

Liver fluke (Platynosum concinuum), a trematode
This infection is most common in the Southern continental U.S.A. and Hawaii and most often infects stray and ranging feral cats.  Infection occurs from intermediates hosts such as snails, lizards, and toads.  Infection causes hepatic and portal fibrosis, biliary epithelial hyperplasia, chronic debilitation, and icterus.  Effective drugs include bithionol, praziquantel, albendazole, and niclosamide.
 

Dichlorvos (Task®) -- Dichlorvos inhibits the enzyme acetylcholinesterase, thus inhibiting the metabolism of the neurotransmitter acetylcholine.  This causes an excess of cholinergic activity in the worm, ultimately causing worm death.  It is used orally in the treatment of round and hook worm infection in dogs (doses range from 27-33 mg/Kg) and cats (11 mg/Kg).  Its use is contraindicated with constipation/fæcal impaction (due to a risk of rupture of the intestinal wall) and with liver dysfunction.  Dichlorvos is also used as a flea treatment in dogs and cats.
 

Pyrantel (Nemex®) and oxantel -- Used for rounds, hooks, and stomach (cats).  The dose is 5 mg/Kg for dogs and 10 mg/Kg for cats.  One dose is followed in three weeks by a second dose.

Diethylcarbamazine -- Used in round, lung, and heartworm (discussed below) infections.  Doses range from 55-110 mg/Kg for rounds in both dogs and cats, and 80 mg/Kg for lungworm b.i.d. for 3 days.

Levamisole (Tramisol®, Levasole®, and Ergamisol®) and butamisole -- These drugs are also immunostimulants (which may help the host immune response in killing the infection) and may also decrease carbohydrate metabolism within the helminth (thus reducing its energy supply).  Doses vary greatly, depending upon the age and weight of the animal and the specific helminth being targeted.

Praziquantel (Droncit®) -- This drug produces paralysis by enhancing calcium influx into the helminthic muscles.  It may also inhibit function of helminthic suckers and cause focal vacuolisation/disintegration of helminthic cellular membranes.  It is used primarily for tapes in doses ranging from 23 to 34 mg per treatment in cats.
 

Drugs that inhibit ATP formation -- Through inhibition of oxidative phosphorylation, these anthelminthics inhibit cellular ATP formation, thus depriving the cell of necessary forms of energy.
Nitroscanate -- used primarily for rounds and hooks.
Niclosamide (Nicloside®, Cestocid®, Fenesal®, Lintex®, Vermitid®, Yomesan®) -- flukes and tapes
Disophenol -- standard dose is 10 mg/Kg, subcutaneously
Quinacrine (Atabrine®) -- Also intercalates DNA, thus inhibiting helminthic cellular division.  Used primarily for giardiasis and trichomonal infections.  Doses are 6.6 mg q12h for 5 days for dogs and 11 mg/Kg s.i.d. for 5 days for cats.
 
Drugs that inhibit glucose uptake -- These agents inhibit glucose uptake from dietary sources, thus depriving the helminth of dietary energy sources.
Benzimidazoles -- These drugs inhibit microtubular polymerisation, to decrease glucose transport from the alimentary canal to the systemic circulation of the helminth.  They may also inhibit oxidative phosphorylation, thus inhibiting ATP formation.
 
Febantel (Vercom®) -- a prodrug that is converted in vivo to mebendazole -- used for hooks, rounds, whips, and tapes.  Doses are 15 mg/Kg daily for young dogs and 10 mg/Kg daily for adult dogs and cats for three days.

Fenbendazole (Panacur®) -- used for rounds, hooks, whips, and tapes -- 25-50 mg s.i.d. to b.i.d. for 3-14 days.

Thiabendazole (TBZ) -- 30-70 mg/Kg b.i.d. for up to 1 month (dogs)

Albendazole (Valbazen®, bovine) -- 25-50 mg/Kg b.i.d. for 5-21 days.

Others of the class -- mebendazole, oxfendazole, flubendazole
 

Bunamidine (Scoloban®) -- disrupts glucose uptake, exact mechanism is not known -- used for tapes at doses of 25-50 mg/Kg on an empty stomach.
 
Miscellaneous Anthelminthics
Piperazine -- An older anthelminthic, it is available over-the-counter in many co-ops and pharmacies.  It is thought to work through enhancing GABA neurotransmission, thus reducing neuronal activity by enhancing the inhibitory effect of GABA, thus causing worm paralysis.  A wide range of doses has been reported for piperazine.

Dithiazanine (Telmicid®) -- MOA is unknown.  Doses range from 6.6 to 11 mg/Kg daily for 7 to 10 days.
 

Drugs Used in the Treatment of Heartworm
Prophylactic Drugs -- These agents should be given at least one (1) month prior to the onset of the mosquito season and continue until one (1) month after the end of the season.  Depending upon the local, many veterinarians prefer to dose year-round, to minimise the risk of infection.
Ivermectin (Heartgard®) -- Enhances the release of the neurotransmitter GABA, thus enhancing GABA-mediated CNS depression in the helminth.  It does not readily penetrate the blood brain barrier of most dogs (NOTE the contraindication below), thus has minimal side effects in most animals.  It primarily acts as a microfilaricidal agent (more effective in preventing heartworm than in treating infection), but also possesses ecto- and endo-parasiticidal activity.  Dose for prophylaxis is 6 mcg/Kg/month.  If it is used in an attempt to treat active heartworm, the dose may range from 50-300 mcg/Kg, but note that the risk of adverse effects is greater.
 
Ivermectin is contraindicated for use in collies, since this breed exhibits a less well-developed blood brain barrier, and the drug may achieve toxic concentrations in the dog, producing profound CNS depression, leading to death.
 
Milbemycin (Interceptor®) -- Interferes with transmission of GABA (presumably enhancing it).  Used only for the prevention of heartworms at doses ranging from 0.5-1.0 mg/month.  It is also effective in treating hook and round worm infection.  Milbemycin may be used in collies.
 
Drugs used to treat active heartworm infections
Thiacetarsemide (Caparsolate®) -- an arsenical agent that is adulticidal in mature heartworms by causing acute arsenical poisoning in the worm.  It is available only for intravenous administration (2.2 mg/Kg b.i.d. for two days) and produces severe hepatotoxicity accompanied by vomiting, renal toxicity, and phlebitis with sloughing at sight of administration.  Caparsolate® should be refrigerated and should not be used if it has turned orange in colour or if a precipitate has formed.

Melarsomine (Immiticide®) -- Similar to thiacetarsemide, but not as toxic and available for intramuscular administration.  Dose is 2.5 mg/Kg once daily for two days.

Horse Bots -- Stomach infection of Gasterophilus, the larvæ of bot flies.  Produces mild gastritis.  Treated with trichlorofon, dichlorvos, and ivermectin

Habronema -- stomach worm, produces catarrhal gastritis and severe, tumour-like lesions.  Positive diagnosis often does not occur until post-mortem.  If detected, treatment is with ivermectin.

Pinworms -- these infections most often occur in the small intestine.  Eggs are laid around the anus and produce a crusty, yellow-white mass.  Most broad spectrum drugs, such as the benzimidazoles, are effective.

Parascaris -- Equine roundworms.  These infections are relatively common, producing what is often referred to as "summer cold", characterised by lethargy, fatigue, and occasional colic.  Foals are often infected on birth (the eggs are common in pastures, paddocks, and stalls).  Treatment is comprised of dosing at eight weeks old and every 6-8 weeks thereafter until yearlings with piperazine, thiabendazole, or any broad-spectrum anthelminthic.

Strongyles -- Blood, Palisade, or Red worms.  Infection occurs via ingestion.  The worms mature in the large intestine and migrate to the cranial, mesenteric artery, producing thrombosis and/or arteritis, anæmia, weakness, emaciation, and diarrhœa, which may proceed to colic or gangrenous enteritis.  Treatment most often comprises ivermectin for larval stages and oxbendazole or fenbendazole for any stage of maturity.  NOTE that treatment with organophosphate anthelminthics (trichlorfon or dichlorvos) may precipitate colic, especially if given with a phenothiazine-type tranquilizer.  Benzimidazole-resistant strains should be treated with piperazine, pyrantel, dichlorvos, or ivermectin.

Strongyloides -- The larvæ may be transmitted through milk.  Treatment is with thiabendazole, cambendazole, or ivermectin.

Tapes -- Three species of tape worms may infect horses, each specific for either the stomach, small intestine, or cæcum, producing gastrointestinal effects, unthriftiness, and anæmia.  Treatment is with pyrantel, niclosamide, or dichlorophen.

Trichostrongylus -- small stomach or hair worm.  Produces a chronic catarrhal gastritis with nodular areas of thickened mucosa with congestion and increased mucous secretion.  Treatment is with the benzimidazoles or ivermectin.
 

Stomach Worms -- Affecting primarily cattle include Hæmonchus (barber's pole, large stomach, or wire worms), Osteragia (medium or brown stomach worm), and Trichostrongylus (small stomach worm).  All may produce persistent watery diarrhœa alternating with constipation, anæmia, and lower jaw œdema (bottle jaw).  A newer drug used in the treatment of Osteragia infections is doramectin (Dectomax®).

Cooperia -- a small, red worm primarily affecting the small intestine of cattle, produces diarrhœa, anorexia, and emaciation.

Bunustomum -- Primarily in cattle, produces anæmia, weight loss, and diarrhœa alternating with constipation.

Strongyloides -- An intestinal threadworm of cattle, produces anæmia, weight loss, diarrhœa (containing large amounts of blood and mucous).

Chabertia -- An intestinal worm of sheep, produces colonic congestion, ulceration, hæmorrhage, soft fæces, and unthriftiness.

Liver Fluke -- Affects primarily sheep, the fluke enters by ingested herbage and is released in the duodenum, where it penetrates the intestinal wall and gains entry into the peritoneal cavity and enters the liver.  As the fluke grows, it destroys hepatic tissue.  Albendazole is more widely effective, however clorsulon is effective against immature flukes and is given by oral drench at a dose of 7 mg/Kg.
 

Specific Large Animal Anthelminthic Drugs
Many of the drugs used in large animals are the same as those used in small animals, or possess the same mechanism of action.  Agents used more in large animals include the following:

Trichlorfon -- an organophosphate insecticide (see dichlorvos, above)
Cambendezole -- a benzimidazole anthelminthic (see benzimidazoles above)
Dichlorophen -- uncouples oxidative phosphorylation (see above)
Morantel -- a depolarising neuromuscular blocker (see pyrantel above)
Doramectin -- an avermectin similar in action to ivermectin (see above)
Clorsulon -- inhibits the enzymes 3-phosphoglycerate kinase and phosphoglyceromutase, to inhibit the glycolytic pathways of the helminth, thus depriving the worm of its energy source.  It is used primarily in the treatment of ovine fluke infections.
 

Alternate Grazing of different species -- for instance, sheep and cattle may be alternated on different pastures.

Rotational Grazing of different ages -- for instance, older and younger cattle may be alternated on different pastures.

Alteration of land between grazing and cropping -- for instance, this year use it as land for crops, and next year use the same land as grazing land.

Advantages of incorporation -- provides a continuous, low level drug administration, decreases the risk of pasture contamination

Disadvantages of incorporation -- erratic consumption, subtherapeutic concentrations, tissue residues, drug resistance over time.

In some instances, prophylactic control treatment may also be used to prevent infection.  An example of this control treatment is the occurrence of Osteragia infection in the Northern U.S.A.  Infections typically occur in the Spring, with a huge increase of Osteragia in pastures in mid-July (as mature worms are passed through the fæces to the pasture).  Therefore, two or more anthelminthics may be administered every 3-5 weeks when the cattle first go to grass in the Spring, to prevent the initial infection.

Oftentimes, young cattle may require treatment post-weaning and these animals may have to be segregated during first grass grazing.  Dosage forms most commonly used in cattle include a continuous release bolus (for example morantel released for 60-90 days; oxfendazole, albendazole, or levamisole released over 21-30 days), paste, or bolus.
 

Sheep are especially susceptible to infection post-parturient, due to a relaxation of resistance that occurs with the birthing process.  Therefore, many owners opt to treat prophylactically for one month ante- and post-partum.  Sheep in general have a lower resistance to infection and clinical disease is more common in this species.  Frequent treatment is required, especially during the first year of life.  Unfortunately, sheep are also consistently more susceptible to adverse effects.  All of these factors contribute to the higher level of physical stress from which sheep seem to suffer.
 

Sows also exhibit periparturient relaxation of the immune system. Therefore, sows and gilts are often treated 10 days before breeding and 10 days prior to farrowing.  Boars are typically treated at six month intervals.  Additionally, weaners and feeders are treated before placement in clean pens.