Internal Parasites

parasiteascaridadult.jpgASCARIDS

These worms are a zoonotic risk to people.

These worms, commonly referred to as roundworms, are among the largest and most familiar of nematode parasites infecting domestic animals. Toxocara cati and Toxascaris leonina infect cats and Toxocara canis infects dogs. These parasites are contracted directly from an animals mother via placental migration or from the milk, or by ingesting parasite eggs. The eggs of these worms are very hardy and can remain in the environment for long periods of time. Once into the host animal these parasites migrate through the body before finally coming to rest in the small intestine. This migration takes from 1-2 months depending on the worm involved. If T. canis eggs are eaten and hatch in the host stomach they then migrate through the gut wall, enter the portal blood stream and wander about in the liver parenchyma for a while. They then enter the caudal vena cava and are transported to the heart and lungs. When they reach the lungs they break out of the capillaries and enter the alveoli. At this point they molt into a third stage larva and ascend the bronchial tree and trachea to the pharynx. They are then swallowed, go through two more molts and end up in the small intestine. This pattern of migration is termed tracheal migration. However if the larvae fail to break into the lung alveoli they return to the heart via the pulmonary veins and are scattered throughout the body by the systemic circulation. This is called somatic migration. This differentiation of route is very important. In young dogs most larvae will follow the tracheal route and end up in the intestine. In adult dogs most will end up in the tissues as arrested second stage larvae. If the egg is ingested by an inappropriate host such as a human the larvae will also end up in the host tissue. If a small animal such as a mouse eats the eggs it ends up as a larval stage in the mouse tissue. Then if that mouse is eaten by a dog the encysted larval forms develop in the dogs intestine. This is referred to as mucosal migration. Migration and encystment of second stage larvae in inappropriate (paratenic) hosts and transmission via predation is also seen in T.cati and Toxascaris leonina. The encysted second stage larvae in the host tissues are hormonally triggered in the third trimester of pregnancy to migrate to the mammary tissue and uterus where they are passed to the unborn pups. These parasites are mature by the time pups are three weeks of age. Nursing bitches can be infected with immature forms shed in the feces of their pups. Thus it can be seen that ascarids are very well developed parasites with several modes of transmission! Although there are minor differences, T.cati and Toxascaris leonina behave in a similar fashion.

Ascarid Life Cycle

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Clinical Signs

Parasites depend upon their host for survival. Thus it is in the parasites best interest not to kill its host. However large infestations of ascarid worms can cause significant problems. Large numbers can cause obstruction of the small intestine. They can migrate into and obstruct the bile duct. The tracheal migration route can result in damage to the liver and/or lungs. An owner may notice a general unthriftiness and poor quality hair coat. With heavy infestations worms may be vomited or passed in the stool. In the majority of pets with ascarid infections no outward signs are apparent. In these cases detection and diagnosis requires a microscopic examination of the feces. Fecal material is mixed with a concentrated salt solution. The worm eggs float to the surface of the solution and can be collected for microscopic examination. The appearance of these eggs is specific to the type of worm carried by the host. Once eggs are identified appropriate treatment can be initiated.

Visceral Larva Migrans

Human Toxocarosis results from paratenic host second stage larvae somatic migration. In man the migrating larvae develop nodules which occur primarily in the liver, kidneys, brain and lungs. They can also occur in the eyes causing granulomatous retinitis. They are associated with a sustained eosinophilia and clinical signs based on organ involvement. Children are at the greatest risk of infection. The typical epidemiological situation involves a toddler soil contaminated with ascarid eggs. As mentioned previously these eggs are hardy and exist for a long time in soil. Soil in the vicinity of habitual canine defecation sites and public parks visited by large numbers of dogs tend to be highly contaminated. Although T.cati is less of a risk than T.canis, it has also been associated with Visceral Larva Migrans. Therefore it is advisable to keep toddlers from playing in areas where dogs and cats habitually defecate. For this reason dog and cat feces should never be used to fertilize vegetable gardens! The bagging and responsible disposal of dog and cat feces helps to lessen environmental contamination with ascarid worm eggs. The raccoon roundworm, Baylisascaris, can also be transmitted by dogs. Fortunately this form of transmission is rare. Baylisascaris tends to cause a neurotropic form of Larval Migrans with the nervous system being the target for granulomas. This can cause death when these parasites end up in the wrong host…such as humans. Raccoons tend to use defined areas or defecation, raccoon latrines. As the round worm eggs are very hardy in the environment any feces, especially in raccoon latrines, should be approached with extreme caution.

Treatment

Most dogs and cats will be infected with roundworms at some point in their lives. As pups and kittens worms are passed from their mothers and as adults they are picked up from eating feces, contaminated soil and via predation of infected small mammals. There are many safe and effective anthelmintic products available for treating roundworms. Pups and kittens can carry egg producing adult worms as early as three weeks of age. The mothers of nursing offspring are also likely to be shedding ascarid eggs at this time. In order to minimize environmental contamination testing, and if required, worming should begin at an early age and continue at regular intervals throughout a pets’ life.

Many breeders treat pups/kittens with Strongid-T (Pyrantel) or Piperazine products before they are placed in homes. These are relatively effective worming agents for ascarids but do not address other types of worms. However at WBVC we are finding that many young pets wormed with products continue to harbour ascarid worms. At WBVC we routinely use broader spectrum products for routine parasite control. These are relatively safe, easy and effective.  We also advise periodic fecal parasite examinations as not all internal parasites are controlled by one product. For pets at risk we are advocates of continual monthly parasite control for common internal and external parasites. Once a month use of an appropriate topical or oral medication can achieve this goal. The treatment/control of choice will depend on the individual pet and the geographical region. Ask your veterinarian for advise pertaining to your situation.

 

CESTODES

These worms are commonly referred to as tapeworms. Although there are many types of tapeworms, some of which can be a serious problem to both pets and man, those found in the Pacific Northwest generally cause little harm or inconvenience to dogs and cats. They are long, segmented worms that anchor themselves to the wall of the small intestine. They form segments (egg cases) that move down the bowel with feces. These segments, approximately 1/8 inch long and flat, come out with the feces and crawl about on the pets hair around the anus. They dry out and end up looking like a grain of brown rice. Once in the environment these desiccated egg cases crack open dispersing minute tapeworm eggs. The eggs are ingested by intermediate hosts (fleas and small mammals) and the lifecycle repeats itself. Some tapeworms can complete their lifecycle in less than two weeks! Control requires treatment of the pet, elimination of infected fleas and restriction of hunting activity. At WBVC we generally treat adult tapeworm infestations with Drontal Plus or Milbemax plus flea control and client education.

           

 

Dipylidium caninum Life Cycle

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Taenia spp. Life Cycle

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TRICUROIDS.

Tricuris vulpis is a nematode commonly called whipworm or threadworm. The adults, fine curved worms, are found in the large intestine. The lifecycle is simple and direct. Eggs are passed in the stools and an infective first stage larva develops in about a month. This larval form does not hatch from the egg unless it is ingested by a dog. Egg laying adults can be found in the large intestine 10-12 weeeks after ingestion. As with ascarids, the eggs are quite resistant so that dogs living in infested environments tend to become reinfected. Whipworm infections are usually asymptomatic but heavy infections can cause recurrent diarrhea, often with mucus and flecks of blood. As with other intestinal parasites diagnosis depends on identification of eggs passed in the feces. Due to the hardiness of the eggs and the long prepatent period it is advisable to administer a second treatment one month after the first. As for ascarids, the broad spectrum  are effective products for treating whipworm.

        

 

ANCYCLOSTOMATOIDS.

Ancyclostoma caninum, commonly referred to as hookworm, is occasionally seen in our area. When it does appear it is usually in dogs imported from other areas. Adult worms occupy the small intestine and eggs are passed in the feces. Environmental contamination plays a major role in the transmission of hookworm. The eggs hatch outside the host and infective can enter the host via ingestion,direct penetration of the skin or, as with ascarids immature stages can be passed directly to pups. Clinical signs vary from subclinical to per acute depending on the number of parasites and the health of the host. Any dog showing signs of anaemia and contact pattern skin irritation should be checked foe hookworm infestation. Diagnosis is made by via fecal floatation and egg recognition. The environment can be treated using sodium borate or sodium hypochlorite. Animals infected with hookworm can be safely and effectively treated with appropriate broad spectrum worming agents. Severely debilitated dogs will need to additional supportive care as required.

 

GIARDIA.

As with other protozoans Giardia is a single celled organism. It is a tear drop shaped cell that is adapted for attachment to the mucous epithelial cells of the small intestine. This trophozoite phase is occasionally found in the feces of infected animals, but mature Giardia cysts are more commonly seen. Contaminated water is the most common source of infection. Clinically animals with Giardia vary in symptomology from aclinical to severe enteritis and weight loss. “Beaver Fever”, as it is commonly called, can also cause enteritis in man. This protozoan is rarely found with simple fecal floatation techniques. Direct wet smear examination of fresh feces or a fecal concentration technique yield the best results. Effective treatment can be accomplished with a course of metronidazole. Historically there was a vaccine to help prevent infection with Giardia. However this was found to be less effective than hoped and is no longer available.

 

COCCIDIA.

There are many species of coccidia all of which are quite host specific. Isosporan coccidia such as I.felis have a relatively simple life cycle. Infective oocyts are shed in the feces. These can be ingested by the cat or a paratenic host such as a mouse. When the cat eats the mouse it becomes infected with infected cysts from the mouse tissue. The immature forms invade the host gut cells where the asexual phase of the life cycle takes place. It is followed by the sexual phase resulting in oocyte formation and the destruction of host cells. Toxoplasma gondii has a more complex life cycle. Various stages of this parasite are capable of infecting both the definitive and intermediate hosts. They can cause extra-intestinal as well as intestinal infection in the definitive host. In addition it can passed transplacentally to the unborn. Diagnosis is via fecal floatation and oocyst identification. These organisms can be carried asymptomatically but they can also be associated with diarrhea. On rare occasions rapidly acquired, heavy infestations can be fatal before any oocyts have been shed in the feces. Coccidial infections respond well to treatment with sulpha based antibiotics.

Rabbits can be troubled with coccidiosis.  In rabbits there are intestinal and hepatic (liver) forms of coccidial parasitism. Eimeria species affect rabbits. E. stiedae causes hepatic coccidiosis. Hepatic coccidiosis is not uncommon in wild populations however most commercial rabbitries are free this parasite. Pet rabbits presenting with this condition usually are, or have been, housed in high density non commercial situations in the presence of endemic E.steidae. The intestinal form can be caused by various Eimeria species including E. intestinalis and flavescens (the most pathogenic), E. irresidua, magna, and media (which are moderately pathogenic) and E. perforans and neoleporis (which are less pathologic). Other species have also been reported. Treatment is as for dogs and cats and attention to clean housing and husbandry is essential.

Human Toxoplasmosis: T.gondii is capable of extensive parasitism of tissues outside the gut. Following acute infection, cysts persist for long periods of time in many body tissues. Humans can contract T.gondii from infected cat feces or from ingesting uncooked meat containing toxoplama cysts. If pregnant women become infected it is possible for the fetus to be infected transplacentally.

 

INTRAVASCULAR PARASITES

 

DIROFILARIA IMMITIS.

Commonly termed heartworm, is a major concern in many areas of the world including the central interior of British Columbia; the Okanagan. Although the Lower Mainland area has all the necessary factors to support endemic heartworm populations it does not do so at this time. Canines are the species most at risk however heartworm incidence in cats is increasing and it has been found in man. Mosquitoes are the intermediate host (vector) in the transmission of this disease. Thus climate and environmental factors must be favourable to mosquitoes for heartworm to be endemic. It is not a problem in areas where mosquitoes do not thrive. Large adult worms reside in the right heart, pulmonary arteries and caval veins. They release microscopic microfilaria into the circulating blood. When the animal is bitten by a mosquito microfilaria are ingested along with the blood mea. In 10-30 days infective third stage larvae enter the mosquito’s salivary glands. When it takes its next meal these tiny larvae enter the animal. The tiny larvae pass through the second and third larval stages in the hosts connective tissue. In about four months, after the final molt, immature adults migrate to the right heart via the venous circulation. Once in the heart the worms mature and produce microfilaria.

Heartworm Life Cycle

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The presence of large adult worms cause physical obstruction plus pathological changes in the pulmonary arterial tree. Infected animals fatigue easily, cough and appear unthrifty. Diagnosis is done by testing a small blood sample for antigen detection. At WBVC we use the SNAheartworm.jpgP test for detecting adult infection. This is more accurate than testing for microfilaria as some animals can have adults without circulating microfilaria; occult heartworm infection. Even with the most up to date testing methods an animal can be carrying early stages of the disease for six months before detection is possible! Infected animals can be treated, in hospital, with injections of Caparsolate or Immiticide. This is followed up with elimination of microfilaria using oral Interceptor. This procedure is not without risk, time consuming and costly. Prevention is the preferred route of action. At WBVC we advise routine testing and the use of heartworm prevention products such as Revolution during periods of risk. As these products also help control roundworms, whipworms and hookworms many people use this once a month prevention throughout the year .Revolution and Triflexis (Sentinel is not currently available) also control fleas, mites and lice). Milbemax can also be used for internal parasite control in cats.


5383 West Boulevard,
Vancouver, B.C.
Canada, V6M 3W4
westboulevardvet@gmail.com

Phone: 604-266-7421
Fax: 604-263-6740

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