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Internal and External Parasites

Internal and external parasites have the potential to cause a great deal of harm in both our pets and their human counterparts. This pamphlet will describe the common parasites encountered by our pets and preventing parasitic infections. Some of these parasites are zoonotic – they have the potential to infect humans. In general small children, the elderly and immunocompromised individuals are at the highest risk of contracting these parasites.


The most common intestinal parasites are: roundworms, hookworms, whipworms (dogs only), tapeworms, coccidia and Giardia. Most of these parasites, with the exception of whipworms, cause problems primarily in young animals; although animals at any age may be infected. The majority of these parasites can be identified on a fecal flotation but egg shedding is cyclic, therefore a negative fecal flotation is not a guarantee that the pet is parasite-free.

Roundworms: Pets are infected with roundworms through ingestion of eggs, ingestion of rodents/birds infected with roundworms, and from their mother in utero or through nursing. Roundworm eggs are fairly resistant and can survive for years in the soil. Once ingested, roundworms lay eggs in the small intestine and are shed through the feces. The roundworm juvenile may also migrate to the liver and/or lungs or encyst in muscle tissue. Common clinical signs of roundworm infestation are: vomiting, soft stool, pot belly, general unthriftiness or respiratory problems. Roundworms may be seen in the vomit or stool, they resemble strands of spaghetti. Roundworms are zoonotic, they have the potential to infect humans. Roundworms in humans go through visceral or ocular migrans – a migration to various organs or the eye.

Hookworms: Our pets become infested with hookworms through four routes: ingestion of infective larvae, through the placenta before birth, during nursing or through skin penetration by infective larvae. Adult worms lay eggs in the small intestine, which are shed through the feces, hatch in the soil, and then the infective larvae penetrate the skin or are ingested. Hookworms may cause tarry diarrhea, weight loss, anemia and unthriftiness. Hookworms cannot be seen with the naked eye in stool or vomit. Hookworms also have a zoonotic potential – they can cause cutaneous larval migrans (burrowing into the skin by the infective larvae).

Whipworms: Whipworm eggs must be ingested from infected stool and only infect dogs. The adult worms lay eggs in the large intestine and the eggs are shed in the stool. Few eggs are produced and shedding is intermittent, these characteristics make it more difficult to pick up whipworm eggs on a routine fecal flotation. Whipworms feed off of blood causing anemia, hemorrhagic diarrhea and weight loss. Whipworms infect primarily mature dogs. The eggs are very resistant and may survive months to years in the soil. Whipworms have not been shown to have any zoonotic potential.

Tapeworms: Tapeworms are transmitted through ingestion of an intermediate host (fleas, rodents). Tapeworm segments are passed in the stool and may attach to the fur around the anus, they resemble grains of rice. Tapeworms may cause intestinal upset. The zoonotic potential is low for most tapeworms, but they may cause mild intestinal upset in humans.

Coccidia: Ingested infective oocysts alone, or ingestion of infected intermediate hosts (rodents, etc.) are the two routes of transmission of this organism. Coccidia invades the intestinal cells and the oocysts are shed in the feces. Coccidia causes watery stools, decreased appetite and lethargy. The oocysts are hardy and may remain in the environment for up to one year.

Giardia: Infected feces or contaminated water must be ingested for Giardia transmission. This organism resides in the small intestine and cysts are passed in the stool 5-10 days after infection. The cysts are resistant to many environmental factors and may be hard to eradicate. Clinical signs of infected animals may range from no apparent signs to loose stools and weight loss. Giardia can be difficult to diagnose and may require specialized testing to confirm infection. Giardia is a potentially zoonotic organism and can cause intestinal disorders in humans.

Prevention/Control of Intestinal Parasites

Good personal hygiene habits (washing hands) after playing with pets or contact with soil are important safeguards against the spread of parasites.

Environmental Control

Pick-up and dispose of fecal material in your yard. Feces can be bagged and put in the trash, burned or flushed down a toilet. This should be performed at least weekly, with daily pick-up being preferable. Following any treatment for parasitic infestation, stools should be picked-up and disposed of immediately.

Sandboxes should be covered when not in use as they may serve as an outdoor litter box.

In some cases, more extreme measures such as removing several inches of topsoil or laying concrete over infected areas may be necessary to prevent reinfestation.

Parasite Control for Your Pet

The easiest means for protecting against most intestinal parasites is through administration of heartworm preventative, which also has an intestinal dewormer in it. Interceptor for dogs and Revolution or Interceptor for cats are given monthly to control roundworm, hookworm and whipworm infestation. Year round administration of these agents provide excellent coverage for most intestinal parasites. Kittens and puppies require more rigorous deworming protocols and are typically given intestinal dewormer every two weeks for several doses depending upon fecal flotation results, clinical signs, etc.

Fecal samples should be checked on all puppies and kittens at least one time and many patients will require multiple fecal flotations to monitor treatment. It is recommended to check stool samples on adult animals that are predatory, spend a large amount of time outdoors, are not currently receiving any type of intestinal parasite control or live with individuals at higher risk for contracting disease (small children, the elderly and immunocompromised individuals).

Rigorously deworm pregnant animals as well as their offspring during the first few weeks of life.

Heartworm Disease

Heartworms are spread by mosquitoes who have bitten infected animals. The incidence of heartworm disease is greater in dogs than cats, but cats are susceptible as well. In fact, 1/3 of heartworm positive cats are indoor-only pets. The infected mosquito will bite the animal, transmit the juvenile stage of the heartworm under the skin of the bitten animal.

The juvenile heartworm then goes through several life stages, migrating through the tissue into blood vessels and eventually find their way to the heart or blood vessels leading to the lungs. This process takes 5-6 months in dogs. Overall, dogs carry higher numbers of heartworms than cats do. The worms cause inflammation in the heart, pulmonary vessels and lung tissue.

The heartworms also affect the efficiency of the heart and alter blood flow through the heart and lungs. Heartworm disease is very common in this area because we have such a high mosquito population. Heartworm disease cannot be transmitted directly between dogs/cats and must go through the mosquito. Humans are not susceptible to heartworm disease. Heartworm disease can cause the following signs:

Exercise intolerance


Difficulty breathing

Sudden death (cats)

Weight loss

Asymptomatic – many animals won’t show any

signs until the disease is very severe and has

been present for a long period of time.

Preventing Heartworm Disease

Heartworm disease is easily preventable. Monthly heartworm preventative actually kills certain juvenile stages of the worm that the animal has been exposed to over the previous month. Interceptor, Sentinel, Heartgard, and Revolution are the most common heartworm preventatives. We use Interceptor most frequently in dogs and Revolution in cats.

To be effective, heartworm preventative must be given consistently during heartworm season, in our climate this is May-December. Unfortunately, our weather is extremely variable and this must be taken into account when administering the preventative.

Year round heartworm preventative (discussed previously for intestinal parasite control) is a good option to not only control intestinal parasites, but to eliminate the need to closely monitor when to start and stop preventative for the year. The American Heartworm Society recommends year-round preventative and yearly testing. Test results in cats are less consistent and testing is not routinely recommended for the feline patient.

Another way to decrease the likelihood of heartworm disease is to keep your pets inside at dusk and dawn, which tend to be the times of the day mosquitoes are at their worst. Advantix for dogs is a flea and tick control agent that also touts mosquito repellant qualities. This is a topical agent applied monthly.

External Parasites

The two most common parasites in this category are fleas and ticks. These parasites are most prevalent in the spring, summer and fall. They are both vectors (carriers) of disease in both humans and pets.


Fleas are perhaps the most dreaded of external parasites because they are so hard to get rid of once you have an infestation in your house or yard. Fleas feed on blood from their host (cat, dog, rabbit, rodent, etc.) and their feces are largely comprised of this blood. The flea eggs incubate for two days, then larvae hatch from these eggs and feed on the feces from the adult fleas.

The larvae molt two times over the next eight days then form a cocoon for approximately two weeks. Adult fleas emerge from these cocoons and find a new host. Most of the eggs and fecal matter of the fleas fall off into the environment (carpet, bedding, sofa, etc.) so the majority of the flea life cycle occurs off of the host. Because of this particular life cycle, fleas are hard to control as most products only treat one or two stages of the flea life cycle.

Fleas are prevalent in our area any time there is not a hard frost on the ground. During the winter months, rabbits/rodents may still carry fleas and have the potential to infest your pet. If your pet spends time boarding or is around other pets it makes sense to use flea control year around.


Ticks are prevalent in wooded areas but can be found in many outdoor environments. Ticks climb vegetation, etc. and

easily transfer to a host (dog, cat, rodent, etc.). Once

on the host; female ticks take blood meals, engorge,

fall off the host and lay eggs. Once the eggs hatch,

the tick goes through several larval and nymph stages.

The whole tick cycle (depending upon species) can

take over a year to complete. Ticks are important

carriers of several diseases. Most commonly, they

carry Borrelia burgdorferi, the organism that causes

Lyme disease in humans and dogs. Most tick species

are searching for hosts in the spring, summer and fall,

although some may even be active in the winter months.

Flea/Tick Control Products

Sentinel (dogs): this is Interceptor (heartworm preventative) plus a flea control agent. Sentinel is a flavored tablet given once a month for heartworm prevention, intestinal parasite control and as a means of flea control. This product prevents the development of flea eggs so an infestation will not occur on your pet or in your house. It is possible, with this product, to see a small number of fleas on your pet. This is adequate for pets that spend little time outdoors and around other pets. This is not a good choice for dogs that suffer from flea allergy dermatitis or dogs that are at high risk for tick exposure.

Advantix (dogs): this is a spot-on product applied monthly to kill adult fleas and ticks. It also claims to have repellant qualities against ticks, mosquitoes, gnats, etc.

Advantage (cats): monthly spot-on product that kills adult fleas.

Capstar (dogs and cats): tablet that kills all fleas currently on the pet. Capstar only works for 24 hours. This is a great product for controlling infestations used in conjunction with a topical product.

Frontline Plus (dogs): monthly spot-on that kills adult fleas, flea eggs, flea larvae and ticks. The kill time is 24-48 hours.

Frontline Plus (cats): see Frontline Plus for dogs.

Revolution (dog and cat available): monthly spot-on that kills adult fleas, flea eggs, flea larvae, ticks, ear mites, and Sarcoptes (skin mite). Revolution is also a heartworm preventative. Revolution is excellent for indoor/outdoor cats. Tick control with Revolution does not appear to be as good as with Advantix or Frontline.

Mites and Lice

Mites and lice are not as commonly seen as ticks and fleas. The most common mite is Otodectes or ear mites. These mites are more prevalent in cats and can be spread from cat to cat very easily. Poor environmental conditions and/or suppressed immune function typically play a role in most other mite/lice infestations.

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