Parasitic diseases are some of the scariest and least understood diseases that Animal Health Services clients encounter. Because many external parasites are microscopic and internal parasites live in our pets’ bodies, many pet owners think they don’t have to worry about them because they can’t see them. This line of thinking couldn’t be further from the truth! Most parasites, directly or indirectly, can harm your pet whether you can see them or not. Way too often, pet owners lose their beloved pets because they don’t want to spend the extra money on parasite prevention. What they fail to realize is that parasite prevention is much less costly than treatment for the diseases they pass along. Plus, a pet’s health and well-being reduces when infected by a parasite.

Internal Parasites vs. External Parasites

Parasites can be classified into two major groups: external and internal. Most people are familiar with external parasites, like ticks and fleas, because they can be seen with the naked eye. They are less familiar with mites and lice, which usually require a microscope to see.

Internal parasites are almost never seen. If they are visible, it is usually because a pet has a horrendous amount of worms and they are shedding live worms via stool or vomit. Typically, these internal parasites are diagnosed via eggs in a fecal test or blood test. If a pet has a tapeworm, they will shed segments of the worm.

Wildlife: The Most Common Source of Parasites

Most external parasites are spread through contact with infected animals or the environment. In our area of Arizona, wildlife (coyotes, rabbits, mice, etc.) are the most common sources of parasites.

Heartworm is a particularly dangerous parasite spread more often around wildlife. This parasite is carried by mosquitoes (and yes, we do have mosquitoes here!). They are transmitted from mammal to mammal through mosquito bites, and once the worms develop and reach a new animal, they migrate for up to 6 months before ending up in the heart. By the time a worm reaches the heart or nearby blood vessels, it’s anywhere from 6 to 14 inches long! Heartworm disease will often cause dogs to experience heart failure. Cats are actually considered abnormal hosts for heartworms, and of those that become infected, 30% die suddenly with no prior warning signs.

Demodectic mites are one common exception. These mites are not considered contagious, so nearby wildlife doesn’t increase a pet’s risk of contracting them. These mites live in the pores of a dog’s skin, and they seem to have a symbiotic relationship with dogs, because they can be found in healthy dogs, too. However, if they are allowed to overgrow, these mites can cause severe skin problems, especially in puppies, so don’t let them become a serious problem.

Testing for and Treating Parasites

There are a lot of different parasite treatments available, but no single treatment covers all types of parasites. Medications used for preventing heartworm disease are also effective at treating a large majority of the parasites we most commonly encounter. Unfortunately, even these medications do not treat all types of parasites. The Companion Animal Parasite Council, responsible for monitoring parasite incidences around the country, recommends all pets to have stool samples checked two to four times every year. In our part of Arizona, we typically recommend twice-yearly samples.

The American Heartworm Society, which monitors heartworm disease, recommends annual heartworm screenings and year-round heartworm preventative medication for dogs. They also recommend year-round preventative medication for cats, regardless of whether they are primarily indoor or outdoor cats. Because cats are considered abnormal hosts, there unfortunately is no accurate test for heartworms in cats.

By testing for parasites regularly, not only do we check for potential breaks in medication, but we also make sure we are treating all parasites your pet may have acquired since their last test.

Your Pet’s Parasites and You

We often come into contact with parasites from our pets because of our close associations and close living quarters with our furry family members. Because we are abnormal hosts, many parasites do not survive in our bodies. However, many others can. The Center for Disease Control (CDC) is becoming increasingly concerned about the spread of pet parasites to humans because of the rise in incidence of human cases.

There are a whole host of parasites that can affect humans, but because of our limited exposure to the jungles of the Amazon and the swamps in Florida, we’re not as likely to be exposed to the parasites that are more common in these regions. However, pet owners are potentially exposed to multiple parasites every day just through our interactions with our pets.

Some common external parasites affecting humans include mites, fleas and ticks. Internal parasites we can contract include hookworms, tapeworms, roundworms, heartworms and whipworms. There are certainly health concerns associated with all of these parasites. However, the CDC has recently become particularly concerned about the transmission of roundworms to people, as a human roundworm infection can lead to symptoms as severe as blindness and brain damage. This means preventing parasites in your pets protects your entire family.

Parasites in Your Pet’s Environment

Once your dog or cat has had parasites, often your home and yard are seeded with parasite eggs. Many parasite eggs have the ability to survive harsh temperatures and the strongest cleaning chemicals. Unfortunately, this often means that even after being treated for parasites, many cats and dogs will become reinfected from eggs in the home or yard.

To put things in perspective, a recent study performed on two puppies with roundworms found that each puppy would shed a couple of million eggs per week. This study also showed that normal environmental conditions (drying, heat, freezing) did not kill the eggs.

The general recommendation for decontaminating the environment is to cover your entire yard with 6 to 12 inches of straw and burn it or to remove 12 to 18 inches of dirt. Preventing parasites in the first place with twice-yearly fecals and appropriate preventive medications is much easier, of course.


  1. Have your pets tested for intestinal parasites twice every year and for heartworm once a year.
  2. Keep your pets on oral and topical parasite prevention medications all year.

To learn more about parasite prevention, or to find out which parasite prevention medications are best for your pets, call Animal Health Services at 480-488-6181.