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Why Internal Parasites in Dogs are Becoming More Common

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The subject of “worms” isn’t a favorite one to linger on for most. But according to experts, it might be time for pet parents (and veterinarians) to devote some focused attention to these common—and harmful—parasites. 

Despite nearly four decades of veterinarians recommending monthly heartworm prevention and the widespread availability of safe, effective treatments for intestinal parasites, recent studies point to an alarming rise in the number of dogs infected with heartworm and hookworm (1, 2).

The bottom line is that effective parasite protection is becoming increasingly important. To stem the spread of infection, pet parents and veterinarians need to work together to prioritize not just heartworm disease prevention, but protection against a wide variety of parasites, including tapeworms, whipworms, roundworms, and hookworms.

Internal Parasites in Dogs Are on the Rise

anxious dog at vet

Canine heartworm infection is rapidly increasing in much of the United States, despite veterinarian recommendations on prevention and testing, a recent study reveals (3). Heartworm disease is a serious (and potentially fatal) condition caused by worms which can grow to be up to a foot long and live in the heart and blood vessels of the lungs of infected pets. Many dogs hide any symptoms until they are in heart failure. 

From 2013 to 2016, two separate analyses showed heartworm infection rates increased anywhere from 15.3 percent to 21.7 percent nationwide (4, 1). During this time, the number of dogs receiving preventive medication remained unchanged. Approximately two-thirds of the dogs in the United States received no heartworm prevention each year.

Southern states continue to see the highest rates of infection, but over the last decade, studies are tracking a troubling increase occurring along most of the Atlantic coast, central United States, and western states (5). 

In some regions, pet parents may only use heartworm prevention or dewormers seasonally, during the warmer months of the year. However, a recent U.S. study reveals that whipworms and roundworms actually peak in winter, while hookworms are more prevalent in the summer and early fall (2). These findings underscore the importance of year-round parasite protection for pets, especially since whipworms, hookworms, and roundworms are the three most common gastrointestinal parasites in dogs. The same study also noted that hookworm prevalence increased by 47 percent from 2012 to 2018. 

Most pet parents would never knowingly place their dogs at risk, but skipping that dose of dewormer or missing the odd one here and there could be related to increasing rates of heartworm and hookworm. 

“Even the one-third of pet owners who are using preventive medication may not be following their veterinarian’s recommendations for all-year protection or may only be using a heartworm treatment that does not protect against intestinal parasites and tapeworms,” explains Elanco Animal Health’s Dr. Jason Drake, a veterinarian and diplomate of the American College of Veterinary Microbiologists in parasitology.

Parasites and Pets are on the Move

two dogs at animal shelter

The most dramatic increases in parasites are being seen in areas with historically lower parasitic infection rates. Rapid increases in heartworm, for example, are being seen in the northern and western U.S. (5). One important reason for this geographic shift may be the relocation of rescue dogs by animal rescue organizations from the southeastern and Gulf Coast states where parasitism is relatively high, to areas where many parasites haven’t been as common. 

According to a recent survey, only approximately one-third of rescue organizations test, treat, or provide heartworm prevention prior to transporting dogs (6). In addition, it’s estimated that between 14.6 percent to 48.8 percent of dogs in animal shelters or rescued following natural disasters have heartworms (7). 

“It takes six months for a dog to test positive for heartworms following infection, so even if a dog tests negative, it could be moved to a new location, then test positive months after relocation,” says Drake. 

Changes in climate and weather may also make parasite survival and transmission easier in areas where this wasn’t as likely in the past. One reason these changes are so concerning is that heartworms resistant to currently available preventives have been identified in dogs from the Mississippi River Valley. As dogs are relocated to new areas, it is quite possible these resistant worms are also being moved into new areas. 

Common parasite vectors like ticks, fleas, and mosquitoes are also becoming more common in areas of the country—or for longer periods of the year—than has been seen historically, bringing tapeworm, heartworm, and other vector-borne diseases with them. 

While the findings are concerning, the good news is there are steps you can take to protect your pet from both internal and external parasites.

Year-Round Parasite Protection

vet holds dog

When it comes to your pet’s health, parasite protection should be a crucial conversation to have with your veterinarian. Even if your veterinarian has mentioned parasite control during your pet’s visit, you might not have retained all of the information.

It’s important to understand how parasites spread, the risk they pose, and the consequences of skipping doses of medication. Here are some important facts about parasites you should know:

  • Mosquitoes spread heartworms to dogs
  • Whipworm and roundworm eggs can survive for years in the soil after deposited in the feces by an infected dog
  • Tapeworms can infect dogs through fleas, rodents and rabbits in your own yard, or a raw meat diet
  • Many of these parasites (like roundworms, hookworms, and some tapeworms) can also be spread from animals to people.

“The gross-out factor is a powerful tool,” says Drake. “People who understand that their pets are constantly being exposed to parasites from within their environments and that many of the early signs of infection aren’t highly visible will likely improve their compliance or adherence to parasite control programs.”

Since heartworm, intestinal parasites, ticks, and fleas can affect pets in all seasons,  both the CAPC and FDA call for year-round, broad-spectrum parasite protection. 

Your veterinarian can outline a plan and recommend products that make sense for your canine companion. 

Some heartworm preventives also provide protection against a variety of intestinal parasites. For example, Interceptor® Plus (milbemycin oxime/praziquantel) is a chewable tablet that prevents heartworm disease and treats and controls adult hookworm, roundworm, whipworm, and tapeworm infections in dogs. For external parasite protection, Credelio® (lotilaner) is an effective treatment to kill adult fleas, treat and prevent flea infestations, and treat and control tick infestations for one month in dogs.

Each time your veterinarian screens your dog for intestinal parasites during a wellness visit, you are also being put to the test as a pet parent. A negative test result is cause for celebration. You should congratulate yourself for a job well done, but remember to keep up the good work. Your dog will most likely be continuously exposed to new infections, so it’s best to remain vigilant, especially as parasite prevalence shifts. 

Credelio Indications

Credelio kills adult fleas and is indicated for the treatment and prevention of flea infestations, treatment and control of tick infestations (lone star tick, American dog tick, black-legged tick, and brown dog tick) for one month in dogs and puppies 8 weeks and older and 4.4 pounds or greater.

Credelio Important Safety Information

Lotilaner is a member of the isoxazoline class of drugs.  This class has been associated with neurologic adverse reactions including tremors, incoordination, and seizures. Seizures have been reported in dogs receiving this class of drugs, even in dogs without a history of seizures. Use with caution in dogs with a history of seizures or neurologic disorders. The safe use of Credelio in breeding, pregnant or lactating dogs has not been evaluated. The most frequently reported adverse reactions are weight loss, elevated blood urea nitrogen, increased urination, and diarrhea.  For complete safety information, please see Credelio product label or ask your veterinarian.

Interceptor Plus Indications

Interceptor Plus prevents heartworm disease and treats and controls adult roundworm, hookworm, whipworm, and tapeworm infections in dogs and puppies 6 weeks or older and 2 pounds or greater.

Interceptor Plus Important Safety Information

Treatment with fewer than 6 monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention. Prior to administration of Interceptor Plus, dogs should be tested for existing heartworm infections. The safety of Interceptor Plus has not been evaluated in dogs used for breeding or in lactating females. The following adverse reactions have been reported in dogs after administration of milbemycin oxime or praziquantel: vomiting, diarrhea, decreased activity, incoordination, weight loss, convulsions, weakness, and salivation. For complete safety information, please see Interceptor Plus product label or ask your veterinarian.

Disclaimer: The author received compensation from Elanco US Inc., the maker of Interceptor Plus and Credelio, for her services in writing this article. 


  1. American Heartworm Society – AHS Announces Finding of New Heartworm Incidence Survey. Retrieved from: https://heartwormsociety.org/newsroom/in-the-news/347-ahs-announces-findings-of-new-heartworm-incidence-survey
  2. Drake, J., Carey, T. Seasonality and changing prevalence of common canine gastrointestinal nematodes in the USA. Parasites Vectors 12, 430 (2019) doi:10.1186/s13071-019-3701-7
  3. Drake J, Heinz-Loomer C, Rotenberry A, Wiseman S. Increasing incidence of Dirofilaria immitis in dogs in USA 2013–2017. In: Proceedings, 63rd Annual AAVP Meeting, Denver, CO, USA; 2018.
  4. Drake, Jason & Wiseman, Scott. (2018). Increasing incidence of Dirofilaria immitis in dogs in USA with focus on the southeast region 2013–2016. Parasites & Vectors. 11. 10.1186/s13071-018-2631-0. 
  5. Self, S.W., Pulaski, C.N., McMahan, C.S. et al. Regional and local temporal trends in the prevalence of canine heartworm infection in the contiguous United States: 2012–2018. Parasites Vectors 12, 380 (2019) doi:10.1186/s13071-019-3633-2
  6. Simmons K, Hoffman C. Dogs on the move: factors impacting Animal Shelter and Rescue Organizations’ Decisions to accept dogs from distant locations. Animals. 2016;6:11. http://www.mdpi.com/2076-2615/6/2/11.
  7. Drake, J., Parrish, R.S. Dog importation and changes in heartworm prevalence in Colorado 2013–2017. Parasites Vectors 12, 207 (2019) doi:10.1186/s13071-019-3473-0

Credelio and Interceptor are trademarks of Elanco or its affiliates. 

© 2020 Elanco.  PM-US-20-0567

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