States with the highest cases of parasite-borne diseases in cats and dogs

Certain US states saw elevated rates of parasite-borne illnesses in cats and dogs last year, according to 2024 data from the Companion Animal Parasite Council, a nonprofit organization focused on bringing awareness to the risks posed by parasites. Although parasite-related illness rates in dogs and cats varied by state, these organisms affect all states, including during the winter months.

“Parasites don’t follow a calendar,” Kelly Cairns, DVM, MS, DACVIM (SAIM), vice president of medical excellence and education at Thrive Pet Healthcare, said in a news release.¹ “We see dogs and cats dealing with parasite-related illnesses throughout the year, even if they don’t go outside much. Sometimes, it just takes one little mosquito slipping in through an open door or a flea hitching a ride on someone’s pant leg for your furry friend to end up with more than just an itch.”

Heartworm disease infection rates

Dogs

Last year, 1 in 15 dogs tested positive for heartworm disease in Mississippi, making this the state with the highest infection rate. Louisiana, Alabama, and Arkansas followed closely behind, each with a rate of 1 in 30 dogs testing positive. According to Thrive, cases of heartworm have risen in Texas, South Carolina, and Georgia.¹

Mosquitoes are vectors for heartworm, which affects a dog or cat’s heart and lungs. Symptoms of heartworm in dogs include coughing, shortness of breath, fatigue following moderate activity, or weight loss. Still, some canines may not exhibit any signs until the disease has progressed.

Cats

Although less common, cats are also at risk of contracting heartworm, with infection being possible through a single mosquito bite. In the US, approximately 1 in 500 cats test positive for heartworms annually.¹ In 2024, Texas, Mississippi, Alabama, and Louisiana showed higher rates of heartworm in cats, with 1 in 200 of these animals testing positive for the parasite.¹

Cats that have been infected with heartworm may exhibit nonspecific symptoms that can resemble other feline diseases. Symptoms include vomiting, coughing, difficulty breathing, decreased activity and appetite, and weight loss.^1,2

Like dogs, some cats may be asymptomatic, with some being able to naturally eliminate the heartworms, while others may die suddenly without ever showing signs of illness.² According to the FDA, felines with heartworm disease seldom show signs of heart failure^.2

Lyme disease rates

Last year, West Virginia reported the highest infection rate in the US for Lyme disease, where 1 in 6 dogs tested positive. Other states at high-risk for the disease include Vermont, Maine, and Connecticut, with 1 in 8 dogs testing positive. Trailing closely were Pennsylvania, Massachusetts, and New Hampshire, with 1 in 10 dogs showing positive Lyme disease results.¹

In dogs, Lyme disease is the most prevalent tick-borne disease. Symptoms include joint stiffness, lethargy, and loss of appetite.

Anaplasmosis rates

With 1 in 3 dogs testing positive for anaplasmosis, Maine and Vermont had the highest rates of the disease in the nation. Following these 2 states were Connecticut, New Hampshire, and Massachusetts, with 25% of dogs showing positive results.¹

Affecting dogs, cats, and humans, the disease is spread by ticks. Symptoms of anaplasmosis in cats and dogs include fever, lethargy, joint pain, and loss of appetite. In humans, symptoms mimic those of the flu.

Ehrlichiosis rates

Cats

Ehrlichiosis is another tick-borne illness, with the disease being poorly understood.³ In cats, ehrlichiosis affects their blood cells. Ehrlichia, the bacteria that cause the disease, can be transmitted to cats within 3 hours of when the tick attaches itself. Infected cats can experience a range of varying symptoms, including³:

• Fever
• Lethargy
• Decreased appetite
• Pale mucous membranes
• Bruising and bleeding
• Vomiting and/or diarrhea
• Joint pain
• Uveitis
• Enlarged lymph nodes
• Weight loss
• Difficulty breathing
• Enlarged spleen, causing an enlarged appearance of the abdomen
• Nervous system tissues

Dogs

One in 6 dogs tested positive for ehrlichiosis in Arkansas, making it the state with the highest number of cases. Missouri and Oklahoma followed, with 1 in 10 dogs and 1 in 11 dogs, respectively, having the disease.¹

Canines with ehrlichiosis will undergo 3 stages of the disease: acute, sub-clinical, and clinical or chronic.⁴ In the acute stage, dogs may experience fever, swollen lymph nodes, respiratory distress, weight loss, bleeding disorders, and neurological disturbances. Patients who progress to the sub-clinical phase show no clinical signs. Following the sub-clinical phase, untreated dogs will advance to clinical ehrlichiosis, during which they are likely to develop issues like anemia, bleeding episodes, lameness, eye problems, neurological problems, and swollen limbs.⁴

With approximately 1 in 4 cats and 1 in 7 dogs being affected by fleas, officials with Thrive expressed the importance of year-round prevention in a recent news release. “A tick can survive for more than 200 days without food or water, and fleas can continue to multiply even in your living room,” Cairns said.¹ “That’s why year-round prevention makes a big difference.”

References

1. Parasite hot zones: States where pets are most at risk. News release. Thrive Pet Healthcare. April 2, 2025. Accessed April 3, 2025. https://prnmedia.prnewswire.com/news-releases/parasite-hot-zones-states-where-pets-are-most-at-risk-302418633.html
2. Keep the worms out of your pet’s heart! The facts about heartworm disease. December 22, 2022. Accessed April 3, 2025. https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease#:~:text=Cats%20with%20heartworm%20disease%20may,show%20signs%20of%20heart%20failure.
3. Jones L. Ehrlichiosis in cats. PetMD. May 25, 2022. Accessed April 3, 2025. https://www.petmd.com/cat/conditions/infectious-parasitic/ehrlichiosis-cats
4. Williams K, Llera R, Ward E. Ehrlichiosis in dogs. VCA Animal Hospitals. Accessed April 3, 2025. https://vcahospitals.com/know-your-pet/ehrlichiosis-in-dogs

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