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Heartworms are a constant danger to your pet and precautions such as a heartworm preventative should be used year round. Read on to learn more about prevention and treatment. 

White Dog
What are Heartworms?

Heartworms are a parasite transmitted to your dog by mosquitoes. When your pet is bitten by an infected mosquito, they will become infected with heartworms. The adult heartworm is an internal parasite that lives in the heart and pulmonary artery of the dog. They are 1-12 inches long. round and white. ​  

  The adult male and female worms found in the pulmonary artery and heart of a dog produce offspring that are released into the bloodstream.  These are called microfilaria.  Microfilaria need to be ingested by a mosquito feeding off the blood of an infected dog to complete the cycle. These ingested microfilaria develop in the mosquito into infective heart worm larvae in 10-14 days. The mosquito bites an animal and transmits the larvae while feeding. Once the animal is bitten the larvae enter the dog's blood stream, migrate through the animal's body from the bite to the pulmonary artery and the heart of the animal where they develop into adults.  Different life stages require different treatments. 


In canines the initial symptoms may be a persistent cough and lethargy. The disease will then progress to a point where the patient will lose weight and develop a pot belly (ascites) due to fluid buildup in the abdomen. At this point heart damage has been done. The kidneys and liver are also involved, resulting in lung disease and congestive heart failure. 

Although uncommon heartworms have been detected in cats. Heartworms in cats many times go undetected or may manifest as feline asthma. In other cases spontaneous death may result. 



Dogs - Initially many dogs are asymptomatic (have no symptoms) and the best diagnostic test is the occult blood or snap tests. The test reacts to antigens (proteins) produced by the worms and is very sensitive. An antigen is something the body recognizes as foreign to itself.

Cats - There is a snap test for heartworms in cats. There is no treatment as of yet other than symptomatic. There is a preventative available. 


Dogs - Start at 6-8 weeks of age

          Monthly tablets- very safe, must give whole chew/tablet


          Six or twelve month Proheart Injectable

Cats - Able to start at 6-8 weeks of age

          Monthly tablet 

Thunderbird Veterinary Hospital Policy 
Regarding the Dispensing of  Heartworm Preventatives

The following policy is provided to help answer questions regarding the dispensing of heartworm preventative for your pet.


Legal Requirements



All heartworm preventatives are labeled prescription drugs, requiring veterinarians to follow federal and state regulations for prescribing and dispensing them. These regulations require a valid veterinarian-client-patient relationship, which necessitates a PATIENT RECORD and a RECENT PHYSICAL EXAMINATION.


Medical Guidelines


Heartworm preventative cannot be dispensed without a heartworm test to puppies over 5 months of age. Dogs that have been receiving heartworm preventative regularly on a year round basis should be screened routinely every year. If a dog is on a monthly preventative, a one month lapse can pose a risk for heartworm infection. In general, if our records indicate a lapse in giving heartworm preventative we may require a heartworm test prior to refilling a heartworm preventative. If there is a lapse of greater than 5 months, a test will be required to refilling a heartworm preventative prescription. If the lapse is less than 5 months in most cases the prescription will be refilled but a retest will be required prior to filling a second prescription. 


These guidelines are to prevent a potentially life-threatening reaction when heartworms  have already developed. Each year we detect a few heartworm infections in dogs that have been receiving preventative therapy. These are usually due to known or suspected noncompliance in administering the preventative and or incomplete consumption of the preventative. 




At Thunderbird Veterinary Hospital, our primary concern is to dispense heartworm preventative medication in a manner that ensures your pets SAFETY and PROTECTION, while complying with state and federal laws. To achieve that goal we adopted the following guidelines for pets receiving heartworm preventatives.

         1) A current patient record must be on file

         2) The pet must be screened for heartworms every year to screen for resistance

         3) During a wellness exam for a new patient we require 

             oral authorization or  written documentation from the patient's ​

             previous veterinarian, or the patient's medical record supporting ​

         ​ ​​   appropriate/current heartworm preventative treatments.​​​​​​​​​​​​​​​​​​

        ​​ 4) A current heartworm test is required before preventative therapy can be 

​             initiated. This test may need to be repeated before renewing the

​             prescription, depending on the class of medication used and the​

            record of administration.    

​          5) Any deviation ​​​from these guidelines must occur within the context         ​

             of a veterinarian-client-patient relationship.       

          6) Year round prevention is required because the mosquito season is 

​​              variable and unpredictable in Oklahoma.​



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