As a pet owner, you must have heard of heartworm, a blood parasite that is transmitted through mosquito bites and resides in the pulmonary artery upon infection. Compared to the expensive and lengthy treatment for heartworm, even after successful deworming, it may leave indelible damage to the cardiovascular, pulmonary, liver, and kidney organs. Preventing heartworm is simple and effective, and it is currently the only heart disease that can be prevented. The value of prevention over treatment is fully reflected in this disease. Remember to prevent heartworm to avoid worrying.
Heartworm
Heartworm (scientific name Dirofilaria immitis, commonly known as Heartworm) is a roundworm parasite. When a mosquito feeds on the blood of an infected animal, microfilariae (larvae of heartworm) will take the opportunity to hitch a ride. After 10-14 days, they can grow to be infectious and will sneak into the next host through mosquito bites to form an infection. They move along the blood vessels to the pulmonary artery to get nutrients and continue to develop. Adult worms can even grow to over 30 centimeters. After the heartworm matures, it begins to mate and reproduce. Usually, 7-9 months after infection, the next generation of microfilariae will proliferate to a detectable level.
Generally speaking, heartworms can survive in most hosts for 3-5 years or more, long-term residing between the pulmonary artery and the right heart. In addition to the worms themselves directly damaging the heart valves and vascular endothelial hyperplasia, leading to impaired function of the right heart and lungs, excessive worm infection, dead worms, and the host’s immune response can also cause other complications such as hemolysis, thrombosis, caval syndrome, and liver and kidney dysfunction. Common symptoms include coughing, shortness of breath, reduced physical strength, fainting, abdominal distension (ascites), pale mucous membranes, and hematuria, all related to the above events.
Prevention is Better than Cure, Say It Three Times
In a large-scale heartworm survey in Taiwan in 2017, the prevalence rate of dogs was 22.8%, which means for every 4-5 dogs in Taiwan not protected against heartworms, one is infected. Although dogs kept outdoors have a higher prevalence rate, reducing contact with vector mosquitoes in the environment is also helpful. However, even those kept entirely indoors with occasional outings have prevalence rates of 7.5% and 10%, which means even if they never go out, they still need to be protected against heartworms. After all, mosquitoes can enter indoors through elevators or pipes.
Therefore, whether kept indoors or outdoors, do not take chances. The most important thing is to start prevention from a young age (6-8 weeks old), whether it is topical drops or oral tablets. Since some preventive drugs only protect against external parasites, make sure to choose products that include heartworm prevention and contain macrocyclic lactones. For sheepdogs that are more sensitive to this ingredient or pregnant and nursing female dogs, pay extra attention to the safety test reports of the products. Regularly administer these preventive drugs, which can effectively kill microfilariae and third/fourth-stage larvae at low drug concentrations without causing significant toxicity to the body.
Mosquitoes can enter anywhere, and even if they never go out, they still need to be protected against heartworms, and it should start from 6-8 weeks old!
Heartworm Testing and Diagnosis
Suitable Timing for Testing
- Suspected symptoms of heartworm infection (coughing, intolerance to exercise……)
- Not regularly preventing heartworms, preparing to start medication
- 4 months after changing anti-parasite preventive drugs or brands
- Routine physical examination (even if medication is given on time, the effectiveness of prevention is not guaranteed to be 100%)
How to Diagnose Heartworm
- Unless it is less than seven months old, even if infected, it has not yet reached the detectable stage. Otherwise, whether you want to confirm or exclude heartworm infection, it is recommended to perform both antigen testing and microfilaria screening as a double insurance.
Antigen Testing (Blood Rapid Test)
It is the most sensitive diagnostic tool for heartworm today, which uses serology to detect the proteins secreted by mature female worms (currently there is no effective method to detect male worms).
Microfilaria Screening
Examine thick blood smears under a microscope to see if there are active heartworm larvae.
Auxiliary Examinations
After confirming heartworm infection, other non-diagnostic examinations are also very important to understand the severity of the infection and the overall feeling of other organ systems, which helps in formulating treatment plans and assessing the patient’s tolerance based on the results of examinations before and after treatment.
Thoracic X-ray
It can confirm the cardiovascular outline and pulmonary infection status, and exclude other cardiopulmonary diseases that may cause similar symptoms. When there are more worms, the carrying parts, peripheral pulmonary arteries, pulmonary artery trunk, and right heart, will expand or bulge; in addition, the lungs may also show thickening of bronchitis or interstitial infiltration due to inflammation or embolism.
Cardiac Ultrasound
The worms will appear as two parallel lines under ultrasound, and this “equals sign” image is quite recognizable. However, in mild infections, the worms are mostly distributed in the peripheral pulmonary arteries, and only when there are enough will they fill the main trunk of the pulmonary artery and the right heart that are within the scanning range. Even if the sensitivity to heartworm detection is not high, it is still recommended as an assessment of cardiac function and structural damage.
The arrows point to heartworms in the pulmonary artery.
Other Examinations
Blood tests may find evidence of increased eosinophils or basophils due to parasitic infections, accompanied by anemia caused by chronic inflammation or hemolysis. Serum biochemistry often shows increased liver index, hyperglobulinemia related to immune reactions, increased acute renal index, and hypoalbuminemia. In severe cases, there will also be hemoglobinuria in the urine test.
Heartworm Treatment
After a positive diagnosis of heartworm and completion of other examinations to assess, if there are severe complications, symptoms will be stabilized first, and it is possible to consider removing heartworms as much as possible through surgical methods, and then follow the internal treatment guidelines of the American Heartworm Association to kill the remaining adult heartworms and microfilariae in multiple modes, while strictly limiting exercise. Although the embolism of dead worms during the treatment process cannot be avoided, it can minimize related complications.
Limiting Exercise
Regardless of the severity of heartworm infection, the most important principle in the treatment is to limit exercise.
Surgical Removal of Heartworms
Surgery will make an incision from the right neck, and use forceps to enter the right atrium from the jugular vein to remove heartworms. Therefore, mild infections where the worms only exist in the peripheral pulmonary arteries and microfilariae are not applicable, and it is also impossible to completely remove all heartworms. Subsequent internal treatment is still needed. Severe infections may affect the return of blood flow and cause caval syndrome, which is a higher surgical anesthesia risk level. However, removing the worms can alleviate symptoms and subsequent treatment of dead worm embolism. Under careful assessment, it is a treatment option.
Internal Medication Treatment
The American Heartworm Association recommends a treatment course that includes three injections of adulticide Melarsomine, oral antibiotics Doxycycline, and steroids to reduce adverse reactions to treatment, as well as preventive drugs for larvae to attack heartworms at various stages of growth. With a pincer attack, there is a 98% success rate in treatment. Finally, remember to retest for microfilariae and antigens to be negative, and the one-year treatment course is considered complete.
| Days after confirmed infection | Treatment content |
|---|---|
| Day 0 | Confirm infection, start limiting exercise, use steroids to alleviate clinical symptoms |
| Day 1 | Administer heartworm preventive medication and observe for adverse reactions |
| Days 1-28 | Oral antibiotics Doxycycline for four weeks to inhibit heartworms and reduce worm death lesions |
| Day 30 | Administer heartworm preventive medication |
| Day 60 | Administer heartworm preventive medication |
| First dose of Melarsomine adulticide injection | Strictly limit exercise and give steroids for four weeks |
| Day 90 | Administer heartworm preventive medication |
| Second dose of Melarsomine adulticide injection | Third dose of Melarsomine adulticide injection |
| Day 91 | Strictly limit exercise and give steroids for four weeks |
| Day 120 | Microfilaria screening (microscopic examination of thick blood smears) |
| Day 365 | Antigen testing (blood rapid test) |
Can Cats Get Heartworms?
YES, the answer is affirmative. Cats in Taiwan have a 3.1% positive rate for heartworm antigen and a 6.7% positive rate for antibodies. Although cats are not the main host for heartworms and have some resistance, the number of worms infected is also small, possibly only 2-3, many of which may die before reaching the pulmonary artery. The chances and extent of infection seem to be much less than dogs, and one might think they can breathe a sigh of relief. However, this is not good news. The results are often more serious than imagined, and even a single worm can be life-threatening.
Since cats are not ideal hosts, heartworms may not be able to reach the heart smoothly, and a high proportion of abnormal migration in the body can cause damage to the whole body arteries or the nervous system. Symptoms usually appear when the worm reaches the lungs and dies. The acute and severe inflammatory response caused by heartworm-related respiratory diseases is quite similar to feline asthma or allergic bronchitis, including persistent shortness of breath, chronic cough, hemoptysis, and even sudden death without warning.
Diagnosis is difficult, and the prevalence of heartworms in cats may be underestimated. Clinical examination mainly relies on testing antigens and antibodies in the blood. However, these tools have their own limitations. For example, antigen testing that detects mature female worms can confirm the diagnosis, but it can cause false negatives because there are only male worms or the female worms are not yet mature; antibody testing can confirm the antibodies produced by having had heartworms, but it cannot determine whether the infection is still ongoing at the time of examination. Therefore, to diagnose heartworm infection in cats, both antigen and antibody testing are necessary, and they need to be combined with X-rays, cardiac ultrasound, medical history, and clinical symptoms to increase the accuracy of diagnosis.
In cats, asymptomatic heartworm infection may heal on its own and may not require treatment. Monitoring the condition and following up again after 6-12 months is possible; if symptoms appear and there are lung lesions on X-rays, steroids can be given to alleviate symptoms; very low doses of adulticide melarsomine are toxic to cats and are not recommended; continued use of macrocyclic lactone preventive drugs can reduce the number of worms in the body, but the allergic reaction to the death of worms is still unknown. Since no internal treatment has been shown to clearly extend survival time, in principle, surgical removal of worms is better than destroying them in the body, but most situations are still limited by the insufficient number of infections and are not applicable.
If cats are infected with heartworms, there is currently no effective treatment, and regular prevention is still the key!
References
- (2017) Prevalence and epidemiology of canine and feline heartworm infection in Taiwan, Parasit Vectors.
- (2020) Current Feline Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs, AHS.
- (2018) Current Canine Guidelines for the Prevention, Diagnosis, and Management ofHeartworm (Dirofilaria immitis) Infection in Cats, AHS.
Original article by KPTer, if reproduced, please cite the source: https://www.kaipet.com/en/ultra-complete-guide-diagnosis-prevention-treatment-heartworm-disease-dogs-cats
