Dirofilaria are long threadworms whose larvae are transmitted by various species of mosquitoes. Depending on the species, they colonize either the skin or the large blood vessels of the lungs, and in severe cases, the heart. This is why they are known as heartworms. Dirofilaria and the mosquitoes that transmit them are mainly found in the Mediterranean region, Eastern Europe, and North America. The disease is brought to Germany by travelers and through the importation of dogs from abroad. Due to the climate-related spread of mosquitoes to the north, it is becoming increasingly important.
A distinction is made between cutaneous dirofilariasis (affecting the skin) and cardiovascular dirofilariasis (affecting the heart and circulatory system). These are caused by two different pathogens. Dirofilaria repens develops exclusively in the subcutaneous tissue, while Dirofilaria immitis migrates to the blood vessels of the lungs and heart.
The life cycle of dirofilaria occurs in two host animals. The first host is the mosquito, and the second is dogs or wild animals such as foxes and martens. Dirofilariasis cannot be transmitted from animal to animal without the mosquito. There are now over 70 species of mosquitoes known to transmit dirofilaria. Some of these have already been detected in Germany.
When a mosquito bites an infected dog, it ingests Dirofilaria larvae during its blood meal. Within approximately two weeks, the infectious larvae develop in the mosquito through various larval stages. These are transmitted to the next animal during the blood meal. The higher and more stable the ambient temperature, the faster this development takes place. Climate warming is therefore also playing into the hands of dirofilariasis here.
In dogs, the larvae initially reside in the subcutaneous tissue. There, they develop into young worms within approximately two months. In the case of Dirofilaria immitis, these migrate further into the blood vessels. They prefer to settle in the small vessels of the lungs. There, they mature into adult heartworms over a further four to five months. These migrate to the larger veins near the heart and sometimes into the right ventricle. Heartworms can grow up to 31 centimeters long and live for up to seven years. Skin worms can reach a length of up to 17 centimeters and also live for several years. The female worms produce new larvae. These are then ingested by the next biting mosquito via the blood.
Cats, ferrets, and humans can also contract dirofilariasis. It is therefore a zoonosis (a disease that can be transmitted from animals to humans). Here, too, worms develop that can migrate to the heart. However, no new larvae are usually produced.
An infection with skin worms manifests itself through lumps in the subcutaneous tissue. As the worms migrate, the lumps can disappear and reappear in other places. Skin inflammation with itching, hair loss, or abscesses (encapsulated collections of pus) may occur.
The most common symptom of heartworm infection is a chronic (long-lasting) cough. This starts off mildly and worsens over months or years. It is accompanied by slowly increasing shortness of breath, which is mainly noticeable during exertion. You will not notice the symptoms for a long time when you are resting. Heartworms destroy red blood cells through mechanical friction, which can lead to anemia (low blood count). Signs of this include pale mucous membranes, fatigue, and listlessness. Stamina continues to decline steadily. If heartworms constrict the pulmonary vessels, this can lead to a life-threatening embolism (blockage of blood vessels) in the lungs with shortness of breath and dizziness. If the worms reach the right ventricle, right heart failure can develop. This manifests itself in water retention in the limbs and abdomen, causing the abdomen to swell. In addition to general malaise and severe shortness of breath, this can lead to sudden cardiac death in extreme cases. If heartworms pass through the right ventricle and into the vena cava (the large hollow vein leading to the heart), they can obstruct blood flow to the heart. The resulting vena cava syndrome can quickly lead to loss of consciousness and a life-threatening condition.
Declining performance and reluctance to move are often attributed to the natural aging process. In animals from foreign animal welfare organizations, their nature and normal urge to move are unknown. As a result, cardiovascular dirofilariasis is often only considered at a very late stage.
In cats, ferrets, and humans, depending on the age and size of the worms, there are often no or only minor symptoms. However, there are also cases of severe disease.
After a general examination, your vet will perform various blood tests. These can detect changes in the blood count as well as dirofilariasis and/or its larvae. If the heartworm test is positive, an X-ray of the chest and an echocardiogram (ultrasound examination of the heart, see corresponding article) should be performed. A detailed examination of the lungs, heart, and blood vessels is crucial for assessing the severity of the disease and choosing the appropriate treatment.
Due to the long development cycle of dirofilariasis, it can only be detected in the blood six months after infection. Dogs imported from affected areas should therefore be examined immediately after arrival and again six months later.
As soon as heartworms are detected, your dog will be treated with a worming medication against the larvae every four weeks. This can be in the form of tablets or spot-on preparations (applied to the skin). For heartworm, this is done for four months, and for skinworm, for at least six months to a year. Dirofilaria is home to bacteria that cause blood vessel inflammation. A four-week course of antibiotics is therefore recommended. One to two months after the initial treatment against the larvae, a medication is administered to kill the adult worms. It is injected deep into the lumbar muscles. As this is painful, your dog may be given a short anesthetic. A second injection is given four weeks after the first. A third injection is administered 24 hours after the second. Depending on the number of worms and the extent of lung damage, anticoagulants and anti-inflammatory drugs may also be used.
Keep your dog calm throughout the entire treatment and for one month after the third injection, as exercise increases blood flow. This can cause the dead worms and larvae to clump together, leading to embolisms.
In cases of severe heartworm infestation, the simultaneous death of many worms carries a high risk of embolisms and allergic reactions, including allergic shock. If heartworms have already reached the right ventricle or vena cava, killing the worms with medication can be life-threatening for your dog. They can then only be removed surgically.
In cats with no or mild symptoms, the disease is allowed to heal on its own. Lung changes are treated with an anti-inflammatory drug.
Skin nodules can be surgically removed. Skin inflammation is treated with anti-inflammatory and, if necessary, antibiotic ointments.
The prognosis for heartworms depends on the severity of the infection. The longer the infection has been present, the larger the worms are and the higher the risk of complications and subsequent damage. Dogs with mild infestations and early treatment have a good prognosis. There is usually no subsequent damage. However, if lung or heart damage is already present, the prognosis is very poor.
The prognosis for cutaneous dirofilariasis is good with treatment.
The best prevention is to avoid traveling with dogs and cats to areas where heartworms and the mosquitoes that transmit them are present. If this is not possible, you must protect your pet with a monthly deworming medication against the larvae during your trip. It is important to repeat the treatment one month after your return.
Also use a mosquito repellent during the trip. However, some dog breeds cannot tolerate all active ingredients against heartworms. Never use a product for dogs on cats or vice versa. This can be fatal for cats. Therefore, always consult your veterinarian. They will be happy to help you choose the right product.
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