Dirofilariasis is a parasitic disease caused by nematodes of the Dirofilaria genus. The symptoms depend on the type of roundworm and can affect the skin or be caused by damage to the lungs and heart. Dirofilaria is transmitted by mosquitoes, particularly in the Mediterranean region and North America. Dogs that have been in affected areas are therefore particularly at risk. Unfortunately, however, in regions of Germany, e.g. B. in the Freiburg area, carriers of dirofilariasis detected.
What are Dirofilaria and how are they transmitted?
Dirofilariasis is divided into skin-affecting (cutaneous) dirofilariasis, caused by Dirofilaria repens, and cardiovascular (cardiovascular) dirofilariasis, caused by D. immitis. In addition to dogs, foxes and less frequently cats and ferrets can also be affected. Vectors of D. repens and D. immitis are native (endemic) particularly to the Mediterranean region and parts of Asia and Africa. D. immitis is also found in North America and Australia, e.g. T. also in Central and South America.
Dirofilaria develop in the mosquito from the first larva to the transmissible third larva. The development of the first larval stages depends on fairly stable warm ambient temperatures. Due to global warming, temperatures are also becoming milder in northern regions, which is why Dirofilaria and their intermediate hosts, the mosquitoes, are increasingly spreading towards Germany. So far, however, they are not endemic in this country and are therefore rare.
The mosquito transmits Dirofilaria as a larva during the bite into the subcutaneous tissue of the dog. From there, the larvae migrate through the blood vessels to their target tissue, where the adult worms reproduce. For D. immitis, these are the large blood vessels of the lungs. In severe infestations, the worms also migrate further into the right ventricle, which has given D. immitis the name heartworm. Adult female heartworms are about 1 cm across and up to 30 cm long. Male D. immitis remain smaller.
In D. repens, too, the female worms are larger than the males. The adult skinworms reach a length of about 15 cm and live in the subcutaneous tissue. The young first larvae (microfilariae) that develop after mating are distributed via the blood vessels throughout the dog’s body and are ingested by the mosquito when it bites. These microfilariae are also detected by the veterinarian in a blood test. In the mosquito, they develop to the third instar larvae, when they can in turn be transmitted from the mosquito to other dogs.
What are the typical symptoms of cutaneous dirofilariasis?
Infection with the skin worm D. repens often causes no symptoms. Symptoms of cutaneous dirofilariasis in dogs are swelling or nodules in the subcutis. Rarely, inflammation can develop, which becomes noticeable through itching, hair loss or abscesses.
What are the typical symptoms of cardiovascular dirofilariasis (heartworm disease)?
Heartworms live in the large blood vessels in the lungs, narrowing them and reducing blood flow to the affected area. Dead D. immitis, in particular, severely impedes blood flow and can then trigger an embolism, i.e. a complete vascular occlusion. The tissue is no longer adequately supplied and dies. In addition, the heartworms can destroy the red blood cells through mechanical friction, which leads to anemia.
Cardiovascular dirofilariasis in dogs causes symptoms such as coughing, shortness of breath and reduced resilience. In most cases, the symptoms of cardiovascular dirofilariasis only appear under stress. So when the dog is in motion and making more or less effort. At rest, symptoms are often only noticeable with a high worm infestation or not at all.
When the worm reaches the right ventricle via the pulmonary vessels, it expands and becomes overloaded. Then symptoms of right heart failure can occur:
- Fluid retention in the abdomen (ascites), causing the abdominal girth to increase
- Water retention in the limbs (oedema)
- shortness of breath
- refusal of feed intake
- weight loss
- in extreme cases sudden cardiac death
If the heartworms get through the right ventricle into the vena cava, they can trigger a life-threatening vena cava syndrome. The vena cava allows blood to flow back into the right ventricle. If worms constrict the large vein, not enough blood can flow to the heart. Signs of vena cava syndrome are pale mucous membranes, loss of consciousness and shortness of breath.
In some cases in older dogs it is assumed that declining performance and reluctance to exercise are simply due to the aging process and that dirofilariasis is therefore diagnosed very late. Signs of dirofilariasis can also initially go undetected in dogs from animal shelters abroad. The reasons for this are that the character of these dogs is initially unknown and the animals are often housed in small kennels without much movement. In this way, intermediaries and the new owners can e.g. B. assume that the dog simply has little urge to move due to its nature. If the new owners then move their animals more and more, the first symptoms can be noticed.
What therapy options are there?
Canine dirofilariasis requires therapy for several reasons: Heartworm causes irreversible damage to the lungs and heart, which increases with the duration of the infestation. In addition, dogs that have D. immitis or D. repens microfilariae circulating in their blood vessels are a source of infection for other dogs, cats, and even humans if the vector is present. The treatment of dirofilariasis is aimed at killing the worms (fast-kill method) or rendering the females infertile (slow-kill method). This is done with medication and depends on the Dirofilaria species and the severity of the infestation.
In the case of a severe heartworm infestation, the simultaneous death of many worms carries a high risk of vascular occlusions and allergic reactions, including allergic (anaphylactic) shock. Therefore, before initiating treatment, the veterinarian will assess the severity of the infestation with a cardiac ultrasound. If the heartworms have already reached the right ventricle or the vena cava, killing the worms with drugs can be life-threatening for the dog. The prognosis for such severely affected dogs depends on the general condition and is always cautious. One treatment option is surgical removal of the worms from the heart.
Dogs suffering from cardiovascular dirofilariasis who are being treated with medication should exercise as little as possible. A lot of exercise leads to increased blood flow, which in turn promotes clumping of dead worms and microfilariae in the vessels. This increases the risk of developing embolism and complications.
When should you go to the vet?
Every dog that is brought to Germany from endemic areas and has not received any prophylaxis or where there are uncertainties in this regard should be examined by a veterinarian for dirofilariasis about 6 months after entering the country. Dogs from animal shelters abroad should be presented to the veterinarian twice, the first time shortly after entry and again 6 months later. An infection can only be detected if the infestation has existed for a longer period of time, as this is carried out by detecting microfilariae that circulate in the blood.
After 6 months, adult dirofilaria have mated in the target tissue and their larvae, the microfilariae, become detectable. If dirofilariasis is suspected, the veterinarian will take blood and examine it under a microscope for the presence of microfilariae or have this examination carried out by a specialized laboratory. The best time of day for a blood test to detect microfilariae is in the evening, as this is when they are most active and most reliably detectable.
Reasons to go to the vet immediately and indications of major organ damage from dirofilariasis in dogs are severe coughing, possibly accompanied by retching, and exercise intolerance. The prognosis is all the better, the faster dirofilariasis is diagnosed and treated. Long-term heartworm infestation can cause major damage to the lungs or heart muscle that can no longer heal.
The veterinarian can use a cardiac ultrasound to determine whether the worm infestation has already caused tissue damage to the heart. The costs for veterinary examinations and the treatment of dirofilariasis in dogs depend on the necessary measures and are based on the fee schedule for veterinarians.
How to prevent dirofilariosis?
Avoiding endemic areas is the safest way to prevent dirofilariasis in dogs. If the dog has no contact with the mosquitoes that transmit it, an infection with D. immitis and D. repens is ruled out.
If the dog cannot be avoided in an endemic region, preventive treatment against Dirofilaria should be carried out, especially during the mosquito season in the target area. Drugs that already kill larvae, but also adult worms, come into question. Such prophylactic worm treatment should only be carried out after a prior examination by the veterinarian and should be individually tailored to the dog. Caution should be exercised in Collies, their mixed breeds and other MDR1-defective dogs, in which some drugs can produce severe neurological effects.
In addition to medicinal prophylaxis, the dog should also be protected against mosquito bites. In the particularly active phases of mosquitoes, i.e. at dusk, dogs should only stay indoors, e.g. B. are protected accordingly by insect screens. The animal’s sleeping place can also be equipped with appropriate protective nets.