Heartworms (Dirofilariae) are vector-borne parasites that require two hosts to complete their life cycle: the dog and the mosquito. As many as 30 species of mosquito can transmit heartworms and it is a serious and potentially fatal disease.
Pathogenesis
D. immitis requires the mosquito as an intermediate host and is integral in enabling the completion of its life cycle. The female mosquito bites an infected dog and ingests microfilaria (progeny shed by adult heartworm) that live in the small vessels of the blood stream. The microfilaria develop over 10-30 days in the mosquito's gut before becoming infective larva, then re-entering the mosquito's mouthparts to complete their maturation when they enter the dog's blood stream post-bite.
The larvae migrate through the bloodstream to the heart and adjacent blood vessels, maturing to adults and reproducing microfilaria within 6 months.
Adult heartworms can be found in the heart, pulmonary artery and adjacent blood vessels. Female adults can be 6-14 inches long and 3 mm wide, and one dog may carry as many as 300 heartworms when diagnosed. Adults may live up to five years, during which they will have produced millions of microfilaria.
Heartworm is not transmitted horizontally between dogs, and therefore the spread of disease coincides with the season when mosquitoes are most active.
Clinical presentation
It may take years before dogs show any clinical signs, and therefore the disease is rarely seen in dogs under a year due to the length of the larvae to mature into adults. This does indicate that by the time clinical signs are seen, the case is fairly advanced. Clinical manifestations are associated with cardiac, blood pressure, and hypoperfusion abnormalities due to the bulk of the adults and microfilaria clogging up the cardiovascular system. Hypoperfusion and hypoxemia of the lungs, liver and kidneys are most pronounced, and present as:
- Soft, dry cough
- Lethargy, weakness, listlessness
- Exercise intolerance
- Anorexia
- Weight loss
- Anaemia
- Manifestations of hepatopat – icterus, cirrhosis
- Manifestations of congestive heart failure – ascites, oedema
- Thromboembolism, pulmonary hypertension
Severity of disease depends on the worm burden, the length of infection, and location of the worms.
Diagnosis
Clinical signs (as above)
Blood smear examination for microfilaria: a specific test but not sensitive, nor representative of stage nor burden of infection
Serology (ELISA) – antigen serology is considered the standard in canine medicine
Advanced imaging – chest radiographs are used to assess the extent of cardiopulmonary damage. Adult heartworms are very echogenic on ultrasound/echography, so can provide an indication to adult worm burden.
Haematology – pancytopenia, particularly an anaemia of chronic disease
Biochemistry – increased liver enzymes and creatinine
Treatment
It is important to note that not all dogs infected with heartworm are great candidates for rapid treatment protocols, and there remains a high degree of risk with treatment that depends on the worm burden and severity of clinical signs. By the time clinical signs are present, the disease and damage is likely to be advanced, and the animal must be stabilised before a treatment plan is determined.
Stabilisation treatments may include:
- Glucocorticoids
- Diuretics
- Vasodilators
- Positive inotropic agents
- IVFT
When treatment addressing the adult worm burden begins, note that the adult worms begin to die and start to decompose, at which point parts are carried to the lungs where they lodge in small blood vessels and are eventually reabsorbed by the body. This resorption can take weeks to months, and most post-treatment complications, such as thromboembolism, are caused by these fragments – it is the most dangerous period and therefore physical exertion must be avoided.
Prior to treatment, it is useful to determine the microfilarial ‘load’ through the use of a modified Knott’s test because high burdens will increase anaphylaxis risk.
An example treatment protocol for heartworm:
Day 1
Doxycycline: 10mg/kg SID for 30 days. The bacteria Wolbachia inhabits the adult D. immitis, and therefore concurrent treatment to remove that risk with antibiotics should be considered. As a result, treatment with doxycycline renders the heartworms more susceptible to treatment.
Ivermectin: 0.2-0.3 mg/kg SC. Ivermectin is not a treatment for D. immitis but is considered a preventative, and useful in the protocol.
Day 15
Ivermectin: 0.2-0.3 mg/kg SC
Day 30
Melarsomine: 2.5mg/kg SC/IM. Merlarsomine dihydrochloride is an adulticide targeting the adult worms.
Day 60
Melarsomine: 2.5mg/kg SC/IM
Day 61
Melarsomine: 2.5mg/kg SC/IM
Prednisolone may reduce the risk of thromboembolic complications if given simultaneously. If burdens are high, then oral prednisolone can be used from Day 30 (or when the first adulticide dose is given). The dose must be tapered as such:
- 5mg/kg BID for 1 week
- 5mg/kg SID for 1 week
- 5mg/kg every 48 hours for 2 weeks
The period of greatest risk of post-treatment complications is 7-10 days after adulticide treatment, but may occur at any time. The most significant factors that contribute to these post-treatment complications are:
- Severity of cardiopulmonary tissue damage and disease
- Adult worm burden
- Levels of physical exertion by the patient
Prognosis
The prognosis is typically dependent on the adult worm burden and the severity of clinical signs at presentation. If tackled early, most animals have a good to moderate prognosis. If tackled late, then risks of cardiovascular episodes and post-treatment complications provide a moderate to poor prognosis.
Prevention
A regular monthly prevention protocol for all dogs in high-risk areas should be employed. There are multiple products available and macrocyclic lactones are the agents of choice e.g. ivermectin and moxidectin.