Dirofilaria Dog

1. Epidemiology/microbiology

· Dirofilaria immitus most common cause of heartworm and endemic in US.

· Canids are definitive host, but found in a number of other species.

· Endemic in North and South America, coastal Africa, southern Europe & Asia, cases increasing every year.

· Transmission

o Reservoir of disease in domestic and feral dogs.

o Mosquito vector (more than 70 species have been shown to transmit).

o Environmental temperature above 57 0F (13 0C) for development of infective L3 larvae.

· Life Cycle

o Life cycle of 7-9 months

o Female mosquitoes ingest microfilariae from host.

o Microfilariae transform into L1 larvae after entering mosquito.

o Two moults over 2-4 weeks (depending on temperature) into L3.

o L3 deposited onto dog’s skin in a droplet of hemolymph when feeding.

o Enter through bite would and moult to L4 within a couple of days.

o L4 migrate through body and from 50-70 days after infection undergo moult to juvenile worms.

o Worms penetrate circulation, mature over several months in pulmonary vessels. Produce microfilariae around 180-210 days.

o Mature adults have a life expectancy of 5-7 years and vary in length from 15-18cm (males) to 25-30cm (females).

· Pathogenesis

o Damage to pulmonary arteries and lungs – eosinophilic pneumonitis and endothelial damage.

o Wolbachia also implicated in disease process but mechanism unknown.

o Severity of disease related to interaction with host, exercise seems to make clinical signs appear more quickly.

o As worms die lodge in small distal pulmonary arteries – PTE. Leads to haemorrhage and fibrosis – Pulmonary hypertension.

o Leads to cor pulmonale and right-sided heart failure.

o Caval syndrome occasionally reported in large worm burdens due to obstruction of right atrium.

o Glomerulonephropathy is also reported.

2. Clinical findings

· Frequently asymptomatic, especially if inactive.

· Mild cough often noted first, followed by exercise intolerance and unthrifty appearance.

· Advanced disease – PTE, ascites, lethargy, haemoglobinaemia.

3. Diagnostic investigations

· Radiography – reversed D cardiac silhouette, caudal lobar pulmonary arteries enlarged and tortuous. May be accompanied by pulmonary parenchymal disease.

· Microfilarial detection – direct examination using ‘Knott’s procedure’. Negative in up to 80% of cases due to single sex infections, immune response, heartworm preventative. Positive from 6 months after infection.

· Serology - Antigen detection – sensitivity 95-100%, specificity 100%, as long as 3 or more female worms. False negatives in all male infections and with low number of females. Detected by 5-7 months after infection.

4. Treatment/prognosis

· Medical management:

o Melarsomine – arsenical-based drug, effective against adults and filarial stages from 4 months. Two dose protocol only kills 90% of adults, 3 dose protocol 98%

o Macrocyclic lactones – ivermectin, milbemycin, moxidectin, selamectin. Preventatives. Administered for 2 months before an adulticide to eliminate juvenile stages and allow maturation of adults for melarsomine to be effective. Pre-treat with antihistamines/glucocorticoids.

o Doxycycline – for Wolbachia.

· Prevention – macrocyclic lactone heartworm preventatives.

· Prognosis – varies with severity of worm burden and clinical signs from excellent to grave.

· Public health – rare reports of human infections.

Feline Heartworm Disease

· Same pathogen can infect cats, distribution the same as canine disease.

· Life cycle the same as in dogs up to the immature adult stage, at this point in most cats the juvenile worms die 3-4 months after infection resulting in an abbreviated infection. In a very small number of cats the worms mature to adults and live 2-4 years.

· Pathogenesis:

o Acute vascular and parenchymal inflammatory response to the worms (whether or not disease progresses).

o Can lead to severe lung injury and sudden death in some cats (10-20% of cats with adult infections).

o Can develop chronic respiratory disease (whether abbreviated or adult infection).

· Clinical findings – coughing or dyspnoea (64%), intermittent vomiting (38%), 28% no clinical signs. Respiratory distress, ataxia, collapse, seizures, sudden death rarely reported.

· Diagnosis – difficult.

o Radiology – Vascular enlargement (64%), hyperinflation.

o Echocardiography – worms visible in 64%

o Antigen testing – positive in 55% usually don’t get adult worms so will be negative.

o Antibody testing – positive in 82%

o Microfilarial detection – negative in 90%

· Therapy – no adulticide proven to be of benefit, therefore use long-term macrocyclic lactones and steroids.

· 10-20% mortality within 4 years.

· Prevention – as dogs.

Literature:

· Case report of an adult Dirofilaria worm being found in the spinal canal of an adult cat leading to hind limb paraparesis (JFMS 2013).

	Created by Andy Kent over 10 years ago