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 9 years ago