Dirofilaria - Dog & Cat

Andy Kent
Note by Andy Kent, updated more than 1 year ago
Andy Kent
Created by Andy Kent almost 6 years ago



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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).

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