Parasites of the Cardiovascular System

DOES NOT include Chagas

What is the casual agent of heart worm disease?

dirofilaria immitis

What super family does dirofilaria immitis belong to?

filarial nematode (filarioidea)

What are the definitive hosts of dirofilaria immitis?

domestic dogs

What are the accidental hosts of dirofilaria immitis?

domestic cats, ferrets, humans

What are the physical characteristics of dirofilaria immitis?

white, long, plain nematodes between 12-31 cm long (female worms are longer than males)

Where do dirofilaria immitis usually accumulate in dogs and cats?

pulmonary arteries

What is the lifespan of dirofilaria immitis?

adult worms life 5-7 years and microfilariae may stay viable up to 2.5 years in dogs (shorter life span in cats)

What is the first step of the dirofilaria immitis life cycle?

microfilariae of an infected dog are picked up by a mosquito upon feeding

What stages do microfilaria molt within the mosquito? For how long?

from L1 stage to the L3 stage takes 10-14 days; development/molting is temperature dependent and takes the majority of a mosquitos life span (~3 weeks)

Where do microfilaria become the sausage stage and molt?

malpighian tubules

How do L3 enter the circulatory system?

labium bends at feeding releasing hemocoel (hemolymph) fluids that contains L3 which enter via puncture wound of stylett

In what stage do worms migrate toward thorax between muscle fibers?

L3-L4

In what stage do worms migrate intramuscularly or in blood vessels?

juveniles

How long after infection does it take for L3 to molt into L4 in the subcutaneous tissues? How long do they stay as L4?

3-4 days to molt and they stay as L4 for about 45-65 days

By _____ days, most L4 are in the abdominal tissues.

21

By _____ days, most L4 are in the thoracic tissues.

41

At ______ days, L4 molt to juveniles, which penetrate muscles and veins, then begin entering the heart and pulmonary tissue via vasculature.

50-70

What occurs by day 70-120 (2.5 - 4 months) post-infection?

developing larvae have moved to the small vessels of the lungs (juveniles)

What happens to juveniles in the pulmonary arteries?

grow 10-fold and begin to occupy larger vessels and then the right heart

When do juveniles become sexually mature? What occurs during this time?

6-7 months, after this time microfilariae (and circulating antigen) are present in blood if both male and female worms are present and mate

What do several products available for dogs contain at high concentrations to repel and kill mosquitos?

permethrin

What are the primary species of mosquitoes?

culex, aedes, anopheles

What are the primary species of mosquitoes that cause natural infection in large animals?

C. quinquefasciatus and Aedes albopictus

What is the natural infection rate in wild mosquitoes?

~7-10%

What is the natural infection rate of mosquitoes within flight range of heartworm positive animals?

~75% (3-7 miles)

What is wolbachia?

obligate, intracellular, gram-negative, endo-symbiotic bacteria

What component does wolbachia have in the pathogenesis of filarial disease?

wolbachia surface protein (WSP) induces a specific IgG response in hosts infected by D. immitis; hypothesized that wolbachia contributes to pulmonary and renal inflammation through WSP so eliminating wolbachia reduces the immune component of pathology

What pre-treatment kills wolbachia? What impact does this have?

doxycycline; enhanced adulticide efficacy, reduced pathological sequelae, doxy halts production of additional microfilariae, and prevents exposed microfilariae from developing to adults in new host after development in mosquito

True or False: All dogs with heartworm have disease whether or not symptoms are present or not.

True

When does disease begin to occur?

shortly after immature worms first enter the pulmonary arteries

What life stages of D. immitis cause pathology?

migrating larvae, juvenile and adult worms, dead adults and microfilariae

What are some examples of lesions seen with D. immitis infection?

mechanical damage, inflammatory damage, immune-mediated reactions, thromboemboli

What does severity of disease depend on?

number of adult worms (more is worse), duration of infection (longer is worse), individual host response (more robust is worse), and worm condition (dead worms are more pathogenic because they form thromboemboli)

What is pathognomonic of heartworm disease in necropsy?

roughened, stippled appearance

Vessel thickening and loss of elasticity causes increase pressure on pulmonary artery, right heart and vena cava. What does this lead to?

chronic passive congestion, pulmonary hypertension, right heart enlargement, congestive heart failure, ascites, hepatomegaly

What is class I heart worm disease?

asymptomatic to mild disease, with no clinical or radiographic signs and no laboratory abnormalities. Subjective signs such as loss of condition, decreased exercise tolerance, or occasional cough may be seen

What is class II heart worm disease?

moderate disease, characterized by occasional cough and mild-to-moderate exercise intolerance, slight loss of condition, increased lung sounds, and mild to moderate radiographic changes, including right ventricular enlargement. Laboratory results may show anemia & proteinuria

What is class III heart worm disease?

severe disease variably characterized by anemia, weight loss, exercise intolerance, resting tachypnea, severe or persistent coughing, dyspnea, hemoptysis, syncope, and ascites. Severely abnormal radiographs show right ventricular hypertrophy, pulmonary artery enlargement, and diffuse pulmonary densities. Laboratory results indicate marked anemia, thrombocytopenia, and proteinuria. Electrocardiographic evidence of right ventricular hypertrophy is often present

What is class IV heart worm disease?

sudden onset with collapse, hemoglobinuria, and respiratory distress. This syndrome is usually fatal if treatment is not started immediately (poor to grave prognosis); caval syndrome

What are signs of caval syndrome? Think acute heart worm disease.

large numbers of worms obstruct blood flow though tricuspid valve and cause hemolytic anemia & regional heart failure, rapid onset, hemoglobinuria, dx confirmed by echo, death usually within 12-72 hours, poor candidates for treatment, tx=surgical removal of worms via jugular vein

What are the characteristics of feline dirofilariasis?

susceptible hosts but more naturally resistant to infection with adult Dirofilaria immitis than are dogs; adult numbers are low, stunted, and live only 2 years (contrasts to 5 in dogs) medial life span being 1.5 years

What can be seen with feline dirofilariasis?

typical amicrofilaremia or transient microfilariae, pulmonary intravascular macrophages

True or False: Cats with only a few worms are considered to be heavily infected.

True

Why is clinical important of D. immititis amplified in cats?

even a small number of heart worms are life-threatening, although often asymptomatic

What is the 1st life cycle phase of HW disease in cats?

high mortality of large numbers of juvenile hw arriving in pulmonary vasculature 3-4 months post-infection (heartworm associated respiratory disease)

What is HARD? (heartworm associated respiratory disease)

acute vascular and parenchymal inflammatory response resembling asthma or allergic bronchitis during the first stage of the hw life cycle in cats

What is the 2nd life cycle phase of HW disease in cats?

sporadic death of adult heartworms; even one degenerating adult can result in acute fatal pulmonary inflammation and thromboembolism → sudden death

What are the clinical signs of acute feline hw?

respiratory distress, ataxia, seizures, collapse, circulatory shock, sudden death; asthma-like shock, acute pulmonary infiltrates with eosinophilia

Why are acute diseases seasonal in the fall in Louisiana?

associated with maturation; Mf production by females and/or mortality of young or adult worms acquired in spring-summer

What are the clinical signs of chronic feline hw?

often asymptomatic, respiratory signs such as cough or dyspnea and or vomiting (reportedly due to death of juveniles when entering the heart)

What is characteristic of human dirofilariasis?

abortive results in radiographic changes referred to as “coin lesions” which have been misinterpreted as representing neoplasia and can lead to unnecessary thoracic surgery

What is antigen testing?

preferred method (~20% may be mf negative) detects female uterine antigen via ELISA (earliest detection about 5 months post-infection), semiquantitative

What is microfilariae testing?

performed in tandem with antigen testing (no recommended alone), concentrates mf (modified knott’s or filtration test), earliest detection about 6 months post-infection

How long after successful adulticide treatment is circulating Ag still detectable?

up to 6 months, occasionally longer

Is there a relation of mf number to worm burden?

No, 10% mf are in peripheral circulation and 90% in splanchnic circulation (liver, lung, spleen)

What is nocturnally sub-periodic?

mf numbers increase slightly in the late afternoon

What is acanthocheilonema?

non-pathogenic reconditum which reside as adults in the subcutaneous tissue produces mf

What are Dr. Baker’s “5 Rs” of reconditum?

rare, reduced, rectangular, recurved (rounded), and rapid

What are the physical characteristics of D. immitis?

thicker, tapered head, straight tail, high number of aimlessly moving microfilariae

What are the physical characteristics of acanthocheilonema reconditum?

thinner, rectangular head, rounded (recurved) tail “button book”, low number of progressively moving microfilariae

What is inferred when you have an antigen positive but microfilaria negative patient?

unisex infection (males only), immune-mediated clearance of microfilariae, dog is on monthly preventative, tested after microfilaricide treatment, immature adults not yet producing microfilariae (5-6 months post-infection)

What is inferred when you have an antigen negative but microfilaria positive patient?

microfilariae are not D. immitis, adults are dead and antigen cleared but microfilariae still circulating, antigen blocking from antigen-antibody complexes (heat sample to release), microfilarial contamination of lysing solution, dye, or filter chamber, dog was transfused with microfilaremic blood, prenatal transfer of microfilariae, antigen destroyed due to improper storage or mistreatment of sample, hypergammaglobinemia (gamma globulins have non-specifically bound antigen)

What can provide information on disease severity?

thoracic radiographs

What can be seen on echocardiography?

body wall of adults is highly echogenic, distinctive, short, parallel-sided images with the appearance of “equal signs”

Because of potentially lower worm burden, what is more difficult to diagnose in cats?

single-sex infections and infrequency of microfilaremia

What is true regarding microfilaria tests in cats?

<20% of cats with mature heartworm infection are microfilaremic

What is true regarding antibody tests in cats?

approximately 15-25% of cats with adult infection are antibody-negative and on the other hand, 80-90% of antibody-positive cats do not harbor mature worms

What is true regarding antigen tests in cats?

only mature female worm infections (typically 7-8 months post-infection) are detected so a positive test indicated that the cat harbors or recently harbored mature female heartworms

Due to one third of adult heartworm infections in cats only consisting of male worms, what does this do to the antigen test?

they are not detected using antigen test (only females detected) so a negative test does not conclusively rule out adult heartworm infection nor will it detect juvenile worms that are responsible for HARD