Exam 2 - Lecture 16 Flashcards
Lecture 16
Dirofilaria immitis - Canine Heartworm
-Large filamentous worms living in pulmonary artery and vasculature of dog, cats & other mammals (pet
ferrets):
-Adult males, 12‐20cm w/ coiled tails
-Females 25 to 31cm,
-Non‐remarkable morphology
-Reproduction is viviparous:
-Vermiform embryonic stage called microfilaria
-Diagnostic for infection status
Dirofilaria immitis Life cycle
-Dogs infected by mosquitoes harboring infective stage larvae
-The larvae migrate in SQ tissue:
-Colonize the pulmonary artery ~ 70 to 100 days post‐infection
-Become reproducing adult worms
-Microfilariae begin circulating ~6 months post‐infection (pre‐patent period)
-Female mosquitoes feeding on
microfilaremic dogs complete the lifecycle
-Development to infective stage in the mosquito, ~ 14 days @ > 57o F
-Transmission to new host
Canine Heartworm
-Enzootic in the United States
-Cases found in all lower 48 states, and Hawaii
-Temperature (> 57o F) & sufficient precipitation
-Development of infective larva & maturation of mosquito vector
-Seasonal & perhaps year-round transmission
-Life cycle dependent on infected animals for transmission of microfilariae to mosquito hosts
Mosquito Vectors
-Tennessee:
– Culex pipiens
– Cx nigripalpus
– Aedes trivittatus
– Ae japonicus
– Ae vexans
– Anopheles quadrimaculatus
-Other mosquito species:
– Aedes albopictus (FL, GA, LA,)
-Factors Affecting Transmission:
– Vector efficiency (# L3 / # MF), mosquito strain variability
– Mosquito feeding activity (day, dusk/dawn, night)
* Aedes (dusk/dawn) Culex (night), A. albopictus (aggressive day‐time)
– Microfilarial activity in host
* Coincides w/ mosquito activity?
– Source & average # blood meals taken
– Day/Night‐time temperature variation
* # of days where temperature facilitates development in mosquito
– Prevalence of microfilaremic hosts in area
Canine Heartworm Disease Pathogenesis
-Worms live in pulmonary arteries and right side of heart
-Immune stimulation of host tissues by adult worms & their products
-Obstructive fibrosis, pulmonary endothelial damage, & narrowing of vasculature bed w/ impaired blood flow
-Reduced cardiac output
Radiography
-Pulmonary hypertension induces compensatory hypertrophy w/ enlargement of the right side of the heart
-Reverse D radiographic profile
-Enlarged, thickened and tortuous pulmonary artery
-Pulmonary infiltrates in caudal lung lobes
Canine Heartworm Disease Pathogenesis
-Kidney disease from obstruction of glomeruli:
– Microfilaria
– Antigen‐antibody complexes
– Wolbachia pipientis
-Endosymbiotic gram‐negative bacteria:
– Recognized as an inciting factor in
heartworm associated inflammatory disease
– Present in arthropod vectors
– Transmitted in utero from adult female worms to their microfilariae
– Currently thought to be essential element to HW infection of mosquito host & survival of adult worms in definitive host
Caval Syndrome - Pathogenesis
– Large numbers of adult
worms are pushed through
the right atrium into the
vena cava
– Presence obstructs blood
flow & interferes with action
of tricuspid valve
– Blood passing through the
worm‐clot is hemolyzed
– Jaundice, hemoglobinemia,
& hemoglobinuria
– Fatal course of disease if not
attended to promptly 13
DIC “death is coming” with Severe Cases - Pathogenesis
– Disseminated Intravascular Coagulation
– Disorder associated w/massive
activation and consumption of
proteins involved in coagulation
– Uncontrolled hemorrhaging:
-Secondary host response triggered
by various infectious disease events
-Heartworm induced pulmonary
thromboembolism can be an facilitating factor!
Heartworm - Clinical Significance
-The most significant veterinary health issue in companion animal medicine
-Cases continue to appear in pet dogs seen in every veterinary practice:
-Despite improved diagnostic methods
-Highly effective preventive drugs
-Increased client awareness
-Apparent geographic spread into new localities:
– Intensity of infection density along Mississippi embayment & Gulf Coast
– Hawaii
– San Francisco area
-Cases in Alaska, all are associated w/ relocation or travel to endemic localities
– None autochthonous (resulting from local transmission)
Cases for Heartworm in the US
-7,188 cases noted in 2022 survey of TN veterinarians:
– 1.75 case/50 pet dogs tested
– State‐wide prevalence ~1.75%
– U.S. prevalence ~1.15%
– TN accounts for ~3.4% of all U.S.
cases
– Louisiana @ ~ 6%
– Texas @ 22%
– Alaska @ ~0.01%
-Cases in virtually all counties served by veterinarians:
– 321 cases/40,803 (~0.79%) Knox Co
– 16 cases/603 (2.6%) Claiborne Co
– 24 cases/571 (4%) Campbell Co
– 266 cases/2,338 (11.38%) Hardeman
Co
Insidious Disease & Economic-Emotional Burden
-Treating Canine Heart Worm Infections cost pet owners money
– 1.75% state‐wide prevalence; AVMA estimate 1,636,181 TN pet dogs
– 28,633 cases @ $1000/case (AAHA estimate) = ~$28,633,167
-Source of emotional anguish
-Potential cause/excuse for pet surrender
-100% Preventable
-Cost of prevention ($180/year) = $5,153,940 or ~ 18% of treatment
costs
-Consider annual cost of tobacco smoking @ $4,690/year (person)
Heartworm Preventives - Historical heartworm prophylaxis
-DEC + Oxibendazole
-Targets L 3 at molt to L 4
-Given by mouth SID
-Has “plus” for enteric nematode parasite control
-DEC is extremely microfilarcidal
-Annual testing mandatory to prevent anaphylaxis & death
Heartworm Preventives
-Monthly administration:
-Kills infective L 3 and early L4 larvae acquired over the previous 30 days since last treatment
-Most have some measure of GI & ectoparasite parasite control
What is ProHeart 6?
-Heartworm prevention
– Moxidectin (0.17mg/kg)
