Exam 2 LC16: Canine HWD
What is the bacterial endosymbiont present in all D. immitis parasites?
Wolbachia pipientis - present in all life stages of HW
What is the dx stage of HW?
adults and microfilaria
On what state of HW does tx work?
adults only
Currently all USDA approved antigen tests detect what specific antigen?
female uterine antigen
Can microfilariae develop into adults in the same dog?
no - IH is required for development
Does a weak positive HWT tell you anything? What about a strong positive?
no - it positive no matter the intensity - just means detected either lower or higher level of antigen
Adult D. immitis occur primarily in what location in the body?
pulmonary arteries - secondarily in right heart
What is the scientific name of HW?
Dirofilaria immitis - Dirofilariasis
Where do adult HWs naturally mautre?
w/i pulmonary arteries - NOT in right side of heart - adult worms can occur in RV in heavy infections or due to backflow after dog dies
What is the PPP of D. immitis?
6-7 months
When can you dx HW?
6-7 months post-infection
What is the youngest age you should test dogs for HWD?
7 months due to PPP
How do Wolbachia spp. contribute to HWD pathogenesis?
- Wolbachia are released in large #s at death of HW and during microfilariae production - release of bacteria and endotoxins exacerbates lung injury
How do you dx D. immitis?
- detect antigen in blood (mainly produced by female worms) - recover microfilaria in blood *most dogs are diagnosed before clinical signs are present bc of annual screening
When do you retest for HW?
- in 6 mo. - *ANUALLY - pet in highly endemic area - new pet not on HWP - non-compliant - after winter "off" - changing products
Adult HWs cause progressive pulmonary hypertension due to pathological changes in the pulmonary arteries. What pathological changes are seen during the progression to pulmonary hypertension?
- inflammation of vascular endothelium → thickening of vessel → turbulent blood flow - plasma leakage into lung → pulmonary edema - *pulmonary hypertension → RV hypertrophy & right heart failure
Pathology - adult HWs: Caval syndrome is a rare but life threatening pathologic outcome of HWD. Describe what caval syndrome is and what causes it
- large mass of worms in RV, RA and vena cava (back up) - caused by exposure to large # infected mosquitoes over short period of time
What are the 5 causes of false negative antigen HWTs?
- low worm burden (sensitivity) - immature females (recent infection; prepatent) - male infections only (no antigen tests reliably detects males) - not allowing test to warm to room temp (if tests require refrigeration) - immune complexes binding antigen
Summary of D. immitis stages and locations
- microfilariae: blood → mosquito - L3: mosquito → SC - L4: SC - adults: pulmonary arteries and righ theart
Describe the pathology caused by microfilaria
- obstructing fine capillaries - immune response to dying microfilariae may form immune complexes and impair kidney function , resulting in proteinuria
What are the clinical signs seen with severe HWD?
- pale MM - bounding jugular pulse - murmurs (right sided - most often assoc. w. tricuspid) - ascites - enlarged liver and/or spleen - hemoglobinuria
What are the major clinical signs of caval syndrome?
- partial obstruction of blood flow through tricuspid valve/interference w/ valve closure (causes bounding jugular pulsations, hemoglobinuria and hemoglobinemia due to RBC lysis) - sudden onset w/ no prior hx suggestive of HWD *life-threatening w/ rapid fatality
When would you use a heat-tx for HW testing?
- pets w/ clinical signs indicative of HWD but negative Ag and microfilaria tests - discordant results (Ag and microfilaria test results don't match)
In ~20% of HW positive dogs will not have circulating microfilariae. Why?
- prepatent infections (immature worms) - unisex infections (only male or only female) - drug-induced reduction (MLs) - immune-mediated clearance
Besides antigen and antibody tests, what are other diagnostic methods used for HW?
- rads - echo *more for staging dz
Why do we seen immune complex binding antigen, which interferes with HWT results?
- shelter dogs - dogs on "slow-kill" tx regimen
What are the PE findings of HW?
- tachycardia - weak pulses - cough - difficult breathing
Pathology: Why do dead adult worms cause even more inflammation?
- thrombus formation around worm => embolism that can lodge in other areas - vascular fibrosis around worm (mummification)
What are you testing for in regards to tick and HW testing?
- tick: antibodies - HW: antigen *some can be combo tests
What are the causes of false positive antigen HWTs?
- well type tests due to inadequate washing - cross rxn with other antigens (Spirocerca lupi) - residual circulating antigen following tx of HWD - other unknown antigens or test errors
What parasite species produces microfilaria that look like D. immitis microfilaria?
Acanthocheilonema reconditum - not cross reactive for HWT (does not produce FP)
Besides pulmonary hypertension, what other pathological signs are caused by adult HW?
- worm obstruct vessels, heart chambers, and valves - inflammatory mediators induce immune complex glomerulonephritis (type III hypersensitivity) - caval syndrome
How rapidly clinical signs of HWD occur in dogs can be quite variable and are due to...
- # worms - *duration of infection - animal size - exercise level of animal
What are the clinical signs of HWD?
- *fatigue; exercise intolerance - *cough - cyanosis - heart murmur - tachycardia - ataxia - syncope
What are the natural DH and IH for HW?
- DH: dogs - IH: mosquitoes
Describe the lifecycle of D. immitis
- Female D. immitis in pulmonary arteries deposit microfilariae - mosquito bites dog and acquires microfilaria - microfilariae develop into infective L3 - mosquito bites dog and deposits infective L3 SC where molt to L4 - L4 migrate to thorax and molt to immature adults w/i 2-3 months post-infection - immature adults enter circulation between 70-90 days post-infection - mature D. immitis mate and deposit microfilariae 6-7 months post-infection
What are the 3 tests for microfilaria?
- Knott's test - Filter techniques (Difil Test) - direct smear
T/F: The color intensity of a positive antigen test result cannot reliably be used to determine the level of worm burden
True - does not distinguish heavy vs low infection - blue spot no matter the intensity = positive result
T/F: Using HWP on-label requires annual HWT. This test does NOT have to be negative
True - just have to test
On what stage of HW do preventives work?
L3 and L4 larvae
What is the infective stage of HW?
L3 larvae
T/F: HW preventives prevent HWD, but do NOT prevent getting heart worms/larvae
True
T/F: L3 and L4 are NOT found in the circulatory system. They migrate through tissues
True
T/F: There are often no clinical signs of HWD noted during the PPP
True
T/F: You can start to see pathological signs caused by heart worms before an animal shows up positive on a HWT
True
T/F: a differential for an enlarged right heart is HWD, but this is not specific to HWD
True
What is the most common presentation of HWD?
annual wellness visits
A 2 year old lab, not on HWP, tests negative on a snap test during annual exam. Can you tell the owner that their pet's HWT is negative?
tested negative for HW antigen, but dog may have microfilaria present - start HWP now and retest in 6-7 months
What should you do if a dog is negative for microfilaria but antigen positive?
this is very common - retest using a different antigen test - microfilaria test are unreliable