Heartworm Flashcards
Heartworm lifecycle
- a mosquito bites an infected animal, ingesting heartworm microfilariae (pre-L1)
- microfilariae mature into heartworm larvae (L3) inside mosquito (10-14 days)
- infected mosquito bites a dog, transmitting the larvae (L3)
- Larvae enter the dog’s bloodstream, migrate to the heart and lungs, grow to a foot long and become sexually mature (7-9 months)
- Adult heartworms can live within the heart and lungs for 5-7 years
in which canadian provinces in heartworm most commonly found?
- ontario (76%)
- manitoba (14%)
- quebec (7%)
where is heartworm found in north america, mostly?
South-eastern united states
Is the risk of heartworm in Canada changing in dogs/cats?
- No evidence that risk of infection in Ontario has changed
how likely is a dog to get infected with heartworm in ontario? what is the overall prevalence? (2010 data)
- overall “prevalence” = 0.13%
- > 51% infections acquired in Ontario
- others cases are imported
what proportion of heartworm positive dogs in ontario were not on heartworm prevention in the previous year?
83%
where in ontario do most heartworm cases occur?
~80% cases south of highways 402/401/403
How many heartworm-positive dogs have clinical signs, ie how prevalent is disease?
~12%
prevalence of heartworm in cats compared to dogs in ontario
prevalence of infection is generally ~5-15% prevalence in dogs (typically 1-2/year in Ontario)
how long do heartworm parasites live in cats?
adult parasites typically live for 2-3 years
burden of heartworm in cats vs dogs and disease
burden typically less, but one parasite can cause disease
what tests do we run for heartworm in cats and with what time spacing?
antigen and antibody tests typically run simultaneously
Risk of heartworm infection depends on:
- geographical location
- travel
- lifestyle
- use of preventatives
drugs used for heartworm prevention in dogs
ivermectin
milbemycin
selamectin
moxidectin
heartworm product containing ivermectin for dogs
heartguard-30 plus
heartworm products containing milbemycin for dogs
interceptor, interceptor plus
nexguard spectra
credelio plus
heartworm product containing selamectin for dogs
revolution
heartworm product containing moxidectin for dogs
advantage multi
proheart 6
simparica TRIO
old heartworm product that kills if given to a positive dog
decacide, contains drug diethylcarbamazine
which heartworm dugs for dogs are licenced as safe to administer to HW+ animals
Revolution
Advantage multi
which heartworm drugs for dogs are topical?
revolution, advantage multi
what heartworm drugs for dogs are all-in-one combinatino products?
nexguard spectra
credelio plus
simparica TRIO
what drugs are used in heartworm preventatives for cats?
milbemycin
selamectin
moxidectin
sprinomectin
what products for heartworm in cats contain milbemycin?
interceptor, milbemax
what products for heartworm in cats contain selamectin?
revolution, revolution plus
what products for heartworm in cats contain moxidectin?
advantage multi
vitrecto
what products for heartworm in cats contain eprinomectin?
nexguard combo
which heartworm medication in cats are licensed as safe to administer to HW+ animals?
revolution
advantage multi
which heartworm products for cats are all-in-one products
revolution plus
vitrecto
nexguard combo
what drugs are in heartguard plus? what animals is it for? what does it prevent?
ivermectin + pyrantel (Heartgard Plus)
- dogs only
> safe at licensed dosage in Collie breeds - ivermectin: Dirofilaria immitis
- pyrantel:
> Roundworms (Toxocara /Toxascaris)
> Hookworms (Ancylostoma /Uncinaria)
what drug is contained in interceptor? what animals is it used for, and what parasites? safe in collie breeds? what other life stage does it kill?
milbemycin
- for dogs and cats
protects from:
-dirofilaria immitis (dogs and cats)
-Toxocara spp. (dogs and cats)
- Toxascaris leonina (dogs only)
- ancylostoma spp. (dogs and cats)
- trichuris vulpis (dogs)
> safe in collie breeds
microfilaricidal at heartworm-preventative dosage
what drug in contained in interceptor plus? what does in protect against? what species is it for
milbemycin and praziquantel
- for dogs only
-same as interceptor, + tapeworms:
- dipylidium caninum, taenia pisiformis, echinococcus spp.
what drug is in revolution? what does it protect against in dogs and cats? safe for collies?
selamectin (topical), for dogs and cats
protects against:
- dirofilaria immitis (both)
- toxocara spp. (cats, + aid in control for dogs)
- ancylostoma spp. (cats)
- fleas (both)
- otodectes (both)
- sarcoptes (dogs)
- dermacentor variabilis (aid in control for dogs)
- rhipicephalus sanguineus (dogs)
-safe for collies and in heartworm + animals
what drug is in advantage multi? what does it protect against? safe for?
moxidectin + imidacloprid, for cats and dogs
-dirofilaria immitis
-toxocara spp.
- toxascaris leonina (dogs only)
- ancylostoma spp.
-fleas, mites, etc.
-safe for collies and in heartworm + animals
what is proheart6 used for and what does it contain? how long does it protect for? adverse stuff?
moxidectin
-dogs only
-dirofilaria immitis
- ancylostoma caninum
- uncinaria stenocephala
-6 months of preventive activity
- adverse reactions known (GI, hepatic, neuro…)
products “for everything” for dogs, and drugs they contain
Nexguard spectra
- milbemycin + afoxalaner
Simparica TRIO
- moxidectin + serolaner + pyrantel
Credelio plus
- milbemycin + lotilaner
what life-stage do monthly heartworm preventatives generally kill?
-approved for L3 and early L4 stages
> kill larva that have been developing for the last ~4 weeks
When should dogs/cats be places on monthly preventive medication?
-drugs approved with 1 month “reach back” activity:
Start: (Ontario transmission starts ~June 1):
● one month after earliest start date of transmission season (Ontario = July 1)
Finish: (Ontario transmission stops ~October 8):
● within 1 month following end date of transmission season (ontario = Nov 1) for all except credelio plus, interceptor plus, and nexguard spectra
● 2 months following end date (Credelio Plus)
● 6 months following end date (Interceptor Plus, nexguard spectra)
when is year round heartworm prevention reccomended?
if transmission occurs >6mo of the year
If a puppy/kitten is born during the heartworm-transmission season, by what age should it receive its first preventative treatment for heartworm?
- 8 weeks
> off label, monthly products have 7-8 weeks of reachback
=> so, by 8 weeks of age
what methods should be used to screen dogs for heartworm? when?
Antigen + Microfilaria tests:
- some dogs antigen negative but microfilaria positive
- antigen + microfilaria in blood ~7 months following infection
when?
- 7 months after end of transmission season (ie. May-June in Ontario)
what is the minimum age that a dog should be tested for:
a) heartworm antigen?
b) heartworm microfilariae?
7 months for both
when should dogs be tested for heartworm?
- first time seen (if > 7 months old) to establish baseline data
Thereafter:
- licensed use of heartworm preventatives requires annual testing
- testing less frequently than once a year = off label:
> acceptable if evidence to justify (ie. low risk); requires informed consent
In cases of noncompliance or changing the
brand or type of heartworm preventive, the dog
should be antigen and microfilaria tested prior
to starting or changing products.
risk of going off label with heartworm medication:
drug companies will typically not cover adulticide treatment if infection occurs
How frequently should you test for heartworm in canada? why?
- In Canada, justification for annual testing of dogs on preventive medication, especially if good owner compliance, is questionable:
- low risk of infection
- preventives work extremely well
- very low predictive value of positive test
- Evaluate risk of infection on an annual basis
- test if:
(a) travel to higher risk area,
(b) poor/no compliance - Annual testing in high risk areas (e.g. southeast USA)
In May 2023, you examine a dog that received selamectin monthly from June 1 to November 1 in
2021 and 2022 (good compliance):
- in May 2022 dog tested heartworm antigen -ve
Has never travelled outside Ontario BUT will travel to Texas for 2 weeks in January 2024.
What heartworm testing and preventive treatment would you recommend for the next 12 months ?
-treat after Texas: one treatment 4 weeks after their departure date (to Texas)
> Testing in May/June 2023 can be done to stay on label, but risk is low so don’t have to if client does not wish to and there is informed consent (due to good compliance and location)
> Test in May 2024 would not detect any heartworm infection picked up in Texas (<7 mo time period), so that would not be a factor for deciding to test at this time
In June 2023, you examine a puppy born in
Ontario in December 2022:
- has never received heartworm preventives
- will not travel outside Ontario
What heartworm testing and preventive treatment would you recommend for the next 12 months ?
-Don’t need to test puppy as it was born outside of heartworm season and is only ~6 mo old, which is too young to detect antigen/MF
- Test in May/June 2024
- can prescribe HW prevention starting July 1 (to Nov 1, or different depending on medication selected)
What do we do if a dog tests antigen positive without clinical signs?
- Repeat antigen test with different blood sample (different company too, to rule out processing errors)
- Microfilaria detection, using a concentration method (eg. knott’s)
- Review travel and compliance history
- no clinical signs, so rads and US will not be useful
what additional tests should we do for a dog with clinical signs of heartworm?
- repeat antigen test with different blood sample
- Microfilaria detection test
- review travel and compliance history
- radiographs
- echocardiography
most D. immitis and infected with what? how do we deal with this in a positive case?
Wolbachia, an intracellular bacteria
-if dog is HW+, treat with doxycycline at 10mg/kg BID for 4 weeks
>reduces bacteria numbers in all parasite stages for ~12 mo
> reduces pulmonary pathology after melarsomine treatment.
> gradually suppresses microfilaremia if given to HW+ dogs
> reduces parasite transmissibility in mosquito after ingestion of microfilariae
how do we treat dogs infected with heartworm?
(i) Treat with heartworm preventive on days 1, 30, 60, etc (if risk of infection).
(ii) Treat with doxycycline daily from days 1-28.
(iii) On days 61, 90 and 91 treat with melarsomine.
(iv) On days 61 and 90: prednisone at decreasing dosages for 4 weeks
3-dose protocol used for all infections except caval syndrome
> melarsomine at 2.5mg/kg, IM, on days 61, 90, and 91
(used to do 2 treatments 24h apart)
what to be mindful of when using melarsomine for HW; protocols?
Be extremely careful that 100% is given IM (epaxial muscles)
- strict cage rest for 1 month following each treatment
- recognize that treatment protocol is not 100% efficacious
- antigen and microfilaria test 9mo after last treatment to determine if treatment eliminated infection
What is the “slow killprotocol” for HW? what is important to note?
monthly reventive medication for 6+ months
starting day 1:
- moxidectin/imidacloprid (advantage multi) monthly for 6-18 months
-doxycycline for days 1-28
-retest for antigen every 3-4 months
-strict cage rest throughout?
Note: ivermectin/pyrantel (Heartgard Plus) used by some but significantly
less efficacious – may take monthly treatments for >5 years!
Note: not endorsed by American Heartworm Society!
how do we manage HW positive cats?
- do not use melarsomine
-if possible, use “symptomatic” therapy (e.g. prednisone)
-surgical removal? - monitor using antibody and antigen tests, and thoracic radiography, every 6-12 months:
> antigen negative within 4-5 months following elimination
> may remain antibody positive for extended period (antibody generated to surface antigen of immature stage)
reccomendation of CAPC for heartworm prevention
- year-round preventatives to maximize compliance
negative implications of increasing the number of preventative treatments for HW from 6 to 12 times per year in Ontario
- cost
- selection for drug resistance in D. immitis
- selection for drug resistance in intestinal parasites
is drug resistance developing in heartworm?
- anecdotal evidence from parts of southern USA
> more + cases in dogs on prevention reported
> resistance to macrocyclic lactones? - resistance to ivermectin and milbemycin documented in Canadian dog that originated from Louisiana
> adults parasites eliminated with melarsomine
>unable to eliminate microfilariae with MLs
implications of drug resistance in heartworm?
- most cases in Mississippi River Valley
- ensure HW treatment protocol eliminates both adults parasites and microfilariae, particularly in dogs originating from SE USA
