Information compiled by Julia Michitsch B.Sc., M.Sc. (Zoology)

About Heartworm Disease

Heartworm (Dirofilaria immitis) is a parasite transmitted by mosquitoes that can cause potentially fatal health concerns in both dogs and cats. Heartworm is present in all 50 U.S. States with Mississippi, Louisiana, Arkansas, Texas and Tennessee being the states with the highest recorded incidence¹. It has slowly made its way up to Canada, with Ontario being named as the province with the greatest risk of infection². In 2019 alone, IDEXX laboratories (one of the companies that perform heartworm testing) reported that there have been 91 positive test results in Ontario ³. 

Prevalence of Heartworm Disease in Ontario

 According to a study conducted between 2007 and 2016² looking at blood samples from dogs, there were 762 positive test results for heartworm in Ontario, with 722 of these occurring in Southern Ontario. Three clusters of highest prevalence were determined based off of these results: the North-Western Region (Rainy River county, west of Thunder Bay), the Southern Ontario Region (encompassing the counties of Haldimand-Norfolk, Oxford, Elgin, Chatham-Kent and Lambton) and the Eastern Ontario Region (Lanark and Renfrew counties, just west of Ottawa). Whereas the total prevalence for all of Ontario was calculated to be 0.12%, Haldimand-Norfolk was recorded at 0.64%, Chatham-Kent at 0.59% and Rainy River at 0.50%. Interestingly, the counties of Prince Edward, Kawartha Lakes, Dufferin, Muskoka, Haliburton, Parry Sound, Manitoulin, Sudbury, Timiskaming, Cochrane and Algoma all had zero positive test results from 2007-2016.

A similar study was conducted by another group of researchers looking at the prevalence of heartworm specifically in cats in both the U.S. and Canada⁴. Positive test results were found in cats in 35 of the U.S. states (more so in the Southern states), but not in Canada. This study found a 3-fold increase in positive results in outdoor cats, and a 2.5 increase in cats that were unhealthy when tested (immune systems were already weakened by other conditions). The increase in positive test results for heartworm observed in dogs in Ontario from 2007-2016 could be related to an increase in frequency of screen tests being conducted². The heartworm detection test is part of the screening panel that also includes tick-borne diseases such as Lyme, Anaplasma, and Ehrlichia – all of which became more of a concern in Ontario over the time that this study was conducted, and as such led to an increase in the number of screen tests completed.

Climate change is also considered to be a potential cause for the increased prevalence of heartworm in Ontario^1,2,5. The increasing seasonal temperatures have allowed mosquito populations to travel farther north than they ever have before, and have allowed them to remain active for a longer season as well¹. Generally, the higher the temperature, the faster mosquitoes will develop from egg to adult, and the faster the heartworm parasite can progress through its lifecycle¹. 

The lifecycle begins when adult heartworms living in an infected animal mate to produce microscopic baby worms called microfilariae (singular:  microfilaria)⁵. Microfilariae circulate in the blood stream of the animal until they are picked up by a feeding mosquito⁵ . The baby worms continue to develop within the tissues of the mosquito until they become third stage larvae (L3), the stage at which they can be transmitted by the mosquito to a new host⁶. The rate at which the baby worms develop into the L3 larvae is temperature dependant, taking as little as 10-14 days when the weather is warm⁶. The L3 larvae are deposited on the skin of the new host while the mosquito feeds⁶. They enter the new host’s body through the bite wound and continue to develop as they make their way through connective tissue and into the blood stream⁶. After 6-7 months of living in the new host, the larvae become sexually mature adults and the cycle starts anew⁵. 

 Adult heartworms can be up to a foot in length and usually live in the heart or lungs of the host animal, or in the blood vessels associated with these organs⁵. Dogs are a natural host for heartworm as their bodies provide a suitable environment for the worms to grow and reproduce⁵. Cats can become infected with heartworm, but generally, the worms do not survive to adulthood⁵. Adult worms can live up to 5-7 years in a dog or 2-3 years in a cat⁵. An infected cat will typically have only 1-3 worms  living in their body (usually juveniles) whereas a dog can have up to several hundred as the adult worms continue to produce offspring over their lifespan⁵. As adult worms can survive for multiple mosquito seasons, the likelihood of transmission from an infected animal to a new host is high⁵. 

 Symptoms of Heartworm Disease

Typical symptoms of heart worm disease in dogs include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss, with advanced disease leading to heart, liver or kidney failure⁵. Life-threatening events such as blockage of the blood vessels in the lungs (pulmonary thrombosis), and Caval Syndrome (cardiovascular collapse caused by sudden blockages within the heart) may occur as the worms increase in number and block blood flow through vessels and organs⁵. Whereas dogs normally don’t show symptoms until there are many worms present, having just 1-2 worms in a cat can make them very ill⁵.  Symptoms in cats can include coughing, asthma-like attacks, periodic vomiting, and lack of appetite or weight loss.  Difficulty in walking, fainting, seizures or fluid build-up in the abdomen can occur occasionally, and in some cases the first sign of disease in a cat is sudden collapse, or even sudden death⁵. Severe complications such as blood clots in the lungs can occur when adult worms die in the cat’s body⁵. Heartworms in cats can also migrate to other areas of the body such as the brain, eye and spinal cord⁵. 

Treatment of Heartworm Disease

Heartworm disease can be successfully treated in dogs, but treatment is expensive, long in duration and quite complex, usually consisting of several steps⁵. Symptomatic dogs are stabilized, and then given melarsomine (the only drug approved for heartworm treatment) in hospital which kills both the adult and juvenile worms⁵. While treatment is going on, the dogs must be restricted in their activity, as physical exertion can increase complications and further the damage done to the heart and lungs by the worms⁵. Most of the worms die quickly, within 1-3 months of treatment being administered, and follow-up blood tests will confirm whether any further treatment is required⁵. Unfortunately, while melarsomine is effective and safe to use in dogs, it is not safe for use in cats⁵. With no approved drug therapy available, treatment of heart worm disease in cats involves stabilizing the patient followed by the implementation of a plan for the long term management of any lung or heart symptoms that arise⁵. In some cases, heartworm infections in cats can resolve on their own – perhaps owing to the fact that cats are not ideal hosts for the worms⁵. Even in cases such as these, the worms still leave permanent damage in the respiratory system of the cat⁵. 

The Importance of Heartworm Disease Prevention

GET YOUR PETS TESTED!

The earlier heartworm disease is detected, the better the chances of treating it successfully and minimizing the damage caused by the worms⁵. As the symptoms of a heartworm infection can take months to become noticeable in pets, regular screening via blood test is extremely important in order to catch the infection early⁵. As a general rule, pets should be tested for heartworm yearly – prior to the start of the mosquito season, and prevention medication. The test works by detecting proteins specific to the adult female worm in the blood.  As discussed earlier, it takes about 7 months after the mosquito transmits the L3 larvae into the host for adults to develop. As such, a test administered prior to the presence of adults will come back negative – masking the fact that the pet is actually infected. This is why puppies under 7 months do not need a blood test in order to start prevention medication, and why pets that have not been on prevention medication before should be tested not only prior to starting medication, but also 6 months later. In cats, since adult worms do not tend to develop at all, an additional type of test may be used which detects antibodies made by the immune system in response to the presence of heartworm larvae in the blood⁵. A chest x-ray or ultrasound may also be used as a tool to screen for heartworm in cats⁵. As mosquitoes can live both outside and inside our homes, pets that are kept indoors are also at risk and should be tested⁵. Pets that are on prevention year-round should be tested yearly as a precaution, to ensure the efficacy of the prevention product⁵. 

USE PREVENTION MEDICATIONS!

There are a variety of medications available to prevent heartworm infection in both dogs and cats. They can be administered orally (as a chew), or topically (applying liquid to the skin to be absorbed) on a monthly basis and all act to prevent the larval stage worms from growing into adults. To date, no vaccine for heartworm has been developed⁵. The heartworm season in Southern Ontario is generally agreed upon to be from May until November – the months during which the temperature is warm enough for mosquitoes to be out and feeding. In order to keep pets safe from contracting heartworm disease, they should be given prevention medications monthly during this time. Remember that mosquitoes can enter and live in our homes as well, so monthly prevention medication is equally important for indoor animals. These medications are highly effective, as long as they are given correctly. If the medication is missed, given late or if the dog vomits or spits out the chew they will be unprotected from infection⁵. 

HELP TO CONTROL THE MOSQUITO POPULATION!

There are several ways that we can help to reduce the number of mosquitoes present in our environment. Mosquitoes can lay 100-400 eggs at a time on the surface of still or slow moving water⁷. The eggs hatch into larvae after 1-2 days, and larvae develop into adults in less than 10 days if the temperature is ideal (between 22-27°C)⁷. In order to prevent mosquitoes from breeding it is important to remove standing water from around the yard. Some ways that this could be done include:  keeping gutters clear, storing flower pots, watering cans, boats and wheelbarrows upside down, replacing water in bird baths, wading pools and outdoor pet dishes at least twice a week, covering garbage, recycling and compost bins – or drilling a drainage hole in the bottom of the bin to allow water to escape, emptying rain barrels if water is over 1 week old (unless the barrel is protected by a fine screen), repairing leaking pipes or hoses promptly,  keeping swimming pools chlorinated and removing water collecting on pool covers, and aerating ornamental ponds⁷. Also, keep natural predators of mosquitoes around – fish, frogs, beetles, dragonflies, waterbugs and birds – as they will help keep the adult mosquito population under control as well as the larvae⁷. 

References

1. Carrozza, A. (2018, April 18). Heartworm: Where are we today? Retrieved November 11, 2019, from https://www.americanveterinarian.com/journals/amvet/2018/april2018/heartworm-where-are-we-today?p=1
2. McGill, E., Berke, O., Peregrine, A.S., & Weese, J.S. (2019). Epidemiology of canine heartworm (Dirofilaria immitis) infection in domestic dogs in Ontario, Canada: Geographic distribution, risk factors and effects of climate. Geospatial Health, 14(1). Doi: 10.4081/gh.2019.741
3. IDEXX – Real-time Pet Disease Reporting. (2019). Retrieved November 18, 2019, from http://www.petdiseasereport.com/content/prevmap.aspx.
4. Levy, J.K., Burling, A.N., Crandall, M.M., Tucker, S.J., Wood, E.G., & Foster, J.D. (2017). Seroprevalence of heartworm infection, risk factors for seropositivity, and frequency of prescribing heartworm preventatives for cats in the United States and Canada. Journal of the American Veterinary Medical Association, 250(8), 873-880. doi: 10.2460/javma.250.8.873
5. Heartworm Society. (1970, August 2). Heartworm Basics. Retrieved November 11, 2019, from https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics.
6. Stilwell, N. (2019, July 24). Get a handle on heartworm disease. Retrieved November 11, 2019, from http://veterinarymedicine.dvm360.com/get-handle-heartworm-disease.
7. Health Canada. (2016, May 13). Mosquitoes. Retrieved November 18, 2019, from https://www.canada.ca/en/health-canada/services/pest-control-tips/mosquitoes.html

At St. David’s Vet Clinic we love being a part of our local community. Each year we host a few annual events where we welcome you and your pets to the clinic to have a bit of fun! Next up is our annual Photos with Santa coming up in just a few weeks time. Read below for our events and be sure to check back on our blog and Facebook page closer to the dates for event details!

St. David’s Vet Clinic is located at 329 Four Mile Creek Road in St. David’s. Our phone number is 905-262-8777. We look forward to meeting you and your pets!

Pet Photos with Santa (December 2019)

Stop by St. David’s Vet Clinic on Saturday December 7, 2019 to visit with Santa and his helpers, enjoy some sweet morsels along with festive refreshments.

We invite you to get a memorable photo taken of your pet with Santa. 

Donations are accepted to enter a draw for a beautiful gift basket full of goodies. All donations raised go to the St. David’s Lioness Guide Dog Program.

NOTL Christmas Parade (December 2019)

Come and walk alongside us (every two years) representing St. David’s Vet Clinic in the Niagara-on-the-Lake Christmas Parade. We will be parading next in December 2020.

All are welcome to join in and enjoy this Holly, Jolly, Good time! We would love to share this festive tradition with all of our 2 legged and 4 legged friends.

Further information will be available closer to the event date.

St. David’s Vet Clinic Children’s Day (April 2020)

Come and meet Dr. Stephanie Hall and her amazing staff at St. David’s Vet Clinic to experience what it’s like to work at a Veterinary Clinic. 

Children’s Day is very popular with young people looking to be a Veterinarian or just have a passion to work with animals in the veterinary field. The activities are Educational, FUN and Interactive.

The kids participate in activities ranging from bandaging their own stuffed animal, checking on a post-surgery patient, viewing x-rays, performing surgery on “Paddington” the teddy bear … and more.

Check our Facebook page for regular updates regarding the event date and registration.

St. David’s Vet Clinic Community BBQ (June 2020) 

Every year in June, when the weather is warm and the sun shines brightly, we look forward to hosting our Annual Summer BBQ in appreciation of all our furry friends, as well as their humans.

Be sure to check our Facebook page leading up to the month of June for information regarding the bbq Date, Time and “Theme”.

We look forward to seeing all you party animals there to enjoy a delicious bbq lunch, casual chit chat and a chance to win a fantastic prize!

Information compiled by Julia Michitsch B.Sc., M.Sc. (Zoology)

 Based on surveillance data from 2017, we have approximately 25 of the 900 known species of ticks in the world living with us in Ontario¹. The top 3 species of ticks identified were:  the black-legged, or deer tick (Ixodes scapularis), the American dog tick (Dermacenter variabilis) and the groundhog tick  (Ixodes cookei )¹.

 While each of these ticks can carry various disease-causing agents, the deer tick and it’s propensity for transmitting lyme disease to both dogs and humans makes it one of the top public health concerns in Ontario¹. In the early 1990’s, Long Point Provincial Park (located along the shore of Lake Erie to the west of Niagara) was the only area in Ontario where the threat of contracting lyme disease from a tick bite existed². Comparing the maps below, we can gain an appreciation of how fast the deer tick population, and hence the threat of disease, has spread across the province in only the last 3 years.

Researchers project that the deer tick population will continue to spread at a rate of approximately 46 km per year until 2022³. Climate change has been implicated as the most important factor influencing this spread, as the warmer temperatures allow the ticks to become established in areas where they could not before³. At this rate, the proportion of the human (and pet) population of Eastern Canada that will inhabit areas with established deer tick populations will increase from 18% in 2010 to over 80% by 2020³. 

 Lyme disease is caused by the bacteria Borrelia burgdorferi. Deer ticks pick up B. burgdorferi as newly hatched larvae in the spring/summer by feeding on the blood of small animals that carry the bacteria (mice, birds, squirrels)⁴. After spending the winter dormant in leaf litter, larvae mature into nymphs in the spring of their second year, and continue to mature through the summer until by fall they are adults, ready to mate and continue the lifecycle⁴. During both the nymph and adult stages, the tick will feed on the blood of larger animals (deer, dogs, humans)⁴. During the feeding process, the tick regurgitates enzymes from its gut that help to keep the blood from clotting⁵. As B. burgdorferi is also present in the gut of the tick, the process of regurgitation causes the bacteria to be transmitted to the new host along with these enzymes⁵. Transmission of B. burgdorferi from the tick to the new host takes a minimum of 48 hours – if the tick is removed prior to this time transmission will fail to occur⁵. Ticks are most active in the spring/summer months, but can remain active into the fall and winter as long as the temperature remains above 4°C ⁶.

Ninety percent of dogs that are infected with B. burgdorferi will not show any symptoms of lyme disease⁵. When symptoms (joint pain, fever) do occur it’s usually weeks to months after infection, and no bullseye rash will develop at the site of the tick bite, as it characteristically does in humans^1,5. The most serious complication of lyme disease in dogs is Lyme nephritis (kidney disease)⁵. This condition results from the dog’s immune system being constantly active in response to the B. burgdorferi infection leading to the production of antibody complexes that build-up in the kidneys over time, causing damage and reducing proper kidney function⁵. While cats can become infected with B. burgdorferi, their immune system seems to be better able to neutralize the infection before they develop any symptoms of lyme disease⁵. In fact, there have been zero reports to date of clinical disease in naturally infected cats⁵. 

 A quick blood test is all that is needed to determine whether a dog has been infected by B. burgdorferi. The blood is screened for the antibodies created by the dog’s immune system to proteins on the surface of the bacteria.

 Dogs will test positive for these antibodies within 3-5 weeks of infection, and can remain positive for over a year while the antibodies continue circulating in the bloodstream⁵. A positive test result does not therefore indicate an active infection, but does confirm that bacteria have been present in the bloodstream at some point in the past. Antibiotics are highly effective in treating lyme disease in dogs, with symptoms generally seen to improve within 48 of the start of the medication⁵. Dogs that test positive for infection may be required to have a urine test done in order to ensure that their kidneys are healthy and functioning properly. 

What can we do to keep our pets safe?

Lyme disease can best be prevented through a combination of tick management techniques and consultation with your veterinarian to determine the best tick prevention products for your pet. 

Tick Management: 

1. Ticks don’t like the sunlight, so keeping grass or brush around the house short will let light in and discourage ticks from inhabiting your yard. Remember that ticks also survive over the winter by hiding and laying their eggs in leaf litter where they are protected from the elements – cleaning up leaves before snow fall will reduce the likelihood of survival for the next generation of ticks. 
2. Avoid walking dogs in grassy or forested areas where ticks are known to reside.  
3. Check dogs for ticks every day. Remove any attached ticks with tweezers by grabbing the tick as close to the site of attachment as possible and pulling upwards in one steady and firm movement. Avoid squeezing the body of the tick with the tweezers, as this could facilitate infection by forcing the stomach contents (and potentially the bacteria) to the mouth of the attached tick. Place the tick in a container/sealed bag with rubbing alcohol after removal. Clean the bite area with rubbing alcohol, or soap and water. 

To help with surveillance efforts, you can bring ticks to your local Public Health office (https://www.niagararegion.ca/living/health_wellness/disease-prevent/ticks/remove-and-test.aspx) – This link provides details on how to package the tick, and where to take it in our area! 

As well, there is an ongoing project called the Canadian Pet Tick Survey organized by faculty at the Ontario Veterinary College. This is the largest systematic pet tick survey to date in Canada, and will provide necessary information on tick populations and the threat of disease across the country. If you discover a tick, please fill out the form at https://www.petsandticks.com/tick-submissions to help in their efforts!

Veterinary Care:

1. Have a yearly blood test done on your pet to check for exposure to lyme disease.
2. There are a variety of options when it comes to tick prevention products. Products can be administered orally or topically and should be given during the season when ticks are most active (May – November), although they can also be taken year round (remember, ticks are active any time of year as long as the temperature reaches 4°C). These products work to prevent lyme disease by stopping the ticks from attaching, and/or transmitting B. burgdorferi to your pet. 
3. There are vaccines available for lyme disease in dogs. These vaccines should be used in conjunction with tick prevention products and only on pets that have not already been exposed to B. burgdorferi (negative blood test). The vaccine will not prevent ticks from feeding on your pet, but will help to increase the speed of the immune response to the bacteria by having antibodies ready in the bloodstream to fight the infection when it occurs. 

It should be noted that ticks can carry a variety of disease-causing agents other than B. burgdorferi  (eg. Anaplasma spp. and Ehrlichia spp.). For this reason, having your pet on tick prevention products that stop the ticks from feeding and transmitting bacteria will protect them from these other diseases as well as lyme disease. 

If you have any questions about tick-borne diseases, or if you’d like to discuss prevention options for your pet, please give St. David’s Veterinary Clinic a call! 

References 

1. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Companion animals and tick-borne disease: A systematic review. Toronto, ON: Queen’s Printer for Ontario; 2017. Retrieved from: https://www.publichealthontario.ca/en/eRepository/Companion_Animals_Systematic_Review.pdf
2. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Vector-borne diseases 2015 summary report. Toronto, ON: Queen’s Printer for Ontario; 2016. Retrieved from: https://www.publichealthontario.ca/-/media/documents/vector-borne-diseases-2015.pdf?la=en
3. Leighton, P.A., Koffi, J.K., Pelcat, Y., Lindsay, L.R. and Ogden, N.H. (2012). Predicting the speed of tick invasion: an empirical model of range expansion for the Lyme disease vector Ixodes scapularis in Canada. J. Applied Ecol. 49: 457-464.
4. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Blacklegged tick life cycle. Toronto, ON: Queen’s Printer for Ontario; 2016. Retrieved from: https://www.publichealthontario.ca/-/media/documents/blacklegged-tick-lifecycle.pdf?la=en
5. Brooks, W. (2018). Lyme disease in dogs. Retrieved from: https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4952009
6. Duffy, D.C. and Campbell, S.R. 1994. Ambient air temperature as a predictor of activity of adult Ixodes scapularis (Acari: Ixodidae). 31(1): 178-80.