Canine Heartworm

By Caitlin McKinney

What is Canine heartworm disease?

We’ve all heard of the term “heartworm” at our veterinary clinics when taking our beloved pet in for their routine annual check-up or puppy vaccinations. But what really is it and how are our veterinarians informing us about it? Canine heartworm is a fatal disease that is prevalent in pets in the United States. Although the name implies that only animals in the dog family are susceptible to the disease, heartworm can infect cats, ferrets, sea lions, and even humans (1). The disease is caused when worms, or filarial nematodes called Dirofiliara immitis, enter the circulatory system of our pets through the bite of an infected mosquito (3). Many consequences can arise when these worms are passed from the mosquito to our furry friend, such as a localization of worms on the pulmonary arteries and other surface blood vessels that can cause eventual life-threatening cardiovascular collapse. With consequences as dire as heart failure, it’s no wonder our veterinarians persistently recommend a preventative deworming medication almost every clinical visit we attend.

Prevalence and transmission of heartworm

Heartworm is vectored by a mosquito, specifically known as the Aedes mosquito. This mosquito is also a known vector of diseases such as dengue, yellow fever, and other viral pathogens, as well as nematode (worm) parasites. Because of this, mosquitos are known as the most dangerous animal in the world. They are found in many climates and can take their blood meal from a variety of vertebrate hosts, such as mammals, birds, reptiles, amphibians, and even fish (3). It is also known as the most common blood-feeding arthropod (invertebrate animal with chitinous exoskeleton). The Aedes mosquito can be active throughout the year in tropical, humid climates, but inseminated females in temperate climates can hibernate through the winter. Thus, their eggs can survive at low temperatures.

 

Figure 1: Incidence of canine heartworm in average number of consumer cases per reported clinic across the United States in 2013.

In the U.S., heartworm has been diagnosed in all 50 states and can still be prevalent in areas that do not have high mosquito populations (4). This is due to infected pets traveling with their owners to areas where heartworm is more common. Additionally, strayed and neglected dogs and wildlife, like wolves, foxes and coyotes, can transmit D. immitis through bite of a mosquito.

Although mosquitos are found throughout the U.S., many actions can be taken to control the population. These include larvicides, removal of standing water, aerial pesticides, bacterial symbionts that interfere with mosquito reproduction, and more simple measures, such as bed nets, window screens, and insect repellent. 

Clinical History

The first documented finding of heartworm was by a man named Chez Iean Wolfe, who found a what he called a “monster” in the heart of a dead horse in 1586 (5). As any curious person does in the 1500’s he made a sketch of the so-called “monster,” which today is known as D. immitis. The first canine case of D. immitis was documented by a nobleman by the name of Francesco Bargo in the 17^th century (5). Bargo identified adult worms in the hearts of his hunting dogs, but thought they were the larvae of a different type of parasitic worm. In 1856 in the southeastern coast of the U.S., canine heartworm was formally described by Joseph Mellick Leidy (5). Unfortunately, even our feline friends could not escape the historical patterns of heartworm, as the first case of feline heartworm was discovered in the 1920s. Finally, like all memorable diseases throughout world history, heartworm disease would not be note-worthy without a human case of infection in 1952 (6).

 

Figure 2: Sketch of D. immitis  in the heart of a horse done by Chez Iean Wolfe.

Heartworm symptoms

Symptoms in dogs may be subtle in early infections and light infections are usually asymptomatic. Lethargy, loss of appetite, weight loss, a mild, persistent cough during moderate exercise are telling symptoms if your pet has a heavy heartworm infection (larger number of D. immitis) (1). As the infection develops, D. immitis can cause blockages of blood flow in the pulmonary arteries and blood vessels which can lead to a life-threatening form of cardiovascular collapse, called caval syndrome (1). Signs of caval syndrome include a sudden onset of labored breathing, pale gums, and dark, bloody urine or coffee-colored urine. Caval syndrome requires the immediate surgical removal of the heartworm blockage. The appearance of a swollen belly is also common due to excess fluid in the abdomen.

Heartworm testing

Heartworm tests that are done by veterinarians require a small blood sample from your pet. By detecting the presence of certain proteins within the worms, the results of the test are very quickly received whether the test is given in a clinic or sent to a diagnostic laboratory. Usually these tests are given in the form of snap tests (Hartgard snap tests), that require just one drop of a blood in a blood sample that’s taken when you take your pet to get spayed or neutered.  To prevent initial infection or further development of infection, pets should be tested annually for D. immitis.

 

Figure 3: Heartgard snap test for heartworm showing positive results.

Treatment

Currently, there is no vaccine for canine heartworm which is why prevention is the of utmost importance. Prevention includes a monthly use of an oral dewormer, called prophylactic oral ivermectin or an injectable form administered every 6 months. This is a cream-like substance that is 99% effective in killing the D. immitis larval worms before maturing into adults, which are the cause of heavy infections in dogs. However, ivermectin has little, if any, effect on adult worms. Eradication of adult worms with this treatment could take as much as two years or more with a proper medication regimen. The other option for eradicating adult worms, an arsenic-based compound called melarsomine dihydrochloride, has been highly controversial due to its toxic properties and complex regimen. In this regimen, it’s important to restrict canine exercise for at least a month because worms can become dislodged from the pulmonary arteries and end up in the lungs, causing respiratory failure.

Could bacteria save the day?

New studies have found that D. immitis are the host to bacteria in the genus Wolbachia, a gram-negative intracellular organism. It’s been found that Wolbachia is essential to the survival and reproduction of female worms, and antibiotic treatment that kills the bacteria stunts larval development within the females. Thus, new combinations of treatments for heartworm have been administered that include a dose of antibiotics along with conventional therapies, like ivermectin (2). This combined treatment is being tested as an alternative to melarsomine dihydrochloride and plays a key role in the advancement of the treatment of canine heartworm in veterinary medicine.

 

Figure 4: Fluorescence microscopy of Wolbachia bacteria.

References

1.     Heartworm disease. (n.d.). Retrieved March 8, 2020, from https://www.avma.org/resources/pet-owners/petcare/heartworm-disease

2.     Kristina Kryda, Robert H. Six, Kelly F. Walsh, Susan J. Holzmer, Sara Chapin, Sean P. Mahabir, Melanie Myers, Tammy Inskeep, Jady Rugg, Blair Cundiff, Aleah Pullins, Michael Ulrich, John W. McCall, Tom L. McTier, & Steven J. Maeder. (2019). Laboratory and field studies to investigate the efficacy of a novel, orally administered combination product containing moxidectin, sarolaner and pyrantel for the prevention of heartworm disease (Dirofilaria immitis) in dogs. Parasites & Vectors, 1, 1.

3.     Loker, E. S., & Hofkin, B. V. (2015). Parasitology: A conceptual approach.

4.     Meredith R. Spence Beaulieu, Jennifer L. Federico, & Michael H. Reiskind, (2020). Mosquito diversity and dog heartworm prevalence in suburban areas. Parasites & Vectors, 1, 1.

5.     (n.d.). Retrieved April 3, 2020, from https://web.stanford.edu/group/parasites/ParaSites2006/Dirofilariasis/History.htm

6.     Dog Heartworms. (n.d.). Retrieved April 3, 2020, from https://encyclopediaofarkansas.net/entries/dog-heartworms-14172/

Bed bugs

 Parasitology | Marian Kinsel | Fort Lewis College

Fig. 1 Bed bug actual size representation. Image courtesy of CDC; Blaine Mathison

What are Bed bugs?

Bed bugs are an infestation caused generally by two parasites, Cimex lectularius and Cimex hemipterus who live their entire life with humans. Bed bugs are a red-brown color, oval-shaped insect that can range in size from 1 millimeter to 7 millimeters in size. Have short antennae, black eyes and structures on their back that resemble wings but are not. Adolescent bed bugs have the same features as an adult beg bug but are smaller and resemble more of a yellow body color. These small insects feed on the blood of their human or animal host, which result in irritation. Bed bugs are considered a public health pest by the Environmental Protection Agency, Centers for Disease Control and Prevention, and United States Department of Agriculture. Currently bed bug infestations and bites are not known to cause or spread disease. Although, infestations should not be taken lightly and should be controlled.

Bed bugs bite their human or animal host and usually feed on them in during their sleep. Individual bed bugs feed on their host for about ten minutes. Larger infestations tend to have a line pattern of bites along an edge of clothing or blanket. Bites are often not felt immediately but can later react to the protein excreted through their saliva. Following the bite, swelling and irritation arise and the bite will be visible. It is difficult to differentiate bed big bites from other bites that could be from mosquitos and other insects. Depending on the person, increased amounts of bed bug bites may be show low or sever reactions. Bed bug infestations need to be found to fully classify the bite as a bed bug bite.

Fig. 2 Life cycle of C. lectularius, a casual agent of bed bugs. Image courtesy of Centers for Disease Control and Prevention

Where are they found?

Bed bugs are present in North and South America, Africa, Asia and Europe. Bed bugs are usually thought to be found in poor areas but are present in all economic classes. Smaller infestations are easier to dispose of than larger infestations, but smaller infestations are harder to locate than larger ones. They are found in the folds of mattresses and sheets and excrete and musty odor. Bed bugs can be thought of to be something else, such as carpet beetles or cockroaches. Carpet beetles are darker than bed bugs, have a smaller set of antennae, and their body shape is rounder. Cockroaches are also a red-brown color and oval-shaped but have long antennae and longer legs than a bed bug. Bed bug infestations are commonly resulted from travel as these insects can be found in motels and hotels, regardless of cleanliness and the price paid for the room/suite. They can be unknowingly be carried on travel items such as luggage, furniture and clothes. Bed bugs are most common in apartment buildings, hotels, dormitories, and areas with increased amounts of different human interaction.

Bed bug control

Since bed bug bites do not lead serious diseases, the best way to treat the infected area is to avoid contact with the area, apply antiseptic lotions and in serious cases; consume an antihistamine. To rid infestations, it is safest to contact a professional. There are currently 300 pesticides that are used for bed bug control, including pyrethrins, pyrethroids, desiccants, biochemicals, pyrroles, neonicotinoids, and insect growth regulators. These pesticides are highly recommended, and some are only recommended to be used by professionals. The easiest way to prevent bed bug infestation is to regularly check new purchases, sleeping areas and to practice general hygiene tactics.

Sources:

1.Centers for Disease Control and Prevention

2.Global Health, Division of Parasitic Diseases

3.United States Environmental Protection Agency

4.Colorado State University Extension

5.New York City Health Department

The CATastrophe that is Toxoplasma gondii

By Claire Nagel

What is it?

            Toxoplasma gondii is a protozoan parasite that causes Toxoplasmosis, a very common parasitic infection distributed all over the world.  Cats and other felines are its definitive host, meaning the parasite reaches sexual maturity and reproduces in the feline.  Rodents and other animals are intermediate hosts, while humans are a secondary host.  It can be transmitted through vertical transmission, fecal-oral transmission, iatrogenic transmission (medical procedures), and trophic transmission.  Toxoplasmosis mostly occurs in immunocompromised individuals, such as those infected with AIDS, or children who got it from their mothers.  Symptoms include fever, fatigue, muscle pain, headache, and sore throat, and can result in encephalitis and/or ocular disease.  It can even cause sudden abortions in sheep.   T. gondii is found globally in most continents, including North America, Europe, and South America.  It is estimated that in the United States and United Kingdom, 16-40% of the population are infected, while in Central and South America and Continental Europe, estimations range from 50-80%.

Clinical History

            T. gondii was first discovered accidently in 1908 by Charles Nicolle, a scientist who was working on the disease Leishmania in the North African rodent, Ctenodactylus gundi.  T. gondii was also identified by Alfonso Splendore at around the same time, but in the rabbit population in Sao Paulo.  It was not until 1948 though that Albert Sabin and Harry Feldman published their discovery on the ‘dye test’, a way to diagnose the infection in humans through the use of serum samples.

How is it Transmitted?

            T. gondii can be transmitted by various routes.  It can be transmitted vertically from mother to baby.  If a woman gets infected while pregnant, the infection can be passed on to the developing fetus.  Diseases of this range from mild to severe.  T. gondii can also be transmitted through fecal-oral transmission by consuming tissue cysts from uncooked or undercooked meat, or from food or water contaminated with oocysts from cat feces.  Trophic transmission occurs when a rodent or bird is eaten by another animal, typically a feline, and iatrogenic transmission can happen during blood transfusions and organ transplants.

Why is it so Special?

            T. gondii is an interesting parasite because it changes the behavior of rodents who are infected.  When a rodent is infected with the parasite, they have a reduced distaste to cat odor.  Uninfected rodents flee when they smell cat urine, but infected rodents are not afraid of the smell and do not run away, increasing the likelihood that they will be eaten by a cat.  Reports show that this attraction to cats can be in part due to the parasite activating the sexual arousal pathways in rats.  It is similar to a male rat smelling a female rat in heat, but he has no idea that what he is actually attracted to is cat urine.

Treatments Available

            There are currently no treatments that completely cure an individual of T. gondii, but there are drugs that are effective at treating it, such as sulfadiazine, pyrimethamine, and spiramycine.  These drugs target the parasite during its tachyzoite stage (rapid multiplication), but have limited effectiveness against the tissue cyst stage of T. gondii.  In most cases, animals and humans who have been infected with Toxoplasmosis develop immunity against the disease in the future.  There are no vaccines currently against the parasite in humans, but there is one vaccine used to prevent the Toxoplasma-induced abortion in sheep.

Conclusion

            Overall, T. gondii is a very weird parasite that has many different effects on their hosts.  It causes flu-like symptoms in humans, abortions in sheep, and an attraction to cat pee in rodents.  Discovered in 1908 in North Africa, T. gondii is now prevalent all over the world.  There currently is no cure to the disease, but with further research, it can be done.

Figure 1. The lifecycle of T. gondii (Hill et al., 2002).

References

Hill D, Dubey JP. Toxoplasma gondii: transmission, diagnosis and prevention. Clinical microbiology and infection: the official publication of the European Society of Clinical Microbiology and Infectious Diseases. 2002;8(10):634-640.

Innes EA. A Brief History and Overview of Toxoplasma gondii. Zoonoses & Public Health. 2010;57(1):1-7. doi:10.1111/j.1863-2378.2009.01276.x.

McConkey GA, Martin HL, Bristow GC, Webster JP. Toxoplasma gondii infection and behaviour - location, location, location?. J Exp Biol. 2013;216(Pt 1):113–119. doi:10.1242/jeb.074153