Chagas disease is an infection caused by a single-celled protozoan organism called Trypanosoma cruzi (a.k.a. American trypanosomiasis).
The disease is a potentially worrying condition because not only is there no known cure, but in addition to infecting dogs it can be transmitted to people.
These trypanosomes have a complicated life cycle that relies on a species of bugs (called “kissing” or “assassin” bugs) to transmit them from host to host.
When trypanosomes enter the bloodstream of their final host, such as a dog, they cause sudden and severe illness that can overwhelm the dog’s heart and lead to death. If he survives this phase, the dog may appear well, but may have sustained damage to the heart muscle, likely leading to cardiac failure in years to come.
Young dogs are most commonly infected by either eating the meat of an infected animal or from an open wound contaminated by an assassin bug’s feces.
With the latter, the first signs of illness are soreness and swelling around the wound, then the dog’s lymph nodes enlarge and he becomes feverish. Over the next few days, the trypanosomes migrate via the bloodstream to their preferred sites of the heart and brain.
This parasite needs to reproduce within the host’s cells, and of these, its favorite are the heart muscle fibers. As these cells fill with young parasites, they swell and eventually rupture, releasing fresh waves of trypanosomes into the bloodstream.
This causes huge damage to the heart, sufficient enough to kill the host. Sometimes the parasite is taken in the blood to the brain, where it reproduces. Again, cells fill with young parasites; the damage can cause seizures or can interfere with brain function such that the dog struggles to walk, shakes uncontrollably or repeatedly falls over.
Should the dog survive, he may appear well for many years, but he will have to cope with a damaged heart that over time will fail.
A summary of the signs is listed below:
- Reluctant to exercise
- Tired all the time
- Seizures or fits
- Sudden death
- Ill thrift (failure to thrive)
- Lack of energy on walks
- Heavy breathing
- Poor appetite
- Heart failure
The normal life cycle of the protozoan Trypanosoma cruzi involves woodland animals, such as raccoons and opossums, and it is only by accident that dogs become infected. Of those, the most at-risk pets are hunting dogs that kill, catch and eat infected wild animals, thereby infecting themselves.
Dogs can also pick up Chagas disease through contact with the infected feces of the assassin bug. A dog’s open wound or mucous membranes provide an entry point upon contact with an infected assassin bug’s feces.
In the early stages of infection, the protozoan is present in the blood and is visible when a fresh blood smear is examined under the microscope.
As the infection progress, the parasite sets up nests of infection within the host’s tissues, and a histologist can examine a biopsy sample of these infected organs. As more time passes, the body mounts an immune response to fight off the parasite and this serological evidence of the presence of T. cruzi can be detected on a blood test.
There is no proven cure for Chagas disease. Several drugs have been shown to have a limited effect in the early stages of infection, but none bring about a complete cure.
The drugs used to treat the human disease have a disappointing effect in dogs, and because of this, and the risk of infection to humans, an owner of a sick dog is faced with the distressing decision of whether to euthanize his pet or not.
While there is no cure for Chagas disease, there are many effective prescription medications available to help dogs’ hearts pump more effectively.
Because dogs pick up this disease by eating infected meat, it is a good idea to stop hunting dogs from picking up carcasses.
Another preventative measure is to use insecticidal products to kill assassin bugs. Also, an infected pregnant dog passes infection to her puppies in the womb, and thus should never breed.
- Exotic Diseases of Animals. Geering, Forman & Nunn. Publisher: Australian Government Publishing Services.
- Infectious Diseases of the Dog and Cat. Publisher: WB Saunders.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Oct. 13, 2018.