Cause and Transmission
Toxoplasma gondii is a protozoan confirmed as the cause of canine toxoplasmosis. Cats transmit environmentally resistant ococysts in feces. Both wild and domesticated cats can transmit this parasite through the feces. Dogs can pass ococysts in feces after the ingestion of feline feces but the organism does not replicate.(1) Insects like flies and cockroaches are thought to be an intermediate host. Toxoplasmosis can also be passed through raw meat, dead animals, or birds. It can also be passed through contaminated water or unpasteurized goat milk.
The pathogenesis of toxoplasmosis shows that three different infectious stages can occur, i.e. tachyzoites, bradyzoites and sporozoites. Transmission of these infectious stages is possible if a healthy dog either eats raw meat containing oocysts of toxoplasmosis or if it is transmitted through the transplacental route, i.e. from mother to fetus. It will then appear in any puppies.
After intake by either route, most commonly by intake through contaminated food, meat or water, these protozoa multiply in the stomach and intestine. Once the infectious stage matures i.e. becomes sporozoites or oocysts, they spread through the entire body through the blood and lymphatic circulation. Clinical symptoms usually appear 20 days after ingesting protozoa in the optimum quantity for multiplication.
Clinical symptoms are associated with the degree of damage caused to the soft tissues, which is directly related to the number of trachyzoites released during multiplication of the number of protozoa. This factor also represents the severity of output toxoplasma can cause in dogs.
As the protozoa multiply in numbers and spread to other parts of the body, toxoplasma can only cause a sub clinical infection, i.e. symptoms are not clinically important and the condition remains untraceable.
Symptoms of toxoplasmosis in dogs include:
- Appetite loss
- Abdominal Pain and Swelling
- Difficulty breathing
- Coordination problems
- Eye infection
- Muscle control loss
- Inflamed retinas, Corneas, Irises
- Tilted head
Most visceral (internal organs) physiological systems are affected including the digestive, circulatory, lymphatic, respiratory and nervous systems. Affected systems cause severe generalized or generally prevalent disease. Young puppies may not survive to even the third degree of infection thus sudden death is noted with acute symptoms which are those that come on suddenly.
Toxoplasmosis is an important differential diagnosis in any do that is presented to a veterinarian as an emergency that has peripheral or central neurologic signs such as acting as if in pain, or if skin areas are painful to the touch.(5)
Clinical symptoms can help in indicating canine toxoplasmosis in dogs. But since the disease has a wide spectrum of clinical symptoms, it is very hard to confirm through clinical elimination techniques.
Laboratory procedures including biological, histological and serological examinations are preferred for confirming the disease. Serological tests as such as ELISA, Hemagglutination, LATEX Assay etc are confirmatory tools, which can confirm presence of specific antibodies for toxoplasmosis. The severity of the condition can also be assessed through the use of serological (enzyme testing) procedures. In addition, tissues can be examined under the microscope for biological and histological changes; the specific stage of infection is usually determined with these tools.
Some drugs such as Sulfadiazine, Pyrimethamine etc can be used, but are rarely warranted for treatment of canine toxoplasmosis. These drugs are effective, but the pharmacological action and side effects of this mode of treatment is yet to be researched. In most clinics across the USA, Clindamycin is considered to be the treatment of choice for toxoplasmosis (10–20 mg/kg PO, IM, IV q12h for 4 weeks).
Zoonotic Importance of Toxoplasmosis
As toxoplasmosis is a highly transmissible condition to other animals and humans, extensive care is needed regarding the handling of pets with this condition. Moreover, disinfection and preventive measures are usually required for humans if any of their companion animals have been confirmed for this disease. In a study, 60% of humans had been confirmed with the presence of specific antibodies in their serum, when they came in contact with pets who were acting as a reservoir for Toxoplasma.
To prevent canine toxoplasmosis, dog's shoudl be kept away from cat litter boxes or any outside sandboxes.Keep your dog away from areas where cats will defecate.
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(1) Toxoplasmosis; North American Veterinary Conference, M.R. Lappin, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, 2006.
(3) J.P. Dubey, Toxoplasmosis of Animals and Humans (CRC Press, 2009) 2nd Edition
(5) Emergency Presentations of 4 Dogs with Suspected Neurologic Toxoplasmosis; Jeremy M. Tarlow DVM Elke Rudloff DVM, DACVECC Marla Lichtenberger DVM, DACVECC Rebecca Kirby DVM, DACVIM, DACVECC