There are over 100 species of Babesia but only a few are found in the U.S. and are transmissible to dogs. Babesia canis, the "large" species of Babesia is one, "Babesia gibsoni ", a smaller Babesia which affects pit bull terriers almost exclusively is another, and a second but unnamed small Babesia has been identified in California. Babesia species continue to be classified and sub-classified worldwide.

How infections happens and what happens next

Infection occurs when a Babesia infected tick bites a dog and releases Babesia sporozoites into the dog's bloodstream. A tick must feed for 2-3 days to infect a dog with Babesia. The young Babesia organisms attach to red blood cells, eventually penetrating and making a new home for themselves within. Inside the red blood cell, the Babesia divests its outer coating and begins to divide, becoming a new form called a "merozoite" which a new tick may ingest during a blood meal. Infected pregnant dogs can spread Babesia to their unborn puppies and dogs can transmit the organism by biting another dog as well.
Having a parasite inside one's own red blood cells does not go undetected by one's immune system. Infected red blood cells are identified and destroyed which kills the Babesia organism but, unfortunately, if many red blood cells are infected this leaves the host with anemia, a lack of red blood cells. Often the host's immune system will begin destroying the uninfected red cells as well. Symptoms include weakness, jaundice, fever, red or orange colored urine. At least 50% of patients will require blood transfusions.
Making matters worse is the fact that animals seem to get sicker than the degree of anemia would suggest so that there is more to this infection than the actual destruction of red blood cells. The severe inflammation that is associated with this parasitism can be overwhelming and completely separate from the anemia. Platelet counts can drop thus impairing normal blood clotting (especially a problem for Babesia gibsoni). An assortment of neurologic signs of can occur with Babesia infection when parasites sequester inside the central nervous system and generate a more localized focus of inflammation. In severe cases there is a lung injury similar to what people with late stage malaria can experience. The new California Babesia species seem predisposed to creating liver disease.
If the acute symptoms are relatively mild or at least non-lethal, a chronic infection can develop. This is usually without symptoms but the dog may continue to be a source of infection to feeding ticks. Relapses can also occur with stress.
Because Babesia is a tick-borne infection, it is not unusual for infected dogs to have other tick-borne infections such as Ehrlichiosis, Rocky Mountain Spotted Fever, and others. These infections may interact to make each other more severe.
Young dogs tend to be most severely infected, especially. Pit Bull terriers.

Diagnosis of Babesiosis

If one is very lucky, the Babesia organisms can be seen on a blood smear. Babesia canis organisms are tear-shaped and occur in pairs. Other Babesia species have several forms in which they appear. Odds of finding the organism are improved by checking freshly drawn blood taken from a capillary source ( a small cut to an ear, for example) rather than from a blood vessel. If Babesia organisms are found, the patient is definitely infected but they are hard to find so an alternative method of diagnosis is needed.
Antibody testing has been problematic as infected animals may have circulating antibodies long after the organism is gone or may have no antibodies circulating while a few organisms remain hidden inside red blood cells.
The current method of diagnosis involves PCR testing. This is extremely sensitive testing and can distinguish 4 different species of Babesia. While only certain laboratories run this type of testing, this is really the best method of answering the Babesia infection question.

Babesia Treatment

Therapy for Babesia is not a benign under-taking. In fact, if a dog is asymptomatic with Babesia, treatment is not worth the side effects. Further, even with treatment Babesia gibsoni, and probably the other small Babesia species, cannot be fully cleared by any of the drugs listed. Female dogs testing positive for Babesia should not be bred.

• Diminazene Aceturate
  This drug is not available in the U.S. but in other countries is the most commonly used treatment. A single injection is needed and is best used on Babesia canis. Side effects include: nausea, blood pressure drop, painful injection, seizures, and some fatal reactions.
• Imidocarb Dipropionate
  This is the only drug approved for Babesiosis in the U.S. A single dose is usually effective for Babesia canis but two given two weeks apart are needed for Babesia gibsoni and the other smaller Babesias. The injection is painful plus causes muscle tremors, drooling, elevated heart rate, shivering, fever, facial swelling, tearing of the eyes, and restlessness. Pre-treatment with an injection of atropine helps palliate these side effects.
• Trypan Blue
  This medication serves to block the parasite from entering red blood cells and may help minimize the symptoms of the infection. Side effects are minimal and it is given as an IV drip.
• Phenamidine Isethionate
  This drug is not available in the U.S. but a similar drug, Pentamidine isethionate, is. It is more effective on Babesia canis.
• Quinuronium Sulfate
  This drug is not available in the U.S. It is similar to malarial treatment. It is given as a series of two injections two days apart generally with marked improvement in the patient by the second injection.
  A combination therapy of quinine, azithromycin, atovaquone, and/or clindamycin are promising and may become prominent in the future.

A vaccine is available against Babesia in France but only seems effective against certain strains. Vaccination is 89% effective in France. The best prevention is aimed at tick control.