Babesiosis is a parasitic infection of red blood cells that is spread by a tick bite. The disease is also known as piroplasmosis (pirum means pear in Greek) because the parasite appears pear-shaped in red blood cells. Human babesiosis is also known as Nantucket fever because the disease was first diagnosed after an outbreak on Nantucket Island, Massachusetts.
Babesia microti causes most human infections in North America. B. gibsoni and B. canis cause babesiosis in dogs, and in horses B. equi and B. caballi are the causative species. B. gibsoni, B. canis, and B. equi have been identified in rare human infections. B. equi has been reclassified as Theileria equi, but the original name is still commonly used.
Babesia organisms invade red blood cells, causing their destruction when the Babesia organisms multiply within those cells.
The white-footed mouse is the primary host for B. microti, the species that most often infects people. This species has also been found in field mice, voles, rats, chipmunks, and cottontail rabbits. Horses are the host for B. equi and B. caballi. Dogs are the host for B. gibsoni and B. canis. In all cases a tick is the vector, meaning it is infected but not affected by the organism.
Hard-bodied, or ixodid, ticks are vectors for babesiosis. The black-legged deer tick is the primary vector for B. microti, and the brown dog tick is the vector for B. gibsoni and B. canis. Many hard ticks can act as vectors for equine babesiosis. Ticks ingest Babesia while feeding off of an infected host, and the parasite enters the tick’s digestive system, where it multiplies within the tick’s intestinal wall. The parasites then travel to the tick’s salivary glands, where they are passed to another host when the tick takes its next meal. Transmission can occur via the larva, nymph, or adult tick.
The deer tick life cycle takes an average of 2 years to complete. Female ticks lay their eggs on the ground in early spring. By summer, eggs hatch into larvae. Larvae prefer to feed on mice, other small mammals, and birds in the summer and early fall. They will then molt into nymphs and become inactive until spring. Cold weather (near or below freezing) inhibits tick activity. During the following spring and summer, nymphs will feed on white-footed mice or other rodents, birds, and other small mammals, and in the fall they will molt into adults. Most people are infected by nymphs. The adult ticks prefer to feed and mate on large animals, such as whitetail deer, in the fall and early spring. Female ticks then drop off of these animals and lay eggs on the ground, and a new life cycle begins (Figure 3).
After being deposited in a new host, the parasites invade red blood cells, multiply, and eventually rupture the membranes of infected red blood cells and invade other red blood cells (Figure 4).
FIGURE 4 Babesia organisms in red blood cells
(From Harvey JW: Atlas of veterinary hematology: blood and bone marrow of domestic animals, St Louis, 2001, Saunders.)
With B. microti, the tick larvae and nymphs feed primarily on the white-footed deer mouse. The adult ticks feed on whitetail deer, which do not become infected with B. microti. The adult ticks mate while they are feeding on the deer, so even though the deer do not become infected, they allow the life cycle of the tick to continue. The parasite passes from larva to nymph to adult tick as the tick matures. Human infections are accidental occurrences. Person-to-person infections, via blood transfusions and transplacental infection, have been reported.
BABESIOSIS IN ANIMALS
Babesiosis in dogs is a cyclical disease, in that recovery from the initial infection shows variable and unpredictable periods of illness, alternating with apparently healthy periods.
The clinical signs vary, depending on the stage of the disease and the age and immune status of the dog. Young animals are more severely affected than older animals. A tick must be attached to a susceptible host for 2 to 3 days before Babesia organisms are passed to the host. There can be three phases to canine babesiosis: acute, subclinical, and chronic.
The acute phase is the initial infection and usually lasts a short time. It is characterized by hemolytic anemia, enlarged lymph nodes, enlarged spleen, vomiting, lethargy, and fever. Most dogs recover after treatment.
The subclinical phase can last months or years. It is characterized by a balance between the Babesia organisms and the immune system of the host, so there is little clinical evidence of disease. The balance can be upset by stress, concurrent infections (especially ehrlichiosis), immunosuppression, removal of the spleen, surgery, or extreme exercise. During this phase, the dog may show only an intermittent fever and anorexia. When the balance is upset, the Babesia organisms will begin to increase in number, and the dog will move into the next phase. Greyhounds are frequently subclinical carriers of babesiosis, and they can spread the disease through blood transfusions or to their puppies transplacentally.
The chronic phase begins if the dog is unable to clear the red blood cells containing the Babesia organisms from circulation. This phase is characterized by a cycle of lethargy, anorexia, and a gradual loss of body condition, seen most obviously along the spine and around the eyes. Other symptoms include coughing or labored breathing, vomiting, constipation or diarrhea, sores in the mouth, edema, abdominal swelling, a rash or bleeding under the skin, blood clotting problems, swollen joints, seizures, weakness, enlarged lymph nodes, enlarged spleen, and depression.
In rare cases, the central nervous system can be involved, resulting in weakness, incoordination, and seizures.