By Dave Miller
Human babesiosis is an important emerging tick born disease. Cattle are the most common agent of human babesiosis in Europe. In the U.S., babesiosis microti, a babesial parasite of small mammals has been cause of many cases of human babesiosis since 1969, resulting in mild to severe disease.
Babesiosis was first found in Romanian cattle in 1888. The first identified infection found caused by babesia in a human was in 1957 in the former Yugoslavia. The first in the U.S. was in 1969 in Massachusetts. The patient was found to have babesia microti. Since then, more and more cases of human babesiosis have been reported. In the U.S., babesiosis is endemic in the Northeast. Additional cases have appeared in the Midwest and West Coast.
Babesiosis is a zoonotic (mammal to human) disease, requiring transmission from an animal reservoir to humans via a tick, the same as Lyme disease and HGE/HGA (as discussed in my previous article on those tick borne disease).
Of more than 70 species worldwide in the genus Babesia, human infections are largely due to the rodent strain B-microti (found in the U.S.) and cattle strain B-divergens and B-bovis (found only in Europe). Several cases in Washington State and California have been described from a hitherto unknown species of babesia, designated as WA-1. This strain is closely related to B-gibsoni, a canine pathogen (bacterium or virus). A fatal case of babesiosis recently occurred in Missouri from a strain (NO-1) that was closely related to the B-divergens strain. Test for B-microti do not detect infections due to these other strains of Babesia.
In the north east U.S., a black legged deer tick (lxodes scapularis) is the principle vector for transmitting
B-microti. This is the same tick that transmits Lyme disease. Babesia species from rodents, primarily the deer mouse, but also field mice, vole, rat and chipmunk, are transmitted to humans during tick bites in endemic areas. Understandably this is more prevalent during the periods of tick activities in the spring, summer and fall. The tick has 3 developmental stages, lava, nymph and adult, with each stage requiring a blood meal for development into the next stage. As larva and nymph, the tick feeds on rodents, but as an adult, the tick prefers to feed on the white-tailed deer. Female ticks are impregnated while obtaining their blood meal on the deer, with the formation of up to 20,000 eggs.
Human infection is primarily produced by the bite of the infected “nymph” during a blood meal. Curtailment of deer hunting has caused increased deer herds in some areas. The proximity of deer, mouse, and the tick create the conditions for increased human infection. The incubation period after a tick bite usually is 1-3 weeks, occasionally maybe as long as 9 weeks. Because the nymph (primary vector), is only 2mm in diameter when engorged. Most patients do not recall a tick bite.
Babesia species in the host vary in size. These are pear shaped, oval or round. Their ring form and peripheral location in the red blood cells of the host frequently lead to their being mistaken for a similar appearing malaria parasite.
Babesiosis is rarely acquired through blood transfusions. In transfusion- associated cases, sources of babesiosis have included platelets and frozen erythrocytes. The incubation period in transfusion- associated disease appears to be 6 – 9 weeks. Transplacental (immunization) transmission has also occurred in rare cases.
The spectrum of disease manifestation is broad, ranging from a silent infection to malaria like disease, which results in severe hemolysis (dissolution or disintergration of red blood cells) and, occasionally death.
The precise mechanism of hemolysis is unknown. The spleen offers a critical host defense against this infection, as suggested by higher incidence and greater severity of babesiosis in asplenic patients.
The spleen traps infected red blood cells, and their ingestion by the macrophages (cells that consume foreign bodies) follows. The disease itself alters cellular immune function.
In the U.S. infection with B-microti in otherwise healthy individuals generally remains sub clinical; however, symptomatic infection is common in patients who are asplenic, older patients, and those with underlying medical conditions, including human immunodeficiency virus infections. Disease manifestations range from asymptomic infection in healthy individuals to severe illness and death in those who are asplenic, elderly, or immunocompromised ( ).
Patients with clinical illness and intact spleens are usually 50 years of age or older, suggesting that age plays a factor in the severity of the clinical response. Previously healthy individuals with babesiosis generally are older than 60, than are patients with babesiosis with antecedent medical problems. Symptoms begin gradually and are nonspecific. Most common symptoms reportedly are fatigue, malaise & weakness, fever, shaking chills and diaphoresis. Physical findings vary. Babesiosis has been associated with shock and acute respiratory distress syndrome.
Dave Miller
Lexington TWP
Dave Miller is a Maine resident, an outdoor writer and a member of the Carrabassett Valley Trappers Association.
