Babesiosis
Dima Youssef
James W. Myers
INTRODUCTION
Babesia infections have probably been occurring for centuries, but the first documented case of babesiosis in humans was in a splenectomized farmer in Yugoslavia who was diagnosed with a B. bovis in 1957. Clinical infection varies from a silent infection to a severe, life-threatening disease.
Babesiosis parasites:
Tick-transmitted by Ixodid ticks or hard-bodied ticks (Ixodes dammini [Ixodes scapularis] and Ixodes ricinus).
Human infection usually follows a tick bite although two-thirds of patients may not recall the bite.
Note that it may occasionally occur after a blood transfusion or transplacental/perinatal infection. Overall, the risk of acquiring babesiosis from a blood transfusion is very low.
A transfused unit of packed red blood cells is about 0.17%. Platelet transfusion risk is even lower.
Those cases of transfusion-acquired babesiosis in the United States occurred with an incubation period varying from 17 days to 8 weeks.
Zoonotic disease
Infects a wide variety of vertebrate hosts
The parasites are intraerythrocytic.
They are called piroplasms due to the pear-shaped forms.
Babesia microti is the most common species infecting humans.
Both humoral and cellular factors are involved in immunity to babesiosis.
EPIDEMIOLOGY
Babesiosis in Europe (30 cases) occurs only in splenectomized patients (minus one case) and is caused by Babesia divergens, a cattle pathogen. Forty-two percent mortality rate is much greater than the 5% rate found in the United States found with B. microti.
The tick vector of B. divergens is Ixodes ricinus and that of B. microti in England is I. trianguliceps.
Two or more divergens-like pathogens occur in the United States (please see Table 25-1 for a comparison of species.)
A 73-year-old man who had had a splenectomy and had a fatal case of babesiosis. Indirect immunofluorescent antibody testing showed that the patient’s serum had strong reactivity with Babesia divergens, which causes babesiosis in cattle and humans in Europe, but that it had minimal reactivity with B. microti and WA1. Known as MO1.
Table 25-1 Comparison of Species
B. divergens
B. duncani type organisms (WA, CA) and other species
B. microti
Splenectomy as a risk factor
Yes
Usually not but varies.
No
Ixodes tick vector
Ixodes scapularis
Varies
Ixodes ricinus
Host
Cattle
Varies
Mice
Treatment duration
6 weeks. (2 weeks after clearance)
Not well defined
10 days normally
Exchange transfusion used
Common
Perhaps
Rare
Atovaquone-azithromycin combination
No pentamidine and TMP-SMX might be an alternative.
Possible
Yes, for milder disease
Quinine-clindamycin regimen
Yes
Yes. Probably the best approach
Yes
The second case was a 56-year-old man from Kentucky. He had a history of splenectomy, a Caribbean cruise 9 months earlier, and deer and rabbit hunting 4 weeks previously, but he had no recollection of a tick bite. He presented with a fever (temperature, 40°C), a hemoglobin level of 13.7 g per deciliter, a platelet count of 43,000 per cubic millimeter, and hemoglobinuria.
A few cases of babesiosis in the western United States have been reported previously; all occurred in Washington or California. They were either tick borne or transfusion related.
Rarely reported in other parts of the world, including China, Taiwan, Egypt, South Africa, and Mexico.
CHARACTERIZATION OF THE ORGANISM
Invertebrate Hosts
Ixodid ticks are the vectors for Babesia spp.
B. microti can only infect ticks from the genus Ixodes.
Several tick vectors can carry more than one Babesia species.
The nymphal stage of I. dammini and its interaction with Peromyscus leucopus (white-footed mouse) is essential for the maintenance of B. microti.
The adult stages of I. dammini feed primarily on deer (Odocoileus virginianus) and then lay eggs.
The eggs hatch in the summer, and the larvae feed primarily on mice during August and September.
These infected larvae molt to become nymphs in the spring.
The nymphs feed on hosts from May through July.
Vertebrate Hosts
Almost any mammal that serves as a host for a Babesia-infected tick is a potential reservoir.
Small terrestrial mammals to subhuman primates to humans for B. microti and from cattle to various rodents and to humans for B. divergens.
Most white-footed mice (P. leucopus) in babesiosis-enzootic areas are parasitemic for life.
Events in the Vertebrate
The longer the tick is attached, the more likely the sporozoites will be transmitted.
Sporozoites invade the lymphocytes and then differentiate into multinucleate schizonts. These differentiate into merozoites, which lyse the cell to infect the erythrocytes.
The merozoite invades the host erythrocyte through a process of invagination, forming a parasitophorous vacuole.
Within the host erythrocytes, most merozoites become trophozoites and divide by binary fission; more merozoites get produced, and additional erythrocytes get infected.
Four parasites can form at the same time, leading to a Maltese cross form.
Rapid reproduction destroys the host cell and leads to hemoglobinuria in the host.
Some trophozoites can become potential gametocytes that develop into gametes in the tick prior to leaving the erythrocytes within the tick gut.
Host Immune Responses
Both humoral and cellular factors are involved in immunity to babesiosis.
Parasitemia levels begin to decline approximately 10 days after infection after degeneration inside the erythrocyte and clearance by the spleen.
Intraerythrocytic killing at the resolution stage requires T lymphocytes.
NK cells might be mediating protection in the early stages of infection.Related posts:
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