Babesia
Babesia, also called Nuttallia, is an apicomplexan parasite that infects red blood cells and is transmitted by ticks. Originally discovered by Romanian bacteriologist Victor Babeș in 1888; over 100 species of Babesia have since been identified.
Babesia comprises more than 100 species of tick-borne parasites that infect erythrocytes in many vertebrate hosts.
Babesia species infect livestock worldwide, wild and domestic vertebrate animals, and occasionally humans, where they cause the disease babesiosis. In the United States, B. microti is the most common strain of the few that have been documented to cause disease in humans.
Classification
Babesia is a protozoan parasite found to infect vertebrate animals, mostly livestock mammals and birds, but also occasionally humans. Common names of the disease that B. microti causes are Texas cattle fever, redwater fever, tick fever, and Nantucket fever. The disease it causes in humans, babesiosis, is also called piroplasmosis.Babesia microti, however, is not part of the genus Babesia. Due to historical misclassifications, the protozoan has been labeled with many names, including Nuttallia, and was renamed from Babesia microti to Theileria microti based on evidence from 2006. Its genetic sequence, published in 2012, shows that the species belongs to neither Babesia nor Theileria, but instead to a separate genus. Another "western" group is also separate from core Babesia.
The avian Babesia species are characterized as having ring and amoeboid forms, and fan-shaped or cruciform tetrad schizonts. Developing parasites have only been reported in red blood cells.
History
For centuries, the animal disease was known to be a serious illness for wild and domesticated animals, especially cattle. In 1888, Victor Babeș first identified the causative agent in Romania and believed it to be due to the bacterium he named Haematococcus bovis. He documented the disease by describing signs of a severe hemolytic illness seen uniquely in cattle and sheep.In 1893, Americans Theobald Smith and Fred Kilborne identified the parasite as the cause of Texas cattle fever, the same disease described by Babeș. They also identified the tick as the transmitting agent, a discovery which first introduced the concept of arthropods functioning as disease vectors.
It was believed to be a disease that only affected nonhuman mammals, but in 1957, the first case of babesiosis was seen in a human. The person had been splenectomized, as were all people diagnosed with babesiosis until 1969, when the first case of babesiosis was diagnosed in a person who still had their spleen. This proved the parasite was a potential pathogen in anyone.
Genetics
Babesia species show host specificity, allowing many different subspecies of Babesia to emerge, each infecting a different kind of vertebrate organism. While B. bovis and Babesia bigemina prefer to infect cattle in tropical environments, they can infect other animals, such as the white-tailed deer. Therefore, while the organism has the capacity to display host specificity, and thus increase transmission effectiveness, it can still infect a variety of hosts. It achieves this through mutations and natural selection. In different environments, individual protozoa may develop mutations, which when they increase the protozoa's fitness, allow the population to increase in number. This specificity explains why Babesia species have such great genetic diversity.Babesia selfishly persists long-term in the host's system: The host gains no benefit from the parasite invasion and only suffers. This allows the parasite to exploit all resources offered by the host, to increase in number, and to increase the rate of transmission. Too lethal an infection results in the host's death and the parasite is unable to spread, which is a loss from an evolutionary standpoint. Different species of Babesia are able to withstand the stress of the host's immune system. Infection typically stimulates the innate immune system, and not the humoral immune system. This results in control of the infection, but also persistence and not clearance of the parasite.
Genomics
The genome of B. microti has been sequenced and shows that the species does not belong to either Babesia or Theileria, but instead to a separate genus., it is known that the mitochondrial genome is linear like other sequenced Apicomplexa mitochondrial genomes, although it was initially reported that it was circular.Partial RNA sequencing of canine piroplasms has identified a number of additional species.
Lifecycle
The lifecycle of B. microti, which is typical of parasites in the genus, requires a biological stage in a rodent or deer host. It is transmitted by ticks of the family Ixodidae between these hosts. To begin, the tick as the definitive host becomes infected itself, as it takes up gametocytes when attached for a blood meal. It also introduces the Babesia into the intermediate host when taking a blood meal. As Babesia enter the animal's red blood cells, they are called sporozoites. Within the red blood cell, the protozoa become cyclical and develop into a trophozoite ring. The trophozoites moult into merozoites, which have a tetrad structure coined a Maltese-cross form. Trophozoite and merozoite growth ruptures the host erythrocyte, leading to the release of vermicules, the infectious parasitic bodies, which rapidly spread the protozoa throughout the blood. Rather than producing more and more trophozoites, some of the merozoites produce gametocytes. The gametes are fertilized in the tick gut and develop into sporozoites in the salivary glands. These are the sporozoites the infected tick introduces when it bites an intermediate host.Even as an incidental host, the phase changes that occur in the parasite are the same within humans as in the biological hosts. Babesia can be diagnosed at the trophozoite stage, and can also be transmitted from human to human through the tick vector, through blood transfusions, or through congenital transmission.
Image:Babesia life cycle human en.svg|thumb|right|500px|Lifecycle of ''Babesia''
Seasonality
Temperature
Cold weather completely interrupts transmission. The emergence of tick-borne diseases has been found to coincide with climate change. The correlation between climate change and the incidence of tick-borne diseases is not known to be strong enough to count as a major factor.Humidity
High humidity and rainfall accommodate ticks carrying Babesia. This may explain why B. bigemina infection in cattle in the hilly region of Meghalaya has increased. The lifespan and number of generations of B. microplus correlate with increasing the longevity of larvae and the number of annual generations. Warm, dry weather interferes with the Babesia lifecycle within the tick. Warm, wet weather increases the intensity of infestation—the population is able to thrive due to the relatively fluid environment, making water and nutrients more accessible.Transmission
Babesia species are spread through the saliva of a tick when it bites. Already at its nymphal stage, a tick bites into the skin for a blood meal. The tick, if not removed, stays attached for three to four days, with longer periods of feeding associated with a higher probability of acquiring the parasite. The parasite can survive in the tick as it molts through its various developmental stages, resulting in all tick stages being potentially infectious. Some species of Babesia can be transmitted from a female tick to its offspring before migrating to salivary glands for feeding. B. microti, the most common species in humans, has not been shown to transmit transovarially.Ticks of domestic animals that transmit Babesia and cause much disease include the very widespread cattle ticks, Rhipicephalus ''microplus, and R. decoloratus. These ticks have a strict one-host feeding cycle on cattle, so the Babesia can only be transmitted by the transovarial route.
In the Americas, Ixodes scapularis is the most common vector. This hard tick, commonly known as a deer tick, is also the vector for other tick-associated illnesses, such as Lyme disease. Many species of Babesia only infect nonhuman mammalian hosts, most commonly cattle, horses, and sheep. B. microti and B. divergens are the two main pathogenic species in humans. Their reservoirs are theorized to be the white-footed mouse, voles from the Microtus genus, and the white-tailed deer. These woodland species are hypothesized reservoirs because although they are known to harbor the disease, complete reservoir competence has not yet been shown.
Most cases of transmission between humans are attributed to a tick vector. As of 2003, the Centers for Disease Control and Prevention acknowledged more than 40 cases of babesiosis contracted from transfusions of packed red blood cells, as well as two infections documented from organ transplantation. PRBC transfusions that cause infections were identified through testing the blood donor for B. microti antibodies. The occurrence of Babesia transmission through PRBC blood transfusions puts pressure on governmental organizations to heighten standard measures for screening blood donations.
Transmission is also possible through congenital transmission. As symptoms may not appear, many women may not be aware they are infected during pregnancy, so a measurement of congenital transmission rate is not known at this time.
Currently, no vectors for avian Babesia have been identified, but they are assumed to be ticks. Babesia'' species require competent vertebrate and invertebrate hosts to maintain transmission cycles.