Babesia (also called Nuttallia) is an Apicomplexan parasite that infects red blood cells, transmitted by ticks. Originally discovered by the Romanian bacteriologist Victor Babeș, over 100 species of Babesia have since been identified.
Babesia species infect livestock worldwide, wild and domestic vertebrate animals and occasionally humans where it causes the disease babesiosis. In the United States, B. microti is the most common strain of the few which have been documented to cause disease in humans.
Babesia is a protozoan parasite found to infect vertebrate animals, mostly livestock mammals and birds, but also humans. Common names of the disease, which Babesia microti causes are Texas cattle fever, redwater fever, tick fever, and Nantucket fever. The disease it causes in humans, babesiosis, is also called piroplasmosis.
Due to historical misclassifications, the protozoan has been labeled with many names, including Nuttallia; the microbiological community changed the name Babesia to Theileria based on evidence from 2006. The sequence published in 2012 shows, that the species belongs to neither Babesia nor Theileria but instead to a separate genus.
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.
Until 1957 it was believed to be a disease that only affected nonhuman mammals, when the first case of babesiosis was seen in a human. The person had been splenectomized, as were all people diagnosed with babesiosis until 1969. That was when the first case of babesiosis was diagnosed in a person who still had their spleen, and proved the parasite was a potential pathogen in anyone.
Babesia show host specificity, allowing many different subspecies of Babesia to emerge, each infecting a different kind of vertebral 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 their fitness allow the population to increase their numbership. It explains why there is such great genetic diversity for this organism.
Babesia selfishly persists long-term in the host's system: the host gains no benefit from the parasite invasion but rather suffers. This allows the parasite to exploit all resources offered by the host, to increase in number, and the rate of transmission. Too lethal of an infection would result in the host´s death and the parasite unable to spread, 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 humoral immune system. This results in control of the infection, but also persistence and not clearance the organism.
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. As of 2013[update] 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 piroplams has identified a number of additional species.
The lifecycle of B. microti, which is typical of parasites in that 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 themselves as it takes up gametocytes when attached for a blood meal. It also introduces the Babesia into the intermediate host (cattle eg ) when taking a blood meal. As Babesia enter the animal´s red blood cells (erythrocytes) 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 which 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 congenital transmission (an infected mother to her baby). .
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 has been called not strong enough to count as a major factor.
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. life span and number of generations of Babesia microplus correlate with increasing the longevity of larvae and number of annual generations. Dry warm weather interfers with the Babesia life cycle 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.
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, though, 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 (Boophilus) microplus, and R.(B.) 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 (Peromyscus leucopus), microtus voles (Microtus spp.), and the white-tailed deer (Odocoileus virginianus). 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 (CDC) acknowledged more than 40 cases of babesiosis contracted from transfusions of packed red blood cells (PRBC) and two infections documented from organ transplantation. PRBC transfusions that cause infections were identified through testing of the blood donor for B. microti antibodies. The occurrence of Babesia transmission PRBC through blood transfusions puts pressure on governmental organizations, such as the CDC, to heighten standard measures for screening blood donations.
Transmission is also possible through congenital transmission (an infected mother to her baby). As symptoms may not appear many women may not be aware they are infected during pregnancy, and therefore a measurement of congenital transmission is not known at this time.
Of the species to infect humans, B. microti is most common in the Americas, whereas B. divergens is the predominant strain found in Europe. Endemic areas are regions of tick habitat, including the forest regions of the northeastern United States and temperate regions of Europe. Ixodidae, the tick vectors of B. microti, also transmit the better-known Borrelia burgdorferi, causative agent of Lyme disease. For reasons that remain unclear, in areas endemic to both Lyme disease and babesiosis, Lyme disease transmission prevails and is more predominant in the region. Prevalence of babesiosis in malaria-endemic regions remains unknown due to the likelihood of misdiagnosis as malaria. As the disease results in a high number of asympomatic individuals, many populations can possess high seroprevalence without much documentation of illness. For example, in Rhode Island and Nantucket, seroprevalence has been measured to be 20–25%. Prevalence of babesiosis is mostly documented during the months of May to September when tick activity in endemic regions is high.
Signs of infection with B. microti usually arise one to eight weeks after a bite from an infectious tick. Infections from B. divergens have a shorter latent period, usually ranging from one to three weeks. The severity of B. microti infections varies. For 25% of cases in adults and 50% of cases in children, the disease is asymptomatic or mild with flu-like symptoms. In other cases, symptoms are characterized by irregular fevers, chills, headaches, general lethargy, pain, and malaise. In severe cases effects of parasitic multiplication like hemolytic anemia, jaundice, shortness of breath, and hemoglobinuria are documented. Immunocompetent individuals with healthy spleens often recover without treatment.
Splenectomized patients are more susceptible to contracting the disease and can die within five to eight days of symptom onset. They suffer from severe hemolytic anemia and occasional hepatomegaly and splenomegaly has been documented. Parasitemia levels can reach up to 85% in patients without spleens compared to 1–10% in individuals with spleens and effective immune systems.
Complications include acute respiratory failure, congestive heart failure, and renal failure. Infections can be fatal in 5–10% of hospitalized patients, with increased risk of death in the immunosuppressed, the elderly, and those coinfected with Lyme disease. B. divergens infections have a much higher fatality rate (42%) and present with more severe symptoms. Infected individuals suffer from hemoglobinuria followed by jaundice, a persistently high fever, chills, and sweats. If left untreated, B. divergens infections can develop into shock-like symptoms with pulmonary edema and renal failure.
As a protozoan parasite, the most effective way to identify Babesia infection is through blood sample testing.
Babesia species enter red blood cells (erythrocytes) at the sporozoite stage. 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. This tetrad morphology seen with Giemsa staining of a thin blood smear is unique to Babesia, and distinguishes it from Plasmodium falciparum, a protozoan of similar morphology that causes malaria. 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. It is important to pay attention to particular morphologies of Babesia in blood smears, because of its substantial similarity to the malarial parasite Plasmodium falciparum. This has resulted in many patients suffering from babesiosis being misdiagnosed. The few distinguishing factors for Babesia include protozoa with varying shapes and sizes, the potential to contain vacuoles, and the lack of pigment production. Trophozoites appearing in a tetrad formation within an erythrocyte are also indicative of Babesia.
Despite much study of babesiosis and malaria, misdiagnosis with blood smear can be frequent and problematic. To supplement a blood smear, diagnoses should be made with an indirect fluorescent antibody (IFA) test. IFA testing has a much higher specificity than stained blood smears with antibody detection in 88-96% of infected patients. Diagnostic measures through antibody testing are also particularly useful for identifying serum prevalence in asymptomatic individuals. Due to the transmissibility of Babesia through blood transfusions, IFA testing would be an effective means of screening for the disease in blood donations.
Historically, babesiosis diagnosis was carried out with xenodiagnosis in hamsters for B. microti and in gerbils for B. divergens. this diagnostic technique has been abandoned for faster diagnostic measures.
Several methods are available to manage and treat babesiosis in animals.
In humans many spontaneously recover, having only experienced mild symptoms undiagnosed as the disease. This is almost always seen in B. microti infections, which are generally more common in the United States. For B. divergens and more severe B. microti infections, the standard treatment historically for symptomatic individuals was oral or intravenous clindamycin with oral quinine. With the results of research completed in 2000, however, treatment regimens have been increasingly leaning towards oral atovaquone with oral azithromycin. The latter are preferred, as they are equally effective in all but the most severe cases and exhibit fewer associated adverse reactions. In severe cases, blood exchange transfusions have been performed to lower the parasitic load in an individual. Other measures include addressing and correcting abnormal clinical signs.
Prevention in animals
In 1906, efforts were made to eradicate the tick vector of bovine babesiosis in the United States. This eradication was recorded as being successfully completed four decades later.
Effective control can be achieved by vaccination with live attenuated phenotypes of the parasite. The vaccines have a number of drawbacks, so better, safer vaccines are still being researched. In recent[when?] years, a number of parasite proteins with immunogenic potential have been discovered. Through polymerase chain reaction, genetic sequencing, and bioinformatics analysis of the genes, a high degree of conservation (98–100%) was found among Brazilian isolates of B. bovis and the T2Bo isolate. Thus, these genes are considered for inclusion in a recombinant cocktail vaccine for cattle babesiosis caused by B. bovis.
Prevention in humans
The most effective public health measure for Babesia is to avoid tick exposure. This can be through personal prevention such as avoiding tick-infested areas (especially during high tick season between May and September), remaining covered with light clothing, searching for ticks after being outdoors, and removing discovered ticks from the skin. Other measures include applying diethyltoluamide (DEET), a common repellent that is effective against ticks and insects. (For people who react adversely to DEET, alternative insect repellents should be used.) On a state level, if health departments are particularly motivated, tick elimination is a possibility. In 1906, efforts were made to eradicate the tick vector of the bovine disease form of babesiosis in the United States. This eradication was recorded as being successfully completed four decades later.
Complete eradication through vector control would be a long-term project, which would significantly reduce the prevalence of both babesiosis and Lyme disease, but, as public health departments are often short on funding, preventive measures are more recommended.
Due to the relatively low prevalence of the human disease and the presence of several reservoirs, babesiosis has not been a candidate for vaccines. In regions where ticks of domestic animals are routinely controlled with chemical acaricides to reduce incidence of infection with B. bovis and B. bigemina, the risk to humans from these parasites will be reduced.
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Figure 2. Panel of computer-generated electronic images of photomicrographs of Babesia-infected erythrocytes on a Giemsa-stained smear of peripheral blood...
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