Intoduction

Babesia spp. are protozoan parasites of domestic and wild animals.  Species infective to humans are Babesia divergens, B.bovis and B. microti.   European cases are mainly due to B. divergens although B. microti has also been reported.  However, almost all of the North American cases are due to B. microti.

Life cycle

Human babesiosis is a zoonosis, acquired by tick bite when individuals accidentally interact with the natural life cycle of the parasite.

Babesia divergens/bovis

When the tick bites, sporozoites are injected into the blood stream and penetrate the erythrocytes. In contrast to the malaria life cycle, there is no tissue stage for Babesia divergens.  Babesia multiplies in the red cell by budding in contrast to schizogony in Plasmodium species.  The red cell ruptures and daughter parasites invade new erythrocytes for further asexual multiplication.  Some of the sporozoites injected by the tick vector follow a different path of intra-erythrocytic development, growing slowly and "folding" to form accordion-like structures which are destined to undergo further development in the tick vector.  Within the intestine of the tick, the accordion-like stage eventually fuses with another, to form a zygote. Further development outside the intestine occurs in a variety of tissues, the salivary glands and ovaries being especially important for transmission.  Sporozoites in tick salivary glands are injected into the mammalian host at the next blood meal. Trans-ovarial transmission of Babesia divergens also takes place so that newly hatched tick larvae are already infected. Trans-stadial transmission to nymph and then to adult stages can then take place.

Babesia microti

In the small mammal host of Babesia microti, sporozoites from the tick vector first enter lymphocytes and undergo merogony, the daughter parasites of which then enter erythorcytes.

Babesia microti does not undergo trans-ovarial transmission, but once a larva has become infected from a mammalian host it is able to pass on the infection trans-stadialy to the nymph.

Clinical disease

Babesia divergens/bovis

Patients who are particularly at risk are those who have had a splenectomy.  The patient may feel vaguely unwell at first but by the time the diagnosis has been made, is usually very ill, with fever, prostration, jaundice, anaemia and haemoglobulinuria. Nausea, vomiting and diarrhoea have also been recorded.

Babesia microti

B. microti is able to infect immunologically intact individuals.  Most human infections are subclinical. Where clinical illness develops, the incubation period is 1 to 3 weeks, occasionally up to 6 weeks. The illness usually begins gradually, with anorexia and fatigue, plus fever (without periodicity), sweating, rigors and generalised myalgia. Physical examination may reveal only fever, but may also show mild splenomegaly and sometimes mild hepatomegaly.

Laboratory diagnosis

Definitive diagnosis depends upon finding parasites on blood film examination which can be detected 2 to 4 weeks after a tick bite.  Hamster inoculation and serology have also been used for diagnosis.

1.  Microscopic Examination

Babesia divergensbovis

Babesia divergens are pear shaped, oval or round and may exist in pyriform pairs. There may be 1 to 8 parasites per red cell.  Ring forms especially may be confused with malaria parasites, especially Plasmodium falciparum .  However, in contrast to Plasmodium species,  Babesia does not form pigment, does not cause alterations in red cell morphology and does not exhibit the Maurer's clefts of Plasmodium falciparum , the Schuffner's dots of Plasmodium vivax, or the James's dots of Plasmodium ovale.

• The "Maltese cross form" is unique to Babesia but in its absence it may be very difficult to distinguish young ring forms of Plasmodium falciparum from Babesia. The absence of pigment cannot be relied upon as young rings of Plasmodium do not exhibit pigment.  Babesia are smaller than malaria parasites, and in some of the larger rings there is white vacuole containing erythrocyte stroma, instead of the pink vacuole seen in malaria.  Babesia does not form schizonts.

Babesia microti

Ring, rod shaped, pyriform, amoeboid, and "Maltese cross" forms are seen. In heavy infections different stages may be noted in the same red cell. Intra-erythrocytic stages measure approximately 2 x 1.5¥m. In very high parasitaemias, extracellular merozoites are found singly or as a syncytial structure. Peak parasitaemia varies between less than 1% to approximately 10%

2.  Serodiagnosis

The Indirect Fluorescent Antibody Test (IFAT) is available for both B. divergens and for B. microti and is the most useful serological test for early diagnosis.

3.  Animal Inoculation

This is not routinely used for diagnosis but B. microti grows well in hamsters and antibodies can serve as a confirmatory test.