07/11/2017
Babesiosis;
Synopsis
Babesiosis is a tick-borne protozoan disease affecting domestic and wild animals and humans worldwide. Babesiosis is caused by parasites of the genus Babesia (B.) that belong to protozoan piroplasms.1 The disease is named after the Romanian bacteriologist Victor Babeş. Babesiosis is also known as piroplasmosis (from Latin pirum, meaning “pear”, and plasma, “image, formation”). Babesia spp. are common blood parasites of mammals. Human babesiosis is uncommon, but more cases in people have been reported recently, likely because of rising awareness. Babesiosis is found in dogs worldwide, but only rarely in cats.
Agent Properties
Several Babesia spp. have been detected in domestic cats in different regions of the world, including South Africa, Sudan, Zimbabwe, Israel, India, Thailand, Brazil, France, Poland, and Germany. However, only few Babesia organisms have been characterised.
Most reports about babesiosis in cats come from South Africa, where it is mainly found in the coastal areas. There (as well as in other parts of Africa, e.g. the Sudan), infection is caused mainly by B. felis, a small Babesia spp. that causes severe anaemia and icterus.2,3 Most other small Babesia spp. in domestic cats, like the B. cati found primarily in India, are less pathogenic.
Babesia leo, another small species, is antigenetically and genetically similar to B. felis and common in lions of the Kruger National Park, South Africa, and in Swaziland4, but also prevalent in domestic cats in these areas. A small babesia was also seen in blood smears of cats in Rio de Janeiro, Brazil, but the species was not identified.
Sporadic cases of canine Babesia spp. infections in domestic cats have been reported in Europe, such as B. canis canis from Spain and Portugal, a B. microti-like species from Portugal, and a B. canis-like species from Poland.3 B. canis presentii has been detected in Israel, and B. canis vogeli is common in stray cats in Thailand.5-7
In addition to the small B. leo, several large Babesia spp., including B. herpailuri and B. pantherae, have been discovered in wild felids, in lions, cheetahs, and the Florida panther. They can be transmitted experimentally to domestic cats, but their infectivity and pathogenicity under natural circumstances is unknown.3
Several new species (e.g. B. venatorum) have been discovered in the last years. A new species, named B. hongkongensis was discovered in kidney sections of a free-roaming cat in Hong Kong.8 The level of genetic divergence within Babesia spp. suggests that new subspecies might represent an “expanding universe”.9
Life cycle
Babesioses are vector-borne diseases usually transmitted by ticks. The vector (or vectors) for Babesia spp. in cats has (have) not yet been identified.
Babesiae replicate in erythrocytes, where they produce merozoites. These structures appear as inclusions attached to each other at their ends, thereby forming tetrads. These so-called Maltese Cross formations are essentially pathognomonic of babesiosis. Ticks are infected by ingesting merozoites during feeding, and replication of the parasite within their salivary cells results in sporozoite formation. When infected ticks feed, the sporozoites are regurgitated and fed back into the bloodstream of the host.
Epidemiology
B felis is the most important pathogenic species in cats. It is mainly found in the coastal areas of South Africa.2,3 It usually occurs in cats less than three years of age, without any predilection for s*x or breed.10,11
Apart from the usual transmission by ticks, Babesia spp. have been transmitted iatrogenically, e.g. through blood transfusions. Virulence is determined primarily by the parasite species involved.
Clinical signs
Severe disease has been described in cats infected with B. felis in South Africa, where feline babesiosis is diagnosed regularly.2 This species has not been reported in Europe so far. Feline babesiosis caused by other Babesia spp. presents rather as a chronic, mild disease.5
Common clinical signs are anorexia, lethargy, weakness, and a rough haircoat.10,12 Unlike in dogs, fever and icterus are uncommon. In most instances with fever, a concurrent illness is diagnosed.12 Most clinical signs are secondary to haemolytic anaemia that results from the infection of erythrocytes by the piroplasms. Cats usually cope with the anaemia and may show only mild clinical signs.
Complications of babesiosis include renal failure, pulmonary oedema, hepatopathy, and CNS signs.12 Concurrent infection with Mycoplasma hemofelis, feline leukaemia virus, or feline immunodeficiency virus may contribute to the clinical presentation and severity of disease.
The typical laboratory findings in cats with babesiosis are a consequence of haemolytic anaemia, which is usually regenerative, macrocytic, and hypochromic. Haemolysis can be caused by both extravascular and intravascular erythrolysis.11 Anaemia is most pronounced approximately three weeks after an experimental infection.13 Blood smears can show increased polychromatophils, Howell-Jolly bodies, nucleated erythrocytes, and anisocytosis.12 Erythrophagocytosis by monocytes is also observed,12 and intra-erythrocytic parasites can sometimes be detected.11 Secondary, immune-mediated haemolytic anaemia with anti-erythrocyte antibodies can be occasionally seen, leading to a positive Coomb’s test and auto-agglutination.11
Infected cats usually show no changes in the white blood cells. Thrombocytopenia is common in other species but is an inconsistent finding in cats.11,13
In serum biochemistry, ALT activity is elevated in most cases, whereas ALP activity is generally within reference values.2,11 Total bilirubin concentration is commonly increased,11 most likely as a result of haemolysis, but secondary hepatocellular injury may be a contributing factor.2 Feline babesiosis does usually not alter urea, creatinine, cholesterol concentrations, or blood pH.14 Polyclonal gammopathy has been observed in cats with hypergammaglobulinaemia also leading to increased total protein concentrations.11
