Babesiosis: A Dangerous Tick-Borne Disease
Affecting Cattle
Babesiosis, commonly known as
redwater, is a parasitic disease caused by an intra-erythrocytic protozoan
belonging to the Babesia genus. It is primarily transmitted by hard ticks of
the Ixodidae family. Unlike many parasitic diseases, Babesiosis tends to affect
adult cattle more severely than young ones, often resulting in subclinical
infections in the latter. The disease is characterized by symptoms such as
fever, haemoglobinaemia, haemoglobinuria, anaemia, and, in severe cases, death.
Causes and Epidemiology
Babesiosis in cattle is primarily
caused by four significant species of Babesia: B. bovis, B. bigemina, B.
divergens, and B. major. Among these, B. bovis, B. bigemina, and B. divergens
are more prevalent, with B. major being less significant. The spread of Babesia
spp. is influenced by local climate conditions and the behavior of tick
vectors.
Tropical species: B. bovis and B.
bigemina. These species are found in regions such as Australia, Africa, South
and Central America, Asia, and the southern parts of Europe. In Australia and
the Americas, the tick Boophilus microplus acts as the primary vector, while in
Africa, other Boophilus and Rhipicephalus species are involved. Boophilus ticks
are one-host ticks, meaning their entire life cycle takes place on a single
animal, with only the engorged female dropping to the ground to lay eggs. Some
African vectors, like R. evertsi, are two-host ticks, and R. appendiculatus is
a three-host tick. B. bovis infection generally results in low parasitaemia in
venous blood, but it can be high in capillaries, leading to blood sludging.
Severe effects on the blood coagulation system are observed, making B. bovis
the most pathogenic species among bovine Babesia. On the other hand, B.
bigemina causes higher venous parasitaemia but primarily leads to febrile
reactions and straightforward haemolytic anaemia.
Temperate species: B. divergens
and B. major. Babesia divergens is commonly found in areas with permanent
pasture in northwestern Europe and is transmitted by the three-host tick Ixodes
ricinus. Babesia major, which is limited to southeastern England and some
islands off the coast of The Netherlands, is transmitted by Haemaphysalis
punctata. B. divergens exhibits similar behavior to B. bigemina, causing high
parasitaemia, fever, and severe haemolytic anaemia. However, its impact on
blood coagulation systems is less severe compared to B. bovis. The epidemiology
of B. divergens closely relates to the ecology of its tick vector, I. ricinus.
In Europe, I. ricinus is generally active between May and November, with
population increases during spring and autumn, although in northern climates,
it may only have one peak in midsummer. Outbreaks of babesiosis are frequently
observed about two weeks after favorable weather conditions. Limited research
has been conducted on B. major's epidemiology, but reported isolations have
occurred in May and June.
Signs and Symptoms
Early signs of Babesiosis in
cattle include slight dullness, high fever (40.5–41°C or 105–106°F), pipe-stem
diarrhoea, haemoglobinuria, slight dehydration, and sunken eyes. After 24–36
hours, mucous membranes tend to become pale, pulse rate increases, animals slow
down, appetite and thirst decrease, urine becomes dark and reduced in quantity,
and faeces may return to normal. In the late stages of the disease, rectal
temperature is often subnormal, mucous membranes become blanched, appetite
diminishes, constipation becomes apparent, heart rate significantly increases,
and pregnant cows may experience abortion.
Necropsy Findings
During necropsy, affected animals
may exhibit a very blanched carcass, and jaundice might be present. The liver
is often swollen and pulpy, with dark and enlarged kidneys. The bladder
contains red-brown urine, and ecchymotic haemorrhages can be observed under the
epicardium and endocardium.
Diagnosis
The diagnosis of Babesiosis
requires considering the history of recent movement to tick-infested pastures.
Clinical babesiosis is unlikely to be observed in cattle younger than nine
months old. Although these cattle can get infected and show febrile reactions,
their resulting parasitaemia remains low, and haemoglobinuria tends to be mild.
In areas with high tick populations, most cattle are infected at an early age
and develop immunity over time, resulting in enzootic stability. In the early
stages of the disease, haemoglobinuria might not be present, necessitating a
careful examination of stained blood smears for accurate diagnosis. Once
haemoglobinuria becomes evident, the parasitaemia is more noticeable.
Differential diagnosis involves ruling out other conditions causing
haemoglobinuria, such as anaplasmosis, eperythrozoonosis, leptospirosis,
postparturient haemoglobinuria, and bacillary haemoglobinuria.
Treatment and Control
The treatment of Babesiosis in
cattle involves two primary aspects. First, treatment with babesicides is
administered, and secondly, supportive therapy, including blood transfusion and
fluid replacement, may be necessary. Currently, there are limited options for
babesicides. Imidocarb is the only licensed treatment in the UK, given at a
dose rate of 1mg/kg body weight. Imidocarb is widely used in South America and
other countries due to its high effectiveness and relatively low toxicity.
However, it leaves tissue residues for several weeks after administration.
Imidocarb can also be used at double the therapeutic dose as a
chemoprophylactic measure, offering protection from infection for up to six
weeks. This approach is often utilized in cattle exposed to ticks or those that
have been vaccinated with live Babesia. The goal is to induce mild infection in
cattle, leading to immunity against clinical illness and further infection.
Live vaccines for B. bovis and B.
bigemina have been available in various countries for many years, particularly
in Australia and South America. These vaccines consist of live organisms made
avirulent by repeated rapid syringe-passage through splenectomized calves. B.
divergens currently lacks a vaccine, although ongoing research aims to develop
inactivated recombinant vaccines for this species, as well as for B. bovis and
B. bigemina.
Severely affected adult cattle
often require blood transfusions, which involve collecting 5 liters of blood
from a healthy, unaffected cow into a 22 percent solution of the anticoagulant
acid citrate dextrose (ACD). This mixture is immediately transfused into the
recipient animal. Single transfusions without cross-matching of blood types
usually yield successful results, but repeated transfusions can lead to
compatibility issues with blood antigens.
In conclusion, understanding the
causes, symptoms, diagnosis, treatment, and control measures of Babesiosis is
crucial for managing and preventing the disease in cattle populations.
