The role of Ticks and Tick-borne diseases in livestock sector (Theileriosis, Babesiosis, and Equine Piroplasmosis)

Ticks play a major role in transmitting infectious diseases. Emerging or re-emerging infectious diseases are important global problems of great concern to humans as well as to animal health, with many pathogens being able to infect multiple species. With increasing antimicrobial resistance among bacterial pathogens, there has been an increase in the occurrence of zoonotic diseases, sometimes causing widespread outbreaks with considerable domestic animal, wildlife and human morbidity and mortality.

Therefore, infectious diseases have been recognized as an increasing threat to the general public and animal husbandry. Accelerating climate change carries serious threats for public health and society. Global warming and the unstable climate are playing an ever-increasing role in driving the global emergence, resurgence and redistribution of infectious diseases. Globalization and climatic abnormalities have allowed parasites to invade into new geographic areas or establishing ranges in common localities, giving rise to epidemics and epizootics worldwide.

Ticks are a major group of arthropod vectors, characterized by the diversity of pathogens they transmit, by their impact on human and animal health, thus have a great significance on economic, veterinary, and human health perspectives because of their capacity to transmit a variety of diseases. Ticks are widespread in tropical and subtropical regions worldwide, including Africa, South and Central America, the USA, southern Europe, Asia, and Australia, due to suitable climatic conditions. Where the warm and humid lowlands considered an ideal habitat for ticks and endemic for TBDs.

Besides the physical injury inflicted on the animal host, ticks transmit a number of pathogens that can cause morbidity and mortality of livestock if untreated, resulting in huge economic losses. Uganda suffers an aggregated annual loss (direct and indirect) of over USD 1.1 billion in the TTBDs (Ticks and Tick-borne diseases) complex. East Coast fever (ECF) caused by a protozoan haemoparasite, Theileria parva, is the most prevalent and economically important tick-borne disease (TBD) in Uganda and its vector, the brown ear tick (Rhipicephalus appendiculatus) widely distributed. Other prevalent TBDs in Uganda include anaplasmosis, babesiosis and heartwater.

Theileriosis

Both Theileria and Babesia are members of the suborder Piroplasmorina. Although Babesia are primarily parasites of RBCs, Theileria use, successively, WBCs and RBCs for completion of their life cycle in mammalian hosts. The infective sporozoite stage of the parasite is transmitted in the saliva of infected ticks as they feed. Sporozoites invade leukocytes and, within a few days, develop to schizonts. In the most pathogenic species of Theileria (eg, T parva and T annulata), parasite multiplication occurs predominantly within the host WBCs, whereas less pathogenic species multiply mainly in RBCs. Development of the schizont stage of pathogenic Theileria causes the host WBC to divide; at each cell division, the parasite also divides. Thus, the parasitized cell population expands and, through migration, becomes disseminated throughout the lymphoid system. Later in the infection, some of the schizonts undergo merogony, releasing merozoites that infect RBCs, giving rise to piroplasms. Uptake of piroplasm-infected RBCs by vector ticks feeding on infected animals is the prelude to a complex cycle of development, culminating in transmission of infection by ticks feeding in their next instar (trans-stadial transmission).

Bovine theileriosis is a tick-borne hemoprotozoan disease of cattle caused by several Theileria species and among them T. parva, the cause of East Coast fever and T. annulata, the causative agent of tropical theileriosis is the most pathogenic and economically important, which cause acute disease resulting in high levels of mortality. Theileriosis is seen mainly in Africa and India, where it is characterized by a high mortality rate and is known as East Coast Fever. Theileria parva affects cattle and water buffalo (Bubalus bubalis). African buffalo (Syncerus caffer) and cattle are important reservoir hosts for this organism. Theileria annulata affects cattle, yaks and water buffalo and is transmitted by ticks of the genus Hyalomma. Theileria sergenti is found in Korea and Japan. Theileria mutans, a parasite of cattle in the southwestern United States, is relatively nonpathogenic. A mild form of theileriosis, locally called “January disease,” occurs in cattle in Mozambique. This condition is caused by the subspecies T. parva bovis. Corridor disease is caused by T. parva Lawrence, and Theileria annulata is the cause of Mediterranean Coast fever or tropical theileriosis. T lestoquardi, T luwenshuni, and T uilenbergi are important causes of mortality in sheep, and T equi sometimes causes clinical disease in horses. Theileria are obligate intracellular protozoan parasites that infect both wild and domestic Bovidae throughout much of the world, and have complex life cycles involving both vertebrate and invertebrate hosts. Theileria organisms are transmitted to susceptible cattle by the ticks Hyalomma anatolicum, Hyalomma lusitanicum, and Amblyomma hebraeum. Tropical theileriosis and East Coast Fever are disease transmitted by Ixodid tick of the genus Hyalomma and Rhipicephalus, respectively.

East Coast fever

Caused by Theileria parva, is an acute disease of cattle. It is usually characterized by high fever, swelling of the lymph nodes, dyspnea, and high mortality. It is a serious problem in east and southern Africa.

T parva sporozoites are injected into cattle by infected vector ticks, Rhipicephalus appendiculatus, during feeding. Ticks acquire infection by feeding on infected cattle or African buffalo (Syncerus caffer), which carry the infection but do not show signs of disease. Both cattle- and buffalo-derived T parva are highly pathogenic when transmitted to cattle, but the latter do not develop to the piroplasm stage and therefore are usually not transmitted by ticks from infected cattle.

Tropical Theileriosis

T annulata is the causal agent of tropical theileriosis, which is widely distributed in north Africa, the Mediterranean coastal area, the Middle East, India, countries of the southern former USSR, and Asia. It is transmitted by several species of ticks of the genus Hyalomma. T annulata can cause mortality of up to 90%, but strains vary in their pathogenicity.

The kinetics of infection and the main clinical features of the disease are similar to those produced by T parva, but unlike East Coast fever, anemia is often a feature of the disease. Characteristic signs include fever and swollen superficial lymph nodes, and if the disease progresses, cattle rapidly lose condition. Animals that recover from infection are immune to subsequent challenge.

Clinical findings

East Coast fever/corridor disease (T. parva) is characterized by fever, generalized lymphadenopathy, anorexia, loss of condition and, in some animals, nasal discharge or diarrhea. Petechiae and ecchymoses may be found on the conjunctiva and oral mucous membranes, and milk yield usually decreases in lactating animals. Terminally ill animals often develop pulmonary oedema, with severe dyspnea and a frothy nasal discharge. Tropical theileriosis (T. annulata) generally resembles East Coast fever, but these parasites also destroy red blood cells, causing anaemia and, in some cases, jaundice or haemoglobinuria. Petechiae are often found on the mucous membranes, and hemorrhagic diarrhea may be seen in the late stages. Other species of Theileria tend to be carried asymptomatically, although some can cause anaemia or other clinical signs, especially when there are exacerbating factors such as coinfections. There is no evidence that the species of Theileria found in ruminants affect humans.

Diagnosis of Theileriosis in Cattle

Diagnosis is based on clinical signs and detection of parasites in lymph node aspirates.

Serology is only of value in detecting previous infection in recovered animals.

Confirmation of disease caused by T parva and T annulata relies on microscopic examination of Giemsa-stained smears of lymph node needle aspirates for the presence of schizonts in infected leukocytes. The intra-erythrocytic piroplasm stages are also readily detected in stained blood smears. Piroplasms assume various forms, but typically they are small and rod-shaped or oval. The schizonts and piroplasms of T parva and T annulata are morphologically similar.

Definitive diagnosis can also be confirmed using antigen-specific ELISAs or PCR on lymph node aspirates.

Treatment and Control of Theileriosis in Cattle

Bovine theileriosis has global economic significance thus prevention is the best method to control losses related with the disease. Among Several control methods the most practical and widely used method is the chemical control of ticks with acaricides (Amiraz^®, Mobedco Manufacturing Co) but this needs to be applied at regular intervals to be effective. However, tick control practices are not always fully effective and hence vaccination is the most sustainable option. Since there is difference in breed of cattle to tick resistance the selection of tick resistant cattle breeds is also proposed as a sustainable approach for controlling infection in developing world.

Only a single compound, buparvaquone (Butex Inj.^®, Mobedco Manufacturing Co), is available for treatment of the diseases caused by Theileria parasites. Treatment is effective when applied in the early stages of clinical disease but may require more than one dose and often accompanied by anti-inflammatory drugs and antidiuretics, if there is evidence of pulmonary edema. Treatment is less effective in the advanced stages, when there is extensive destruction of lymphoid and hematopoietic tissues. Development of resistance to buparvaquone has also been reported for T annulata.

Babesiosis

Babesiosis, also known as piroplasmosis, tick fever, red water, Texas fever, splenic fever, tristeza, etc., is caused by various tick-borne, intraerythrocytic, protozoan parasites of the genus Babesia order Piroplasmida, phylum Apicomplexa. These parasites are capable of inducing disease in a wide range of vertebrates. The disease is usually characterized by fever, hemolytic anemia, hemoglobinuria, and in severe cases, death. Babesiosis is economically the most important arthropod-borne disease of cattle worldwide with vast areas of Australia, Africa, South and Central America and the United States continuously under threat.

Bovine babesiosis is a tick-borne parasitic disease that results in significant morbidity and mortality in cattle. The economic losses can be considerable, especially when animals with no immunity are moved into an endemic area. Three species of Babesia cause most clinical cases in cattle: Babesia bovis and B. bigemina are widespread in tropical and subtropical regions, while B. divergens circulates in parts of Europe and possibly in North Africa. A recent study suggested that Babesia sp. Mymensingh may be an additional species capable of causing clinical babesiosis in cattle. Other Babesia that can infect cattle include B. major, B. ovata, B. occultans and B. jakimovi. Bovine babesiosis can be managed and treated, but the causative organisms are difficult to eradicate. Most cattle Babesia do not seem to affect humans; however, B. divergens can cause rapidly progressing, life-threatening hemolytic anemia in people who have had splenectomies.

Transmission

In nature, the babesias are transmitted biologically by ixodid ticks, but other mechanical means of transmission, such as biting flies and fomites that transfer blood to a susceptible host, may theoretically induce infection.

The major vectors for B. bigemina and B. bovis are Rhipicephalus microplus (formerly Boophilus microplus), and in some areas, R. annulatus (formerly Boophilus annulatus). R. microplus and R. annulatus are one-host ticks that complete their life cycle on a single host, and preferentially feed on cattle. Additional members of Rhipicephalus and some ticks in other genera have also been suggested as vectors in some regions. Babesia can be transmitted transovarially. They are stimulated to undergo their final maturation when an infected tick attaches to the host. B. bovis usually becomes infective within 2-3 days after larval ticks attach. It does not persist in R. microplus after the larval stage. B. bigemina matures approximately 9 days after larval attachment, and it is only transmitted by nymphs and adults.

Ixodes ricinus is the major vector for B. divergens. All three of its life stages are thought to be capable of transmitting this organism. Haemaphysalis longicornis transmits B. ovata, while B. occultans is thought to be transmitted by Hyalomma marginatum, Hy. rufipes and possibly other members of this genus. The vectors for B. major are thought to include Haemaphysalis punctata and possibly other members of this genus. B jakimovi might be transmitted by a member of the genus Ixodes.

Cattle that have recovered from acute babesiosis can remain asymptomatically infected, and recrudescence of parasitemia can occur at irregular intervals. Persistent infection with B. divergens, with periodic waves of parasitemia, was also detected in some experimentally infected sheep. Babesia can be transmitted directly between animals in blood, for instance during transfusions, and possibly when smaller amounts of blood are transferred on reused needles or field surgical instruments or by biting flies. Transplacental transmission has been demonstrated for B. bovis and B. bigemina in cattle, but seems to be infrequent.

Clinical findings

Clinical signs usually appear 2-3 weeks after a bite from an infected tick. After inoculation with contaminated blood, the incubation period can be as short as 4-5 days for B. bigemina and 10-12 days for B. bovis.

Babesiosis is characterized by fever, which can be high, and varying degrees of hemolysis and anemia. Anemia may develop rapidly. The resulting clinical signs can include pale mucous membranes and increased respiratory and heart rates, as well as a decreased appetite, a drop in milk production, weakness, lethargy, and other signs related to anemia or fever, including abortions or temporarily decreased fertility in bulls. Jaundice is sometimes apparent, especially when the clinical signs are less acute, and hemoglobinuria and hemoglobinemia are common in animals infected with B. bigemina. B. bovis can cause additional clinical signs via changes in red blood cells (RBCs) that result in their accumulation in capillaries, including those of the brain. This can result in neurological signs (e.g., incoordination, teeth grinding, manic behavior), and may cause or contribute to other serious syndromes such as respiratory distress. However, they may occur if anemia results in brain anoxia. “Pipestem” diarrhea is reported to be common in the early stages of babesiosis caused by B. divergens, from changes in intestinal and ruminal motility. Terminal recumbency, dehydration and constipation may occur in the late stages of babesiosis. In animals that survive, the anemic crisis generally passes within a week. The survivors may be weak and in reduced condition, although they usually recover fully.

The severity of babesiosis can vary considerably between individuals, and cattle younger than 9 months are usually infected without clinical signs. Mild illnesses, with mild fever, anorexia and an uneventful recovery, are also reported to be common in animals infected with B. divergens. A few congenitally infected calves were reported to have signs of babesiosis, including neurological signs. In one case, a clinically affected calf was born to a dam with no apparent history of babesiosis. Some calves seem to be infected in utero but asymptomatic at birth.

Diagnosis of Babesiosis in Cattle

Babesiosis is often diagnosed by identifying the parasites in blood or tissue smears stained with Giemsa. Fluorescent dyes such as acridine orange can aid in parasite identification, and immunostaining techniques have been described. B. bigemina and B. divergens can be found in normal venous blood samples, but B. bovis is more likely to be recovered from capillary blood. Samples should be taken from capillaries in the ear or tail if the latter organism is suspected. At necropsy, recommended samples include the kidney, myocardium, liver and lung. The brain (cerebral cortex) can also be sampled in animals with neurological signs. Diagnosis is unreliable if an animal has been dead for more than 24 hours, but parasites can sometimes be found in blood from the lower leg.

Babesia are identified under oil immersion. The World Organization for Animal Health (WOAH) recommends a 10x eyepiece and 100x objective lens, at a minimum. Babesia are found within RBCs, and all divisional stages – ring (annular) stages, pear-shaped (pyriform) trophozoites either singly or in pairs; and filamentous or amorphous shapes – may be detected simultaneously.

PCR tests can be used to diagnose clinical cases and distinguish species of Babesia. Other genetic tests, including loop mediated isothermal amplification (LAMP) assays and a PCR-ELISA have been published for some organisms.

Treatment and Control of Babesiosis in Animals

In endemic areas, sick animals should be treated as soon as possible with an antiparasitic drug. Imidocarb (Imicarb^®, Mobedco Manufacturing Co) is used most often. Where it is available, diminazene aceturate (Diaminavet Plus^®, Mobedco Manufacturing Co) can also be used in cattle infected with B. bigemina or B. ovata. However, it is reported to be less effective against B. bovis and B. divergens. Treatment is most likely to be successful if the disease is diagnosed early, and may fail in very sick animals. Blood transfusions and other supportive therapy may also be necessary.

Babesiacides

A variety of babesiacidal drugs have been used to treat bovine babesiosis; however, only diminazene aceturate and imidocarb dipropionate are still in common use. These drugs are not available in all endemic countries, or their use may be restricted. Manufacturers’ recommendations for use should be followed. For treating cattle, diminazene is administered at 3.5 mg/kg, IM, once. For treatment, imidocarb is administered at 1.2 mg/kg, SC, once. At a dosage of 3 mg/kg, imidocarb provides protection from babesiosis for approximately 4 weeks and may also eliminate B bovis and B bigemina from carrier animals.

Supportive treatment

Supportive treatment is advisable, particularly in valuable animals, and may include the use of anti-inflammatory drugs, corticosteroids, and fluid therapy. Blood transfusions may be lifesaving in very anemic animals.

Tick control

Tick control, via acaricides (Amiraz^®, Mobedco Manufacturing Co) or management practices, can be useful in reducing tick burdens, which can lower transmission rates. However, this can lead to naive cattle populations, with consequent risk of outbreaks of disease should tick populations increase. Chemical tick control cannot be relied upon to prevent transmission of Babesia, and outbreaks often occur after introduction of susceptible cattle to endemic areas despite acaricide use. Acaricide resistance is also an increasing problem. However, acaricidal tick control before moving animals from tick-infested areas is useful to prevent the introduction of ticks and babesiosis to tick-free areas.

Eradication of the tick vector is rarely feasible on individual premises but may work on a regional level in well-coordinated programs.

Using Resistant Breeds

Bos indicus–based breeds are commonly used to minimize production losses associated with ticks and babesiosis.

Vaccination

Vaccination using live attenuated strains of the Babesia parasites has been used successfully in countries such as Argentina, Australia, Brazil, South Africa, and Uruguay. The vaccine is available in either a chilled or frozen form. One vaccination produces adequate immunity for the typical productive lifespan of animals raised in commercial settings. Commercial vaccines based on recombinant antigens are not yet available.

Equine Piroplasmosis

Equine piroplasmosis (EP) is a tick-borne disease of equines (Horses, mules, donkeys and zebra) caused by the intraerythrocytic protozoan parasites Babesia caballi and Theileria equi of the order Piroplasmida. Theileria equi was previously designated as Babesia equi but compelling evolutionary, morphological, biochemical, and genetic evidence supports its reclassification as a Theileria. Theileria equi is a small parasite and is more pathogenic than B caballi. Equine piroplasmosis is found in Africa, Europe, Asia, South America, Central America, and the southern US. It is transmitted by ticks of the genera Rhipicephalus, Dermacentor, and Hyalomma.

Transmission

This disease is a tick-transmitted disease of equids and its presence requires competent arthropod vectors. Infected animals may remain carriers of these blood parasites for long periods and act as sources of infection for other ticks. The introduction of carrier animals into areas where competent tick vectors are prevalent can lead to an epizootic spread of the disease.

Theileria equi sporozoites inoculated into horses via a tick bite invade the lymphocytes and these intra-lymphocytic forms undergo development and eventually form Theileria-like schizonts; merozoites released from these schizonts invade RBCs and transform into trophozoites, which grow and divide into pear-shaped tetrad (‘Maltese cross’) merozoites.

Twelve species of ixodid ticks in the genera Dermacentor, Rhipicephalus and Hyalomma have been identified as transstadial vectors of B. caballi and T. equi, while eight of these species were also able to transmit B. caballi infections transovarially.

Transmission is also possible through mechanical vectors and blood-contaminated instruments (e.g. contaminated needles). Intrauterine infection, particularly with T equi, is also relatively common.

Clinical findings

Incubation period of equine piroplasmosis associated with T. equi is 12 to 19 days and approximately 10 to 30 days when caused by B. caballi.

The clinical signs of equine piroplasmosis are often nonspecific, and the disease can easily be confused with other similar haemolytic conditions presenting fever, anaemia and jaundice. Theileria equi tends to cause more severe disease than B. caballi. Piroplasmosis can occur in peracute form which is rare form of disease with only clinical observation being moribund or dead animals, acute form which is most common form of disease cases characterized by fever that usually exceeds 40°C, reduced appetite and malaise, elevated respiratory and pulse rates, congestion of mucous membranes, production of a dark red urine; faecal balls that are smaller and drier than normal, affected animals may appear unthrifty; anemic or icteric, subacute form is similar to acute form but accompanied by weight loss in affected animals and intermittent fever, mucous membranes vary from pale pink to pink, or pale yellow to bright yellow; petechiae and/or ecchymosis may also be visible on the mucous membranes, normal bowel movements may be slightly depressed and the animals may show signs of mild colic, and chronic form the chronic cases usually present nonspecific clinical signs such as mild inappetence, poor performance and a drop in body mass. Documented case fatality rates vary from 10–50%. Most animals in endemic areas survive infection.

Diagnosis of Equine Piroplasmosis

Equine piroplasmosis is often diagnosed by identifying the parasites in blood or tissue smears stained with Giemsa. Identification of the agent by microscopic examination of blood demonstration of parasites in stained blood; using Giemsa staining method, thick blood smear technique also used in instances where the parasitemia is very low.

B. caballi more often is associated with sludging or a packing of parasites in capillaries and small vessels of different organs, resulting in dysfunction of these organs with a low parasitemia in the peripheral blood circulation. B. equi infections are more frequently characterized by higher parasitemias, red cell lysis, and death due to anemia. B. caballi parasitemias rarely exceed lo%, whereas a 90% B. equi parasitemia is not uncommon.

Serological tests may be used to diagnose clinical cases and to detect chronic carriers. The tests most commonly used are the complement fixation test (CFT) and the competitive enzyme-linked immunosorbent assay (cELISA). An immunofluorescence assay (IFA) is also available, but is more subjective to interpret in the laboratory and less commonly used in the U.S. The CFT, cELISA, and IFA are available for each specific EP-organism, T. equi and B. caballi. The CFT detects antibodies as early as 8 days after infection, however titers decline around 2-3 months post-exposure. Thus, the CFT is a reliable test in acute infections or cases of recent exposure but has low sensitivity in cases of chronic infection. The cELISA detects IgG antibody, which takes longer to develop post-infection, but the assay has shown high sensitivity and specificity for identifying chronically infected animals.

Indirect fluorescent antibody test (IFAT) has been successfully applied to the differential diagnosis of T. equi and B. caballi infections and Enzyme-linked immunosorbent assay (ELISA) have been used.

Treatment and control of Equine Piroplasmosis

EP is most commonly introduced into an area by means of carrier animals or infected ticks. Thus, movement of equids requires testing (by IFA or ELISA).
Reducing exposure of equids to ticks by repellents, acaricides and regular inspection for animals and premises, control and eradication of the tick vector; including removal of nearby vegetation that could harbor ticks.
Any detected EP-positive animals should be quarantined from surrounding horses and vectors. Special care in possible mechanical infection of horses with contaminated blood. There are no vaccines available.

In conclusion, the treatment protocol for Equine Piroplasmosis is same as for babesiosis in cattle.

References