Tropical Theileriosis
Also known as
Mediterranean theileriosis
Description
This is a disease of cattle in the Mediterranean and Middle East, from Morocco to western parts of India and China, Asia, and the former USSR. It is a tick-borne disease, caused by a microscopic protozoan parasite, Theileria annulata. The vector ticks are of the genus Hyalomma.
Theileria annulata, the parasitic protozoan, lives in blood cells and can cause mortality in up to 90% of infected cattle. Several different strains of these protozoa vary in their pathogenicity.
The mode of infection and main clinical signs are similar to those produced by T. parva, the species that causes East Coast fever, but unlike East Coast fever, anemia is often a feature of tropical theileriosis.
Protozoa in the genus Theileria are all tick-borne parasites found in many mammals. More than a dozen species occur in cattle, water buffalo, sheep and goats. Some cause few or no clinical signs, but others can cause serious illnesses with high morbidity and mortality rates, especially in susceptible animals that have never encountered them.
The two organisms with greatest economic impact in cattle are Theileria parva (which causes East Coast fever/corridor disease) and T. annulata, which causes tropical
theileriosis. In addition to causing direct losses, highly virulent species of Theileria can limit the movement of livestock between countries and are a constraint on the importation of new breeds or improved stock.
The widely distributed T. orientalis/ T. buffeli group is classed among the less pathogenic species of Theileria, but members of this group have caused a number of outbreaks among cattle in New Zealand, Australia and other countries in recent years.
The most prominent signs in cattle are fever and lymph node enlargement, but there is a wide range of clinical signs, especially in enzootic areas (regions where a disease occurs regularly).
Tropical theileriosis was once considered benign, in comparison to East Coast fever (caused by a related parasite) but with the introduction of European breeds into these regions, it became a bigger problem with major economic impact, since the new cattle had no immunity and were more adversely affected. Animals native to endemic areas seem more tolerant to the disease, and buffalos especially, appear less susceptible.
T. orientalis has caused a number of outbreaks in cattle since 2010, especially in New Zealand and Australia. This illness tends to be more serious in young calves and pregnant or recently calved animals. One study estimated overall case fatality rate at about 17% and cumulative mortality more than 0.5% during outbreaks caused by the Ikeda genotype of T. orientalis on New Zealand farms. Some farms were more severely affected, with cumulative mortality rates greater than 5%.
In severe infections due to Theilelia parasites, fever temperatures are generally higher than in any other cattle disease. Fever from 105 to 107.6 degrees is common in the first days, in acute stages. Then from day 5 to day 10 (from clinical onset), temperature drops to a more normal range (100.5 to 103 degrees F) and the animal starts eating again, but the disease continues to progress.
From day 10 to day 15, the animal declines, with subnormal temperature (98 to 100 F), anemia, jaundice, and heart failure. These animals rarely recover, even with intensive treatment.
Lymph nodes are commonly enlarged and there may be periodic bleeding from the nose. The animal has difficulty breathing and loses weight. Other signs, which may or may not be present’ include blood-tinged diarrhea, sometimes with obvious blood clots, grinding of teeth, circular raised patches of hair all over the body, hemorrhages in the mucous membranes of the eyes or vagina, and some degree of anemia.
Animals that recover from infection are immune to subsequent challenge with the same parasite. Treatments and control are similar as those for East Coast fever.
Signs
- Fever
- Swollen superficial lymph nodes
- Weight loss
Cause
At least 15 species in the genus Theileria infect domesticated ruminants. The two most virulent organisms in cattle are Theileria annulata, which causes tropical theileriosis, and T. parva, which causes East Coast fever.
East Coast fever is often called corridor disease when the causative organisms are found in African buffalo. Although there are some differences in how cattle-adapted strains or buffalo-adapted strains behave initially in cattle, both cause the same clinical signs, and buffalo-derived strains can adapt to cattle.
Other related organisms known to infect cattle include T. velifera, T. aurotragi, and T. mutans, While all of these species are much less virulent than T. parva or T. annulata, the T. orientalis group can cause oriental theileriosis, which is also called Theileria-associated bovine anemia (TABA).
At one time, three separate organisms (T. orientalis, T. buffeli and T. sergenti) were recognized in different parts of the world, but today they are thought to be the same species, which is usually called either T .orientalis or T. buffeli.
Cattle, yaks, water buffalo and African buffalo can be infected with T. orientalis. T. mutans has been found in cattle, water buffalo, African buffalo and bushbuck, while T. velifera and has been detected in cattle and African buffalo. T. taurotragi has been recognized in cattle, eland and bushbuck, but African buffalo do not seem to be susceptible to this organism.
Theileria are transmitted by ticks acting as biological vectors (the immature parasite develops within the tick itself), and can be transmitted via ticks that carry the parasite through several stages of the tick’s own life cycle. The protozoa enter the body as sporozoites in the saliva of a feeding tick.
Ordinarily, T. parva and T. annulata only mature after an infected tick attaches to a host, and the tick must be attached for a few days before these organisms are transmitted. However, T. parva can develop to the infectious stage in ticks on the ground if environmental temperatures are high. These organisms may enter the host within hours of attachment.
Once inside the mammalian host, these parasites undergo a complex life cycle involving the replication of schizonts in leukocytes (white blood cells) and development of piroplasms in erythrocytes (red blood cells). Piroplasms infect ticks when they feed on the animal’s blood. Theileria can also be transmitted mechanically via blood (such as on reused needles) and possibly by biting flies and sucking lice.
Ruminants, including animals that recover, can carry some species of Theileria for months or years. Trans-placental transmission (from dam to fetus) has been documented for several species of Theileria in cattle. The frequency of this vertical transmission might differ, depending on whether the dam is acutely infected or simply a carrier. T. orientalis has been found in colostrum, but attempts to infect calves orally with colostrum have been unsuccessful.
The incubation period for tropical theileriosis is approximately 1-3 weeks.
In cattle and water buffalo, East Coast fever/ corridor disease (T. parva) is characterized by fever, loss of appetite, weight loss, and, in some animals, nasal discharge and/or diarrhea. Milk yield usually decreases in lactating animals. Corneal opacity and skin lesions have been reported occasionally.
Terminally ill animals often develop pulmonary edema, with severe difficulty breathing and a frothy nasal discharge. Some animals have poor productivity after recovery, with stunted growth. T. parva can also cause a fatal condition called “turning sickness,” in which infected cells block capillaries in the central nervous system and cause neurological signs.
Tropical theileriosis (T. annulata) generally resembles East Coast fever, but these parasites also destroy red blood cells, causing anemia and, in some cases, jaundice or bloody urine. Petechiae (tiny purple, red or brown spots) are often found on the mucous membranes, and bloody diarrhea may be seen in the late stages. Some animals abort.
In at least two cases, newborns appeared normal at birth but developed clinical signs within a few days, probably from infections acquired in utero. Neurological signs have been documented in some terminally ill water buffalo, but turning sickness does not seem to be a feature of tropical theileriosis in cattle.
Other species of Theileria tend to be carried by some animals without sitns, although some can cause anemia or other clinical signs, especially when there are factors that make the disease worse, such as coinfection with other diseases.
The Ikeda genotype of T. orientalis has recently caused a number of outbreaks in cattle, with clinical signs of fever, lethargy, lack of appetite, lymph node swelling, anemia, jaundice, diarrhea and reproductive losses including stillbirths and late term abortions.
Although many animals recover, some cases are fatal. Similar cases, accompanied by generalized depigmentation of the skin, were reported in water buffalo infected with T. orientalis, and T. taurotragi was recently suggested to be the cause of a neurological disease (locally called ormilo disease), among cattle in Tanzania.
The most pathogenic species in cattle and small ruminants are all exotic to the U.S. and must be reported to state or federal authorities immediately upon diagnosis or suspicion.
The severity of theileriosis varies with the species of Theileria, strain and dose of the organism, and host factors such as immunity and concurrent illnesses. There are breed-related differences in susceptibility in cattle.
The effect of coinfections with less pathogenic species of Theileria seems to be complex. While coinfections may increase severity of some illnesses, some organisms seem to protect cattle from T. parva. In endemic regions, outbreaks of East Coast fever and tropical theileriosis tend to occur mainly when susceptible animals are moved into the area.
These diseases may also be seen in situations where exposure has changed, such as when tick control measures have stopped.
The fatality rate for untreated East Coast fever/ corridor disease can approach 100% in susceptible taurine, zebu or sanga cattle. In contrast, morbidity rates can be close to 100% in indigenous cattle but the mortality rate is usually low.
The mortality rate for tropical theileriosis is reported to be 40-90% in newly introduced cattle but only about 5% in some indigenous animals.
Prevention
Theileriosis is not transmitted by direct contact. Prevention measures include controlling movement of cattle, culling of infected animals, along and with measures to prevent the parasites’ establishment in ticks.
Tick control should be considered, but tick resistance to parasiticide products may be increasing.
There are various options for controlling ticks on domestic animals, including topical application of parasiticidal chemicals in dip baths or spray races (a structure the cattle walk through while being sprayed) or pour-on formulations, spraying parasiticides on walls of cattle pens, and smoothing the walls of cattle pens with mortar to stop ticks molting there.
Selection of certain types of attle for their ability to acquire immune resistance to ticks is another strategy.
In endemic areas, exposure can be reduced with acaricides (pesticides to kill ticks) and other methods of tick control such as rotational grazing. The transfer of blood between animals must be avoided.
Vaccines are available for East Coast fever and tropical theileriosis in some countries. Attenuated live vaccines are used for tropical theileriosis, but animals are vaccinated against East Coast fever by concurrently administering several strains of T. parva and an antibiotic (usually a long-acting tetracycline). This generally results in a mild infection followed by immunity, and the animal becomes a carrier.
The possibility of introducing live vaccine organisms into areas where these parasites are not endemic is a concern, however.
Selection of certain types of cattle that have more resistance to these parasites is another strategy. Endemic stability (in which animals are affected at a low levels or are not as susceptible to the disease) may be feasible in endemic areas.
Vaccination against tropical theileriosis can be used in cattle breeds that are susceptible to infection. Live attenuated vaccine are being used in many countries like India, Iran, Turkey etc.
These vaccines are a basically a white blood cell infected with Theileria annulata at the schizont stage and then passaged (moved through several new hosts or tissues) for attenuation (reduction of its ability to cause the actual disease).
Treatment
An efficient treatment with parvaquone, then buparvaquone became available in many countries from the mid-1990s. Buparvaquone, halofuginone and tetracycline and butalex and oxytetracycline have all shown to be effective in sick animals.
Combinations of drugs have been used in some outbreaks. Antiparasitic drugs are most effective in the early stages of theileriosis, and severely affected animals may die despite intensive care. The protozoan organisms can persist in the animals that recover. Drug-resistant organisms have also been reported.