Babesiosis

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Babesiosis

Also known as Texas fever, cattle fever, redwater fever, Spanish fever, splenic fever and cattle tick fever

Description

Babisiosis includes several diseases caused by certain types of protozoa that can affect cattle, sheep, pigs and horses. In cattle the disease in tropical and sub-tropical climates (including South and Central America, Africa and Australia) is caused by Babesia bigamina and B. bovis, while in temperate regions like northwestern Europe, Spain and the UK, B. divergens is the main protozoan involved.

Babesiosis is a hemolytic disease (meaning it destroys red blood cells) caused by the protozoan parasites transmitted through the saliva of infected ticks.

The protozoa that cause this disease arrived in North America in the 1600s, brought by cattle imported from Spanish colonies of the West Indies and Mexico. It spread rapidly throughout the southern United States because the ticks that are part of the life cycle of these parasites came along with the animals.

The disease spread north as infected herds were driven to cattle markets. In 1868 there was an outbreak among cattle in Illinois. Midwestern farmers soon realized this highly fatal disease was associated with longhorn cattle driven north by South Texas ranchers. The Texas cattle appeared healthy, but Midwestern cattle that were allowed to mix with them or use a pasture recently vacated by the longhorns, became ill and often died.

Farmers called the disease Texas fever or Texas cattle fever because of its connection with Texas cattle. To protect their own herds, the states along the cattle trails passed quarantine laws routing cattle away from settled areas or restricting the passage of herds to winter months, when there was less danger from Texas fever. In 1885 Kansas outlawed the driving of Texas cattle across its borders.

By 1877, northern ranchers were determined to stop southern herds from contaminating their own cattle.  Sometimes they sent armed parties to turn back herds coming north through Missouri, Kansas, Arkansas, and Indian Territory.

In 1893 researchers in the federal Bureau of Animal Industry discovered the pathogen and found that the disease is caused by a microscopic protozoan that inhabits and destroys red blood cells. They also discovered that the protozoa were spread by cattle ticks. After sucking blood from an infected animal, the tick drops off to lay eggs. Weeks after the original tick dropped from its host, its progeny were infecting other cattle. Several different species of tick are now known to spread babesiosis.

Identification of the pathogen (the protozoan) and vector (the tick) still did not explain the apparent good health of Texas cattle that carried the disease. Newer research found that calves in a tick-infested area are born with partial resistance to infection, due to antibodies in colostrum from their dams. This resistance lasts a month or more.

In regions where the disease is widespread, calves suffers mild disease at an early age after their immunity from colostrum wanes, then develop enough immunity to keep from being overwhelmed but usually not enough to rid themselves of the pathogen. By the time these animals reaches adulthood, they keep a sort of balance with the protozoan parasites and are able to live in reasonably good health while remaining a carrier.

Strong immunity occurs after natural infection, and if the infection recurs repeatedly, the immunity may be permanent. If the infection is not repeated, immunity wanes after about 6 months. Different types of cattle are more susceptible than others. Zebu and Afrikander cattle have more resistance to ticks and to their protozoan parasites than the British and European breeds.

The disease may have seasonal variations if the tick population varies with the climate. Tick activity is highest in warm weather. Mortality in cattle without prior exposure to the disease ranges from 70 to 90%.

Babesiosis is still a serious threat to livestock in many parts of the world. In the U.S. it has been eliminated by a vigorous program of cattle dipping, which eradicated the tick vector. Before the disease was eradicated, nonimmune cattle were protected by elaborate federal quarantine laws separating southern cattle from other cattle in railway cars and stockyards. Northern cattle taken south for breeding purposes could be immunized by receiving injections of small amounts of blood from infected animals.

In the southern U.S, eradication efforts directed against Boophilus ticks began in the early 1900s. A quarantine zone in south Texas along the US/Mexico border was established following eradication of the disease in the U.S.

The permanent quarantine zone, spanning approximately 85% of the total shared border between Texas and Mexico, helped limit reintroduction of the vector and disease, and reduced the incidence of babesiosis to a few sporadic outbreaks. A strict regimen of monitoring and surveillance coupled with rapid response and systematic application of containment procedures (under joint jurisdiction of the Texas Department of Agriculture and USDA) has been effective in enforcing the permanent quarantine zone, preventing the spread of babesiosis into the United States until recently.

Ticks that parasitize only one host are easier to eradicate than ticks that parasitize several host species. The presence and increase of exotic game animals in the Southwest helped facilitate spread of the tick vector. When coupled with expanding populations of native deer that can host the tick vector, the prevalence of Boophilus ticks presents an increased risk of the return of babesiosis to the U.S.

Signs

    Sudden onset of high fever, depression, loss of appetite, cessation of rumination (no longer chewing the cud), weakness, drop in milk yield, brick-red mucous membranes soon becoming pale due to anemia, pipe-stem diarrhea that later changes to constipation, dark or bloody urine, abortion in pregnant cows.

Causes

The parasitic protozoa destroy red blood cells—resulting in anemia, fever and death.

Ticks are the natural vector; the protozoa persist in the ticks and pass part of their life cycle through the ticks. Boophilus ticks are the primary vector in the U.S.

Two closely related species are capable of transmitting these pathogens--Rhipicephalus (Boophilus) annulatus (often called the cattle tick), and R. microplus (often called the southern cattle tick). Both of these tick species and their associated pathogens were introduced to the Western Hemisphere on livestock brought by early explorers and settlers from different parts of the world. No other tick species in the U.S. are capable of transmitting the protozoa that cause Texas cattle fever.

The cattle tick originated in the Mediterranean region where climates are relatively temperate, while the southern cattle tick is from the tropics of the Indian sub-continent. Both ticks were both successful in adapting to the southern climates of the U.S., as well as similar climates in Mexico, Central and South America.

The ticks remain on the same animal through the larval, nymph and adult stages until the blood-engorged females drop off the host animal. These females lay 2,500-4,000 eggs (which contain the protozoa), and then die. The males remain on the animal to mate with more females. It takes 20 days from the time the larvae arrive on the host animal until the first females start dropping off, with most of them off the host by day 25. Animal movement during this period allows ticks to be dropped into new locations.

A successful hatch of young ticks depends on moderate temperatures and high relative humidity--more common to tree and brush-covered areas than open meadows.

Cattle are the preferred host, and in the early 1900’s when cattle were basically the only host in the southwestern U.S. these ticks were much easier to control. Today the high population of white-tailed deer and several exotic ungulates--including nilgai antelope, which is the largest Asian antelope—also serve as hosts. The nilgai is native to India (and a host animal for the southern cattle tick in India), but this imported exotic species has gone wild in parts of Texas (approximately 15,000 of them now live in South Texas).  Thus the ticks and the nilgai are thriving together.

Prevention

Prevention depends upon control and elimination of the ticks that transmit the disease. There are no protective vaccines in the U.S. although vaccines have been developed in South America and Australia.

Treatment

Mild cases may recover without treatment.

Sick animals can be treated with an antiparasitic drug. Treatment is most likely to be successful if the disease is diagnosed early; treatment may fail if the animal has been weakened by anemia.

Treatment in the UK involves administration of imidocarb dipropionate, along with supportive fluid therapy or blood transfusions if the disease is severe. The only licensed treatment for babesiosis is Imidocarb. Consult your veterinarian for use of this drug in the U.S.

About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.

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