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Biliary fever in horses is quite similar to biliary in dogs, redwater in cattle and malaria in humans – with some very, very important differences, of course. A recent survey showed that the number of equine biliary cases treated, exceeded the number of infectious respiratory diseases and cases of African Horse Sickness combined. So equine biliary is probably more common than we originally thought!
Equine piroplasmosis, equine babesiosis and equine theileriosis are some of the different synonyms for equine biliary fever – but for the purposes of this article, I’ll refer to it as biliary. In South Africa, biliary in horses is caused by two organisms; Theileria equi (previously Babesia equi) and Babesia caballi.
These organisms are protozoa which are transmitted primarily by ticks. The protozoa are so tiny, that they live inside red blood cells (which in themselves are very small). Of the two organisms, Theileria equi is by far the most common cause of biliary in South Africa. It also causes a more severe form of biliary, and it has also displayed a certain amount of resistance to the drugs routinely used to treat equine biliary fever.
T. equi is transmitted primarily by the red-legged tick (Rhipicephalus evertsi evertsi). Under normal circumstances, 5-20 days after being bitten by an infected tick (not all red-legged ticks are necessarily infected with T. equi), the first signs are seen. These are: Fever (normally higher than 40OC); loss of or reduced appetite; higher than normal respiration (> 20-25 breaths per minute) and pulse rates (> 45-60 beats/minute); stool may also be smaller and harder than normal.
Probably the most important symptom, anaemia (low red blood cell number), can be masked by congested (reddish) mucous membranes – this is occasionally the response to the fever, especially in the acute cases. The “actual” mucous membrane colour may vary from pale-pink to pale-yellow – this is due to the parasite within the red blood cells causing its destruction.
Sometimes the urine may be a dark yellow to orange-brown, due to the broken down red blood cells that have been digested, processed and then excreted by the body. Colic and slightly swollen distal limbs are also symptoms that may occasionally occur. Fever and paleness are indeed the most important symptoms, but they aren’t the only symptoms. Therefore (here comes the stuck record...) please get your vet to confirm the diagnosis.
So why are outbreaks of equine biliary, unlike what we see in cattle and dogs, relatively uncommon? And in general, why are mortalities relatively low among horses suffering from biliary, when compared to the mortalities seen in dogs, cattle and humans (malaria)?
The reason is believed to be that horses often become carriers of biliary (especially Theileria equi). As a consequence, their immunity is stimulated on a regular basis, but during times of stress such as foaling, another disease and travelling, immunity is suppressed temporarily allowing the T. equi parasites to replicate (reproduce and increase in numbers) and then cause illness.
Unfortunately, foals may contract biliary fever while in the mare’s uterus, and will essentially be born ill, with biliary fever. Apart from the above circumstance, foals generally have a non-specific, and quite effective, immunity until they are about six months of age. The exact reason for this “non-specific” immunity is unknown. It is related to the immunity of the mare, but not dependent on it.
Treating an uncomplicated equine biliary case should have satisfactory results. However, the horse that has biliary that has progressed to jaundice, severe anaemia, very dark urine, poor response to routine or recognised treatment medicines, undoubtedly requires veterinary attention.
The recognised treatments for equine biliary fever are diminazene which is available at co-ops and farm shops, or imidocarb which is available from your vet. Always closely follow the directions of use as stated in the package insert. If there is any doubt whatsoever, do not hesitate to contact your vet.
(References available from the author)
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