Research into parasitic diseases produces more questions than answers, finds Peter Green MRCVS
Liver fluke is a large parasite that lives in the liver of grazing animals. Animals catch the infection on damp or marshy pasture where the intermediate host, a small snail, thrives. Cattle, sheep, goats and deer can be severely affected, and farmers regularly have to dose their stock to treat liver fluke.
In one survey in Ireland, 9.5% of horses at an abattoir were infected with liver fluke; in Spain, 60% of horses were found to be positive on a blood test. We know how cattle and sheep get the infection: they ingest the early stage of the parasite as they graze. It seems obvious that horses are infected this way, too.
Vets in Dublin have recently tried to infect horses experimentally with liver fluke by feeding them the infectious stage of the parasite. None of the horses got liver fluke, either on blood tests or at subsequent post mortem. Using the same batch of infectious flukes, cattle and sheep succumbed to the disease, even at much lower doses.
So, how do horses get infected? Maybe they are only susceptible as foals, or perhaps infection takes years to develop? Maybe only a proportion are genetically susceptible? Further research is required.
The parasitic disease equine piroplasmosis (EP) is common globally. This tick-borne disease is caused by parasites that invade and destroy blood cells. Two species of the parasite, Babesia caballi and Theileria equi, cause similar signs: fever, anaemia, dehydration and lethargy.
The number of tick species capable of transmitting EP in the UK is increasing, due to changing climatic conditions, as the ever-present Ixodes ricinus is joined by other ticks that were formerly rare.
Severely infected horses may die. Even with treatment, there is strong evidence that infected horses become carriers for life and may show long-term effects such as poor performance.
We have been taught that chronic, low-level piroplasmosis in carrier horses will flare up into acute severe disease when horses are stressed, but vets in Israel have published work contradicting this.
They looked at more than 50 infected horses that underwent either major surgery or competed in an 80km endurance race. Both experiences are known to be stressful; horses carrying other infections, like Salmonella, often become sick with acute disease when stressed in this way.
The vets examined the horses before and afterwards, measuring levels of Theileria in the blood and looking for signs of EP flare-up. Nothing changed, signalling that other factors are involved.
Further reading: Liver fluke; Veterinary Parasitology 281, 109094, EP; Tick & Tick-borne Diseases 11, 101384
Ref Horse & Hound; 18 June 2020