Page 921 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

896 CHAPTER 5

VetBooks.ir achieve given the frequently non-specific nature of Aetiology/pathophysiology
Sporadic cases of bacterial infection of the bili-
clinical, clinicopathological and histopathological
findings. Importantly, in cases showing prolonged
from calcium bilirubinate and calcium phosphate)
persistence of serum markers of hepatic insult, it ary tract with, or without, cholelithiasis (formed
should be considered that continued exposure to a are occasionally seen as a cause of liver disease in
hepatotoxic agent may be more likely than treatment horses. It has been speculated that duodenal reflux
failure, and additional efforts to identify and remove via the bile duct leads to ascending biliary infec-
exposure to possible toxic agents are an important tion, although given such presumed aetiology it is
clinical response. unclear why pancreatitis is not reported in horses
via similar mechanisms. Nevertheless, frequent
Prognosis isolation of Enterobacteriacea from liver biopsy
Prognosis of cases of hepatotoxicity varies depend- specimens in cholangiohepatitis cases lends some
ing on the duration, extent and severity of toxic credence to this theory. Other causes of bacterial
damage. Irreversible changes including necro- hepatitis include rare cases of hepatic abscessa-
sis, megalocytosis and bridging fibrosis are the tion and also Tyzzer’s disease caused by Clostridium
most concerning prognostic findings, although a piliforme infection in foals.
biopsy score encompassing broader histopatho- The Flaviviridae family contains the genera pegi-
logical findings may enable a more accurate prog- viruses and hepaciviruses, which are both known to
nosis to be made. Delayed recognition of severe cause hepatitis in other species including humans.
hepatic pathology due to the generally subclinical Three Flaviridae have been investigated in horses
nature of early-stage hepatic insults is a common with liver disease including two pegiviruses known
reason for attributing a poor prognosis to cases of as equine pegivirus and Theiler’s disease-associated
liver disease at the time of initial diagnosis. In this virus, and the hepacivirus known as non-primate
respect, further investigation of horses with clini- hepacivirus. Additionally, equine parvovirus and
copathological evidence of liver disease should be kirkovirus have come under aetiological suspicion
encouraged regardless of the presence or absence in cases of equine liver disease. The clinical rel-
of associated clinical signs. Unfortunately, in cases evance of these viruses remains unclear and under
of PA toxicity, the disease is frequently not identi- investigation.
fied before the stage of irreversible hepatic insuf- Hepatic migration of larval stages of nematodes
ficiency, as earlier stages of the disease may remain such as Parascaris equorum, P. univalens, Strongylus equi-
subtle or subclinical. Other hepatotoxic insults may nus and S. edentatus may be associated with multifocal
be better associated with the potential for revers- liver injury, ultrasonographic changes and increased
ibility and hepatic regeneration, with examination biochemical markers of liver disease. Nematodes
of biopsy specimens having the best predictive rarely cause liver injury severe enough to result in
value for success. clinical signs although acute haemorrhagic necrosis is
reported.
INFECTIOUS HEPATITIS Fasciola hepatica (liver fluke) infection is an occa-
sional cause of hepatitis in horses, especially in some
Definition/overview geographic areas. Donkeys might be predisposed as
Infectious hepatitis is commonly recognised in other higher prevalences are often reported. Obstruction
species and, to a lesser extent is also known to affect of bile ducts and associated chronic inflammation
horses. Various infectious agents including bacteria, are important contributors to their clinical effects.
viruses and parasites may be implicated in causing Hepatic cystic echinococcosis (CE) refers to infec-
liver disease. As previously mentioned, liver disease tion of the liver with hydatid cysts, the intermediate
will frequently be seen as an outbreak in multiple stage of the tapeworm Echinococcus granulosus equinus.
horses on the same premises and therefore conta- Domesticated hunting dogs that are fed on raw horse
gious causes should be considered alongside dietary/ offal (e.g. fox hounds) are the main definitive host for
toxic factors. the parasite. Although the disease is international,