Veterinary Services

An increase in the number of reports of hypomagnesaemia was noted this month with ten holdings affected. This compared with one the previous month and none in April 2010. While this will have been due in part to the good weather causing rapid growth of grass that was low in magnesium, cases also occurred in housed cattle. Perth diagnosed the condition in two suckler cows that died following a short period of tremors, recumbency and convulsions. Both cows were housed, bedded on straw and fed poor quality silage. Assay of serum taken antemortem from one of the cows found a magnesium value of 0.2 mmol/l (reference range 0.8 – 2.0 mmol/l) while assay of aqueous humour from the other cow estimated a magnesium value of 0.4 mmol/l (reference range >1.0 mmol/l). Hypomagnesaemia is also recognised in younger cattle and Ayr diagnosed hypomagnesaemia in a well grown four-month-old Limousin bull calf that died shortly after it was found twitching in lateral recumbency.  The calcium to magnesium ratio in bone was elevated at 143 (reference range 30 – 70).

Hypovitaminosis A was responsible for blindness in five Holstein bulls from a dairy farm in Midlothian. The animals, aged twelve to fourteen months, were from three separate groups with six to eight animals in each. The blindness became apparent when two of the bulls, which had been fattened for bull beef, were being loaded to go for slaughter. Examination found the affected animals’ pupils to be fixed and dilated but there were no other clinical signs. Blood samples identified low vitamin A values of 0.24 to 0.37 µmol/l (reference range 0.87 - 1.75 µmol/l). The batch of mineral mix that had been used on farm had just finished and so it was not possible to determine if this had been deficient in vitamin A.

Generalised and systemic conditions

Malignant catarrhal fever (MCF) was diagnosed on a dairy farm in Argyll where eight cows from a herd of 200 died over a two month period. All affected animals showed pyrexia, respiratory signs and one appeared blind and hypermetric. Postmortem examination of one case was carried out on farm and another cow was submitted to Ayr. Histopathological examination of tissues from both cases identified a perivascular mononuclear cell infiltrate, a finding considered strongly suggestive of MCF. In both cases ovine herpesvirus 2 (OvHV-2) was detected by PCR.  MCF is usually a sporadic disease in cattle but outbreaks have been reported (Otter. A et al, Vet. Rec. 151, 321-324). The cows were housed when this outbreak started and the only contact with sheep had been a period of twelve hours approximately five weeks previously. Further investigation is proceeding on this farm as approximately 30 other cows with similar clinical signs have appeared to recover. Blood samples from thirteen of these have tested negative for OvHV-2 on PCR.

Botulism was diagnosed by Dumfries in a group of 18-month-old dairy heifers that were bedded on poultry litter. Two deaths had occurred and a third heifer was recumbent. One carcase was submitted for postmortem examination and type D toxin was recovered from rumen and small intestinal contents.

Diagnoses of bovine neonatal pancytopenia (BNP) continued across Scotland this month. A definitive histological diagnosis can be made where the haematopoietic cell population is less than 25% of that expected. However it is now considered that there is a spectrum of disease where bone marrow pathology is abnormal but does not fit these defined criteria. One such case was a nine-day-old Limousin calf submitted to St. Boswells with gross evidence of bleeding. Histopathology of bone marrow identified erythroid hyperplasia with some myeloid activity, however there was a lack of megakaryocytes which would have accounted for the haemorrhage described. This calf was younger than those that typically die of BNP and SACCVS considered that it may have gone on to develop more typical bone marrow changes if it had lived for a few more days.

Typically most outbreaks of salmonellosis due to Salmonella Dublin occur in late autumn and early winter however nine outbreaks were diagnosed across Scotland this month, the highest number since August 2010 when 13 were diagnosed. Clinical signs associated with the outbreaks included abortions, high mortality in neonatal calves and diarrhoea in adult cattle. Aberdeen diagnosed an outbreak of salmonellosis due to the second most common serovar Salmonella Typhimurium. AHVLA was informed and the client was made aware of the zoonotic risks. The isolate was recovered from faecal samples from two six-year-old suckler cows with diarrhoea. It was also recovered from tissues from a three-day-old calf submitted from the same farm. On postmortem examination there was evidence of navel-ill, joint-ill, peritonitis, pleuritis and enteritis.

Alimentary tract disorders

Review of Bovine Coccidiosis Diagnoses

SAC C VS continued to see an increase in the number of diagnoses of bovine coccidiosis in 2010 when 209 outbreaks were diagnosed compared with 160 in 2009. An outbreak is defined as one or more diagnoses in a single herd within one month. There has now been an upward trend for several years with coccidiosis accounting for 2.6% of diagnosable submissions in 2006 compared with 4.7% in 2010.

For submissions that the ages of affected animals were recorded the numbers of diagnoses for each age group are shown in Figure 1 (see top right-hand side). On beef farms the peak of diagnoses occurs at one to two months of age. Spring-born suckler calves grazing contaminated pastures are likely to have driven the seasonal trend in 2010 evident in Figure 2 (see top right-hand side).  A rise in diagnoses can be seen from February (2.3%) up to a peak in June (9.5%).  Examination of the dairy calf data in Figure 1 reveals the peak of diagnoses occurring at age two to three months. This likely reflects the age at which small groups of dairy calves are mixed. 
 
Marked intestinal pathology was seen at necropsy of a six-day-old Aberdeen-Angus heifer calf submitted to Perth. This was the only animal to be affected and the calf had died after a short period of recumbency. There was a localised area of swelling and haemorrhage in the mid-jejunum causing a partial intestinal obstruction. The intestine was distended with fluid proximal to the obstruction and empty distal to it. Histological examination of sections of jejunum found changes consistent with severe necrosis of the small intestine affecting the entire thickness of the wall and extending to the serosa. SACCVS considered that the extensive damage would have severely compromised the intestinal barrier allowing release of bacteria and endotoxins into the circulation and abdominal cavity. The underlying cause was not apparent but possibilities included toxin production or ischaemic damage. 

Respiratory tract conditions

Edinburgh diagnosed bacterial pneumonia in a one-day-old Aberdeen-Angus calf. At necropsy approximately four litres of cloudy yellow fluid were found in the thoracic cavity and the right lung was covered by a layer of fibrin.  The right cardiac lung lobe was consolidated and the remainder of the lungs were only partially inflated.  Mannheimia haemolytica was isolated from lung and spleen tissues.  SAC C VS considered that the calf may have become infected transplacentally in utero.

Reproductive tract conditions

 
24 outbreaks of abortion due to Bacillus licheniformis were seen this month. This compares with just seven seen in April 2010. SACCVS suspects that this increase is due to the sudden onset of severe weather with heavy falls of snow last November and December that necessitated the early introduction of feeding and extended the winter feeding period on many farms. In one outbreak investigated by Edinburgh, the affected spring-calving suckler herd had only one live calf from its first six births.  Two foetal submissions were received and Bacillus licheniformis was isolated in pure growth from stomach contents of both.  Histopathology of placenta confirmed suppurative placentitis and there was no evidence of any other infectious causes of abortion.

Five foetuses were examined at Dumfries from a large suckler herd of 450 cows where there had been more than ten abortions. One of the foeti had a severe fibrinous peritonitis (Figure 3 - see top right-hand side) plus a few petechiae on the conjunctiva.  A pure growth of Mycoplasma bovis was isolated from all foetal tissues and stomach contents.  Histopathology was consistent with a severe bacterial process with systemic involvement.  Bloods were collected from cows that aborted and those that calved normally.  All proved seropositive for M. bovis indicating widespread exposure but providing no information on when this occurred.  There was no other evidence of M. bovis infection in the herd such as swollen joints, nasal discharges, mastitis, calf pneumonia or ear infections.