Calf watch: Brucella abortus excluded in peri-natal death
A current investigation into causes of calf loss in extensively managed north Australian beef herds aims to establish causes of death in the immediate pre- and post-birth periods. It has been difficult to establish causes of calf death using traditional methods of observation, because calving cows are hard to find in large paddocks. Calf carcasses are similarly difficult to locate, owing to rapid decomposition and predation. Calf Watch is a current DPIR Livestock Industry Development project, which uses birthing sensors inserted into the vagina of pregnant cows; an electronic calving alert allows researchers to locate calving sites when the sensors are expelled, collect dead calves for autopsy and tag live calves for further tracking.
In September 2019, researchers detected a calving cow, shortly after receiving the sensor alert. The foetus was only partly expelled through the pelvis, and staff manually delivered the dead calf. The carcass was submitted to the regional veterinary officer for autopsy, where the following findings were noted:
- evidence of some foetal distress, bright red gums and a very large, swollen tongue
- the calf was normally developed and at term
- unclotted blood was easily collected via direct puncture into the heart
- lungs were solid and did not float in water, indicating that the calf had not taken a breath
- kidneys and liver were moderately decomposed
- the heart, lungs, spleen, gastrointestinal tract and brain were significantly less decomposed.
Laboratory findings showed evidence of amniotic fluid inhalation (foetal or calf-bed fluids), indicating distress of the calf while it was still in the uterus. This finding is suggestive of dystocia (meaning the calf was stuck, and unable to be delivered without assistance) as the cause of death during the birthing process. However, the lung also showed evidence of inflammation, and possible involvement of bacteria. Therefore, culture of the lung, and exotic disease testing was requested at the Australian Animal Health Laboratory (AAHL) for Brucella abortus infection. B.abortus is a bacteria that may cause pneumonia in a calf foetus, and because it is not known in Australia, is a potential cause of infectious emergency animal disease.
Testing was also undertaken to rule-out viral causes of abortion. The calf blood was tested for antibodies to Bovine Viral Diarrhoea Virus (also known as Pestivirus) and several arboviruses (insect-borne infections) as shown in Table 1:
Histology (examination of tissue cells) ruled out infections that commonly cause problems in NT beef cattle and calves, such as Leptospirosis, vibriosis, Tritrichomonas fetus, Listeriosis, Yersinia pseudotuberculosis and Neospora caninum infections. The bacterial culture grew small amounts of two bacteria, including Mannheimia haemolytica, which is a cause of “Shipping Fever” pneumonia in adult cattle. Given the large amount of fluid that the calf had inhaled, it is likely that the bacterial growth represents a secondary infection of the lungs after death, and not a cause for the neonatal death. The referral testing at AAHL on the lung tissue was negative for B.abortus. This was established using a molecular diagnostic testing method with high reliability.
Investigation into the cause of death in this calf established that this was a simple case of dystocia, where the position of the calf in the uterus made it impossible for a normal, unassisted vaginal delivery to occur. Unless calving cows are intensively monitored, there will always be a proportion of calf deaths in the uterus. This occurs if the calf runs out of oxygen during a prolonged labour and delivery, because of disruption to the maternal oxygen supply through the umbilical cord. Eventually, the unborn calf needs to attempt to take a breath; if the head is still inside the uterus, foetal fluids will be inhaled and the calf will die shortly afterwards. However, despite the benign findings, this investigation is an important and useful example of passive surveillance, where DPIR researchers were able to rule-out an important exotic animal disease, as well as adding to the knowledge base around causes of neonatal calf death in the NT.
Last updated: 12 December 2019