Prepared as part of the author's participation in the 2024 NSW Animal Health Frontline Field Epidemiology Training
Program
Summary
A property at Bugaldie in NSW experienced six late-term abortions in their heifers during the 2024 calving season
from mid-June to early August. Investigation of the sixth affected calf identified hydrancephaly and positive
seroconversion to Akabane virus leading to a diagnosis of clinical Akabane infection.
Akabane transmission in the Warrumbungle region had not been recorded in the period since 2002. It poses a risk of
calf production losses. The absence of an available vaccine in Australia complicates prevention efforts. The National
Arbovirus Monitoring Program (NAMP) in the Coonabarabran region serves as a valuable early warning system, enabling
landholders to monitor all classes of cattle for dystocia during Akabane virus outbreaks.
Situation, Spatial and Temporal Distribution
Late-term abortions were observed in a mob of 140 heifers by the producer during routine calving checks. The first
abortion was identified in the second week of June 2024, and by 5 August, six late-term abortions had occurred, midway
through calving. The producer phoned the district veterinarian to investigate when the sixth aborted calf was found
(Figure 2).
The affected farm, situated near Bugaldie in central western NSW, consisted of six adjoining properties and one
separate property located 7.8 km east of the main property (Figure 1). The enterprise managed a herd of 600
self-replacing Hereford beef cattle along with 2,500 Merino ewes. The only cattle introduced to the herd were stud
bulls, with numbers varying each year. Notably, no female cattle had been removed from the property, except for
agistment on a neighbouring property.
The heifers were grazing naturalised pasture and supplemented with straw hay, dried distiller's grain (DDG) pellets and
Darryll Lea lollies (seconds) at the time of investigation.
Quarterly survey bleeds conducted by the NAMP in the Coonabarabran region revealed 2/12 exposures to Akabane virus in
the sentinel herd during April 2024. The sentinel herd was located approximately 40 km east-southeast of the Bugaldie
property and these were the first detections of antibody to Akabane virus since the herd's establishment in 2015.
Figure 2. Timeline of abortion events
Descriptive epidemiology
Demographics
The case definition applied was: Any late-term abortion or stillbirth in cattle on the affected property at Bugaldie
during the 2024 calving season.
The producer identified six cases that fit the case definition within the herd of 600 (Table 1). The attack rate of
heifers calved at the time of investigation was 6/70 (8.6%) and case fatality rate was 100%.
No cases had been observed by the producer in the cows at the time of investigation.
Table 1. Demographic data
Mob
Number in mob
Number of abortions
Number calved at time of investigation
Heifers
140
6
70 approx.
Cows
460
0
200 approx.
Clinical manifestations
A post-mortem examination was performed on a calf that met the case definition and a blood sample was collected
from the mother.
The post-mortem examination finding was hydranencephaly (Image 1). The brain removed had minimal cortical tissue
but the cerebellum was still mostly intact (Image 2).
Image 1. Hydranencephaly in aborted calf. Longitudinal cut through head.
Image 2. Brain from aborted calf
Laboratory results
Pathology results confirmed Akabane virus infection through positive ELISA on the sera of the dam (HEIFER) and the
pericardial fluid of the aborted calf (CALF) (Figure 3).
Figure 3. Pathology Results from EMAI
EMAI Virology
Akabane Virus Antibody ELISA
Lab No.
Sample ID
Result
Lab No.
Sample ID
Result
0001
HEIFER
Positive
0010
CALF
Positive
Immunoglobulin G (Bovine) AGID Test
Analysis
Immunoglobulin G AGID
µg/ml
Lab No.
Sample ID
Sample Desc.
0010
CALF
Pericardial fluid
<121
Comment(s): Interpretation of IgG (Bovine) AGID results: <121 µg/ml indicates that an infectious
agent cannot be incriminated by the detection of specific antibody. However, it does not exclude the possibility of
incriminating an infectious agent by culture or by antigen detection.
VIROLOGY COMMENT: Very low levels of antibodies against Akabane virus were detected in the pericardial fluid
(#10). Presumably this level did not raise the total IgG level above the normal range.
The landholder reported one further affected calf, this time with arthrogryposis. It was stillborn on 13 August bringing the total affected calf count to seven. This calf was born to a cow and was the only case reported from the cow mob. The cows were
joined from October 2023 to January 2024. No further cases were reported after the seventh case.
Image 3. Arthrogryposis in stillborn calf from the cow mob
Discussion and recommendations
Akabane virus infection is caused by an arbovirus that can result in congenital abnormalities, late-term abortions
and stillbirths when naïve cattle are infected while pregnant. Outbreaks can occur in southern and western NSW when
weather conditions are conducive to the spread of mosquitoes from the Akabane endemic regions of eastern NSW. During
these periods the virus tends to spread west and south to non-endemic regions.
The type of congenital deformity depends on the stage of gestation when infected. Foetuses are generally unaffected
during the first month and the last two months of gestation. Many defects occur during infection at 3-6 months
gestation1. Hydranencephaly is the most prevalent malformation observed in infections up to day 105, while
arthrogryposis becomes the main deformity in cases occurring after day 105. Approximately 30-40% of infections lead to
foetal malformations1.
The textbook congenital malformation rate of 30-40% of infections is substantially higher than the observed 4.2%
defect rate in the heifers at the end of calving. However, the initial infection rate probably played a role in this
discrepancy. The Coonabarabran NAMP sentinel herd results, up to October 2024, reported a 16% Akabane seroconversion
rate, demonstrating that not all animals become infected during the transmission period. Factors such as mosquito
activity likely influenced this infection rate.
An infection rate of 16% in the heifer mob at Bugaldie in relation to the observed 4.2% congenital defect rate aligns
with the expected 40% malformation rate among infected individuals.
Arthrogryposis was not observed in the calves born to the heifer mob. Affected calves in this mob likely suffered
from hydranencephaly. This finding indicates that the period of infection for the heifer mob likely occurred between
mid-October 2023 and mid-February 2024, while they were on Property 1. In contrast, the single case of arthrogryposis
in the cow mob suggests that the infection period for the main property (Property 2) was later, between mid-February
and mid-May, possibly due to mosquito and virus activity moving from east to west (Figure 3).
Figure 3. Timeline of potential infection period
The reason for the lower attack rate in cows compared to heifers remains unclear, but there are several
possibilities. One explanation could be reduced mosquito activity on Property 2, resulting in fewer transmission
opportunities. Alternatively, the cows might have had some level of cross protection from Simbu virus antibodies.
Another possibility is that the virus reached Property 2 later in the season, by which time the cows were further
along in gestation and therefore the calves less susceptible to developing congenital defects. Prior exposure and
seroconversion to Akabane virus is unlikely, given the seronegative NAMP bleeds in the region for the past nine years.
Unfortunately, there is no treatment available for Akabane virus-affected calves. The landholder was advised to
closely monitor both the cow mob and heifers during calving, as arthrogryposis can lead to dystocia.
For live affected calves, humane euthanasia is recommended due to their poor prognosis. It was noted that for calves
with hydranencephaly, euthanasia by blunt force or a penetrating injury (such as via firearm or captive bolt) can be
challenging because of the absence of cerebral tissue. Targeting the brainstem is suggested as a more effective
approach, or alternatively, seeking veterinary assistance for intravenous euthanasia may offer a more practical and
humane solution in such cases.
Prevention of future outbreaks is not possible as there is no vaccine available in Australia. Vector control or
avoidance might be an option, but infection risk is not usually known until after exposure and seroconversion in
sporadic seasons. If vector activity increases in future due to changes in climate then hopefully either animals will
be immune due to previous exposure, or potentially naïve animals can be exposed prior to high-risk periods during
gestation.
Appendices
Case definition
Any late term abortions or stillbirths in cattle on the affected property at Bugaldie during the 2024 calving season.
Acknowledgements
Kendall Lewis - Biosecurity Officer Central West Local Land Services
