Bovine Ephemeral Fever (BEF), also known as "three-day sickness" is an arthropod-borne rhabdovirus1 that occurs sporadically in the North West of NSW. It is generally only seen when seasonal conditions are conducive to the spread of mosquitos and other biting insects.
In late summer and early spring of 2020, multiple cases were seen in the Narrabri and Moree districts. This case report describes two of the cases that were diagnosed early in the outbreak and includes videos of two affected cattle, illustrating the potential differences in clinical presentations.
CASE REPORT / SERIES
On the same day in late February 2020, District Veterinarians were called out to two properties north-west of Narrabri, approximately 50km apart, to investigate reports of recumbent cattle.
Two separate mobs of cattle on this property, housed in paddocks approximately 2km apart, were found to have lame and recumbent cattle.
The heifer mob included 41 heifers, grazing moderate green pick that had grown since recent substantial rainfall two weeks earlier. The green pick was predominantly Pig weed (Portulaca oleracea) and Caltrop or yellow vine (Tribulus terrestris). They also had barley and wheaten hay available that was fed out every second day. This routine had not changed in recent weeks, but the cattle were grazing more green feed and less hay as the pasture became better established. The mob had been yarded one week earlier and vaccinated with 7-in-1 (brand unknown, but covering leptospirosis and major clostridial diseases), drenched with Avomec pour-on (Abamectin 5mg/ml and Triclabendazole 300mg/ml), and given a Vitamin AD&E injection.
On Saturday (day 1), one heifer was noted to be lame on one forelimb. On Sunday (day 2), a second heifer was lame. On Monday (day 3), two heifers were recumbent and unable to stand, and a further four were lame, slow to move, and not coming in to feed at the hay with the rest of the mob. It is not known whether these were the same animals noted as lame on previous days. The remainder of the mob were moved into a different paddock, and the affected heifers were provided with water in buckets.
The cow mob of 48 cows with 2-3-month-old calves at foot was grazing similar pasture and was fed the same hay as the heifer mob. On Sunday (day 2) one cow was noticed standing in the dam (Figure 1). On Monday (day 3), three cows were noted to be lame, slow to move, and drooling.
One further cow, a pet, kept close to the house, was noticed to be lame and slow to move on Saturday (day 1). She was treated with an injection of Tolfedine (dose unknown) and given a bag of 4-in-1 mineral injection containing calcium, magnesium, phosphorous and glucose. By Tuesday (day 4) she was reportedly markedly improved.
Both mobs of cattle were inspected by the District Veterinarian on Tuesday (day 4).
In the heifer mob, there were two recumbent heifers. Both were bright and alert and attempting to stand. Both could get their hind legs under them, and partially rise, but neither could get their forelimbs to bear weight. Both were eating and drinking when provided feed and water. No visible external abnormalities were noted. Both had body temperatures of 37.9 degrees Celsius. Neither had any abnormal heart or lung sounds. Blood was collected from both heifers, and urine was collected from one. An in-field urinalysis stick showed negative results for leucocytes, ketones and nitrates, low positive for erythrocytes, and a urine pH of 8.
Four other heifers were observed in the paddock. They were ambulatory, and grazing, though not enthusiastically. They were all slow to move and lame on their forelimbs.
One recumbent cow was examined in the cow mob. She rose when approached, and moved away, but was very slow and markedly lame in the right forelimb (see Video 1).
The differential diagnoses for this case included Bovine Ephemeral Fever, plant toxicities (including oxalates, nitrates, cyanide) and trauma with or without concurrent hoof pathology.
A mob of 65 heifers was grazing green pick after substantial rainfall over the previous two weeks at a property approximately 50km north of Case 1, west of Moree. The heifers were home bred, and had recently completed their first joining, with the bulls removed six weeks earlier. For the last ten days they had been grazing entirely green pick in the paddocks, predominantly Pig weed (Portulaca oleracea) and Caltrop or yellow vine (Tribulus terrestris), along with other green forbs. Prior to the rainfall they had been on a ration of hay, cottonseed and cotton trash. They had been drenched the week prior with Avomec pour-on (Abamectin 5mg/ml and Triclabendazole 300mg/ml) and had been vaccinated two months previous with 7-in-1 (brand unknown).
On Saturday (day 1) the mob was moved into a new paddock, with similar feed to the old paddock. On Sunday (day 2), several heifers broke into the house yard and consumed garden plants. On Monday (day 3), one heifer was noted to be reluctant to move. She was trucked to the cattle yards where there is a small pen adjoining the yards. In this pen, she became stuck in mud and was unable to rise. When helped out of the mud on Tuesday (day 4), she was noticed to be staggering, and swaying in the back legs, and fell over several times (video 2).
The heifer was brought into the crush for examination and went down in the crush. Her body temperature was 39.6 degrees Celsius. She was bright and alert and attempting to stand. Owing to a history of wooden tongue on the property, her mouth was examined, but no abnormalities were found. Blood was collected.
Differential diagnoses for this case included trauma, unknown plant toxicity, spinal abscess and Bovine Ephemeral Fever.
Both cases returned positive Polymerase Chain Reactions (PCR) for Bovine Ephemeral Fever. No further laboratory tests were completed.
In the north-west of NSW, Bovine Ephemeral Fever outbreaks have been sporadic, with several years between definitive cases. Historically, cases have been seen after periods of substantial rainfall and when insect numbers are high. This observation is consistent with epidemiological studies, which indicate that disease outbreaks often occur in summer, are clustered and short-lived, with the spread depending on the insect vector population1. In February 2020, substantial rainfall was experienced across the north-west for a period of several weeks, leading to a lot of water lying on the ground, and a subsequent rise in mosquito (Culex spp.) and midge (Culicoides spp.) numbers. In Australia, Culex annulirostris has been identified as a biological vector for BEF, while biting midges are also considered a possible vector1.
Spread of the virus by wind-borne carriage of insect vectors is likely, with the spread of insects on the low-level jet stream also considered likely in some outbreaks2. Interestingly, at the same time as multiple cases were being seen across the north-west, from Warialda to Walgett, and south as far as Gunnedah, no cases had been diagnosed in the Northern Tablelands or North Coast regions (pers com. A.Biddle, L.Martin, L.Bolin). Cases had, however, been reported further to the south-west in the Coonamble region in the same week as the two cases discussed here (pers com. J.Kelly).
Additionally, regular bleeding of National Arbovirus Monitoring Program (NAMP) herds in the north-west, conducted in January and early February, returned no positive results for BEF in the sentinel herds, indicating that the disease was not present in the district only weeks earlier. It is possible that the insect vectors were blown down from QLD, where the disease is endemic, rather than coming from the east coast of NSW.
In the weeks following these initial cases, dozens of additional suspected cases were reported by landholders across the north-west. Several deaths, including bulls and calves were reported by landholders, but none investigated to determine a definitive cause of death. The number of cases had seemed to ease by the time of writing in late March 2020.
The lameness experienced by cattle with BEF is described as a shifting lameness3 affecting one or more legs at a time. This symptom is clearly seen in video 1. It is also reported that some animals become more excitable than usual, suggesting central nervous system involvement, which is evident in video 2, where the movement would be better described as ataxic than stiff or stilted. This animal had also been recumbent and so her gait may have been a result of compartment syndrome or mild nerve damage related to recumbency. Hypocalcaemia is also commonly seen with BEF1, so the gait of the animal in video 2 may also have been a result of this imbalance.
These cases serve as a reminder to veterinarians to consider BEF as a cause of lameness and recumbency even with atypical presentations.