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This article was published in 1992
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Ovine Nasal Zygomycosis - A Descriptive Epidemiological Study
George Perry, DV Walgett
A fatal fungal disease causing asymmetrical swelling of the nose of sheep and disseminated granulomatous lesions in tissues associated with the respiratory tract was diagnosed in merino sheep in north-western New South Wales and south-western Queensland. The causative agent was identified as Conidiobolus incongruus, a saprophytic fungus, which is suspected to cause an initial aerosol infection followed by a haematogenous spread to other organs. The main factor contributing to the occurrence of this disease is believed to be the unusually wet winter of 1990 followed by above average summer rainfall of early 1991 which resulted in periodic local flooding and dense native pasture growth with subsequent increased decomposing ground litter, suitable for fungal growth, on floodplains.
CASE DEFINITION
Ovine Nasal Zygomycosis (ONZ) in sheep, now commonly known as Big Nose, is a condition where affected animals develop nasal swellings which is often unilateral and causes distortion of facial features, sometimes fly-blown, but usually putrid smelling and occasionally discharging.
DIFFERENTIAL DIAGNOSIS
ONZ needs to be differentiated from the following: Bighead, Photosensitisation or Facial Eczema, Facial Infection/abscessation such as following dog attack to the face, Facial Flystrike, Allergic Reactions due to bee stings or snake bites, Nasal grass-seed infections.
CAUSATIVE ORGANISM
Histopathology of affected tissues preserved in buffered formalin revealed the presence of fungal hyphae and the fungus was cultured from fresh lesions when sent chilled and was identified as Conidiobolus incongruus. Literature concerning this Conidiobolus species revealed that:
a. this fungus is recoverable from samples of more thoroughly decomposed leaf mould in nature.
b. various Conidiobolus species are found to affect insects, inhabiting soil, decaying vegetation and straw (yet attempts to recover the fungus from decaying vegetation and flies have in this case failed).
c. this fungus produces colourless aerial hyphae with a colourless frosting of phototrophic conidiophores which are forcibly ejected, thus suggesting aerial infection.
d. this fungus causes deep fungal infections whereas other species of Conidiobolus have been known to cause only superficial fungal infections in animal and awn [sic]
e. the only other cases of infections by C. incongruus recorded have been in man although 4 ex 200 sheep on swampy country near Beaudesert, Queensland, were reported to have developed ONZ in 1987 and were found to be due to C. incongruus.
INCUBATION PERIOD
There appears to be a long subclinical incubation period of at least 5-6 weeks following inhalation of fungal conidiophores before any visible changes to the nasal tissues occur. This is followed by continued but a much more rapid swelling to the maxillary and oral tissues before death from starvation and dehydration occurs within the next two to three weeks.
DESCRIPTIVE EPIDEMIOLOGY
1. Time Factors
The first case reported was in late February 1991 with most cases reported from March to May 1991 although cases have continued to be reported voluntarily at very low levels (1-2 per month) since.
2. Place Factors
The focus of the problem is in the area bounded by Walgett-Mungindi-Thallon-Dirranbandi-Hebel and Brewarrina especially on the floodplains between the Barwon and Narran Rivers. Cases have occurred on at least 50 properties (17 in Qld and 34 in NSW) within the main problem area. There were also some cases reported in outlying areas round Coonamble, Moree and Warialda.
3. Animal Factors
Overall incidence rate for the period from February to May 1991 reported was 0.2%.
| NSW | QLD | TOTAL | |
|---|---|---|---|
| Affected properties | 34 | 17 | 51 |
| Affected cases (sheep) | 410 | 290 | 700 |
| Sheep at risk | 215,000 | 140,000 | 355,000 |
| % Incidence | 0.2% | 0.2% | 0.2% |
During the earlier stages of the investigations, most of the cases reported occurred in weaner sheep. It was felt that as weaners are generally more closely supervised, this resulted in higher ONZ observations for that group. However, following a survey on 13 affected properties in October 1991, there appeared to be no difference in occurrence of ONZ in the different ages and classes of sheep. None of the properties surveyed reported ONZ in every paddock.
| No. with ONZ | Sheep at risk | Incidence Rate* | |
|---|---|---|---|
| Rams | 2 | 630 | .317% |
| Adult Ewes | 73 | 30130 | .242% |
| Maiden Ewes | 57 | 11481 | .496% |
| Weaner sheep | 52 | 19703 | .264% |
| Adult wethers | 61 | 13900 | .439% |
| Unweaned sheep | 4 | 1500 | .267% |
| TOTAL | 249 | 74873 | .333% |
4. Environmental factors
(a) Rainfall
Most properties received very significant rain, leading to significant local flooding, in early winter 1990. The 13 properties surveyed received an average of 300mm for April 1990 compared to a 100 year average of 30mm with above average falls in May (65mm vs 40mm ave) and August (70mm vs 30mm). Furthermore, these properties received good summer falls totalling 240mm in January and February 1991 as against an average of 125mm. These summer rains resulted in repeated local flooding in the area.
However, some properties in outlying areas which have reported ONZ (1 case per 10,000 sheep) experienced a very dry summer before being declared drought affected in March 1991 although most of these properties have river frontage.
Effects of river or local flooding on 13 properties since April 1990 in paddocks where sheep were depastured since 1/1/91 on occurrence of ONZ.
| Where flooding occurred:- | ONZ +ve pdks | ONZ -ve pdks |
|---|---|---|
| In all the paddock | 1 | 2 |
| In some of the paddock | 12 | 8 |
| In none of the paddock | 0 | 3 |
Anecdotal reports have been received of this disease following the 1950's floods and 1974 floods.
(b) Soil
The majority of paddocks where ONZ occurred consist solely of black soil or heavy cracking grey/self-mulching clay soils while others had some of these soil types even if the paddock is predominantly of a different soil type. There have been no reports in hard red 'ridge' country where sheep have no access to the floodplains. Effects of soil pH and other characteristics were not examined.
Effects of range of Soil types on 13 properties found in paddocks on occurrence of ONZ.
| ONZ +ve pdks | ONZ -ve pdks | |
|---|---|---|
| Soft Red Soils/Sand Hills | 4 | 5 |
| Hard Red Country | 3 | 4 |
| Cracking Grey/self-mulching soil | 13 | 11 |
| Non-cracking Clay/Scalded country | 4 | 4 |
| Swamps | 8 | 7 |
(c) Vegetation
The effects of various tree species, grass/weed/herbages species, and type of vegetation cover on the occurrence of ONZ were examined and no differences could be detected.
CONCLUSION
More complete epidemiological studies to properly determine the factors which contribute to the occurrence of Ovine Nasal Zygomycosis could not be carried out because:
(a) The small number of properties that cooperated with the investigation and subsequent survey resulted in a sample size too small for meaningful analysis.
(b) Not all sheep could be constantly observed due to the extensive nature of husbandry practices, the large paddock sizes, and nature of the terrain.
(c) The very low incidence rate of the disease.
Despite these limitations, it appears that, apart from the presence of suitable fungal spores, the unusually wet conditions in areas susceptible to flooding resulting in significant amounts of decaying vegetation and in turn providing an environment suitable for fungal growth is the prime factor involved in the outbreak of ONZ.
Soil type is most likely not be a causative factor as it is most likely to be associated with the type of soil usually found on the floodplains rather than an association with the environment necessary for fungal growth as C. incongruus has been found in the more decaying leaf mould in Colorado (USA), an area more noted for mountains rather than floodplains. Colorado is also an area more noted for the cold winters than for the hot summers usually experienced in the affected area here, and hence the disease could not be assumed to occur only following summer rains rather than winter rains. The unusually high winter rainfall recorded probably contributed to the build-up of suitable decaying vegetation matter suitable for the fungal growth.
Although C. incongruus appears to be a difficult organism to recover from the environment and very limited references are available on the presence of this particular organism in nature, it is regarded in some quarters to be a common saprophytic fungi. I do not agree with this as the fungus has been readily cultured from animal tissues samples in the laboratory and will grow, albeit slowly at first, on a range of growth media on plates.
Any diseased animals and the fungus need to be handled with care as it zoonotic and it is certainly nothing to snort about.