Avian botulism has been reported in many areas worldwide, particularly in North America, where at times it has resulted in die-offs of hundreds of thousands of waterbirds. By contrast, there are few reported cases in wild waterbird communities in South Africa. Mass die-offs of waterbirds in the Wilderness Lakes, South Africa, were first observed in January 2015, raising concerns about the effect of disease on the status of waterbird communities in this Ramsar site. Surveys of bird carcasses were undertaken between 2015 and 2017. An estimated 1115 individuals from 23 species were affected, with sick individuals displaying symptoms of avian type C, C and D or E botulism. The highest mortality was amongst red-knobbed coot (
The long-term effect of repeated outbreaks of avian botulism on the abundance of susceptible waterbird species in the Wilderness Lakes is of concern. The previous regular high abundance of duck species, particularly yellow-billed duck and Cape shoveler, was the initial reason for the Wilderness Lakes being declared a Ramsar site. The effect of ongoing high disease-related mortalities may, in part, prevent these wetlands from continuing to regularly support globally significant populations of some waterbird species. The containment of the disease must be attempted by the regular collection and removal of dead birds and fish during the outbreak periods of November–June, and carcasses should be disposed of off-site. Further testing of affected individuals should be undertaken and the monitoring of environmental variables and affected individuals continued to improve the understanding of the drivers and progression of the disease.
Avian botulism is a non-infectious bird disease resulting from the ingestion of toxins produced by the gram-positive bacterium
Avian botulism has been reported from several territories worldwide (Rocke
In this study, we document the repeated outbreak of disease affecting waterbirds and fish in the Wilderness Lakes, and discuss its relevance to the conservation of these aquatic ecosystems.
The Wilderness Lakes System (33°59′ to 34°02′ S and 22°35′ to 22°46′ E) is located on the southern coastline of South Africa, and includes the Touw Estuary and three interconnected estuarine lakes (Rondevlei, Langvlei and Eilandvlei) (
Map of study area showing relative position of estuarine lakes within the Wilderness Lakes Complex.
Rainfall occurs year-round, with a mean of between 900 mm/year and 1000 mm/year in the upper catchments (Adamson
Submerged aquatic plants are widespread and abundant in the shallower (< 3 m) littoral areas of the lakes. Dominant taxa are fennel-leaved pondweed (
The Wilderness Lakes System is located in the Garden Route National Park. The lakes and their interleading channels were designated as a wetland of international importance, or Ramsar site, in 1991 (Ramsar Convention Bureau
Surveys of the abundance of all waterbirds were undertaken biannually during summer (January–February) and winter (July–August) from 2005 to 2017. Surveys were undertaken from a boat over two days by four observers using binoculars. The boat route used enabled the observation of all open water areas and exposed sandbanks and mudflats.
The first bird carcasses were observed and reported by members of the public in January 2015 in front of a bird hide on the northern shoreline of Rondevlei. Shortly thereafter, during regular boat-based surveys on the lakes by South African National Parks (SANParks) personnel to assess aspects of water quality and waterbird abundance, the scale and extent of the outbreak became more apparent, with the observation of multiple bird and fish carcasses on Eilandvlei, Langvlei and Rondevlei.
To monitor the carcass occurrence, regular boat-based surveys were undertaken of all the waterbodies between January 2015 and May 2017, following routes that enabled observation of all the open water areas. Twenty surveys were conducted on Eilandvlei over a period of 256 days, 37 on Langvlei over 462 days and 56 on Rondevlei over 528 days. These equate to surveys being conducted, on average, once every 13, 12 and 9 days respectively, although, during the height of the outbreak, surveys were conducted at shorter intervals. Intermittent surveys were undertaken on the Touw Estuary and adjacent Swartvlei system. All observed and accessible bird and fish carcasses were collected and, where possible, identified. Live birds (approximately 30) with paralysis and those that were unable to walk or fly were captured and identified. Those in an advanced state of paralysis and in the process of drowning when captured were euthanised. All collected dead and sick birds, and all dead fish, were removed from the lakes. All carcasses not used for laboratory analyses were disposed of by burying at a site removed from the lakes in an attempt to try and contain the disease.
State veterinarians were consulted to provide guidance on the probable cause of the bird deaths. Three dead and one paralysed yellow-billed duck (
Four frozen whole carcasses of affected and euthanased waterbirds comprising two Cape shoveler (
Tissue samples of major organs and muscles of three disease-affected and euthanased waterbirds, namely, one each of yellow-billed duck, great crested grebe (
Water samples were collected in polypropylene bottles from nine localities (2 × Rondevlei, 2 × Rondevlei-Langvlei channel, 1 × Langvlei, 1 × Langvlei-Eilandvlei channel, 2 × Eilandvlei, 1 × Serpentine channel) on 20 February 2015, during the height of the first disease outbreak. Sample preparations and species identifications were undertaken by the Stellenbosch Office of the Council for Scientific and Industrial Research (CSIR). All water column algae were identified using a compound microscope (Carl Zeis, Germany) at 1250 × magnification and keys of Truter (
Water level data for the lakes were extracted from the unpublished databases of the South African Department of Water and Sanitation. Temperature (°C), salinity, dissolved oxygen (mg/l) and pH of lake waters were measured quarterly by SANParks personnel at 30 cm depth at five localities in each lake using YSI model 30 S-C-T, model 550A O2 and model 60 pH meters.
The standing biomass of submerged plants was determined during May and June from 1992 to 2017. In Rondevlei and Langvlei, assessments between 2000 and 2004 were undertaken biennially. Four littoral transects extending from the inner edge of the emergent macrophyte zone to the 2 m depth contour were randomly positioned around each lake each year. The above-ground portions of aquatic plants were collected at five 0.0625 m2 sample points along each transect with a submerged macrophyte sampler (Howard-Williams & Longman
This article followed all ethical standards for research without direct contact with human or animal subjects.
A total of 1115 dead or disease-affected waterbirds were recorded on Rondevlei, Langvlei and Eilandvlei between January 2015 and May 2017. This constituted 20% of the number of individuals that, on average, occur on these waterbodies. Twenty-three species from 13 families were affected (
Population statistics and number of mortalities of waterbird species affected by botulism.
Family | Species | Common name | Waterbird communities (2005–2014) |
Mortalities (2015–2017) |
|||
---|---|---|---|---|---|---|---|
Freq. | Avg. | % | No. | % | |||
Dendrocygnidae | Fulvous duck | 5 | < 1 | < 1 | 1 | < 1 | |
White-backed duck | 100 | 74 | 1 | 6 | 1 | ||
Anatidae | Cape shoveler | 100 | 261 | 4 | 204 | 18 | |
Yellow-billed duck | 100 | 544 | 9 | 100 | 9 | ||
Red-billed teal | 90 | 56 | 1 | 5 | < 1 | ||
Maccoa duck | 85 | 19 | < 1 | 4 | < 1 | ||
Southern pochard | 100 | 126 | 2 | 4 | < 1 | ||
Egyptian goose | 100 | 153 | 3 | 24 | 2 | ||
Cerylidae | Pied kingfisher | 95 | 3 | < 1 | 1 | < 1 | |
Rallidae | Red-knobbed coot | 100 | 3597 | 60 | 670 | 60 | |
Charadriidae | Blacksmith lapwing | 100 | 26 | < 1 | 1 | < 1 | |
Laridae | Grey-headed gull | 45 | 3 | < 1 | 1 | < 1 | |
Accipitridae | African fish-eagle | 100 | 6 | < 1 | 1 | < 1 | |
Recurvirostridae | Pied avocet | 25 | 3 | < 1 | 1 | < 1 | |
Black-winged stilt | 95 | 33 | 1 | 4 | < 1 | ||
Podicipedidae | Little grebe | 100 | 269 | 5 | 7 | 1 | |
Great crested grebe | 100 | 209 | 4 | 11 | 1 | ||
Phalacrocoracidae | White-breasted cormorant | 100 | 59 | 1 | 8 | 1 | |
Ardeidae | Squacco heron | 10 | < 1 | < 1 | 2 | < 1 | |
Little egret | 95 | 31 | 1 | 2 | < 1 | ||
Phoenicopteridae | Greater flamingo | 15 | 6 | < 1 | 12 | 1 | |
Threskiornithidae | African spoonbill | 90 | 21 | < 1 | 4 | < 1 | |
Glossy ibis | 95 | 14 | <1 | 1 | <1 | ||
Not identified | - | - | - | - | 41 | 4 | |
Note: For waterbird communities surveyed biannually in Rondevlei, Langvlei and Eilandvlei over 10 years (2005–2014) prior to the initial outbreak of botulism. Species names follow Hockey et al. (
Freq. indicates the percentage of times a species was recorded in all surveys; Avg. is the average abundance of species across all 20 surveys; and % is the percentage representation of a species in the community based on average abundance across all 20 surveys. For statistics of waterbird mortalities recorded in the 3-year study period (2015–2017) in Rondevlei, Langvlei and Eilandvlei, No. indicates the number of sick and dead individuals of each waterbird species located in botulism-affected waterbodies; and % indicates percentage representation of a species in the total number of affected individuals recorded in all affected waterbodies.
Freq., frequent; Avg., average.
There is a strong similarity for most species between the percentage of individuals affected per species and the percentage representation in the waterbird community (
The highest number of waterbird deaths over the 3-year study period occurred on Rondevlei (570), followed by Langvlei (473) and Eilandvlei (72) (
The number of disease-affected waterbirds located in Rondevlei, Langvlei and Eilandvlei in the three different botulism outbreak periods encompassing January–June 2015, November 2015 – August 2016 and January–May 2017.
Family | Species | Common name | Rondevlei |
Langvlei |
Eilandvlei |
|||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
2015 | 2016 | 2017 | Total | 2015 | 2016 | 2017 | Total | 2015 | 2016 | 2017 | Total | |||
Dendrocygnidae | Fulvous duck | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | |
White-backed duck | 1 | 0 | 0 | 1 | 5 | 0 | 0 | 5 | 0 | 0 | 0 | 0 | ||
Anatidae | Cape shoveler | 8 | 46 | 0 | 54 | 69 | 64 | 10 | 143 | 7 | 0 | 0 | 7 | |
Yellow-billed duck | 28 | 9 | 0 | 37 | 51 | 1 | 0 | 52 | 11 | 0 | 0 | 11 | ||
Red-billed teal | 1 | 2 | 0 | 3 | 2 | 0 | 0 | 2 | 0 | 0 | 0 | 0 | ||
Maccoa duck | 0 | 2 | 0 | 2 | 2 | 0 | 0 | 2 | 0 | 0 | 0 | 0 | ||
Southern pochard | 1 | 3 | 0 | 4 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | ||
Egyptian goose | 1 | 3 | 0 | 4 | 15 | 3 | 1 | 19 | 1 | 0 | 0 | 1 | ||
Cerylidae | Pied kingfisher | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | |
Rallidae | Red-knobbed coot | 44 | 385 | 0 | 429 | 149 | 37 | 10 | 196 | 39 | 6 | 0 | 45 | |
Charadriidae | Blacksmith lapwing | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | |
Laridae | Grey-headed gull | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | |
Accipitridae | African fish-eagle | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
Recurvirostridae | Pied avocet | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
Black-winged stilt | 0 | 2 | 0 | 2 | 2 | 0 | 0 | 2 | 0 | 0 | 0 | 0 | ||
Podicipedidae | Little grebe | 0 | 4 | 0 | 4 | 1 | 0 | 1 | 2 | 1 | 0 | 0 | 1 | |
Great crested grebe | 3 | 0 | 0 | 3 | 4 | 3 | 0 | 7 | 1 | 0 | 0 | 1 | ||
Phalacrocoracidae | White-breasted cormorant | 2 | 0 | 0 | 2 | 2 | 1 | 0 | 3 | 3 | 0 | 0 | 3 | |
Ardeidae | Squacco heron | 0 | 0 | 0 | 0 | 2 | 0 | 0 | 2 | 0 | 0 | 0 | 0 | |
Little egret | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | ||
Phoenicopteridae | Greater flamingo | 0 | 10 | 0 | 10 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1 | |
Threskiornithidae | African spoonbill | 0 | 2 | 0 | 2 | 2 | 0 | 0 | 2 | 0 | 0 | 0 | 0 | |
Glossy ibis | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | ||
Not identified | - | 9 | 0 | 0 | 9 | 30 | 0 | 0 | 30 | 2 | 0 | 0 | 2 | |
Note: Species names follow Hockey et al. (
Survey dates for the three locations were as follows: Rondevlei 2015: January–June 2015, Rondevlei 2016: November 2015 – August 2016 and Rondevlei 2017: January–May 2017; Langvlei 2015: February–June 2015, Langvlei 2016: November 2015 – June 2016 and Langvlei 2017: January–May 2017; and Eilandvlei 2015: February–May 2015, Eilandvlei 2016: January–June 2016 and Eilandvlei 2017: January–May 2017.
There was temporal variability in the disease outbreak, with the majority (95%) of waterbird deaths occurring in the summer and autumn months of November–April, with no deaths recorded in the late winter months of September and October (
Number of (a) waterbird and (b) fish mortalities recorded in the Wilderness Lakes per month between January 2015 and July 2017.
No mortalities were recorded amongst 44 additional waterbird species (
Population statistics of waterbird species not affected by botulism. For waterbird communities surveyed biannually in Rondevlei, Langvlei and Eilandvlei over 10 years (2005–2014) prior to the initial outbreak of botulism.
Family | Species | Common name | Waterbird communities (2005–2014) |
||
---|---|---|---|---|---|
Freq. | Avg. | % | |||
Anatidae | African shell-duck | 5 | < 1 | < 1 | |
Cape teal | 65 | 4 | < 1 | ||
Hottentot teal | 70 | 6 | < 1 | ||
Spur-winged goose | 70 | 7 | < 1 | ||
Alcedinidae | Half-collared kingfisher | 10 | < 1 | < 1 | |
Malachite kingfisher | 100 | 8 | < 1 | ||
Cerylidae | Giant kingfisher | 10 | < 1 | < 1 | |
Rallidae | African rail | 35 | < 1 | < 1 | |
Black crake | 90 | 3 | < 1 | ||
African purple swamphen | 100 | 13 | < 1 | ||
Common moorhen | 100 | 52 | 1 | ||
Scolopacidae | Wood sandpiper | 40 | 1 | < 1 | |
Marsh sandpiper | 30 | 2 | < 1 | ||
Common greenshank | 40 | 3 | < 1 | ||
Curlew sandpiper | 10 | 2 | < 1 | ||
Little stint | 15 | 4 | < 1 | ||
Ruff | 60 | 19 | < 1 | ||
Ethiopian snipe | 45 | 2 | < 1 | ||
Jacanidae | African jacana | 30 | 1 | < 1 | |
Burhinidae | Water thick-knee | 35 | 1 | < 1 | |
Charadriidae | Ringed plover | 20 | 1 | < 1 | |
Kittlitz’s plover | 75 | 8 | < 1 | ||
Three-banded plover | 70 | 4 | < 1 | ||
Laridae | Kelp gull | 100 | 8 | < 1 | |
Caspian tern | 40 | 1 | < 1 | ||
Common tern | 60 | 7 | < 1 | ||
Wiskered tern | 25 | 1 | < 1 | ||
Accipitridae | African marsh-harrier | 100 | 4 | < 1 | |
Osprey | 30 | < 1 | < 1 | ||
Podicipedidae | Black-necked grebe | 60 | 24 | < 1 | |
Anhingidae | African darter | 95 | 15 | < 1 | |
Phalacrocoracidae | Cape cormorant | 65 | 45 | 1 | |
Reed cormorant | 100 | 177 | 3 | ||
Ardeidae | Grey heron | 95 | 6 | < 1 | |
Black-headed heron | 70 | 1 | < 1 | ||
Purple heron | 100 | 9 | < 1 | ||
Yellow-billed egret | 40 | 1 | < 1 | ||
Cattle egret | 50 | 8 | < 1 | ||
Black-crowned nightheron | 5 | < 1 | < 1 | ||
Little bittern | 40 | 1 | < 1 | ||
Scopidae | Hamerkop | 10 | < 1 | < 1 | |
Threskiornithidae | African sacred ibis | 25 | 1 | < 1 | |
Hadeda ibis | 45 | 4 | < 1 | ||
Motacillidae | Cape wagtail | 95 | 5 | < 1 |
Note: Species names follow Hockey et al. (
Freq., frequent; Avg., average.
Most affected birds were found in areas of dense flooded plants, or on the shoreline, and were frequently grouped in one or more localities on a waterbody. Birds that were on the shoreline, or propped up by emergent plants, were generally hunched over and unable to stand. All affected individuals were either reluctant or unable to fly, and when attempting to do so were only able to propel themselves for very short distances across the water surface or mudflats with their wings. Most displayed paralysis of the neck and were unable to hold up their head. This was particularly prevalent in the longer-necked species such as ducks and cormorants, many of which, when located, were struggling to hold their head above water. When on land, none were able to stand, and when attempting to walk exhibited a floppy stumbling gate for very short distances. Most were unable to walk, and either sat hunched over or lay laterally recumbent with their legs and necks stretched out. In extreme cases, individuals seemed completely paralysed and unresponsive. Most had their eyes either partially or fully closed.
A total of 124 dead fish from eight species were observed in the Wilderness Lakes over the same period when the waterbird deaths occurred. All dead fish, with the exception of one Cape stumpnose (
The number of dead fish located in Rondevlei, Langvlei and Eilandvlei in the three different botulism outbreak periods encompassing January–June 2015, November 2015 – August 2016 and January–May 2017.
Orign | Species | Common name | Rondevlei |
Langvlei |
Eilandvlei |
|||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
2015 | 2016 | 2017 | Total | 2015 | 2016 | 2017 | Total | 2015 | 2016 | 2017 | Total | |||
Alien | Common carp | 0 | 0 | 0 | 0 | 32 | 16 | 27 | 75 | 5 | 9 | 0 | 14 | |
Mozambique tilapia | 0 | 0 | 0 | 0 | 7 | 1 | 0 | 8 | 0 | 0 | 0 | 0 | ||
Indigenous | Spotted grunter | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1 | 6 | 4 | 0 | 10 | |
White steenbras | 0 | 2 | 0 | 2 | 0 | 1 | 0 | 1 | 0 | 3 | 0 | 3 | ||
Flathead mullet | 0 | 1 | 1 | 2 | 1 | 0 | 2 | 3 | 0 | 0 | 0 | 0 | ||
Cape stumpnose | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | ||
Oval moonie | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | ||
Leervis | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 0 | 2 | ||
Note: Species names follow Whitfield (
Survey dates for the three locations were as follows: Rondevlei 2015: January–June 2015, Rondevlei 2016: November 2015 – August 2016 and Rondevlei 2017: January–May 2017; Langvlei 2015: February–June 2015, Langvlei 2016: November 2015–June 2016 and Langvlei 2017: January–May 2017; Eilandvlei 2015: February–May 2015, Eilandvlei 2016: January–June 2016 and Eilandvlei 2017: January–May 2017.
Laboratory assessments undertaken by state veterinarians on four yellow-billed duck indicated minimal to severe autolysis. All of the birds’ gastrointestinal tracts were empty. PCR tests were negative for both Newcastle disease and avian influenza. No significant bacteria isolates were cultured from body fluids. A histological assessment of lung, liver, intestine, pancreas and brain tissue did not indicate any diagnostic significant microscopic lesions. Oedema intermingled with large numbers of bacteria and non-cellular debris within the parabronchi, most probably from aspirated water, was indicative of drowning. Similarly, a post-mortem and histopathological examination of carcasses submitted to the ARC-Onderstepoort Veterinary Institute and Wildlife Pathology Research revealed no specific lesions that could be associated with mass waterbird deaths. Water and intestinal samples were negative on the mouse bioassay for
The single fish examined histologically, a white steenbras, only revealed epithelial hyperplasia of the gills, indicating subacute bronchitis, possibly because of altered environmental conditions, with death likely as a result of respiratory failure.
All of the waterbodies are interconnected; as a result, variability in their water levels is similar. Within the lake system, this variability is depicted by data from Rondevlei (
Water level relative to sea level in Rondevlei from 2002 to 2017. Arrows indicate times of breaching of the Touw Estuary mouth (black = artificial breaching, red = natural breaching). Shaded area indicates the period when botulism-related deaths of waterbirds were recorded in the Wilderness Lakes.
Seasonal variations in water temperature were similar in all waterbodies (
Time series plots of (a) mean water temperature, (b) salinity, (c) dissolved oxygen and (d) pH collected in January, April, July and October of each year from 2004 to 2017 in Rondevlei (blue), Langvlei (red) and Eilandvlei (green). Horizontal dotted lines indicate environmental limits conducive for botulism occurrence for temperature (> 15 °C), salinity (< 5) and pH (7.5 < pH < 9.0) (Rocke & Friend
Salinity in the Wilderness Lakes fluctuated temporally depending on the timing and volume of inflows of fresh river water, saline marine waters and brackish groundwater, the state of the estuary mouth, the movement of water between lakes, and the timing and rate of water loss through evaporation, evapotranspiration and groundwater movement. All of the lakes became progressively less saline until early 2008 after which an extended low rainfall period resulted in increases in salinity, which stabilised mostly within the range of 5–10 between 2010 and 2017 (
Dissolved oxygen in the lakes is naturally variable, with higher oxygen levels frequently occurring in the cooler winter months, and relatively lower dissolved oxygen levels occurring during the warmer summer months. This distinctive variability also occurred during the period of disease outbreak, and with both upper and lower dissolved oxygen levels not being atypical, or exceeding those recorded in the 11 years prior to the disease outbreak (
The pH within the lakes mostly fluctuated within the range of 7–9 (
Little to no algae were detected in water samples collected during the height of the first disease outbreak. The dominant diatom was
Temporal variation in the biomass of submerged aquatic plants was substantial in all three estuarine lakes (
The biomass of submerged plants determined annually in the period of maximum biomass (May–June) in (a) Rondevlei, (b) Langvlei and (c) Eilandvlei from 1991 to 2017.
A diagnosis of avian botulism is frequently based on a combination of symptoms displayed by sick birds, and the absence of specific post-mortal findings (Rocke & Bollinger
The number of observed dead and sick waterbirds is substantial, comprising approximately 20% of the average number of all susceptible waterbird species on the affected lakes. This is quite probably an underestimation, as assessments of total waterbird mortality based only on carcass retrieval can result in the underestimation of actual mortality by between three (Cliplef & Wobeser
One obvious question relevant to the management of the Wilderness Lakes is why has there been an outbreak of botulism here and now?
All birds, except vultures (Cohen et al.
Both similarities and differences occur in the species affected in this and earlier reported botulism outbreaks amongst waterbirds in South Africa. Whereas Van Heerden (
The transfer of toxins to waterbirds is thought to occur mainly through zooplankton or invertebrate food items that have assimilated toxins (Rocke & Friend
The long-term effect of repeated outbreaks of avian botulism on the abundance of susceptible waterbird species in the Wilderness Lakes is of concern. The previous regular high abundance of duck species, particularly yellow-billed duck and Cape shoveler, was the initial reason for the Wilderness Lakes being declared a Ramsar site. The effect of ongoing high disease-related mortalities may, in part, prevent these wetlands from continuing to regularly support globally significant populations of some waterbird species. The containment of the disease must be attempted by the regular collection and removal of dead birds and fish during the outbreak periods of November–June, and the disposal of carcasses off-site. Further testing of affected individuals should be undertaken and monitoring of environmental variables and affected individuals continued, to improve the understanding of the drivers and progression of the disease.
Serotyping
The authors wish to thank Paul Oberholster for identification of algae species, Emily Mitchell for assistance with the histopathology and members of the Garden Route National Park ranger team, notably Davie Antas, Nelson Tyhali, Petrus Mitchell, Donavan McKeith, Charles April and Jonathan Britton, for undertaking the unpopular task of carcass location and disposal. The authors also wish to thank two anonymous reviewers for their helpful comments on an earlier draft of the article.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
R.M.R. initiated and oversaw surveys of dead birds in 2015. He also arranged a visit by state veterinarians. I.A.R. participated in dead bird surveys from 2015 to 2017, undertook water quality and aquatic plant surveys and maintained databases on water heights and estuary breaching. He undertook the data analysis and wrote the article. D.G. arranged for botulism testing. D.Z. collected tissue samples and arranged for some histopathological examinations. R.M.R., D.G. and D.Z. proofread and contributed to the article.
The South African National Parks (SANParks) funded this research. The State Veterinarian Office of the Western Cape Government undertook infield inspections, post-mortem and histopathological examinations, and provided advice and diagnoses. The Wildlife Pathology Research of the National Zoological Gardens undertook post-mortem and histopathological examinations. The Agricultural Research Council-Onderstepoort Veterinary Institute undertook mouse bioassays, bacterial cultures and histopathological examinations. The Stellenbosch Office of the Council for Scientific and Industrial Research (CSIR) identified algae species.
Data are available from the first author upon request.
The views and opinions expressed in this article are those of the authors and not an official position of SANParks or any institute or organisation that undertook tests in support of this study.