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This article was published in 1978
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Comments on the Biology of Feral Pigs in the Braidwood Pastures Protection District
K.G. Hart, B.V.Sc., Veterinary Inspector, Braidwood
Braidwood P.P. District is a mountainous region containing a coastal plain in the east, and then to the west a steep, coastal range, with two further ranges dividing the tablelands between the coastal range and the A.C.T.
Distribution
Feral pigs extend their range on the coast as far north as Milton, and to the southern border of the P.P. Board at Moruya (the coastal range probably extends south to the Victorian border and beyond. Feral pigs are numerous in the Eden P.P. Board). Pigs or pig signs were encountered throughout the Board in 1977, with the main concentrations on the coast and areas adjoining the coastal ranges. Although they are by no means averse to foraging on open country, pigs will harbour mainly in forest areas, particularly where there is an ample water supply such as in swamps and river valleys.
Abundance
Population densities vary greatly and are hard to estimate. Pigs are sedentary in some areas and regular sightings can be made, while in other areas pigs may only pass through perhaps once every 12 months.
Observations through shooting and spotting have been made on a sedentary population ranging over roughly a 5 square km, area, and they indicate a population of between 50 and 100 pigs, giving a density of 10-20 pigs/squ.km. However, this group lives in an ideal area, and most feral pig populations so far observed would be considerably smaller than this.
Feral sows reach puberty at about 8 months of age, providing they are in reasonable condition and have reached about 30 kg. body weight. A mean litter size (4) - 6 has been quoted for sows over 2 years. Observations (by) me (so) far at Braidwood agree roughly with this figure - several litters of 8 have been reported, with the most common siting being 5-6. Sows under favourable environmental conditions will breed twice a year, and with an estimated weaning rate of 4-5 piglets/pregnancy the potential of the feral pig population increase appears to be enormous, Wild dogs, foxes and perhaps eagles may take young pigs, and the only major controls affecting pig numbers at present are environmental ones.
Habits
Pigs in this district have nocturnal feeding habits and tend to lie up in cover during the day. Limited studies of stomach contents in coastal pigs have found plant material only. Pigs eat pastures, Burrawang Palm roots (a great delicacy) bracken fern roots and many other types of grasses and roots. Pigs also cause considerable damage to crops, particularly corn grown on the coast. Pigs in this area apparently rarely eat carrion, although fish heads are considered excellent bait for pig traps.
Pigs need a regular water supply and during dry periods their ranges tend to contract to areas with reliable water, often thickly forested headwaters of creeks or swamps. Pigs are harder to trap and observe during dry spells.
The pigs living in the National Park at South Durras harbour in coastal swamps frequently raid rubbish tips and backyards of the townships in the area. All along the south coast, rubbish tips are visited by feral pigs and regular tracks with fresh pig spoor attest to the frequency of these visits. The importance of this as an exotic disease risk should not be underestimated. Large numbers of visitors from Sydney, Wollongong, Canberra and elsewhere flock to the coast in the summer, and their food scraps in many cases end up on these rubbish tips and are much appreciated by the local feral pigs.
Origins
Feral pig populations in the Braidwood district are usually established by escapes or deliberate releases from local piggeries. Their heterogeneous origins are reflected in the colouring, which ranges from the true feral all-black pig, through black pigs with white or red spots, to pure white and pure red types.
Contact with domestic pigs apparently often occurs in some areas and pig owners tell lurid stories of nocturnal visits by feral boars to their sows.
Diseases
As would be expected, the diseases and parasites from the domestic situation have been carried on into the wild populations. To date only 9 pigs from 2 coastal populations have been post-mortemed, but in spite of the small sample size, the findings are very interesting.
No evidence of T.B. was found in any pig, although a deliberate search was made for the disease.
Serology
All pigs tested were negative for Brucellosis and Q Fever. Leptospira pomona has been mentioned by Keast et al. (1963) as a common serological finding in feral pigs from inland areas, but no reactions were obtained to L. pomona or any other of 8 Leptospiral serotypes in this study.
All pigs examined showed some serological reaction to Erysipelas, usually low and non-significant titres. However, one pig showed a positive titre. The animal was obviously sick and in poor condition, but no gross lesions of Erysipelas were seen on post-mortem.
Parasites
All pigs examined had lice (Haematopinus suis), but a search was not made for other ectoparasites. Liver fluke were found in light infestations in one population living close to cattle, but the other population living in a National Park where no domestic stock are permitted had no gross evidence of liver fluke. Cysts resembling hydatid cysts were found in the livers of three pigs. with the exception of kidney worms (Stephanurus dentatus) and whip worms (Trichuris), all the common nematode parasites of pigs were present in one or both pig populations. These were:
a. Physocephalus sexalatus (stomach worm) - found in light to moderato infestations in most pigs examined.
b. Metastrongylus (lungworms) - common in light to moderate infestations.
c. Macracanthorhynchus hirudinaceus (Thorny-headed worm) - found in one population only, (the more sedentary group in the National Park at Durras). The pigs with a positive Erysipelas titre had also a heavy burden of these worms, with extensive damage to the intestinal mucosa.
d. Ascaris suum - found in light to heavy infestations in both populations.
Sparganosis
This intermediate stage of the tapeworm Spirometra erinacei is apparently very common in feral pigs in New South Wales, and the coastal pigs examined in the Braidwood district were no exception.
Spargana were found in 6 out of the 9 pigs examined, but their absence in 3 of the pigs may have been due to the fact that an exhaustive search for these parasites is extremely time consuming. The sites where spargana were found were under the pleura of the diaphragm, the pericardium, the peritoneum and the fascia over the quadriceps muscles and the subscapularis but they may be found in connective tissue between muscles in almost every region of the body. Spargana resemble nerve tissue closely, but can be readily removed with forceps where nerve tissue remains firmly attached.
(Incidentally, spargana are probably capable of migratory activity when ingested by humans. They cannot withstand temperature of above 60 degrees C, or freezing, so pork should be safe if well cooked or frozen at some stage).
Control
When he was with the Department, Jack Giles put out a lot of excellent material on Feral Pig Control, and I propose to comment only on direct experience in problems encountered in the Braidwood district.
Methods:
1. Shooting/dogging: Found to be largely ineffective, accounting for only small numbers of pigs. Pigs often have to be hunted on foot, particularly on the coast where thick forest cover precludes vehicular access. Shooting will swatter pig populations and render other more effective control methods (trapping and poisoning) very difficult.
2. Trapping: Experience with weldmesh traps with swing gates in the Braidwood coastal areas has shown little success to date, and trapping cannot be considered as a suitable method for pig control although it is useful for catching small numbers of pigs for research purposes.
The trap has to be established on a regular pig train or foraging/drinking area. A reliable operator is required to service the trap, and such people are not always easy to find on the coast where there is a lot of complacency about pigs. The animals quickly become trap shy, particularly where they have little contact with humans or their equipment, and we have found that trapping only works on sedentary pig populations that are used to nosing about rubbish tips, backyards, etc. Traps anywhere near a public place are also subject to vandalism on the coast, and pigs have been taken out of traps on several occasions.
3. Poisoning: The one control method that has been used with success is 1080 poisoning. Because of its unique physical characteristics and its species specific effects, I consider 1080 to be an ideal poison for feral pigs.
Rabbit pellets are normally used for bait, although grain has been successful where pigs are raiding corn crops.
Any poisoning campaign hinges on the establishment of a feeding station, usually based on a favourite feeding or watering area or on a well-used trail. A minimum of 2 free feeds is allowed, with 2 lbs. of pellets per pig (based on pig tracks, sitings, the extent of rooting activity, etc.). Pigs should be cleaning up most of the free feed before the poisoned bait is laid. Poisoned pellets are placed in the evening, and this timing, plus the nature of the bait material and the competition from pigs on a bait station make it unlikely that non-target species would take the bait.
A careful search around the bait station is always made the morning after poisoning, and any excess bait is collected and disposed of. Pigs that have received a bait often vomit, but apparently these animals have already had a lethal dose and die anyway. The Rabbit Inspector is of the opinion that pigs receiving a sub-lethal dose do not vomit. Although 1080 may have a considerable lag period in its biochemical action, dead pigs are often found on or near bait stations, but no dead wildlife have ever been picked up.
1080 Poisoning is definitely the method of choice for wild pig control in the Braidwood area, and in my opinion is the only method capable of making any major impression on the numbers of pigs living in heavily wooded mountain and river areas. At present, poisoning activity is confined to properties with co-operative owners, while large areas of State forests and National Parks provide areas of safe harbour for these pests. However, the controlling authorities of these areas appreciate the menace of their feral pig populations, and efforts are being made to co-ordinate pig control with the Pastures Protection Board. The use of 1080 poison, the major weapon in pig control; should be vigorously defended against the outcries of ill-informed outsiders.
Acknowledgements
My thanks are due to the Braidwood Pastures Protection Board Rabbit Inspector, Mr. J.N. Webb, whose vast knowledge on the ecology of feral pigs and invaluable help in setting up traps and collecting data made this talk possible.
References
BEARUP, A.J. (1953) Life history of a Spirometrid tapeworm, causing Sparganosis in Feral Pigs. Aust. Vet. J. 29:217--224
GORDON, H. McL. et al. (1954) Sparganosis in Pigs in N.S.W. Aust. Vet. J. 30:135-238
HENRY, V.G. (1968) Fetal development in European Wild Hogs. J. Wildl. Mgmt. 32:9660970
KEAST, J.C. et al. (1963) The role of the Feral Pig as a Disease Reservoir. Aust. Vet. J. 39:99
MATSCHKE, G.H. (1967) Aging European Wild Hogs by Dentition. J. Wildl. Mgmt. 31:109- 123
PULIAR, E.M. (1950) The Wild (feral) Pigs of Australia and Their role in the spread of infectious diseases. Aust. Vet. J. 26:99-110