The summer months, with rain, high temperatures and growing grass, is usually the time most associated with parasites. But what about winter?

Dr JG Nel, technical manager at Ascendis Animal Health, says it is true that high internal and external parasite loads are associated with the warmer months. However, parasite control during winter should not be neglected. Certain parasite species can cause serious problems and production losses during winter.

Problems in winter

In the more temperate regions where winters are fairly mild, parasites may be a year-round problem, says Nel.  

“Examples of internal parasites that occur in winter and can lead to production losses include liver fluke (Fasciola sp.), and conical fluke (Calicophoron microbothrium). Certain roundworm species, such as brown stomach worm (Teladorsagia circumcincta and Ostertagia ostertagi) in sheep and cattle, as well as bankrupt worm (Trichostrongylus sp.) in sheep can cause problems in winter, especially in the winter rainfall regions,” he explains.

Nel adds that the appearance of external parasites such as sheep scab (Psoroptes ovis), which is a state-controlled disease, as well as biting and sucking lice (Damalinia sp. and Linognathus sp.) can also increase during the winter months.

He explains that while tick numbers decline in winter, this is also the period during which the immature larva and nymph stages, which cannot be seen with the naked eye, are found on animals and can cause problems in the future. “Nasal bot fly larvae (Oestrus ovis) can also overwinter or survive in the sheep’s nasal cavities during the winter months, eventually causing problems in the spring and summer months.”

Poor grazing and immunity

The reason why some parasites tend to multiply during the winter months, is mainly due to a drop in the nutritional value of natural grazing in winter, and a decrease in animal’s natural immunity if the necessary nutrients are not supplemented. A high parasite load can also impede the utilisation of valuable nutrients during this stressful time.

Nel believes that winter in certain parts of the country is the ideal time for internal parasites, such as liver fluke, to gain a foothold. “As soon as the minimum temperatures in winter begin to drop below 10°C, freshwater slugs, which act as intermediaries, hide in the mud. The immature stages of the liver fluke parasite will then leave the slug to settle on grazing. This way, ruminants become infected, especially in areas with wet grazing.”

He warns that certain roundworm species, such as the brown stomach worm and the bankrupt worm, thrive in cold and wet winter rainfall regions. This time of the year is therefore marked by high parasite infestations.

“Winter in certain areas also creates favourable conditions for some external parasites. External parasites such as lice and mites can increase if the host environment, especially sheep, and environmental factors are favourable. Long wool and low temperatures create favourable conditions in which these parasites can multiply and survive. High numbers can then occur on the hosts, resulting in severe infestations,” he says.

General disease symptoms  

The disease symptoms of internal parasitic infestations depend on the species with which the animal is infested. Common disease symptoms are decreased appetite, diarrhoea, anaemia (e.g. pale eye mucous membranes), and low protein levels (e.g. bottle jaw).

The last two symptoms are especially noticeable in cases with high roundworm and liver fluke infestations. This can lead to mortalities in sheep.

The disease symptoms of external parasites also vary from species to species. The symptoms of lice and mite infestations are similar. Animals itch and will constantly scratch and bite themselves, which leads to wool being plucked out in the case of sheep. In some cases, lesions become noticeable on the skin and wool losses can occur, which may result in large production losses.

Scope of infestation

Nel says that visible symptoms are just the tip of the iceberg. “When a few of your animals exhibit the same disease symptoms, it is advisable to call in the local veterinarian who will use diagnostic methods to determine which parasites are present.

“Faecal egg flotation and faecal egg sedimentation tests can be performed to determine the internal parasites present. Faecal egg counts can also be done to determine the severity of the infestation. If animals are slaughtered or when a post mortem is done on a dead animal, the digestive tract (from the omasum to the colon) and liver can be examined to detect the presence of internal parasites.

“The skin between the hair or wool can be carefully examined for external parasites such as lice, as well as the larval and nymph stages of ticks. Lesions can also be used to perform a scratch test to determine if animals are infested with mites. Because symptoms are very similar, it is essential to accurately identify external parasites so that the appropriate treatment can be administered,” he says.

Effective treatment

Various treatments are available in the market that can be effectively used to treat internal and external parasites. However, if treatment is to be successful, it is important to accurately identify the parasite as well as the stage in its life cycle.

Nel explains: “Treatments containing triclabendazole can, for example, be used to treat the immature and adult stages of liver fluke. A treatment containing oxyclozanide or resorantel can be used to treat the immature as well as the adult stages of conical fluke.

“Treatments containing any of the macrocyclic lactones (such as ivermectin, doramectin or abamectin) will be effective against sucking lice and mites on sheep, but not against biting lice. The importance of an accurate diagnosis or identification of the specific parasite cannot be overemphasised as it will ensure that the proper treatment is administered. It is also important to know whether the animals should receive only one treatment or whether there should be follow-up treatments.”

Prevention is better than cure

According to Nel, good on-farm management can prevent winter parasite infestations to a large degree. To prevent liver fluke and conical fluke infestation, wet areas, such as marshes and rivers, should be avoided in the autumn months. This prevents contact with the infectious stages the water slug undergoes at this time of year.

He also advises producers to treat strategically in early winter, about two weeks after the minimum temperature has dropped below 10°C, in areas with a higher incidence of liver fluke to prevent infestation in winter. A triclabendazole-containing agent can be used for strategic treatment. Strategic treatment for conical fluke can be done in late winter.

“Strategic liver fluke treatment, with a treatment that is effective against adult stages, can be done in early spring to get rid of adult liver flukes that have survived the winter. This way, the infestation of grazing with liver fluke eggs can be limited and the life cycle interrupted to some degree,” he advises. – Koos du Pisanie, Stockfarm

For more information, phone Dr JG Nel on 012 346 2095.