Farmers in many parts of the country have reported an increase in the incidence of tickborne diseases in young lambs in recent years.
It was therefore fitting that the recent Teagasc hill sheep conference included an in-depth presentation on ticks and tickborne diseases, delivered by Annetta Zintl from the School of Veterinary Medicine, University College Dublin.
Annetta says the greatest threat of tick infestations is in areas with dense vegetation or thick leaf cover.
In Ireland, ticks are frequently found in open hill and rough grazing land
As such, Ireland provides a much more suitable environment for ticks compared to other EU countries, where ticks are confined to wooded areas.
In Ireland, ticks are frequently found in open hill and rough grazing land, especially along the western coast and the Shannon catchment area.
Setting the scene, Annetta said that the tick, Ixodes ricinus, which is also known as the sheep tick, parasitises a broad range of hosts including birds, mammals and even reptiles, with deer generally being the preferred host. She said that sheep are unusual in that they can develop resistance after an infestation.
Ticks that feed and engorge themselves on resistant hosts detach prematurely and usually die, while adult females that survive may produce less offspring.
Ticks feed exclusively on blood and unusually take just three blood meals during their lifetime – once as larvae, once as nymphs and once as adults.
Ticks become active and set out to find a new host once air temperatures rise above 7°C to 10°C
Engorgement or feeding takes between two and eight days for larvae, four and 10 days for nymphs and up to 13 days for adult females – adult males apparently do not engorge.
Between feeds, ticks shelter from adverse conditions in vegetation and leaf litter.
Ticks become active and set out to find a new host once air temperatures rise above 7°C to 10°C. This was evident last spring, when mild weather led to a significant increase in tickborne illnesses in young lambs at a much earlier stage of the year.
Annetta says that while tick bites may cause local irritation and heavy infestations can cause anaemia in the host, ticks on their own typically do not cause serious disease in adult livestock. Significant issues can occur, however, in young lambs aged from two to 12 weeks of age where tick pyaemia is present.
This ailment is caused by a bacterium, staphylococcus aureus, which occurs naturally on the skin and nasal mucosa of the animal.
Tick pyaemia will also lead to immunosuppression
This bacterium gains access through the bite wound and gives rise to abscesses in various parts of the body, in turn causing severe lameness, paralysis of the backend, ill thrift and significant mortality. In some cases, up to 30% of lambs in a group can be affected.
Tick pyaemia will also lead to immunosuppression and an increased risk of other diseases that may be present at a low level becoming established.
Annetta says that in order for a disease to transmit, it must be capable of persisting in the tick for the long periods between feeds. Four tickborne pathogens listed as being important in an Irish context were as follows:
Tickborne fever is thought to be the most common tick-transmitted pathogen in livestock in Ireland. The first symptom of an infection is a high fever in naïve livestock that have been moved to a tick-infested pasture.
Annetta says that following an infectious bite, fever typically develops within a week of exposure and lasts for two weeks. Adults typically make an uneventful recovery and are resistant thereafter.
The exception to this is naïve pregnant animals introduced into endemic areas. In these cases there is a high risk of resultant abortion. In dairy cows, the most likely indicator is a sudden and sharp drop in milk yield.
It can lead to significant disease prevalence and mortality in young lambs, with this often caused by the fact that young lambs cannot keep up with ewes. The disease also causes immunosuppression, making animals more susceptible to other diseases such as respiratory illnesses or other tickborne diseases including tick pyaemia and louping ill.
Annetta says that louping ill, which is an acute viral disease, also infects other domestic animals including cattle, pigs, goats, horses and dogs. Following an infection, the virus multiplies in the lymph nodes and other tissues before being released into the bloodstream. Infected animals show unspecific symptoms such as fever, depression and lack of appetite.
A high percentage of animals will make an uneventful recovery and are subsequently immune to infection. Some, however, can develop a severe disease resulting from the virus invading the brain and spinal cord. This is characterised by muscle tremors, lack of coordination, animals circling, ataxia, paralysis and death.
While bovine babesiosis, commonly known as redwater fever, does not affect sheep, Annetta said it is important to highlight the disease due to the high numbers of mixed livestock enterprises. Infections in cattle are characterised by sudden onset of fever, diarrhoea, anorexia and an insatiable thirst.
The infection leads to destruction of red blood cells, causing severe anaemia and giving rise to the characteristic red colouration of urine.
Untreated animals become comatose and die. The greatest risk, again, is in naïve animals entering endemic areas.
Treatment and control options
Short-acting antibiotics are effective against tickborne fever, but Annetta says there are no treatment options for sheep affected by tick pyaemia or louping ill.
Therefore, the best strategies for managing tick-transmitted diseases in high-risk areas are tick control measures and stable flock management.
Annetta advises farmers with issues to draw up a control plan with their vet
Annetta says plunge dipping provides an immediate and effective treatment route, with ticks killed immediately. Pour-on treatments are easier to administer, but one downside is that they do not kill ticks immediately.
Annetta advises farmers with issues to draw up a control plan with their vet. She says physical control options are also possible, with ticks maintaining a highly localised distribution presence.
This means that tick hot spots can be fenced off, or another option is to graze the least susceptible animals on higher-risk areas or tick-prone pastures.
Replacement animals should be bred on the farm if possible, or sourced locally, and where naïve animals are purchased, high levels of supervision are advised until animals settle into their new environment.