Over the last number of years, interest has been growing steadily in the diseases caused by tick-borne agents in Ireland, writes Suzanne Naughton MVB, technical veterinary adviser, MSD Animal Health.
Ticks are rarely considered serious parasites in their own right. However, it is their ability to carry and transmit a number of disease-causing processes in livestock, as well as humans, which is where their greatest impact on health lies.
Typical Irish rural landscape and weather can lend itself perfectly to the development of active tick habitats. Rougher pastures with good vegetation cover, combined with the mild weather we experience in Ireland, can result in rapidly increasing tick populations, leading to a higher incidence of tick-borne diseases. Seasonal peaks of tick activity occur in spring and autumn. However, as our summers become increasingly wetter and winters milder, tick-borne diseases now commonly occur outside what was once considered the normal seasons.
The most common tick species identified in Ireland is Ixodes ricinus and is considered the most important vector for tick-borne diseases. The I ricinus species of tick involves a four-stage developmental life cycle, with the last three stages involving attachment to a host. This particular species of tick actively seeks host animals by positioning itself on the tips of grass with its front legs outstretched, enabling it to attach to passing animals in a behaviour known as questing. Once a tick is attached, feeding occurs until the tick is completely engorged and drops off the animal.
Besides transmitting important disease agents, ticks can also be an important contributing factor in lack of thrift and hide damage but, most importantly, in reducing the animal’s ability to fight infection through immunosuppression. This, in turn, increases the animal’s susceptibility to secondary infections such as pneumonic pasteurellosis, listeriosis and other bacterial or viral diseases. Disease transmission, as well as attachment to the host, is facilitated by the tick’s saliva. A secretion in the saliva mixes with the host’s skin to form cement, which facilitates a strong attachment rendering the tick difficult to remove. Ticks can remain and feed on a host for up to 13 days in some cases, allowing ample opportunity for disease transmission to occur. Once a tick is attached, there can be a delay of up to 36 hours before disease transmission takes place, meaning prompt removal of ticks can help prevent the transmission of diseases occurring.
What are the most common tick-borne diseases?
Tick-borne fever is a condition caused by the bacterium, Anaplasma phagocytophilum, and is the most common tickborne disease process identified in animals (particularly in cattle). Clinical signs include fever, depression, anorexia, respiratory distress and coughing, as well as abortion in susceptible animals. A sudden and significant drop in milk yield is often seen in the dairy cow. Young or newly introduced stock are often most at risk as a certain level of immunity will develop in the older, home-bred stock. A profound immunosuppression as a result of this infection can increase susceptibility to other secondary disease processes and, particularly in sheep, tick pyaemia. Tick pyaemia, another tick-borne disease, occurs when the tick bite introduces the bacterium Staphylococcus aureus, normally present on the skin surface, resulting in a debilitating disease leading to abscess formation in muscles and joints and, eventually, death. It typically affects lambs from 2-12 weeks of age and treatment involves systemic antibiotics to fight the S aureus infection.
Redwater fever is caused by the parasitic agent Babesia divergins, which infects the host’s red blood cells causing fever, anorexia and anaemia. Acute cases are often characterised by pipestem diarrhea, eventually leading to constipation and portwine coloured urine (haemoglobinuria). The drug imidocarb diproprionate is licensed for the treatment and prevention of redwater fever and can be administered by a veterinary practitioner. The incidence of this disease has reduced dramatically over the last number of years, potentially as a result of improvements in the use of land that would once have held high densities of tick populations.
Louping ill is a viral agent carried by ticks, which mostly affects sheep. The initial stages of infection with louping ill can often go unnoticed. However, in the later stages it can cause severe depression and incoordination, often resulting in paralysis and death. Sheep in endemic areas will often develop immunity. However, young bought-in stock are most at risk of infection and development of clinical signs. There is no treatment for louping ill, although a vaccine is available under special licence. Managing tick populations is the preferred control method.
Q fever is caused by the bacterium Coxiella burnetti and can be a potential cause of late-stage abortion and infertility in sheep. It is also known to cause immunosuppression which can reduce the effectiveness of vaccines administered during the course of this disease. It can cause fever, milk drop syndrome and anorexia as well as subclinical mastitis in dairy cows. It is a known zoonotic agent (ie. transmissible to humans) where typical flu-like symptoms develop such as fever, chills, fatigue and muscle pain.
In order to restrict the occurrence of tick-borne diseases, it is essential to control tick populations in the environment. Reducing the ability of ticks to survive on pasture by removing overlying vegetation, improving drainage and an avoidance of grazing stock on deep matted pasture can all help positively contribute to lowering tick population numbers.
One particular product licensed for the treatment and control of ticks in sheep and cattle is Taktic 12.5% which contains the active ingredient, amitraz.
Amitraz is particularly effective in tick control as it can kill all stages of the tick life cycle. Used as a spray in cattle and as a dip treatment in sheep, Taktic 12.5% can be used as part of a regular treatment programme. Animals should be completely immersed in/sprayed with the diluted solution and all animals in the group must be treated. During periods of greatest risk, cattle should be treated every 9-10 days while for sheep, a single dip treatment will kill ticks and provide protection against reinfestation for up to six weeks.
In conclusion, both climatic and environmental factors in Ireland can greatly influence tick populations, thereby resulting in a significant prevalence of tick-borne diseases in ruminants. Through a greater awareness of the clinical signs associated with some of these diseases, together with employing the control methods as discussed above, the cost of associated production losses and treatment can be significantly reduced.
Zintl A et al. Ticks and Tick borne diseases in Ireland. Irish Veterinary Journal 2017; 70: 4.