Vector borne diseases: when to test and how to read the result

Every vector has a temperature threshold below which it is inactive or unable to support the parasite it carries. Sandflies, the tiny silent insects that transmit leishmaniasis, fly on evenings when the temperature stays above roughly eighteen to twenty degrees. Ticks of the Ixodes genus start questing only a few degrees above freezing, so a mild winter offers no real pause. In mosquitoes the heartworm larva develops only where mean temperatures stay above about fourteen degrees for long enough.

When seasonal averages shift, these windows shift with them. The practical result is twofold: the risk season starts earlier and ends later, and locally acquired cases appear in areas previously regarded as free. For an owner this means two concrete things. First, the protection schedule should be recalculated for the area you live in now, not the one you lived in when the animal arrived. Second, the question for your vet is no longer only which product to use, but which agents are present in your area and in the places you visit during the year.

Canine leishmaniasis is caused by a protozoan transmitted by the bite of a sandfly. The interval between infection and the first clinical signs varies enormously, from a few months to several years, and a proportion of infected animals stay free of signs for a long time while still being able to infect vectors. This is why testing is not triggered by symptoms but repeated periodically in endemic areas.

The least understood point is how the result should be read. Quantitative serology does not return a simple positive or negative but a titre, that is a number. A low titre in a dog without signs indicates contact with the parasite and calls for a repeat test after some months, not necessarily for treatment. A high titre in a dog with compatible signs, such as weight loss, skin lesions, overgrown nails, enlarged lymph nodes or renal changes, carries strong diagnostic weight. Between the two extremes lies a grey zone where further tests are needed, and where comparison with earlier values matters more than the single number.

• When to test in an endemic area: once a year, preferably at the end of the sandfly season, when a summer contact has had time to produce antibodies.
• What to ask for: the numeric titre and the laboratory reference range, because different scales are not comparable with each other.
• What to keep: every earlier report, because the trend over time distinguishes stable contact from progressing infection.
• What not to conclude: a single negative test in a dog exposed only weeks earlier does not rule out infection, because the antibody response takes time.

Heartworm is a worm transmitted by mosquitoes that settles in the pulmonary arteries and the right side of the heart. The test most often used in dogs looks for an antigen produced by adult females, so it cannot turn positive until the parasites have matured. That takes roughly six to seven months from the infective bite. This single rule explains two behaviours that otherwise look illogical.

The first is that testing is done in spring, before starting the new season of preventive treatment, and not at the end of summer: in spring you are looking for infections acquired the previous year, which have now become detectable. The second is that a dog recently arrived from a risk area should be tested and then tested again after the necessary interval, because the first test may fall inside the blind window. Starting monthly prevention in a dog that has never been tested and may be infected is not a neutral choice and should be discussed with the vet beforehand.

Not every tick transmits something, and those that do differ in speed. For several agents transmission requires the parasite to stay attached for a long time, often beyond twenty four or forty eight hours, which makes a daily coat check a genuine preventive measure rather than a ritual. For other agents, though, the documented interval is far shorter, on the order of a few hours, and there speed of removal matters less than preventive protection.

After removing a tick you do not run tests the same day: they would be negative anyway. What is needed is a note of the date, the bite site and, if possible, the appearance of the parasite, then observation over the following weeks. Fever, lameness that moves from limb to limb, sudden dullness, nosebleeds, very dark urine or pinpoint bruising on the gums are the signs that make a targeted test worthwhile, and they should be reported together with the date of the bite.

1. Remove the tick with fine tipped forceps or a purpose made hook, gripping close to the skin with steady traction, without crushing it and without applying any substance.
2. Record the date, the place you were staying and the bite site in the animal's health record.
3. Disinfect the area and watch the spot for about ten days: mild local redness is common, an ulcer that keeps widening is not.
4. Observe body temperature, appetite, energy and gait for at least three weeks, and report any anomaly alongside the date of the bite.
5. If compatible signs appear, ask for a test targeted at the agents present where the contact happened, not a generic panel chosen at random.

These infections exist in cats but behave differently, and tests designed for dogs are less reliable. With heartworm the cat is a host in which the parasite rarely completes development: adults are few, often of a single sex, and antigen detection can be negative even in the presence of disease. The clinical picture resembles asthma, with coughing and laboured breathing, which is why suspicion arrives late. Diagnosis combines several tests with assessment of chest imaging.

For leishmaniasis the cat is considered less susceptible than the dog, but documented cases are increasing in endemic areas, especially in animals whose immune defences are compromised by other viral infections. A cat that goes outdoors in an endemic area, with chronic skin lesions that do not respond to the usual treatments, deserves to have this hypothesis considered rather than dismissed by habit.

A testing plan is worth having when it is repeatable and builds a time series. Three elements are enough: which agents are plausible where the animal lives and where it visits, in which month each test belongs, and where the reports end up. The third is the one most often skipped, and without it the fourth annual check is worth no more than the first, because there is nothing to compare it with.

1. 1

   Define the real exposure

   List the places where the animal spent more than a few days in the past year, including holidays and boarding stays. It is this list, not residence alone, that determines which agents to look for.

2. 2

   Give each test its month

   Heartworm testing before the mosquito season starts, leishmaniasis testing at the end of the sandfly season. Spacing them out avoids repeated pointless sampling and places each test where it is informative.

3. 3

   Record the result with its number

   Negative or positive is not enough: you need the value, the unit and the laboratory. A titre compared with last year's says far more than a titre on its own.

4. 4

   Link events to signs

   Date of a tick bite, stays away from home, onset of lameness or fever: recorded on the same timeline they become a trail that can guide a diagnosis months later.

5. 5

   Review the plan every year

   Moving house, a change of routine or new cases in the area alter the risk profile. An annual review with your vet updates both protection and testing.